Geology 228 Applied & Environmental Geophysics Lecture 9

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1 Geology 228 Applied & Environmental Geophysics Lecture 9 Geomagnetism and magnetic surveys Induced Polarization (IP) Nuclear Magnetic Resonance (NMR)

2 1. The magnetic field of the earth 2. Magnetic prospecting Magnetic survey instruments Proton precessing magnetometer Field procedures 3. Soil and rock magnetism 4. Magnetization and buried magnetic targets Magnetic anomaly of simple geometry Data interpretation 5. Field examples 6. Induced Polarization (IP) 1. Time domain IP and Frequency domain IP 2. Statement on IP for environmental applications 7. Nuclear Magnetic Resonance (NMR) 1. Magnetic spin of hydrogen nuclei 2. Surface NMR: principles and applications

3 Physical Quantity Symbol Definitio n Dimension Unit Abbreviation Expression Remarks Electric Current I I=U/R I Ampere A 1 A=1 A Charge q q=it TI Coulomb C 1 C=1 Asec Potential U U=W/q L 2 MT -3 I -1 Volt V 1 V=1 Nm/(Asec) W-work, in J-Jol Capacitance C C=q/U L -2 M -1 T 4 I 2 Faraday F 1 F=1 A 2 sec 2 /(Nm) Resistance R R=U/I L 2 MT -3 I -2 Ohm Ω 1Ω=1 Nm/(A 2 sec) Conductance Q Q=I/U L -2 M -1 T 3 I 2 Siemens S 1 S=1 A 2 sec/(nm) Induction L L=Φ/I L 2 MT -2 I -2 Henry H 1 H=1 Nm/A 2 Electric Field E E=F/q LMT -3 I -1 V/m F-force, in N-Newton Electric Displacement D D=εΕ L -2 TI C/m 2 Magnetic Field H H=I/2π r L -1 I A/m Magnetic Induction B B=µH MT -2 I -1 Tesla T 1 T=1 N/(Am) 1 γ= 10-9 T=1 nt Magnetic Flux Φ The SI Unit System in Electromagnetic dφ=εdt L 2 MT -2 I -1 Webb Wb 1 Wb=1 Nm/A 1 Gauss= 10-4 T=10 5 γ Magnetic Polarization M M=kH L -1 I A/m Electric Resistivity ρ ρ=1/σ L 3 MT -3 I -2 Ωm Electric Conductivity σ σ=1/ρ L -3 MT 3 I 2 S/m Dielelctric Permittivity ε L -3 M -1 T 4 I 2 F/m ε 0 =1/(36πx10 9 ) F/m Magnetic Permeability µ LMT -2 I -2 H/m µ 0 =4πx10 7 Η/m Relative Mag. Permeability µ r µ=µ 0 µ r dimensionless Magnetic Susceptibility k dimensionless

4 Fundamental Geomagnetism The total field The fundamental field The secondary field Magnetic pole drift Magnetic quiescent diurnal variation

5 The Earth acts like a great spherical magnet, in that it is surrounded by a magnetic field. The Earth's magnetic field resembles, in general, the field generated by a dipole magnet (i.e., a straight magnet with a north and south pole) located at the center of the Earth. The axis of the dipole is offset from the axis of the Earth's rotation by approximately 11 degrees. At any point, the Earth's magnetic field is characterized by a direction and intensity which can be measured. So the geomagnetic field is a vector field. Often the parameters measured are the magnetic declination, D, the horizontal intensity, H, and the vertical intensity, Z. From these elements, all other parameters of the magnetic field can be calculated.

6 The Internal Field (Main Field) The geomagnetic field measured at any point on the Earth's surface is a combination of several magnetic fields generated by various sources. These fields are superimposed on and interact with each other. More than 90% of the field measured is generated INTERNAL to the planet in the Earth's outer core. This portion of the geomagnetic field is often referred to as the Main Field. The Main Field varies slowly in time and can be described by Mathematical Models such as the International Geomagnetic Reference Field (IGRF) and World Magnetic Model (WMM).

