INTRODUCTION OF GEOPHYSICS

Transcription

1 Suez Canal University Faculty of Science INTRODUCTION OF GEOPHYSICS Prepared By Dr. El-Arabi H. Shendi Professor of Applied & Environmental Geophysics 2007

2 Definitions: Geophysics is the application of physics to study of the solid earth. It occupies an important position in earth sciences. It has been taught principally in Earth sciences departments of university. There is an obvious need for it to introduce to engineers and archeologists much more widely than at present. Geophysics developed from the disciplines of physics and geology and has no sharp boundaries that distinguish it from either. The use of physics to study the interior of the Earth, from land surface to the inner core is known as solid earth Geophysics Solid Earth Geophysics can be subdivided into Global Geophysics or pure Geophysics and Applied Geophysics. Global Geophysics is the study of the whole or substantial parts of the planet. Geophysical methods may be applied to a wide range of investigations from ٢

3 studies of the entire earth to exploration of a localized region of the upper crust, such as plate tectonics, heat flow and paleomagnetism. Applied Geophysics is the study of the Earth's crust and near surface to achieve an economic aim, or making and interpreting measurements of physical properties of the earth to determine subsurface conditions usually with an economic objectives ( e.g. discovery of fuel or mineral deposities). Applied Geophysics Comprises the following subjects: 1- Determination of the thickness of the crust (which is important in hydrocarbon exploration. 2- Study of shallow structures for engineering site investigations. 3- Exploration for ground water and for minerals and other economic resources. 4- Trying to locate narrow mine shafts or other forms of buried cavities. 5- The mapping of archaeological remains. ٣

4 6- Locating buried piper and cables Engineering Geophysics: the application of geophysical methods to the investigation of nearsurface physico-chemical phenomena which are likely to have (significant) for the management of the local environment. The discipline of environmental geophysics needs to bring to the attention of policy- makers and planners. The principal distinction between engineering and environmental geophysics is that the former is concerned with structures and types of materials, whereas the latter can also include mapping variations in pore fluid conductivities to indicate pollution plumes within ground water, for examples: - Geophysics can be used to investigate contaminated land to locate polluted areas prior to direct observations using trail pits and boreholes. Large areas can be surveyed quickly at relatively low cost. ٤

5 - The alternative and more usual approach is to use a statistical sampling techniques, the geophysical survey is used to locate anomalous areas and there will be a higher certainly that the constructed trail pits and boreholes will yields useful results. - Geophysics is also being used much more extensively over landfills and other waste repositories. - Geophysics can be used to locate a corroded steel drum containing toxic chemicals. To probe for it poses the real risk of puncturing it and creating a much more significant pollution incident. - By using modern geomagnetic surveying methods, the drum's position can be isolated and a careful excavation investigated to remove the offending (hurt) object without damage. Such approach is cost effective and environmentally safer. - Geophysics investing of the interior of the earth involve taking measurements at or near the ٥

6 earth's surface that are influenced by the internal distribution of physical properties. - Analysis of these measurements can reveal how the physical properties of the earth's interior vary vertically and laterally. - Exploration geophysics developed from the methods used in global geophysics Example: - Exploration seismology used mainly in oil exploration, have been used in academic studies relating to the structure of the earth's crust and upper mantle. - Geophysics measurement within geographically restricted areas are used to determine the distributions of physical properties at depth that reflect the local subsurface geology. - An alternative method of geophysical investing subsurface geology is, of course, by drilling borehole, but these are expensive and provide information only at discrete locations. ٦

7 - Geophysical surveying provides a relatively rapid and effective means of deriving distributed information on subsurface geology. Solid Earth Geophysics Global or pure Geophysics Applied Geophysics Hydro-Geophysics Mining Geophysics Engineering Exploration Environmental Glacio-geophysics ( Geophysics in ( geophysics for Geophysics Geophysics Geophysics (geophysics in Water investigation) mineral glaciology) Exploration) Archaeo- Geophysics (in archaeology)

8 History of Geophysics The beginning of geophysics has been started since: a- Gilbert's discovery which stated that the earth behaves as a great and irregular magnet. b- Newton 's theory of gravitation. * The initial step in the application of geophysics to the search for minerals was taken in 1843 by von warde which used the magnetic theodolite of Lamont to discover magnetic ore bodies. * in 1879 a book by Robert Thalen was published entitled" on the examination of iron ore deposits by magnetic methods". * At that time, the first magnetometer called Thalen- Tiberg magnetometer was manufactured in Sweden. * During the past seventy years, geophysics was used greatly in oil and gas exploration and many geophysical techniques have been developed for the

9 detection and mapping of unseen deposits and structures. * Advances have been rapid during the past decade because of the development of new electronic devices for field equipment and the widespread applications of the digital computer in the interpretation of geophysical data. * Several of the devices now used by geophysicists were developed from methods used for locating guns, submarines and aircraft during the two world wars Examples: 1- Guns were located in France during the First World War by measuring the arrival times of the elastic waves generated in the earth by the recoil of the guns. This lead to the refraction methd of seismic prospecting 2- Submarines were located by transmitting sound pulses underwater and measuring the interval ٩

10 between the emission and return of the pulses; knowing the velocity of sound in water, the distance to the reflecting object could be determined. 3- Radar, which was developed during the Second World War, utilizes radio pulses in similar manner. A modified from of radar has been widely used for navigation purposes in marine and airborne geophysical surveys. 4- Ships, submarines and mines were also detected in both wars by their magnetic properties. ١٠

11 Relation between Geology and Geophysics: * GEOLOGY It involves the study of the earth by direct observations on rocks either from surface exposures or from boreholes and the deduction of its structures, composition and historical evolution by analysis of such observations. GEOPHYSICS It involves the study of the inaccessible earth by means of physical measurements, usually on or above the ground surface. It also includes interpretation of the measurements in terms of subsurface structures and phenomena. * Geophysical studies are quantitative and tangible, whereas geological studies are qualitative and descriptive. Example(1) * In exploration Geophysics for oil, the petroleum geologists extract quantitative information from ١١

12 Geophysical data (such as seismic records, well logs, ). * On the other hand, Geophysicists who are concerned with measurements of physical phenomena are incorporating more geology in order to increase the reliability of the conclusions. Example(2) * The information gained about the sea floor spreading and plate tectonics is due to integrating geophysical and geological information. * Every earth scientist, especially the geologist, should be familiar with the methods of geophysics. This familiarity should enable one to know: a which of the geophysical methods can (or cannot) be of help in a given geological situation. b- The limitations of the geophysical methods. ١٢

13 * The incorporation of the available geophysical in- formation in interpretation of geophysical measurements is very important PHYSICAL PROPERTIES OF ROCKS * The physical properties of rocks that are most commonly utilized in geophysical investigations are: - Density - Magnetic susceptibility - Elasticity - Electrical resistively or conductivity - Radioactivity - Thermal conductivity * These properties have been used to devise geophysical methods, which are: - Gravity method - Magnetic method - Seismic method - Electrical and electromagnetic methods - Radiometric method - Geothermal method ١٣

14 1- Rock Densities: * Any Geologic condition that result in a horizontal variation in density will cause a horizontal variation in gravity or a gravity anomaly. It is therefore the significant parameter in gravity Exploration (i. e. the anomaly source is a local variation in density). * Two problems are faced in connection with this parameter (i. e. density). 1- The maximum density variation between different rocks and between rocks and minerals is approximately (2). This is a very small change compared to the range of magnetic susceptibility ( 10 5) and electrical conductivity ( ). Examples: Clay 1.7 Gravels 1.95 Sand 1.6 sandstone 2.24 limestone 2.11 Dolomite 2.3 ١٤

