ambiguity in earth sciences IESO Geophysics Section Eddy hartantyo, Lab Geofisika FMIPA UGM Pelatihan Tahap II IESO Teknik Geologi UGM Februari 2009 1
Introduction Photos from http://www.eegs.org/whatis/ and personal collection Geophysics Identify Geologic Problem Design Survey Acquire Field Data Data Reduction Model Data Interpret Model Write Report Photos from http://www.eegs.org/whatis/ 2
What is Geophysics? Application of physics to studies of the Earth, Moon, and other planets Includes Earth, Atmosphere, Oceans, Planetary systems What is Geophysics? http://seg.org/you-geo/definition.shtml The study of the earth by quantitative physical methods Often refers to solid-earth geophysics only, thus excluding portions of other subjects Exploration geophysics is the use of seismic, gravity, magnetic, electrical, electromagnetic, etc., methods in the search for oil, gas, minerals, water, etc., with the objective of economic exploitation. 3
Examples of Geophysics Pure and Applied Geophysics Solid Earth Surface Geophysics Applied Geophysics (economic objectives) Exploration Geophysics Engineering Geophysics Environmental Geophysics Groundwater Geophysics Archaeo Geophysics Forensics 4
Applications? http://seg.org/you-geo/definition.shtml Approach to earthquake hazards Plate tectonics, the study of the interior structure of the earth Human's ability to exploit natural resources Engineering Environmental Hydrological Geophysical Methods Respond to physical properties of subsurface media Can you name some of these properties? Passive vs. active methods A very wide range of powerful tools Can you name some? 5
Examples of geophysical methods? Gravity http://www.vibra-tech-inc.com/content.asp?id=magnetics_gravity Examples of geophysical methods? Magnetics http://www.vibra-tech-inc.com/content.asp?id=magnetics_gravity 6
Examples of geophysical methods? Seismics a. Reflection b. Refraction c. Surface or borehole REFRACTION SEISMIC INTERPRETATION Travel-time (ms) 200 175 150 125 100 75 50 25 0 200 175 150 125 100 75 50 25 0 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 2400 2500 distance (m) 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 2400 2500 height (m) 400 350 300 250 10 20 30 40 50 60 70 80 90 100 110 120 150 160 170 180 190 140 400 460 130 200 210 370 410 220 360 380 390 420 450 470 480 BH7/0 430 440 BH 06 BH 1 230 350 240 340 BH 2 250 260 330 270 280 320 290 300 310 BH 9/BH 4 400 350 300 250 200 200 Legend: Soil (soft to hard) - Vp = 784 m/s Bedrock (weathered) - Vp = 1920 m/s Bedrock (fresh) - Vp = 3091 m/s 100 BH 06 Peg/Mark Number Borehole Number 7
Examples of geophysical methods? Ground Penetrating Radar Examples of geophysical methods? Resistivity 8
Geophysical Methods Critical to select correct tool(s) for objective either alone or part of a larger survey Need to understand geologic problem, physical property to be measured If physical principles are misunderstood, then results may be misinterpreted Constraints of Study Geological, Physical, Financial, Temporal Engineering Evaluation Seismic Resistivity EM GPR Magnetics Gravity Borehole Surveys Soil/Rock Engineering Properties 1 3 3 - - - - Excavation Characteristics 1 2 3 3 - - 1 Foundation Evaluation 1 3-3 - - 1 Utility/Rebar Location - 3 2 1 2 - - Roadway Evaluation 1 2 2 1 - - - Electrical Grounding - 1 3 - - - - Geologic Seismic Resistivity EM GPR Magnetics Gravity Borehole Surveys Depth To Rock 1 2-2 - - 1 Faults And Fractures 1 1 2 2 - - 1 Sinkholes and Voids 2 1 2 2-3 2 Karst Areas 2 1 2 2-3 - Stratigraphic Mapping & 1 2 3 3 - - - Hydrogeologic Seismic Resistivity EM GPR Magnetics Gravity Borehole Surveys Water Table Mapping 2 1-1 - - 3 Water Well Location 2 1 1 2-3 2 Seepage Detection 3 1 2 2 - - - Aquifer Thickness 2 2 3 3 - - 1 Saltwater Intrusion Mapping - 3 1 - - - 1 Aquiclude Continuity 3 3 2 1 - - - Environmental Seismic Resistivity EM GPR Magnetics Gravity Borehole Surveys UTS'S/Drums/Utilities - - 1 1 2 - - Landfills and Buried Debris 3 3 1 2 2 - - Soil Groundwater Contamination- 2 1 2 3-2 Plume Delineation - 2 1 2 - - 3 Free Product Detection - - 1 1 - - - Mine Planning Seismic Resistivity EM GPR Magnetics Gravity Borehole Surveys Overburden Estimation 1 2 - - - - - Sand and Gravel Deposits 2 1 2 2 - - 2 Clay Layer Mapping 3 2 1 1 - - 2 9
Geophysical Methods May not yield complete answer or unique solution; Non-uniqueness of forward modeling Many solutions, but which ones are nonsense, which are plausible Interpret within physical constraints & geological framework Geophysics and Geology Geology places constraints on geophysical models Geophysics places constraints on geologic interpretations 10
Planning a Survey - Spatial Aliasing Two and Three Dimensions Location of Profiles 11
Survey Noise Data Processing - Elimination of Noise 12
Displaying Data Displaying Data 13