8 Total Intensity F Declination D Inclination I Horizontal intensity H North component X of the horizontal intensity East component Y of the horizontal intensity Vertical intensity Z

9 H X Y Z = F cos I = H cos D = H sin D = Fsin I = F cos I cos D = F cos I sin D In Connecticut: F ~ nt (gamma) H ~ nt (gamma) Z ~ nt (gamma) D ~ 14 degree w. of true north I ~ 70 degree

15 NORTH MAGNETIC POLE MOVEMENT SOUTH MAGNETIC POLE MOVEMENT û 85û 135û û 145û 150û 155û 80û û -65û û û 235û 75û 240û 245û 250û û 260û û 265û 270û û 275û -70û 135û 140û û û 155û -70û

16 The External Field The Main Field creates a cavity in interplanetary space called the magnetosphere, where the Earth's magnetic field dominates in the magnetic field of the solar wind. The magnetosphere is shaped somewhat like a comet in response to the dynamic pressure of the solar wind. It is compressed on the side toward the sun to about 10 Earth radii and is extended tail-like on the side away from the sun to more than 100 Earth radii. The magnetosphere deflects the flow of most solar wind particles around the Earth, while the geomagnetic field lines guide charged particle motion within the magnetosphere. The differential flow of ions and electrons inside the magnetosphere and in the ionosphere form current systems, which cause variations in the intensity of the Earth's magnetic field. These EXTERNAL currents in the ionized upper atmosphere and magnetosphere vary on a much shorter time scale than the INTERNAL Main Field and may create magnetic fields as large as 10% of the Main Field.

17 Figure G3: The magnetic daily variation for eight days as recorded by a total-field base-station magnetometer at Broken Hill (shown by continuous lines) compared with diurnal functions recovered from the crossover misfit data of an aeromagnetic survey taking place on those days over the Frome area, several hundred km distant. Crossover misfit data have been put into bins, each of length 1 hr. The results demonstrate the usefulness of crossover misfits as a source of information on the magnetic diurnal variation.

18 Regional Magnetic Anomalies Crustal-scale anomalies are associated with crustal structures higher magnetizations found in areas underlain by rocks of Archean age α ( ω ) ds L SIgnificant R( ω ) = contrast S ( ω ) in G the PSmagnetic PR e properties and thickness of continental and oceanic rocks

19 Crustal Magnetic Model

20 Magnetic Anomaly of the United States

21 Local Magnetic Anomalies Near-surface magnetic anomalies associated with mankind activities Metals Fire using R, S : received, source signals G : geometrical spreading P S, P R : radiation pattern, receiver coupling L : propagating path

22 Magnetic Permeability and magnetic susceptibility The magnetic constitutive relation: B = r µ H = µ µ H = µ 0(1 + κ ) H 0 where B - magnetic flux density H Magnetic field µ - Magnetic Permeability µ0 magnetic permeability in vacuum µr relative magnetic permeability κ magnetic susceptibility

24 The total magnetization M is formed by two parts, the remanent (permanent) magnetization M r and induced magnetization M i, i.e., M = M r + M i

25 For the non-ferromagnetic materials, there is a linear relation between the Induced Magnetization M i and the applied magnetic field H, i.e., M = κ i m H Where κ m is the magnetic susceptibility. This relation can also be used for the soft magnetic materials in an approximate sense.

26 Magnetic Susceptibility of rocks, minerals and iron steel more rocks have a wide range: 1 ppm to 0.001; Magnetite ore can be as high as 150; Some minerals are diamagnetic (negative κ); Iron, steel have the values of

29 Magnetic surveys For environmental and engineering purposes, the magnetic surveys are designed to reveal the magnetic anomalies originated from very localized, near-surface sources

30 Proton Precession Magnetometer A bottle of liquid (water, or other fluid with a large number of hydrogen nuclei) surrounded by a suitable coil Accuracy of 0.1 nt (nano-tesla, or gamma, relative accuracy 0.1 / 55000), constrained by the polarization time, bottle of liquid (water, or other fluid with a large number of hydrogen nuclei) surrounded by a suitable coil

32 Notes on Magnetic surveys Make sure no metal in your pockets. Make sure the arrow on the sensor toward North The sensors mounted vertically like a cylindrical drum. Make sure to choose a base station in an open area, with hopefully not close to any surface and buried metal objects (no beeps when you collect the data at the base station). Make sure go back to the base station to collect 3 data points after one hour, to the least every 2-3 hours. Make sure tune the magnetometer to its largest signal strength (enter 53-56) to find the largest signal strength ( ). Make sure hit [read] and [store] to store the readings. Make sure the voltage of the batteries is always above 8.4v, otherwise change the batteries (9 Size D cells of them).