15 2- It is not possible to measure density in situ. A density borehole logger has been used to a limited extent in oil exploration * It is necessary to make density measurements in the laboratory on small samples of outcrops or drill cores. In this case, the laboratory results do not necessarily give the true bulk density of the formation, since the samples may be weathered, fragmented or dehydrated. * Sedimentary rocks have lower densities than igneous and metamorphic rocks. Their densities depend on: composition, porosity and pore fluids, their age and depth below surface (i.e. the density increases with depth and time because the rock becomes compacted and consolidated). For that, the laboratory density measurements should be made, if possible, with the sample in the same ١٥

16 conditions as those prevailing in the formation from which it was removed. * Igneous rocks are denser than sedimentary rocks. * Basic igneous rocks have larger densities than acidic forms. * Porosity is of minor significance in igneous and metamorphic rocks, unless they are highly fractured. Examples: Rock Density Rock Density Granite 2.64 Basalt 2.99 Gabbro 3.03 Acidic igneous rocks 2.61 Basic igneous rocks 2.79 * Density of metamorphic rocks increases with the degree of metamorphism since this process tend to fill pore spaces and recrystallize the rock in a denser form. Rock Density Metamorphic Density form Limestone Shale Sandstone Granite Marble Slate Quartzite Gneiss ١٦

17 * The density of metamorphic rock increases as the acidity decreases. * Non metallic minerals are of lower density than the average of rocks. *Metallic minerals are heavier than this average. Copper 8.7 silver 10.5 Galena 7.5 Laboratory estimation of density : * Density can be determined by direct measurements on rock samples as follows: -The sample is weighted in air and in water. The difference in weights gives the volume of the sample. - Dry density = weight / volume * The density of a rock is quite variable. For that, it is necessary to measure several tens of samples of each rock in order to obtain a reliable mean density. ١٧

18 2- Magnetic susceptibility of rock and minerals (K) * When a magnetizable body is subjected to an external magnetizing field (H), it acquires a magnetization that is lost when the applied field (H) is removed. * Such a magnetization ( j i ) is said to be induced by the applied magnetizing field (H). * ( J i ) is parallel and proportional to the applied field (H) J i = KH * (K) is called the magnetic susceptibility. * A substance is called Diamagnetic if the (K) is negative; it is called Paramagnetic and Ferromagnetic if (K) is positive. Magnetic susceptibility is the significant variable in magnetic, playing the same role as density in gravity. Sedimentary rocks have the lowest average susceptibility and basic igneous rocks have the highest. ١٨

19 Examples: Rock K Rock K Limestone 10x 10 6 shale 50x10 6 Granite 200x10 6 Quartzite 350x10 6 The susceptibility depends upon the amount of ferromagnetic minerals present (i.e. magnetite, ilmenite or pyrrhotite). Laboratory determination of ( k) : The simplest method involves a comparison of the sample with a standard by using a laboratory instrument. It is to compare the deflection produced on a tangent magnetometer by a prepared sample (a drill core or powered rock, K s ) with that of a standard sample of magnetic (K std, FeCI 3 powder in a test tube). "K" is then given by the following equation: K s = K std (d s /d std ) Where d s is the deflection of the sample, d std is the deflection of the standard. ١٩

20 3- Elastic properties of rocks ( Elasticity): * The seismic method utilizes the propagation of waves through the earth. This propagation depends upon the elastic properties of rocks. * The size and shape of a solid body can be changed by applying forces (stresses) to the external surface of the body. * These external forces (stresses) are opposed by internal forces (stain) which resist the changes in size and shape. * As a result the body tends to return to its original condition when the external forces are removed. This property is called Elasticity, The theory of elasticity relates the forces which are applied to the external surface of a body to the resulting changes in size and shapes. The relations between the applied forces and the deformations are expressed in terms of Stress and Strain Stress is a measure of the forces (F) per unit area across a surface element (A) within the material. ٢٠

21 S=F/A when (F) is perpendicular to the area element, the stress is called Normal Stress Normal stress can be classified into Tensile stress if the force is directed away from the material or Compressive stress if the force is directed into the material. Compressive Stress Tensile Stress * When (F) is tangential to the area element, the stress is a Shearing stress ٢١

22 * Strain is a measure of the relative deformation (expressed per unit length or per unit volume) of a body when it is subjected to a stress. * It is the change in size or shape. * A change in shape with no change in volume is called a Shear strain or distortion * A change in volume without change in shape is called a dilatation or contraction. * Strains that are associated with relative change in length in the direction of stresses are called Normal strains. Elastic properties of materials (Elastic or elastic constants) * The elastic properties of a material are described by certain elastic moduli or elastic constants which specify the relationships between different types of stress and strain. ٢٢

23 1- Young's Modulus (E): * If a load (w) is hung on the end of a wire of length (L) and cross- sectional area (A), the wire is elongated by a small length ( L) in vertical direction (Z). * Young's Modulus (E) represents the tensile stress (P z ) tensile strain (e z ) proportionality constant: P z α e z P z = E e z E= p z / e z = ( W/A) / ( L/L) 2- Bulk Modulus (K) * If a body of volume (V) is subjected to a uniform compression stress (P), its volume will be decreased by an amount ( v). * Bulk modulus (K) is defined as the ratio of the pressure to the fractional change in volume. K= P/ ( V/V) ٢٣

24 3-Shear modulus ( Rigidity, µ): * It is a measure of the stress/ strain ratio in the case of a simple tangential stress (Shear), without change of volume. Example: * A pile of cards can be sheared without affecting total volume of cards. µ= (F/A) / ф * The strain in this case is expressed by the angle of deformation. 4- Poisson's ratio ( σ ): * It is a measure of the geometrical change in the shape of an elastic body. Example: * A cylinder of a length (L) and diameter (D) when subjected to a tensile stress parallel to (L), the length ٢٤

25 will be elongated by ( L) and the diameter will be shortened by ( D). The opposite will occur if it is subjected to a compressional stress, the length will be shortened by ( L) and the diameter will be increased by ( D). In either case, σ = ( D/D)/ ( L/L) * For most rock, the value of (σ) is about 0.25, for liquids the value of (σ) attains its maximum possible value of (0.5) as the liquids have no rigidity (µ = 0). * The relations between the elastic moduli are given by the following formulas: E = 9 µ K / (µ + 3K) K= E/3 (1-2σ) µ = E/2 ( 1+σ) σ = (3K-2µ) / (6K+2 µ) ٢٥

26 4- Electrical properties of rock : * Several electrical properties of rock are significant in electrical prospecting which are: A- Natural electrical potential. B- Electrical conductivity (or the inverse, electrical resistivity). C- Dielectric constant. * Electrical conductivity is the most important while the others are of minor significance. A- Natural ( Spontaneous) potentials: * These potential are associated with: - weathering of sulphide mineral bodies. - Variation in mineral content at geologic contacts. - Bioelectric activity of organic material (i.e. in plant roots). - Metal corrosion of underground pipes, cables,.. - Thermal gradient in underground fluids. ٢٦

27 * There are four principal mechanisms producing these potentials. a- Electrokinetic or streaming potential : * It is of mechanical origin, observed when a solution of electrical resistively (ρ) and viscosity (η) is forced through a porous medium. b- Liquid Junction (diffusion) potential: * It is of a chemical origin, due to the difference in nobilities of various ions in solutions of different concentrations. * When two identical metal electrodes are immersed in a homogeneous solution, There is no potential difference between them, If the concentrations at the two electrodes are different, There is a potential difference ٢٧

28 C Shale potential : * It is of chemical origin, occurring when the concentrations at the two electrodes are different. * The combined diffusion and shale potentials are know as the electrochemical or static self potential. d- Mineralization potential * It is of a chemical origin, occurring when two dissimilar metal electrodes are immersed in a homogeneous solution. * These potentials are especially pronounced in zones containing sulphides, graphite and magnetite and have larger values than the other potentials. * The presence of metallic conductors in appreciable concentrations is necessary to produce mineralization potential ٢٨

