Magnetotellurics in Frontier and Reconnaissance Exploration v Karen

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

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 10 k. Hz to. 001 Hz u Used to derive the resistivity structure of the subsurface © Chinook Geoconsulting, Inc 2

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, u 1990’s: Amoco, Sohio, Arco, CGG most work outsourced to contractors and consultants © Chinook Geoconsulting, Inc 3

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 +- 500’ © Chinook Geoconsulting, Inc 4

Typical Resistivity Values © Chinook Geoconsulting, Inc 5

Resistivity Values © Chinook Geoconsulting, Inc 6

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 © Chinook Geoconsulting, Inc to >20 km 7

MT - Detail vs. Recon u Detail: prospect definition v spacing u Recon: =. 5 km on profiles areal coverage v spacing = 1 -5 km on profiles or grids u Communication: GPS sync u Acquisition rate: usu. 5 -10 sta’s/day MT in Nicaragua by horseback © Chinook Geoconsulting, Inc MT helicopter survey in Montana 8

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

Distortion of Magnetosphere © Chinook Geoconsulting, Inc 10

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 © Chinook Geoconsulting, Inc 11

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

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

MT world-wide Western. Geco © Chinook Geoconsulting, Inc 14

MT Data record – Time Series 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, Hx. R and Hy. R. Ex Ey Hx Hy Hz Hx. R Hy. R © Chinook Geoconsulting, Inc TIME 15

How resistivity is computed u Impedance tensor is measured at surface u Compute apparent resistivity (and phase) as a function of frequency ra 2 values computed, rxy and ryx, for the two orthogonal pairs of E and H sensors in horizontal directions u Thus can interpret for strike and dip directions u Two 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 r = resistivity and f = frequency © Chinook Geoconsulting, Inc 17

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 – 1 d, 2 d, 3 d, modeling: fwd and inverse © Chinook Geoconsulting, Inc 18

State of the Art MT Systems 1 u u u Low weight (5 kg); low power cnsmption (. 6 A) Wide frequency range (DC to 30 KHz) Wide dynamic range (120 db, 24 -bit A/D) = v u better S/N; less risk of saturation Internal recording v (32 MB u flashcard, 1 GB hard disk) Recording schedule downloaded from PC © Chinook Geoconsulting, Inc 19

State of the Art MT Systems 2 u GPS-synchronized v no ( 130 ns accuracy) cables or radios u 2 to 8 -channel units, all independent u High reliability (ISO 9001 std), etc. u Fast set-up and deployment v increased u Operating from -40 to +75 C; waterproof; v lightning u Cable-link © Chinook Geoconsulting, Inc production protected available for EMAP 20

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

MT – Onshore Contractors (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) “know about your – Plus more…………. . contractor” GEOSYSTEM u Western. Geco © Chinook Geoconsulting, Inc 22

MT Data Curves Apparent Resistivity 4 Limestone 3 LOG RHO (OHM-M) Basement 2 1 0 • Apparent resistivity • Two curves, xy and yx • Qualitative view of subsurface changes in resistivity • Used with phase data for interpretation Clastics -1 2 Rho. XY 1 Rho. YX © Chinook Geoconsulting, Inc 0 -1 -3 -2 LOG Frequency (Hz) 23

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 © Chinook Geoconsulting, Inc 24

Robust Processing u Improve data quality by – time series editing – removal of outliers – removal of coherent noise – frequency domain editing – use of “quiet” remote © Chinook Geoconsulting, Inc After Before 25

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 © Chinook Geoconsulting, Inc 26

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 u Best jump at all freq’s along a curve correction = TDEM v Acquire EM data at station center v Interpret for near-surface section v Incorporate into MT data and shift MT curve © Chinook Geoconsulting, Inc 27

Advantages and disadvantages of AMT/MT for petroleum exploration: Pros u u u u u Great depth of penetration (10's of kms) Provides information in nonseismic 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 © Chinook Geoconsulting, Inc Cons 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 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 u u u 28

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 = $100 k/km+ for 2 -D u Most data poor to no-record u Alternative = MT, surface geology mapping, Sr isotope dating of limestone © Chinook Geoconsulting, Inc 29

PNG Overthrust u u © Chinook Geoconsulting, Inc 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 DOLOMITE+ANHYDRITE: POTENTIAL RESERVOIR UNIT HIGH RESISTIVITY METAMORPHIC BASEMENT BASALT & SAND DUNES LOW RESISTIVITY SHALE Hi Lo Resistivity VERTICAL EXAGGERATION=2. 0. 31

MT in N. Africa DOLOMITE+ANHYDRITE: RESERVOIR STRUCTURE Resistivity Hi Lo CROSS-SECTION FROM 2 D INVERSION SHOWING (a) RESERVOIR STRUCTURE (b) STRUCTURE AT TOP OF BASEMENT. VERTICAL EXAGGERATION=2. 5. 32

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

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

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

Turkey • Poor seismic • MT shows overthrust • Ties with good seismic reflectors (white lines) • Supported by drilling • Target is Mardin carbonate • Next slide shows sharpboundary inversion with interpretation and seismic picks 36

Turkey © Chinook Geoconsulting, Inc 37

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

Columbia Plateau WA Volcanic-covered province Basalt Covers 35000 sq miles clastics Basement u u u Where is the basin? Flood basalts (Miocene) up to 20000 feet thick Clastic section up to 20000 feet thick beneath basalts Non-seismic area © Chinook Geoconsulting, Inc 39

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