33 Magnetic Survey Design Magnetic surveys are based on the premise that a target is limited in space and has a different physical property (e.g., magnetic susceptibility), from the surrounding formation. Unlike gravity surveying, however, the variation in magnetic susceptibility for various rock types is orders of magnitude greater than the variation in density for the same rock types. Thus, even knowing the types of rocks in a specific area does not provide sufficient information to constrain susceptibilities. Like density contrast, variations in susceptibility tradeoff strongly with other model parameters. Therefore, if susceptibility, or other model parameters, cannot be constrained from different observations, it is difficult to make quantitative estimates of the geologic structure based on magnetic observations alone.

34 Magnetic Survey Design (continue) In a particular survey, we need additional constraints that allow us to use the magnetic observations in a quantitative fashion. This information is derived from other separate data sets, for example, formation layering from seismic and/or density from gravity. Once we have constrained the range of plausible geological models from other observations, we can design a magnetic survey to estimate the spatial extent of the structure and its susceptibility by in-lab forward modeling. In planning the magnetic survey, we will predict the noise from sources not of interest in the survey, estimate the standard deviation of the random (operator and instrument) noise, calculate the shape of the signal (the theoretical anomaly produced by the assumed source), then decide whether the signal generated by the target of interest is above the noise level that allow us to conduct a meaningful interpretation. If the answer is affirmative, then we determine the survey parameters that will produce the best compromise between cost and data quality.

35 Magnetic Survey Routing Lines

36 Geomagnetic Survey Applications Igneous dike intrusion Buried metals Archeological ruins R( ω ) = S ( ω ) G PS PR e Unexplored ordnance R, S : received, source signals G : geometrical spreading P S, P R L : propagating path L α ( ω ) ds : radiation pattern, receiver coupling

38 Top Sensor, Cross-Section Distance from Route 195 (m) 0 Total Magnetic Field (gamma)

39 Five torpedo boats can be clearly seen floating in the Thames River, New London, CT in this 1930s aero photo

41 Induced Polarization Induced polarization is an electromagnetic method that uses electrodes with time-varying currents and voltages to map the variation of electrical permittivity (dielectric constant) in the Earth at low frequencies. Induced polarization is observed when a steady current through two electrodes in the Earth is shut off: the voltage does not return to zero instantaneously, but rather decays slowly, indicating that charge has been stored in the rocks. This charge, which accumulates mainly at interfaces between clay minerals, is responsible for the IP effect. This effect can be measured in either the time domain by observing the rate of decay of voltage, or in the frequency domain by measuring phase shifts between sinusoidal currents and voltages. The IP method can probe to subsurface depths of thousands of meters.

42 detection of disseminated metallic minerals discrimination of clay from silt or sand where formation DC resistivities are similar In nature, the induced polarization (IP) effect is seen primarily with metallic sulfides, graphite, and clays. For this reason, IP surveys have been used extensively in mineral exploration. Recently, IP has been applied to hazardous waste landfill and groundwater investigations to identify clay zones. As with electrical resistivity surveys, vertical or horizontal profiles can be generated using IP. IP can also be used in borehole logging. Constraints: IP cannot be done over frozen ground or asphalt because good contact with the ground is required, like DC resitivitgy. IP is affected by changes in surface relief and lateral changes in resistivity. The electrode array length is about 10 times greater than investigation depth.

43 Method: Induced polarization is the capacitance effect, or chargeability, exhibited by electrically conductive materials. Time-domain IP is done by pulsing an electric current into the earth at one or two second intervals through metal electrodes. Disseminated conductive minerals in the ground will discharge the stored electrical energy during the pulse cycle. The decay rate of the discharge is measured by the IP receiver. The decay voltage will be zero if there are no polarizable materials present. Generally, both IP and resistivity measurements are taken simultaneously during the survey. Survey depth is determined by electrode spacing. The final report products are similar to those of resistivity surveys.