29 B: Electrical conductivity (or the inverse, electrical resistivities) * Electrical current is propagated in rocks and minerals depending on the electrical resistivities of these materials. * The electrical resistance of a material is expressed in terms of its resistivity. Example: If the resistance between opposite faces of a conducting cylinder of length (L) and cross sectional area (A) is (R), the resistively (ρ) is given by: ρ = R A / I If "A" is in meters 2 "L" in meter, "R" in ohm, "ρ" will be in ohm- meter. If these dimensions are in cm,"ρ" will be ohm centimeter, where: 1 Ωm Ω cm. 'R' is given in terms of the voltage (V) applied across the ends of the cylinder and the resultant current (I) flowing through it, by ohm's law: ٢٩

30 R= V/I "R" in ohm "v" in volt and "I" in ampere. A Sample V I The conductivity (σ ) is the reciprocal of resistivity: σ = I / ρ = L/RA= (I/A) / (V/L) = J/E mhos/m or mhos/ cm, where J = current density (ampere/m 2 ), E= electric field (volt/m). Most mineral grains are insulators except metallic ores and clay minerals. ٣٠

31 Electric conduction in these mineral grains being through interstitial water in pores and fissures. The conductivity of a porous rock varies with the volume and arrangement of the pores and the conductivity and amount of contained water. Hard rocks are bad conductors of electricity, but conduction may take place along cracks and fissures. In porous sedimentary formations, the degree of saturation and the nature of the pore electrolytes govern the resistivity. 5 Radioactivity of rocks: * Radioactivity of rocks and minerals are attributed to traces of uranium, thorium and the isotope of potassium (K 40 ) and their radioactive decay products. ٣١

32 * Among the earth's rocks, granites and shale show the largest radioactivity. * In general, the radioactivity in sedimentary rocks and metamorphosed sediments is higher than that in igneous and other metamorphic types, with the exception of potassium rich granites. 6- Thermal properties of rocks: * It is a fact that the temperature increases with depth. Therefore, heat must be flowing upward in the earth. * The amount of heat flow depends on the thermal conductivity of the rocks. * The thermal conductivity is a measure of how easily heat flows through a material. ٣٢

33 GENERAL REVIEW OF GEOPHYSICAL METHODS * The physical properties of rocks have been used to devise geophysical methods that are essential in the search for minerals, oil and gas and other geological and environmental problems. * These methods are: 1- Gravity method 3 - Seismic method 5- Electromagnetic method 7- Geothermal method 2- Magnetic method 4- Electrical method 6- Radiometric method * Geophysical methods respond to the physical properties of the subsurface media (rocks, sediments, water, voids, etc.. ) and can be used Successfully when one region differs sufficiently from another in some physical property. These methods can be classified into two distinct types: ٣٣

34 1- Passive methods: Which detect variations within the natural fields associated with the earth, like the gravitational and magnetic fields, such as gravit, magnetic, some electric and some electromagnetic methods, radioactive and geothermal methods. 2- Active motheds: * These artificially generated signals transmitted into the ground and then modify the received signals in ways that are characteristic of the materials through which they travel. Examples of these methods are seismic and some electrical methods. * Generally, natural field methods (passive methods) can provide information on earth properties to greater depths and are simpler to carry out than artificial source methods (active methods). Moreover, the artificial source methods are capable of producing a more detailed and better resolved picture of the subsurface geology. ٣٤

35 * Geophysical methods may from part of a larger survey and thus geophysicists must be in contact with the whole survey team and particularly to the client. * Few, if any geophysical methods provide a unique solution to a particular geological situation. It is possible to obtain a very large number of geophysical solutions to some problems, some of which may be geologically non-sensical. It is necessary, therefore, always to ask the question: "Is the geophysical model geologically plausible?. If it is not, then the geophysical model has to be rejected and a new one developed which does provide a reasonable geological solution. Conversely, if the geological model proves to be inconsistent with the geophysical interpretation, then it may require the geological information to be re-evaluated. ٣٥

36 It is important that geophysical data are interpreted within geological framework. The various geophysical methods depend on different physical properties. For example: gravity methods are sensitive to density contrasts within the sub-surface geology and so are ideal for exploring for major sedimentary basins where there is a large density contrast between the lighter sediments and the denser underlying rocks. It would be inappropriate to use gravity methods to search for ground water where there is a negligible density contrast between the saturated and unsaturated rocks. If the physical principles upon which a method is based are understood, then it is less likely that the technique will be misapplied or the resultant data misinterpreted. ٣٦

37 The basic geophysical methods are listed below with the physical properties to which they relate and their main uses. Geophysical methods and their main applications Method Physical property Applications Gravity Density p p s s s s s Magnetic Susceptibility m p p s s p p Seismic refraction Elastic moduli, density p p m p s s Seismic reflection Elastic moduli, density p p m s s m Resistivity Resistivity m m p p p p p s p m Spontaneous potential Potential differences - - p m p m m m Induced polarization Resistivity m m p m s m m m m m Electromagnetic Conductance, inductance s p p p p p p p p m EM - VLF Conductance, Inductance m m p m s s s m m EM ground radar Conductivity - - m p p p s p p p Magneto - telluric Resistivity s p p m m P = primary method S = secondary method m= may be used but not necessarily the best approach, or has not been developed for this application, - = unsuitable ٣٧

38 Applications :- 1-Hydrocarbon exploration (coal, gas, oil) 2-Regional geological studies (over areas of 100s of km 2 ) 3- Exploration of mineral deposits. 4- Engineering site investigation. 5- Hydrogeological investigation. 6- Detection of subsurface cavities. 7- Mapping of leachate and contaminant plumes. 8- Location and definition of buried metallic objects. 9- Archaeo-geophysics. 10- Forensic geophysics. * Several geophysical surveying methods can be used at sea ( marine geophysics ) or in the air (aerogeophysics ) * Reconnaissance surveys are often carried out from the air because of the high speed of operation. In such cases the electrical or seismic methods are not applicable, since these require physical ٣٨

39 contact with the ground for the direct input of energy. Geophysical methods are often used in combination. Example: The search for metalliferous mineral deposits often utilizes airborne magnetic and electromagnetic surveying. - prospecting for oil usually includes gravity, magnetic and seismic surveying The importance of such combination appears in the interpretation stage, ambiguity arising from the results of one survey method may be removed by consideration of results from a second survey method. ٣٩

40 Airborne versus ground geophysical methods: Airborne geophysical methods are used in reconnaissance work, but the ground methods are used in more detailed investigations. They are fast and are relatively inexpensive per unit area. Several kinds of surveys can be done at once. They can provide a more objective coverage than ground surveys in many kinds of terrains. For example: several hundred line kilometers of airborne electromagnetic surveying can be done in a day compared with three to five line kilometers per crew in a ground EM survey. The cost of an airborne electromagnetic survey, with magnetic and radiometric data included is likely to be 1/4 to 1/5 the cost of an equivalent ground EM survey ٤٠

41 Airborne survey patterns are reasonably uniform and complete because they do not have the access and traverse problems of ground survey in swamps, dense brush and rugged topography. An airborne survey will give more accuracy than a ground survey in some areas, but it will seldom provide such detail or such sharp signals as a ground survey. 1- Gravity method: It is mainly used for oil exploration. Sometimes in mineral and ground water prospecting. Gravity prospecting involves the measurement of variations in the gravitational field of the earth (i.e. minute variations in the pull of gravity from rock within the first few miles of the earth's surface). ٤١

