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

Magnetotellurics in Frontier and Reconnaissance

Exploration

Karen Rae Christopherson– Chinook Geoconsulting, Inc.

– Evergreen CO USA

MT world-wide

Oil/gas

Minerals

Geothermal

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

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

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’

Resistivity Values

0.01

0.1

1

10

100

1000

10000

Mas

sive

Sul

fides

Sea

Wat

er

Cla

y

Sha

leV

olc'

clas

tics

San

dsto

neH

ydro

carb

on-f

illed

Car

bona

te

Vol

c's

Igne

ous

Resistivity

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

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

MT in Nicaragua by horseback MT helicopter

survey in Montana

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

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

Laying out a coil in Turkey;coils are used to measure themagnetic fields: Hx Hy Hz

GEOSYSTEM

MT Acquisition System

Batteries

Digital AcquisitionUnit

E-Lines

Coils

MT Acquisition

Coils Hx Hy Hz

Amplifiers, digitizer, etc.

Electrodes

E-Lines Ex Ey

ComputerGPS antenna

One station set-up; 2-6 others simultaneously

MT Data recordThis 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 Impedance tensor is measured at surface 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

Thus can interpret for strike and dip directions

afEx Hy

1

5/

2

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 resistivity and f = frequency

500 / f

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

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

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

MT - Contractors

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

MT + EM in Turkey

GE

OS

YS

TE

M

MT Data Curves

2 1 0 -1 -2 -3

-1

0

1

2

3

4

Apparent Resistivity

LO

G R

HO

(O

HM

-M)

LOG Frequency (Hz)RhoXY RhoYX

•Apparent resistivity•Two curves, xy and yx•Qualitative view of subsurface changes inresistivity•Used with phase datafor interpretation

Limestone

Clastics

Basement

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

Robust Processing

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 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

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

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

Pros Cons

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

Egele MT, PNG

MT predictedbase Darai Lsw/in 7%

Drilled by Mobil

Exploration =only surfacegeology and MT

Well

Basement

Limestone

2000

0

clastics

Irou, PNG

MT Geology based on MT and dips

VE=1:1

LIMESTONELIMESTONE

clastics

PNG Overthrust

x

| 11 |

10

| 9 |

8

| 7 |

6

| 5 |

4

| 3 |

2

| 1

-5000

0

FEET

0 5000 10000 15000

2

3

5

10

20

50

100

200

500

10002000

Ohm-M

clastics

limestone

limestone

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

MT in N. Africa

HIGH RESISTIVITYMETAMORPHIC BASEMENT

DOLOMITE+ANHYDRITE:POTENTIAL RESERVOIR UNIT

LOW RESISTIVITYSHALE

BASALT &SAND DUNES

GEOSYSTEM

VERTICAL EXAGGERATION=2.0.

Hi

Lo

Resistivity

DOLOMITE+ANHYDRITE:

RESERVOIR STRUCTURE

GEOSYSTEM

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

-10000

-9000

-8000

-7000

-6000

-5000

-4000

-3000

-2000

-1000

0

1000

Ele

vatio

n (M

ET

ER

S)

SW NE

104105

106107

108109

110111

112

METERS

05000 10000 15000 20000 25000

Ohm-M

AIR

1

10

20

30

50

70

100

200

500

1000

Basement (approx.)

Lo -rho clastics

Volcanics

Higher-rho clastics

Salt Problem

Tr Salt

OligocenePliocene

Jurassic Carbonates and older

WellTop of salt 1800m

GEOSYSTEM

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

GEOSYSTEM

Granite Overthrust - Wyoming

Unexplored - no seismic, no wells

MT shows structure - amount of subthrust

Fast acquisition and interpretation

| | | || |

30000

20000

10000

0

Dep

th(F

EE

T)

Ohm-M

2

5

10

20

50

100

200

500

1000

| 1

2

| 3

4

| 5

6

| 8

9

| 10

11

| 12

13

| 14

15

| 16

FEET

0 5000 10000 15000 20000 25000 30000 35000

Precambrian

|

Tertiary

T/K/J

Granite overthrust

Tr and older

Acknowledgments

Geosystem srl

USA/Italy/UK Lisle Gravity, Inc.