44 Polarizable Ground Current injected into the ground causes some materials to become polarized. There are two microscopic causes of this macroscopic effect. The phenomenon is called induced polarization, and the physical property that is measured is often called chargeability. The figure below illustrates the phenomenon observed. Note how the measured potential exhibits a delayed response when ground is chargeable. Chargeable ground may take several seconds to return to equilibrium after it has been polarized with a current source.

45 Rock / Soil System Grain Pore- Solution Surface Electrical Transport = Flow + Storage

46 What is the Interfacial Polarization? The interface between material with different electrical properties results in charge accumulation under alternating electrical field. Accumulated charges result in polarization (spatially non-uniform charge distribution).

47 Complex Conductivity Flow Storage = D J = = iωεe D t J C σe J * = J C + J D = ( σ + iωε ) E = σ * E

48 Value of the complex dielectric constant ε = ε ' + iε" is the parameter responsible for the observed phenomena in IP measurements

49 Nomenclature σ * 1 = σ * = σ + jσ ρ * = i ωε * Flow σ = ωε Storage ε * = ε + jε ε ω = κ' ε ω = σ 0

50 Complex Conductivity Berea Sandstone 0.01M NaCl ' (S/m) '' (S/m) E-02 1.E-03 1.E-04 1.E-05 σ dc 1.E-03 1.E+00 1.E+03 1.E+06 IP Frequency (Hz) 1.E-03 1.E+00 1.E+03 1.E+06 Flow σ Storage κ = σ ωε 0 κ' 1.E+09 1.E+06 1.E+03 1.E+00 Frequency (Hz) κ 1.E-03 1.E+00 1.E+03 1.E+06 Frequency (Hz)

51 Microscopic effects that cause ground to be chargeable 1)Membrane polarization 2)Electrode polarization 3)Maxwell-Wegner effect

54 Measured IP data There are four types of commonly measured IP data. Two in the time domain. Two in the frequency domain If "small" chargeabilities are assumed, the linear relationship means measured data and chargeabilities recovered by inversion have the same units.

55 Two types of time domain data 1. Dimensionless, where M = (φ s ) / (φ η ), using parameters from the adjacent waveform diagram. This form is difficult to measure directly, but some instruments provide induced polarization measurements in units of mv/v by measuring the decaying portion of this curve at several positions and normalizing these measurements by dividing by the primary voltage (φ η ). The result is sometimes multiplied by 100 so the apparent chargeability can be thought of as a percent. Also, several such measurements (perhaps 10 or more) may be combined, or recorded individually.

56 2. The most commonly measured form of time domain IP is the area under the decay curve, specified by the following equation, using parameters specified in the figure. M 1 t 2 = φs ( t) dt φ t 1 m

57 Two types of frequency domain data 1. Data with units known as percent frequency effect (PFE) require the response to be measured at two frequencies. At higher frequencies, the ground has less time to respond, therefore the signal is expected to be smaller. Below is the equation providing PFE, and a figure illustrating how the data are gathered. PPE = ( ρ ρ ρ a2 a1) a1

58 2. Data with units of phase are gathered by maintaining careful synchrony between transmitted sine wave and the received signal. Then the phase difference between the source and received signals is recorded as a measure of chargeability. Units are usually milli-radians. The following figure illustrates:

59 IP surveys use non-polarization electrodes

61 Rio Nuevo Landfill A test was conducted in1998 to determine the applicability of geophysics for locating buried waste. A series of test boring showed high correlation between IP high s and waste.

62 A full 3D survey was then conducted which detected not only waste, but also pockets of clay fill.

63 Interpretation: some techniques & pitfalls The following are a few comments on interpreting raw chargeability data, listed in no particular order. Chargeability measurements involve dynamic signals that are often 10 to 100 times smaller than signals required to obtain resistivity. Therefore results are quite susceptible to noise of all types. Recall that variations in pseudosections should be "smooth". Spikes or outliers, especially at depth, are likely due to data errors. Stripes on the other hand may be due to individual electrode placements. Remember also that each value in the pseudosection is an apparent chargeability, so intrinsic chargeability of the earth (i.e. the chargeability structure) must be interpreted just as for resistivity surveys. The effects of overburden are the same as those discussed in resistivity, but remember that potentials measured are much smaller, so conductive overburden is more difficult to deal with for chargeability. Occasionally negative apparent chargeability values will be recorded. Intrinsic chargeability can never be negative, but the apparent chargeability can be negative.