42 Different types of rock have different densities and the denser rocks have the greater gravitational attraction. If the higher density formations are arched upward in a structural high, such as an anticline, the earth's gravitational field will be greater over the axis of the structure than along its flanks. Gravity Anticline Gravity anomaly over an anticline ٤٢

43 * A salt dome which is generally less dense than the rock into which it is intruded, can be detected from the low value of gravity recorded gravity recorded above it compared with that measured on either side. Salt dome Gravity Anomaly Over a salt dome * Anomalies in gravity which are sought in oil exploration may represent only one - millionth or even one - ten - millionth of the earth's total field. ٤٣

44 * For this reason, gravity instruments are designed to measure variations in the force of gravity from one place to another than the absolute force itself. * The gravity method is useful wherever the formations of interest have densities which are appreciably different from those of surrounding formations. * Gravity is an effective means of mapping sedimentary basins where the basement rocks have a higher density than the sediments. * Gravity is also suitable for locating and mapping salt bodies because of the low density of salt compared with that of surrounding formations. * Gravity can be used for direct detection of heavy minerals such as chromite ٤٤

45 Magnetic method: * Magnetic method deals with variations in the magnetic field of the earth which are related to changes of structures or magnetic susceptibility in certain near surface rocks. * Magnetic surveys are designed to map structure on or inside the basement rocks or to detect magnetic mineral directly. * In mining exploration, magnetic methods are employed for direct location of ores containing magnetic minerals such as magnetite. * Intrusive bodies such as dikes can often be distinguished on the basis of magnetic observations alone. Electrical methods: * Electrical prospecting uses many techniques, based on different electrical properties of the earth's materials such as: ٤٥

46 - The resistively method is designed to give information about the electrical conductivity of the earth's rocks. - In resistivity method the current is driven through the ground using a pair of electrodes and the resulting distribution of the potential in the ground is mapped by using another pair of electrodes connected to a sensitive voltmeter. - The resistivity method has been used to map boundaries between layers having different conductivities. - It is employed in engineering geophysics to map bedrock. - It is used in groundwater studies to determine salinity. - The induced polarization (IP) makes use ionic exchanges on the surfaces of metallic grains (disseminated sulphides). ٤٦

47 - Telluric current and magneto-telluric methods use natural earth currents and anomalies are sought in the passage of such currents through earth materials. - The self potential method is used to detect the presence of certain minerals which react with electrolytes in the earth to generate electrochemical potentials. - Electromagnetic methods detect anomalies in the inductive properties of the earth's subsurface rocks. - The method involves the propagation of time varying, low frequency electromagnetic fields in and over the earth. - An alternating voltage is introduced into the earth by induction from transmitting coils and the amplitude and phase shift of the induced potential ٤٧

48 generated in the subsurface are measured by detecting coils and recorded. - Electromagnetic methods are used to detect metallic ore bodies. Seismic methods: * There are two main seismic methods, reflection and refraction: 1- seismic reflection method : * This method is used to map the structure of subsurface formations by measuring the times required for a seismic wave, generated in the earth by a near surface exploration of dynamite, mechanical impact or vibration, to return to the surface after reflection from interface between formations having different physical properties. ٤٨

49 S.P. G Reflected Ray Reflector Layer 1, V1 Layer 2, V2 * The reflections are recorded by detecting interments which are called geophones responsive to ground motion. * Variations in the reflection times from place to place on the surface indicate structural features in the strata below. * Depths to reflecting can be determined from the times using seismic velocity information. * Reflections from depths as great as 20,000 feet can be observed from a single explosion, so that in most areas, geologic structures can be determined throughout the sedimentary section. ٤٩

50 * With reflection method one can locate and map such features as anticlines, faults, salt domes and reefs. Many of these are associated with the accumulation of oil and gas. Seismic refraction method: * In refraction method, the detecting instruments recorded the arrival times of the seismic waves when refracted from the surface of discontinuity. S.P. G Refracted Ray Refractor Layer 1, V1 Layer 2, V2 * These times give information on the velocities and depths of the subsurface formations along which they propagate. ٥٠

51 * Refraction method makes it possible to cover a given area in a shorter time and more economically than with the reflection method. Radioactive Method : * This method is used to detect radioactive minerals such as uranium and thorium. Well logging method: * This involves probing the earth with instruments which give continues readings recorded at the surface as they are lowered into boreholes. * The rock properties which are covered by well logging techniques are electrical resistivity, self potential, gamma ray generation density, magnetic susceptibility and acoustic velocity. * Well logging is one of the most widely used of all geophysical techniques ٥١

52 GEOPHYSICAL ANOMALIES * It is the local variation in a measured parameter, relative to some normal background variation is attributed to a localized subsurface zone of distinctive physical property and possible geological importance. * A local variation of this type is known as a geophysical anomaly. Example: * The Earth's gravitational field after the application of certain corrections would everywhere be constant if the subsurface were of uniform density. * Any lateral density variation associated with a change of subsurface geology results in a local deviation in the gravitational field ٥٢

53 * This local deviation from the otherwise constant gravitational field is referred to as a gravity anomaly. * It may be positive (high anomaly) or negative (low anomaly) Positive (high anomaly) Negative (low anomaly) ٥٣

54 AMBIGUITY IN THE INTERPRETATION OF GEOPHYSICAL ANOMALIES * In studying the Earth's hidden features, most problems are of an Inverse type (i.e. deducing the source from the observed anomaly). * The measured physical effect ( e.g. surface variations in gravity, magnetic or electrical fields) can not be interpreted in terms of a unique source occurring at a particular depth inside the earth ( i.e. the same anomaly gives more than one interpretation). * This is because a variety of sources with varying parameters at different depths can theoretically produce the same affect. * A combination of several geophysical methods and the different geological information often yields more information that can help reduce the ambiguity by narrowing down the range of possible solutions. ٥٤

55 NOISE IN THE INTERPRETATION OF THE GEOPHYSICAL DATA * Noises are undesired readings recorded during geophysical measurements and make the interpretation more difficult. * The reliability of geophysical mapping is strongly dependent upon the quality of the field records. * We use the term signal to denote any event on the geophysical record from which we wish to obtain valuable information. Everything else is called noise. * The signal / noise ratio, is the ratio of the signal energy in a specified position of the geophysical record to the total noise energy in the same portion. * Poor geophysical records result whenever the signal/ noise ratio is small. ٥٥

56 * When signal / noise ratio is less then unity, the record quality is usually marginal and deteriorates rapidly as the ratio decrease further. * Some noise can be anticipated on the basis of existing information possible sources of terrain noise (swamps, conductive overburden) may be identified. * Sources of cultural noise- mines, pip lines, and abandoned town sites may be known. * Noise can be attenuated by applying some processing and treatment techniques to the geophysical field data to increase the signal/ noise ratio. ٥٦

57 FIELD GEOPHYSICAL SURVEYING * The field surveying in geophysics can be carried out in the form of profiles or traverses. * These profiles must be, as possible as it can perpendicular to the strike of the causative body. * The distance (interval) between the measuring points ( e.g. stations) depends up the purpose of the surveying ( e.g. regional or detailed studies) Example: 1- In oil exploration, we look for oil traps ( geologic structures) which may be extended for several hundreds of meters or even several kilometers. In this case, the station interval may be of as large as 1 to 2 kilometers. 2- In mineral exploration, we look for mineralized zone of few tens of meters. ٥٧

58 * In this case, the station interval should be as small as possible to cover the target body with enough number of measuring points * The field geophysical measurements can be carried out in more than one profile, parallel to each other to form what is called Grid pattern system. GLOSSERY Anomaly : An irregularity in observed or theoretically calculated geophysical effect caused by a significant change in some physical property ( e.g. density, magnetization, seismic velocity) of rocks. Aquifer: A permeable rock formation that stores and transmits groundwater to wells. Disseminated ore: An ore body in which metal is distributed in small amounts throughout the rock. ٥٨

