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Magnetotellurics in Frontier and Reconnaissance Exploration

MT world-wide. MT - Definition. Passive surface measurement of the earth's natural electrical (E) and magnetic (H) fields Measure changes in E and H w/timeFrequency range 10kHz to .001 HzUsed to derive the resistivity structure of the subsurface. MT - History. First used for academic and geothermalMap plate boundaries, alteration, etc.Use for petroleum starting ~19801980's: many in-house groupsShell, Amoco, Sohio, Arco, CGG1990's: most work outsourced to contractors and consultant197

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Magnetotellurics in Frontier and Reconnaissance Exploration

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    1. Magnetotellurics in Frontier and Reconnaissance Exploration Karen Rae Christopherson Chinook Geoconsulting, Inc. Evergreen CO USA

    2. MT world-wide

    3. MT - Definition Passive surface measurement of the earth’s natural electrical (E) and magnetic (H) fields Measure changes in E and H w/time Frequency range 10kHz to .001 Hz Used to derive the resistivity structure of the subsurface

    4. MT - History First used for academic and geothermal Map plate boundaries, alteration, etc. Use for petroleum starting ~1980 1980’s: many in-house groups Shell, Amoco, Sohio, Arco, CGG 1990’s: most work outsourced to contractors and consultants

    5. Resistivity Contrasts There must be a significant resistivity contrast within the depth of investigation for the method to be useful Contrast of 5:1 or greater Resolution depends on thickness and depth of unit being mapped About 5% of depth e.g. the top of a horizon at 10000’ can be mapped to +- 500’

    6. Resistivity Values

    7. MT - Application - Oil/Gas Reconnaissance or detail High-resistivity (high-velocity) surface (volcanics, carbonates, igneous) Overthrust, fold belts, volcanics Poor or no-record seismic OR Precede seismic, or integrate w/ seismic Near-surface to >20 km

    8. MT - Detail vs. Recon Detail: prospect definition spacing = .5 km on profiles Recon: areal coverage spacing = 1-5 km on profiles or grids Communication: GPS sync Acquisition rate: usu. 5-10 sta’s/day

    9. MT - Source Field High frequencies (>1 Hz) = Spherics thunderstorm activity world-wide Low frequencies (<1 Hz) = Micropulsations Solar wind interacting w/ magnetic field Vary on hourly, daily, yearly cycles

    10. MT - Acquisition Five channels at each station Ex Ey Hx Hy Hz Two to five stations simultaneously GPS sync between stations 24-hour recording/layout/pickup cycle In-field processing and editing

    11. MT Acquisition System

    12. MT Acquisition

    13. MT Data record

    14. How resistivity is computed Impedance tensor is measured at surface Compute apparent resistivity (and phase) as a function of frequency Two values computed, rxy and ryx, for the two orthogonal pairs of E and H sensors in horizontal directions Thus can interpret for strike and dip directions

    15. Depth of Investigation The depth of investigation is a result of the frequency and resistivity of the subsurface Lower frequency = deeper penetration Higher resistivity = deeper penetration Skin depth is an approximate estimate of depth of penetration at particular frequency and resistivity Skin depth (in meters) = where r = resistivity and f = frequency

    16. MT: Current Systems Similar to seismic advances since the 1980’s 24-bit A to D GPS Synchronization Unlimited no. of channels Signal/robust processing Workstations w/ integration of other data 1d, 2d, 3d, modeling: fwd and inverse

    17. State of the Art MT Systems 1 Low weight (5kg); low power cnsmption (.6A) Wide frequency range (DC to 30 KHz) Wide dynamic range (120db, 24-bit A/D) = better S/N; less risk of saturation Internal recording (32MB flashcard, 1GB hard disk) Recording schedule downloaded from PC

    18. State of the Art MT Systems 2 GPS-synchronized ( 130ns accuracy) no cables or radios 2 to 8-channel units, all independent High reliability (ISO9001 std), etc. Fast set-up and deployment increased production Operating from -40 to +75C; waterproof; lightning protected Cable-link available for EMAP

    19. MT - Contractors Geosystem (Italy, US, UK) Phoenix (Canada) Metronix (Germany) Geodatos (Chile) AOA (US - Marine) Zonge (US) Geoinvest (Italy)

    20. MT Data Curves

    21. MT - Processing Remote-reference Coherency check on time series between stations; toss un-coherent data Next: Edit data in time and frequency domain Remove noise from trains, lightning, power stations, etc. Greatly improves data quality

    22. Robust Processing Improve data quality by time series editing removal of outliers removal of coherent noise frequency domain editing use of “quiet” remote

    23. MT - Interpretation PC workstation Editing, viewing of data and parameters Data basing 1-D, 2-D, 3-D modeling: fwd and inverse Convert apparent resistivity vs. frequency to true resistivity vs. depth Colored x-sections and maps Integration w/ geology, seismic, other data Fast turnaround - can be done in the field

    24. MT - Statics Problems Near-surface distortions to electric field created by resistivity variation at surface channels, outcrop, etc. Cause “static” shift in data DC jump at all freq’s along a curve Best correction = TDEM Acquire EM data at station center Interpret for near-surface section Incorporate into MT data and shift MT curve

    25. Advantages and disadvantages of AMT/MT for petroleum exploration: Great depth of penetration (10's of kms) Provides information in non-seismic or poor seismic areas No transmitter required Light-weight equipment --very portable Good production rate (2 - 5 km/day) Better resolution than grav/mag Well-developed interpretation procedure Fast interpretation Little impact on environment Can access almost anywhere Coupling with lateral conductors (e.g. sea) also has to be considered Natural signal can be irregular, and industrial noise a potential problem Resolution less than seismic Data processing and interpretation are complex Static shift of apparent resistivity curves sometimes significant Inversion techniques rely on smooth models, tougher to interpret in complex areas

    26. PNG Seismic Exploration in Papua New Guinea fold belt difficult due to steep dips, remote location, karstified limestone surface Surface limestone = 1-3 km thick Seismic costs = $100k/km+ for 2-D Most data poor to no-record Alternative = MT, surface geology mapping, Sr isotope dating of limestone

    27. Egele MT, PNG

    28. Irou, PNG

    29. PNG Overthrust Limestone thrust over very low resistivity clastics Map depth to base of hanging wall ls Map depth to top of footwall ls Target is folded ss in hanging wall section

    30. MT in N. Africa

    32. Greenland

    34. Turkey

    36. Granite Overthrust - Wyoming Unexplored - no seismic, no wells MT shows structure - amount of subthrust Fast acquisition and interpretation

    37. Columbia Plateau WA Volcanic-covered province Covers 35000 sq miles Where is the basin?

    38. Acknowledgments Geosystem srl USA/Italy/UK Lisle Gravity, Inc.

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