64 Nuclear Magnetic Resonance (NMR) NMR is the single geophysical tool directly inquiring the information of the pore fluids Fluid saturation Fluid type Fluid viscosity Permeability

65 Proton Precession Magnetometer A bottle of liquid (water, or other fluid with a large number of hydrogen nuclei) surrounded by a suitable coil Accuracy of 0.1 nt (nano-tesla), constrained by the polarization time,

Proton Precession Magnetometer A bottle of liquid (water, or other fluid with a large number of hydrogen nuclei) surrounded by a suitable coil Accuracy of 0.

66 Proton Precession Magnetometer A bottle of liquid (water, or other fluid with a large number of hydrogen nuclei) surrounded by a suitable coil Accuracy of 0.1 nt (nano-tesla, or gamma), constrained by the polarization time, bottle of liquid (water, or other fluid with a large number of hydrogen nuclei) surrounded by a suitable coil

67 Proton Precession B

68 Surface Nuclear Magnetic Resonance (NMR) Survey

69 Many atomic nuclei, including protons of the hydrogen atoms in water molecules, have a magnetic moment µ. These nuclei can be described in terms of a spinning charges particle. Generally speaking µ is aligned with the local magnetic field B 0 of the earth. When another magnetic field is applied, the axis of the spinning proton is deflected, owing to the torque applied. When the second field is removed, the protons generated a relaxation magnetic field as they become realigned along B 0 while precessing around with the Larmor frequency: ω 0 =γ B 0, where γ= Hz/nT, the gyromagnetic ratio for hydrogen protons. In SNMR surveys, the measurements use a circular or rectangular loop. An alternating current with a frequency of ω 0 is passed through this loop for a limited time τ, so that an excitation intensity (pulse moment) of q=i 0 τ is achieved. After the current in the loop is shut off, a voltage is induced in the loop by the relaxation of the protons. The initial amplitude of this induced voltage is directly related to the water content; meanwhile, the relaxation time T 2 is directly associated with the porosity or grain size.

70 The protons generated a relaxation magnetic field as they become realigned along local Terrestrial geomagnetic field B 0 while precessing around B 0 after the excitation current turning off, with the Larmor frequency, ω 0 =γ B 0, = Hz/nT X nt = 14,713 Hz In SNMR surveys, the measurements use a circular or rectangular loop. An alternating current with a frequency is passed through this loop for a limited time t, so that an excitation intensity (pulse moment) of q=i 0 τ is achieved. After the current in the loop is shut off, a voltage is induced in the loop by the relaxation of the protons. The initial amplitude of this induced voltage is directly related to the water content; meanwhile, the relaxation time T 2 is directly associated with the porosity or grain size.

71 Properties of Spin When placed in a magnetic field of strength B, a particle with a net spin can absorb a photon, of frequency f. The frequency f depends on the gyromagnetic ratio, γ, of the particle. f = γ B For hydrogen, γ = MHz / T.

72 Nuclei with Spin The shell model for the nucleus tells us that nucleons, just like electrons, fill orbitals. When the number of protons or neutrons equals 2, 8, 20, 28, 50, 82, and 126, orbitals are filled. Because nucleons have spin, just like electrons do, their spin can pair up when the orbitals are being filled and cancel out. Almost every element in the periodic table has an isotope with a non zero nuclear spin. NMR can only be performed on isotopes whose natural abundance is high enough to be detected. Some of the nuclei routinely used in NMR are listed below. Nuclei Unpaired Protons Unpaired Neutrons Net Spin γ(mhz/t) 1 H 1 0 1/ H P 1 0 1/ Na 1 2 3/ N C 0 1 1/ F 1 0 1/

75 Summary: IP and NMR Magnetic components, anomaly type Both need more physical insight and high demand of skills High potential Surface NMR is still in infancy NMR holding the promise for environmental & groundwater research

Geology 228/378 Applied & Environmental Geophysics Lecture 8. Induced Polarization (IP) and Nuclear Magnetic Resonance (NMR)

Geology 228/378 Applied & Environmental Geophysics Lecture 8 Induced Polarization (IP) and Nuclear Magnetic Resonance (NMR) Induced Polarization (IP) and Nuclear Magnetic Resonance (NMR) 1. Time domain