59 Geomagnetic reversal : the earth's magnetic field. A reversal of the polarity of Hydrothermal activity: temperature groundwater. Any process involving high Isostasy : the concept that areas of the crust are in gravitational balance by a mechanism which compensates for the broad topographic variations. Magnetic epoch : A period of the order of one million years during which the earth's magnetic field was predominantly of one polarity. Magnetic event: A short period within a magnetic epoch during which the earth's field had a polarity opposite to that of the epoch. Prospecting: Exploration of an area with the aim of locating minerals, oil gas, water,.etc ٥٩

60 REPRESENTATION OF GEOPHYSICAL MEASUREMENTS * The geophysical data can be represented in TWO forms: Profiles: when the measurements are taken along a single traverse, the measured parameter is plotted on the "Y" axis and the measuring points on the "X" axis. Value Stations * The measurements can also be potted on parallel profiles, called stacked profiles. ٦٠

61 b- contour maps: when the measurements are recorded on a grid pattern system they can be contoured in the form of maps. PROCESSING OF GEOPHYSICAL DATA * The field geophysical data are affected by interference from undesired sources (e.g. noises). * This data must be subjected to different correction and processing techniques before being interpreted. * Rapid advances in digital computer technology made extensive calculations for this purpose are available. ٦١

62 Examples: * Gravity field measurements are affected by latitudes, terrains, drifts and should be corrected for these effects before interpretation. * Magnetic measurements are usually affected by daily variations in the earth's magnetic field and must be corrected. * Some electromagnetic methods are affected by variations in topography and must be corrected before interpretation. INTERPRETATION OF GEOPHYSICAL DATA * Interpretation of geophysical field measurements means that the transformation of digital data into understandable geological forms (e.g. structures, groundwater occurrences, mineral deposits,..) * It can be divided into Qualitative and Quantitative. ٦٢

63 Qualitative interpretation * A first step towards interpretation is the preparation of a contour map on which the intensity values at different stations are plotted and on which the contours of equal values are drawn at suitable intervals. * Contouring of geophysical maps is nowadays often done on automatic plotters using computer programs for interpretation. * Qualitative interpretation means general inspection of the contour map or profile without making any calculations. * Most geophysical anomaly maps are colored using suitable color schemes and color gradations for the areas enclosed between successive contours. * Coloring is a very valuable aid in the qualitative interpretation of a geophysical map in general. ٦٣

64 * Many features of geological interest is first discernible when a map is suitably colored. * An important point in considering the anomalies in an area is the zero level, that is the reading of the instrument at points where the field is the normal undisturbed field. * The qualitative interpretation of geophysical map begins with a visual inspection of the shape and trend of the major anomalies. * Each contour pattern should have its geological counterpart. * After delineation of the structural trends, a closer examination of the characteristic features of each individual anomaly is made. These features are: a- The relative locations and amplitudes of the positive and negative parts of the anomaly. ٦٤

65 b- The elongation and areal extent of the contours which suggests the strike of the corresponding geological feature. c- The sharpness of the anomaly as seen by the spacing of contours (e.g. high horizontal anomaly gradients are often associated with contacts between rocks and with bodies at shallow at depths). d- Circular patterns of contours are associated with circular bodies such as ore body. e- Long narrow patterns are due to long narrow bodies such as dike, tectonic shear zones, isoclinally folded strata. f- Dislocations, when one part of an anomaly pattern is displaced with respect to the other part, are indicative of geological faults. ٦٥

66 Quantitative interpretation * After completing qualitative study it is important to extract some quantitative information (e.g. the important parameter to be estimated is the depth to the anomalous structures). * From the relative spreads of the maxima and minima of the anomaly, the approximate location and horizontal extent of the causative body may be determined. * From the from of the anomaly, the other parameter of the body, its shape and depth may be determined. * The usual procedure in quantitative interpretation is to guess a body of suitable from, calculate its field at the points of observation and compare it with the measured values. * It is then possible to adjust the depth and dimensional parameters of the body by trial and ٦٦

67 error or by automatic optimizing methods until a satisfactory agreement is achieved between the calculated and observed values. * The geometrical parameters must then be translated into structural terms. In the light of know geology. THE PLACE OF GEOPHYSICS IN SOLVING GEOLOGICAL AND ENVIRONMENTAL PROBLEMS 1- In hydrocarbon ( petroleum) exploration: * Petroleum, when in an accumulation, forms only a small proportion of the total fluids present in a rock section, and none of its properties differs sufficiently from those of the salt water. * Since rocks can vary considerably in their physical properties such as densities, magnetic properties, electrical conductivities, and the seismic velocities. It has proved possible to use these variations in ٦٧

68 rock properties to assist in the location of subsurface structures which are favorable for the accumulation of petroleum. * All the geophysical methods concentrate on the discovery of anomalies in the rock which overlie or surround possible petroleum accumulations. * Nowadays geophysical surveys are generally considered to be standard pre- requisites before an exploration drilling program. * Geophysics was first applied to petroleum exploration in the U.S.A in the early 1920's. * Hydrocarbons (oil and gas) are normally found in association with thick sedimentary sequences in major sedimentary basins. * The hydrocarbons are accumulated in commercial quantitative in suitable geological environments called traps. ٦٨

69 * There are many types of traps including tectonic structures such as anticlines, tilted fault blocks, salt domes and stratigraphic traps such as local sand bodies surrounded by clay envelops or local reef developments in limestone sequences. * Geophysical exploration for hydrocarbons normally employs an indirect approach, searching for the traps, depending on the great variations in the physical properties of the earth's rock such as density, magnetic properties, electrical conductivities and seismic wave velocities. * The only techniques which are believed to be directly related to the properties of petroleum itself are the geochemical and radioactive surveys. * All the other geophysical techniques concentrate on the discovery of anomalies in the rocks which overlie or surround possible petroleum accumulations. ٦٩

70 * Exploration is usually carried out in several phases: A-In cases where the subsurface geology is unknown (unexplored areas), the initial reconnaissance may involve gravity and/ or aeromagnetic surveying. * Gravity surveying is capable of identifying areas of thick sediments by their relatively low densities and the large scale negative Bouguer anomalies. * Gravity is also used to determine the subsurface structures by the lateral changes in density. It is employed as a preliminary to the seismic survey enabling areas of maximum interest to be delineated. ٧٠

71 * Gravity method is an ideal technique for detecting the salt domes often associated with oil accumulations, because the density of the salt is low compared with the surrounding sediments. Salt dome * Gravity "highs" are usually due to buried anticlines. Gravity Anticline ٧١

72 * Aeromagnetic surveying can be used to estimate variations of depth to an igneous or metamorphic basement underlying a sedimentary sequence ( i.e. thickness of sediments) and hence to determine indirectly the areas of main sediment accumulation. * Aeromagnetic measurements depends mainly on the great difference in magnetic susceptibility between the sedimentary rocks and the underlying basement rocks. * The aeromagnetic survey is usually used in petroleum exploration more than ground survey for the following reasons: - The speed of the survey. - The possibility of reaching inaccessible area. - Local influences which would affect the accuracy of the ground instrument are avoided. - The aeromagnetic survey provides a rapid and effective method of estimating the depth and shape of the crystalline basement and hence ٧٢

73 approximate thickness of the overlying sedimentary material. *The presence of oilfields may sometimes be directly indicated by the results of aeromagnetic surveys which detect the presence of concentrations of diagenetic magnetite. These concentrations are produced by the reduction of hydrated iron oxides and/or hematite as a direct result of micro seepage from buried oil accumulations. * Once a prospective sedimentary basin environment has been identified, further geophysical surveying normally carried out using seismic methods, especially reflection profiling. * Reconnaissance seismic exploration surveying involves measurements along widely spaced profile lines covering large areas in order to detect regional structural elements. ٧٣

