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

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 2 © Chinook Geoconsulting, Inc

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 3 © Chinook Geoconsulting, Inc

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 ’ 4 © Chinook Geoconsulting, Inc

Typical Resistivity Values 5 © Chinook Geoconsulting, Inc

Resistivity Values 6 © Chinook Geoconsulting, Inc

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 7 © Chinook Geoconsulting, Inc

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 8 © Chinook Geoconsulting, Inc

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 9 © Chinook Geoconsulting, Inc

Distortion of Magnetosphere 10 © Chinook Geoconsulting, Inc

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 11 © Chinook Geoconsulting, Inc

MT Acquisition System Batteries Digital Acquisition Unit E-Lines Coils 12 © Chinook Geoconsulting, Inc

MT Acquisition Coils Hx Hy Hz Amplifiers, digitizer, etc. Electrodes E-Lines Ex Ey Computer GPS antenna v One station set-up; 2-6 or more others simultaneously 13 © Chinook Geoconsulting, Inc

MT world-wide WesternGeco 14 © Chinook Geoconsulting, Inc

Ex Ey Hx Hy Hz HxR HyR TIME 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 the vertical magnetic field, and also shown are the remote reference data, HxR and HyR. MT Data record – Time Series 15 © Chinook Geoconsulting, Inc

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 16

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 17 © Chinook Geoconsulting, Inc

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 18 © Chinook Geoconsulting, Inc

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 19 © Chinook Geoconsulting, Inc

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 20 © Chinook Geoconsulting, Inc

MT – Equipment Manufacturers u Phoenix (Canada) u Metronix (Germany) u Zonge (USA) u KMS (USA) u Etc……. GEOSYSTEM 21 © Chinook Geoconsulting, Inc

MT – Onshore Contractors u WesternGeco (Italy, US, UK, Canada) u CGG (Italy, US, France) u Phoenix (Canada) u Quantec (US/Canada) u Zonge (US) u Geodatos (Chile) u BGP (China) u EMPulse (Canada) –Plus more………….. GEOSYSTEM “know about your contractor” 22 © Chinook Geoconsulting, Inc

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 23 © Chinook Geoconsulting, Inc

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 24 © Chinook Geoconsulting, Inc

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 25 © Chinook Geoconsulting, Inc

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 26 © Chinook Geoconsulting, Inc

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 27 © Chinook Geoconsulting, Inc

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 28 © Chinook Geoconsulting, Inc

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 29 © Chinook Geoconsulting, Inc

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 30 © Chinook Geoconsulting, Inc

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 31

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 32

MT – Geothermal: Newberry Waibel et all, 2012, GRC Transactions 33 © Chinook Geoconsulting, Inc

MT – Geothermal: Newberry Waibel et all, 2012, GRC Transactions 34 © Chinook Geoconsulting, Inc

Salt Problem Tr Salt Oligocene Pliocene Jurassic Carbonates and older Well Top of salt 1800m 35

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 36

Turkey 37 © Chinook Geoconsulting, Inc

Granite Overthrust - Wyoming Unexplored - no seismic, no wells MT shows structure - amount of subthrust Fast acquisition and interpretation 38 © Chinook Geoconsulting, Inc

Columbia Plateau WA Volcanic-covered province Covers sq miles Where is the basin? u Flood basalts (Miocene) up to feet thick u Clastic section up to feet thick beneath basalts u Non-seismic area Basalt Basement clastics 39 © Chinook Geoconsulting, Inc

Acknowledgments u Geosystem srl USA/Italy/UK u Lisle Gravity, Inc. 40