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

MT world-wide Oil/gas Minerals Geothermal

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

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

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 v About 5% of depth e.g. the top of a horizon at 10000’ can be mapped to ’

Resistivity Values

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

MT - Detail vs. Recon u Detail: prospect definition v spacing =.5 km on profiles u Recon: areal coverage v spacing = 1-5 km on profiles or grids u Communication: GPS sync u Acquisition rate: usu sta’s/day MT in Nicaragua by horseback MT helicopter survey in Montana

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

MT - Acquisition u Five channels at each station v Ex Ey Hx Hy Hz u Two to five stations simultaneously u GPS sync between stations u 24-hour recording/layout/pickup cycle u In-field processing and editing Laying out a coil in Turkey; coils are used to measure the magnetic fields: Hx Hy Hz

MT Acquisition System Batteries Digital Acquisition Unit E-Lines Coils

MT Acquisition Coils Hx Hy Hz Amplifiers, digitizer, etc. Electrodes E-Lines Ex Ey Computer GPS antenna v One station set-up; 2-6 others simultaneously

MT Data record This is an actual time series record, showing (from top) Ex, Ey, Hx, and Hy varying with time. Note the correlation between Ex and Hy, and between Ey and Hx. Hz is not shown. Ex Ey Hx Hy

How resistivity is computed u Impedance tensor is measured at surface u Compute apparent resistivity (and phase) as a function of frequency  Two values computed,  xy and  yx, for the two orthogonal pairs of E and H sensors in horizontal directions u Thus can interpret for strike and dip directions  2

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

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

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

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

MT - Contractors u Geosystem (Italy, US, UK) u Phoenix (Canada) u Metronix (Germany) u Geodatos (Chile) u AOA (US - Marine) u Zonge (US) u Geoinvest (Italy) MT + EM in Turkey GEOSYSTEM

MT Data Curves Apparent Resistivity LOG RHO (OHM-M) LOG Frequency (Hz) RhoXYRhoYX Apparent resistivity Two curves, xy and yx Qualitative view of subsurface changes in resistivity Used with phase data for interpretation Limestone Clastics Basement

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

Robust Processing u Improve data quality by –time series editing –removal of outliers –removal of coherent noise –frequency domain editing –use of “quiet” remote After Before

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

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

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

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

Egele MT, PNG MT predicted base Darai Ls w/in 7% Drilled by Mobil Exploration = only surface geology and MT Well Basement Limestone clastics

Irou, PNG MTGeology based on MT and dips VE=1:1 LIMESTONE clastics

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

MT in N. Africa HIGH RESISTIVITY METAMORPHIC BASEMENT DOLOMITE+ANHYDRITE: POTENTIAL RESERVOIR UNIT LOW RESISTIVITY SHALE BASALT & SAND DUNES VERTICAL EXAGGERATION=2.0. Hi Lo Resistivity

DOLOMITE+ANHYDRITE: RESERVOIR STRUCTURE CROSS-SECTION FROM 2D INVERSION SHOWING (a) RESERVOIR STRUCTURE (b) STRUCTURE AT TOP OF BASEMENT. VERTICAL EXAGGERATION=2.5. MT in N. Africa Hi Lo Resistivity

Greenland Nuusaaq peninsula West coast Volcanics at surface Rough terrain Basin structure unknown Recon MT lines Offshore seismic has basement at >7 km Some strat holes

Salt Problem Tr Salt Oligocene Pliocene Jurassic Carbonates and older Well Top of salt 1800m

Turkey Poor seismic MT shows overthrust Ties with good seismic reflectors (white lines) Supported by drilling Target is Mardin carbonate Next slide shows sharp- boundary inversion with interpretation and seismic picks

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

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