74 * Detailed refection seismic surveying involves closely spaced, intersected profile lines in more restricted areas containing the main prospective targets, to delineate the most promising structures. * The main job of seismic interpretation is to involve structural mapping in the search for the structural closures that may contain oil or gas. Geochemical investigations may help to differentiate between those which are hydrocarbon bearing and those which are barren. * The petroleum geologist must be able to relate the resultant sections and maps to the surface geological evidence and the subsurface data furnished by well samples and cores. * Exploration boreholes are normally sited on seismic profile lines so that the borehole logs can correlated directly with the local seismic section. ٧٤

75 * Exploration boreholes are normally sited on seismic profile lines so that the borehole logs can be correlated directly with the local seismic section. *Seismic stratigraphic provides additional criteria on which to select areas for detailed study, for example, the definition of local deltaic of reef faces with an associated high reservoir potential. *Geochemical investigations may help to differentiate between those which are hydrocarbon bearing and those which are barren. *Radioactive survey is a method of surface exploration for oil. It is based on the hypothesis that most crude oils contain radioactive material, some of which notably dissolved radium salts or radon gas, may be carried to the surface by percolation and thus are areas under which oil may lie. ٧٥

76 * Tests for radioactivity may be made on gas samples drawn from shallow surface holes or on soil samples. * These samples are collected along closely spaced profiles or grids covering the area under test, and the relative radioactivity of each sample is then measured and plotted against its map position. * By such measurements radioactivity haloes could be defined over oil fields. 2- The place of geophysics in mineral exploration: * Geophysical methods are extensively used in the search for economically valuable mineral deposits, including non- metallic deposits such as sand, gravel and limestone and metallic deposits such as massive and disseminated sulphides and iron ores. * These deposits differ significantly from their host rocks in their physical properties and consequently give rise to geophysical anomalies of various types. ٧٦

77 * The initial aim of a geophysical survey for ore deposits is to locate mineralized areas of potential interest. * Airborne magnetic and electromagnetic techniques are suitable since large areas can be surveyed rapidly at relatively low cost. * Once possible target area are determined, further information on causative bodies within the anomalous zones is obtained by ground surveys which enable the prospector to determine whether the anomalous bodies are of economic importance. * If ore bodies are present, the ground geophysical data will provide information on their depths, extent and attitude and consequently control the location of exploratory boreholes or trenches. * The return - ratio is very important in geophysical surveys. It is the ratio of the estimated value of the ٧٧

78 ore to the cost of the geophysical work. This ratio must be several hundred to one. A Massive sulphide ores: * They are considered to be a single mass with a cross sectional area of at least 100m2 comprising 50% or more of metallic sulphides. * Such ore may contain magnetic minerals pyrrhotite and magnetite. If these minerals are present in reasonable quantities, the ore will produce large magnetic anomalies. * The electrical conductivity of massive sulphides is very high, in the range S.m -1. * The geophysical methods applicable to the search for such ores those responding to very dense material (gravity), high magnetic (magnetic) and conductive materials (electrical and electromagnetic). ٧٨

79 * Airborne prospecting techniques for massive sulphides usually exploit the property of high conductivity ( i.e. electromagnetic). * Airborne prospecting techniques for massive sulphides usually exploit the property of high conductivity ( i.e. electromagnetic methods are used). * The survey aircraft usually also carries a magnetometer to provide additional information at little extra cost. * Ground geophysical surveys employ electrical and electromagnetic methods. Self potential methods are cheap and effective if the correct subsurface conditions exist and the ore body lies at a depth of less than a bout 30m. * Gravity surveying is a secondary ground exploration tool because of the high cost and ambiguities in interpretation. ٧٩

80 * It provides accurate estimates of ore tonnage on the basis of the total mass anomaly. * Although electrical and electromagnetic methods are the major exploration techniques, they suffer from drawback that anomalies may result from non- economic sources such as graphite or water filled shear zones. * It is possible to eliminate such non economic sources by using a combinations of electrical, magnetic and gravity methods. B- Disseminated sulphides ores : * Disseminated sulphide deposits are those bodies in which sulphides are scattered as specks and veinlets throughout the rock and constitute not more than 20% of the total volume. ٨٠

81 * Magnetic method is not an effective tool in the exploration for disseminated sulphides because their magnetic susceptibility is low. * The electrical and electromagnetic methods appear to be the most suitable survey techniques. * The conductivity of a disseminated sulphide ore body is highly variable because of the irregular dispersion of the sulphides throughout the host. Consequently, Resistivity and electromagnetic anomalies are encountered. * Since electrical conduction through the metallic sulphides is not electronic, but electrolytic through the host rock, disseminated sulphides produce strong induced polarization anomalies. So that, the induced polarization method is the most appropriate to detect such bodies. * The physical properties of economically important sulphides such as chalcopyrite ores are not great ٨١

82 different from zones of disseminated uneconomic minerals such as pyrite. Hence, the economic importance of a deposit cannot be judged solely from its IP response and further geological and geochemical surveying need to be executed prior to any costly drilling program. Iron ores: * The most widely exploited physical property of iron ores in geophysical exploration is their magnetic susceptibility. * The ratio of magnetite to haematite must be high for the ore to produce significant magnetic anomalies, as haematite is non magnetic. C- Geophysical in Hydrogeology: * Many geophysical methods find application in locating and defining subsurface water resources. * The magnetic method is rarely used, but it can be used to locate faults and shear zones which could ٨٢

83 affect the pattern of ground water flow and determine the basement configuration underlying the alluvial deposits. * The gravity method is widely used in regional reconnaissance surveys to delineate the from and extent of porous sedimentary deposits such as buried valley fill, determining the configuration of the bedrock surface over an area of recent surface cover. Observed Gravity Anomaly * The gravity method has also been used to determine groundwater volumes from anomalous mass calculations. ٨٣

84 * Seismic refraction method is widely used in hydro geological investigations. They provide direct information on the level of the water table since an increase in water content causes a significant increase of seismic velocity. * The technique of fan shooting may be adapted to the location of buried channels and gravel filled valleys which are important sources of groundwater in regions of largely impermeable bedrock. Impermeable rock Shot point Buried Valley Fan shooting Impermeable rock * The most widely used geophysical methods in hydrogeology are the electrical techniques. ٨٤

85 * Resistivity surveys are routinely employed in ground water exploration to locate zones of high conductivity corresponding to saturated strata at depths down to 400 ms. * Resistivity surveys may also provide indications of ground water quality. * The Resistivity of the rock is controlled by the volume of water present and will decrease as the salinity of the water increases. * In a homogeneous aquifer, it is possible to distinguish fresh from saline ground water and even to trace the subsurface flow of contaminated ground water resulting from polluted water has a distinctive resistivity. D- Geophysics in engineering geology: * Geophysical methods are frequently used in an initial site investigation to determine subsurface ٨٥

86 ground conditions prior to excavation and construction work. * Both seismic refraction and vertical electrical soundings are routinely employed in the determination of overburden thickness for foundation purposes. * Magnetic surveys are occasionally used to delineate zones of faulting in bedrock, and may be employed in the location of buried, metallic, man made structures such as pipelines or old mine working. * Micro-gravimetric method may be used to detect subsurface cavities, buried valleys, faults within bedrock, underground workings and various archaeological features. * Resistivity method is used to detect the presence of the subsurface voids which constitutes highly resistive zones. ٨٦

87 * A recent ground based radar transmitter can be used successfully to detect the subsurface voides. It provides a shallow penetration continuous profile of the subsurface similar to a seismic section. * It may be noted that all the above survey techniques find application in archaeological investigations, where they may be used in the delineation of buried buildings, walls, tombs and other artifacts. * Geophysical techniques have a major role in offshore engineering activities such as: the construction of harbors, tidal barrages and offshore platforms, the laying of submarine pipelines, and dredging. * Such offshore constructions usually require detailed information on the nature of the sea bed and the thickness of any unconsolidated sediment layers. ٨٧

88 * Dredging, which may be carried out either to establish and maintain a navigation channel in the approaches to a harbour or to extract sand or gravel from offshore banks, requires information on the thickness and distribution of sediment layers. E- Geophysics in the investigation of the Earth's crust. Mantle 2900 Km 2400 Km Solid Crust 5-65 Km Solid core Fluid Core 1100 Km * The crust is defined as that part of the Earth lying above the Mohorovici discontinuity ( i.e. Moho, after a Yugoslavian seismologist Andrijia ٨٨

89 Mohorovicic 1909) which is the boundary between the crust and the mantle, below which the velocity of compressional seismic body waves increases abruptly to about 8.0 km/s. * It is composed of a series of lithospheric plates in relative of rocks. * Large scale seismic refraction surveys, using explosions as seismic sources, have been carried out to study crustal structure in most continental areas. * Such surveys show that continental crust is typically Km. Thick and it is internally layered. These layers are: Sedimentary layer Continental Crust Oceanic Crust Upper mantle Moho ٨٩

90 * Upper crust : which has seismic velocities in the range 5.8to 6.3 km/s and may represent mainly granitic or granodioritic rocks. * Lower crust : which has seismic velocities in the range 6.5 to 7 km/s and may represent igneous and metamorphic rocks, including gabbro and basic granulite. * Marine seismic refraction surveys show that the thickness of the ocean crust is 6 to 8 km, composed of three layers with different seismic velocities which are: Layer Thickness (km) Velocity (km/s) Rock type 1 0 : : 2.5 Sediments 2 1: 2 4 : 6 Pillow lava : : 7.0 Dolerite dykes and gabbro * Gravity surveying estimated regional variations of crust thickness on the basis of variations in the level of the Bouguer anomaly field. ٩٠

91 F- Geophysics in the investigation of the Surface Earth's interior Lithosphere Moho 70 Km Zone of low S-Wave velocity Astenosphere Astenosphere Zone of slowly increased S-Wave velocity 400 Km Zone of rapidly increased S-Wave velocity Mesosphere * Most of the our methods for studying the interior of the Earth are geophysical in nature. * Our knowledge about the earth's interior is gained from large earthquakes whose waves pass through the entire earth. * These knowledge comes from the behavior of these waves as they travel through the earth ( i.e. ٩١

92 includes the changes in the velocities and paths of the waves as they travel through different kind of rocks and from solids to fluids). * The key to our understanding of the earth's interior is the knowledge of seismic wave velocities, because from this we learn what kind of materials lie at depth and how these materials are distributed. * Earthquakes produce compression waves ( P- waves) and shear waves or secondary ( S- waves), together called body waves because they travel through the earth. * P- waves vibrate in the direction of wave propagation, and S- waves vibrate at right angles to the direction of propagation. * P- waves travel faster than S- waves and therefore at recording station first. ٩٢

93 * From the focus of an earthquake, P and S waves spread outward in all directions. * The velocity of waves depends on both the elasticity and the density of rocks through which the waves travel. * Elasticity is a measure of the degree to which a rock deforms when subjected to stress. It generally increases with depth. * Density also increases with depth. * Greater elasticity allows seismic waves to travel faster, greater density slow them down. ٩٣

94 Layers of the Earth 1- Crust: * In crust, there is a general increase ٩٤

95 * The Moho discontinuity is a boundary between different types of rocks and is marked by sharp increase in the velocities of both P and S waves. 2-Mantle: * It has the greatest share of the earth's volume, extending from a depth of about 20 Km. to 2900 Km. * The mantle can be subdivided, based on seismic wave behavior, into a number of layers: a- lithosphere: * It is the most important in the theory of plate tectonics. * It comprises the top part of the mantle and all the crust, about 70 to 100 Km. thick composed of strong, brittle rocks. * The lithosphere is broken up into about two dozen sections called plates, which are shifting position ٩٥

96 with respect to one another over the earth's surface. b- Astenosphere: * It lies below the lithosphere and extends from about 70 Km. under the oceans and 100 Km. under the continents to a depth of about 700 km. * It is a weak material in contrast to the stronger lithosphere. * It is characterized by low seismic velocities, particularly in the top part from 70 km (100 Km. under continents) to about 400 km. * In this section, S-wave velocities decrease from 4.7 km/s to 4.2 km/s. P-wave velocities also decrease. This zone is called the low velocity zone. * The decease in velocities is interpreted to mean that partial melting occurs in the low velocity zone such ٩٦

97 decrease is called attenuation. A weak material has greater attenuation than a strong material. Amplitude Attenuation Example: * The vibration of a bell: A good bronze bell low attenuation and will vibrate when struck because bronze is a strong material. But, a ball made of lead is weak and has a high attenuation when struck. * The low velocities and high attenuation of seismic waves in the astenosphere indicate that the astenosphere is not nearly as rigid as the overlying lithosphere. * The significance of the partially molten astenosphere is that the lithosphere can slide over it. This movement is a vital part of the theory of plate tectonics. ٩٧

98 C- Mesosphere: * It is be the lower part of the mantle (400 to 2900 km). In it, rocks are dense and highly elastic and seismic wave velocities increase. * At its base lies a thin transition zone in which S waves die out quite rapidly. 3- core: * It lies below the mesosphere and the transition zone, little is known about the core. * It plays no role in the movement of lithospheric plates, but it is the source of the earth's magnetic field. * The P- seismic waves show a sharp drop in velocity when they reach the core and that their velocities increases as they travel through it, out with slower velocities than in the mantle. Scientists conclude ٩٨

99 that the core is of much greater density than the mantle. * There is a discontinuity at about 5100 km. velocities increase there, the wave behavior has led seismologists to postulate a solid inner core. * S- waves cannot travel through fluids because they act to change the shape of a body. Water and air can have their volumes changed by contraction or expansion, but they cannot have their shapes changed. * S- waves are not refracted down into the core but die out at the core-mantle boundary. This convincing evidence for a molten outer core. Continental crust versus oceanic crust * The structure and composition of oceanic crust is relatively simple compared with continental crust. * The igneous part of the oceanic crust consists of basalt, rich in iron and magnesium. ٩٩

100 * The oceanic crust is uniform in thickness, being about 10 km thick. * Continental crust may be thin as 20 km and as thick as 70 km under mountain ranges. It averages about 35 km thick. * The rock in the upper 10 to 20 km. Have the average composition of the igneous rock granodiorite. Downward, the continent is composed of the common metamorphic rock gneiss( i.e. metamorphic equivalent of granodiorite). * Continental and oceanic crust also differs in density. Basalt is denser than granodiorite. For that, the average density for continental crust is 2.7 gm/cc and for oceanic crust is 3.0 gm/cc. * About 65% of the earth's surface is underlain by oceanic crust and about 35% by continental crust. Although the oceans cover approximately 71% of ١٠٠

101 the earth's surface, part of the ocean waters lie over the edges of the continents and thus over continental crust. Oceanic crust is relatively young, not greater than 200 million years. Continental crust range to as old as 3.8 billion years. PLANNING AND COORDINATING GEOPHSICAL WORK * There is a special need to coordinate geophysical work with geological investigations because they are so interdependent. * A geophysicist chooses field methods and traverses on the basis of interpreted geology. * A geologist uses geophysical information in making an interpretation. ١٠١

102 Preparing for geophysical surveys A preliminary considerations 1- Geophysical exploration models: * These models depend on the information gained from: a- Geology of the area. b- Contrasts between physical properties. c- Probable range in depth of occurrence. 2- Objectives: * It is important that the objectives of a geophysical survey should be clear at the beginning. * The geophysical survey produced poor results for the following reasons: 1- Inadequate and / or bad planning of the survey. 2- Incorrect choice or specification of technique. 3- Insufficient experienced personnel conducting the investigation. ١٠٢

103 * For cost effective, experienced geophysical consultants are employed for survey design, site supervision and final reporting. ١٠٣

104 * The objective will be to do the work within the best of some sequences such as: a- Limits in cost. b- Time. c- Scheduling. Survey constraints (limitations): 1- Finance: how much is the survey going to cost and how money is available? * The cost of the survey will depend on: Where the survey is to take place? How accessible the proposed field site is? What scale the survey is to operate? The more complex the survey in terms of equipments and logistics, the greater the cost is likely to be. It is important to remember that the geophysics component of a survey is a part if an exploration program and thus the costs of the geophysics ١٠٤

105 should be viewed in relation to those of the whole project. The factors that influence the various components of a budget also vary from country to country and form job to job. Some of the basic elements of a survey budget are given in the following table: Staffing Management, technical, support, administration, etc. Operating costs Cash flow Equipment Including logistics Assets versus usable cash For data acquisition and data reduction analysis computers and software whether or not to hire or buy. Insurance To include liability insurance as appropriate Overheads Development costs Contingencies Administration, consumables, etc. Skills, software, etc. Something is bound to go wrong at some time, usually when it is most convenient. ١٠٥

106 The main people to be involved in a survey are: Geologists, Geophysicists, Surveyors. Vehicles and equipments will need maintaining, so skilled technicians and mechanics may be required. Everybody has to eat and it is surprising how much better people work when they are provided with well prepared food: a good cook at base camp can be a real asset. Due considerations should be paid to health and safety and any survey team should have staff trained in First Aid. Local labor (workers) may be needed as porters (carriers), guides, translators, guards. In some countries, access to a survey site in dry season may be possible whereas during the rains ١٠٦

107 of the wet season, roads may be totally impossible. Also access to land for survey work can be severely hampered during the growing season with some crops reaching 2-3 meters high. some survey such as seismic refraction and reflection may cause a limited amount of damage for which financial compensation may be sought. Consideration has to be given to the transport of the geophysical and other equipments. It may even be necessary to make provision ( arrangement ) for a bulldozer to excavate a rough road to provide access for vehicles. Other constraints (limitations) are those associated with politics, society, and religion: ١٠٧

108 Political limitations: This means gaining permission form land owners tenants ( (الم ستا جرين for access to land and communications with clients which often requires great diplomacy. It is important to have a permission from the appropriate authority to carry out geophysical field work. Examples: permissions from a local council if survey work along a major road is being considered. Permissions from the local harbour master in case of marine surveys to safe other shipping. Social limitations: In designing the geophysical survey, the questions must be asked" Is the survey technique socially and environmentally acceptable? ١٠٨

109 It is always best to keep on good terms with the local people. Treating people with.(فواي د) respect will always bring dividends Each survey should be socially and environmentally acceptable and not cause a nuisance (problems). An example is in not choosing to use explosives as a seismic source for reflection profiling through urban areas or night. Instead, the seismic vibrator technique should be used. Another example: an explosive source for marine reflection profiling would be inappropriate in area associated with a fishing industry because of possibly unacceptable high fish kill. Religious limitations: Religious traditions must be respected to avoid difficulties. The survey should take into account local social customs such as: ١٠٩

110 Muslims like to go to their mosques on Friday afternoon and are thus unavailable for work then. Similarly, Christian workers do not like to work on Sundays or Jews on Saturdays. Geophysical survey design: A- Target identification: Geophysical methods locate boundaries across which there is a marked contrast in physical properties. Such a contrast gives rise to geophysical anomaly which indicates variations in physical properties relative to some background value. ١١٠

111 ١١١

112 o The physical source of each anomaly is termed the geophysical target. Some examples of targets are trap structures for oil and gas, mineshafts, pipelines, ore bodies, cavities, groundwater, buried rock valleys. o In designing a geophysical survey, the type of target is of great importance. Each type will dictate (tell) to a large extent the appropriate geophysical method (s) to be used. ١١٢

113 Examples: - Consider the situation where saline water intrudes into a near- surface aquifer, saline water has a high conductivity ( low resistivity ) in comparison with fresh water and so is best detected using electrical resistivity or electromagnetic conductivity methods. - Gravity method would be in appropriate in this case because there would be no density contrast between the saline and fresh water. - Similarly, seismic methods would not work as there is no significant difference in seismic wave velocities between the two saturated zones. - Also, the shape and size of the target is important to know. In the case of a metallic ore body, a mining company might need to known its lateral and vertical extent. This comes from the amplitude of the anomaly (I.e. its maximum peak-to peak value). ١١٣

114 B- optimum line configuration: There is an important question in this case:" How are the data to be collected in order to define the geophysical anomaly? Two concepts need to be introduced, namely: profiling and mapping. a- profiling : * It is a mean of measuring the variation in a physical parameter along the surface of a two dimensional cross section. * The best orientation of a profile is normally at right angles to the strike of the target. Indication of ١١٤

115 geological strike may be obtained from existing geological maps, mining records, etc.. The length of the profile should be greater than the width of the expected geophysical anomaly to define a background value. Data values from a series of parallel lines or from a grid can be contoured to produce a map on which all points of equal values are joined by isoclines A great care has to be taken over the methods of contouring or else the resultant map can be misleading. ١١٥

116 C- Selection of station intervals: * The point at which a geophysical measurement is made is called a station and the distances between successive measurements are station intervals. * The success of a geophysical survey depends on the correct choice of station intervals. It is a waste of time and money to record too many data and equally wasteful if too few are collected. * How is a reasonable choice of station intervals to be made?. This requires some idea of the nature and size of the geological target. * Any geophysical anomaly found will always be larger than the feature causing it. For example: to find a mineshaft with a diameter of two meter, an anomaly with a width of at least twice this might be expected. Therefore, it is necessary to choose a station interval that is sufficiently small to be able to resolve the anomaly. ١١٦

117 * Reconnaissance survey tend to have coarser station intervals in order to cover a larger area quickly and to indicate zones over which a more detailed survey should be conducted with a reduced station interval and a more closely spaced set of profiles. ١١٧

118 * The figure (A) shows a typical electromagnetic anomaly for a buried gas pipe. The whole anomaly is 8m wide. If a 10m sampling interval is chosen, then it is possible either to clip the anomaly as shown in figure (B) or to miss it entirely (fig. C). * The resultant profile with 2 m and 1m sampling intervals are shown in figures (D) and (E) respectively. * The smaller the sampling interval, the better the approximation is to the actual anomaly. * The loss of high- frequency information, as in figures (B) and(c), is a phenomenon known as know as spatial aliasing. * Another from of spatial aliasing may occur when gridded data are contoured, particularly by computer software. For example figure 1.8 A shows a hypothetical aeromagnetic survey. This ١١٨

119 map was complied from contouring the original data at line spacing of 150m. * Figures (B) and (C) were contoured with line spacing of 300m and 600m respectively. * The difference between the three maps is very marked, with a significant loss of information between figures (A) and (C). * The higher frequency anomalies have been aliased out, leaving only the longer wavelength (lower frequency) features. ١١٩

120 * In addition, the orientation of the major anomalies has been distorted by the crude contouring in figure (C). * The spatial aliasing can be removed or reduced using mathematical functions, which provide ١٢٠