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

Magnetotellurics in Frontier and Reconnaissance

Exploration

Karen Rae Christopherson– Chinook Geoconsulting, Inc.

– Evergreen CO USA

1© Chinook Geoconsulting, Inc

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’


Typical Resistivity Values


Resistivity Values

1

10

100

1000

10000

Cla

y

Sh

ale

Vo

lc'c

las

tic

s

Sa

nd

sto

ne

Ca

rbo

na

te

Vo

lc's

Ign

eo

us

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


Distortion of Magnetosphere


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

GEOSYSTEM11

© Chinook Geoconsulting, Inc

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 or more others simultaneously


MT world-wideWesternGeco


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


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

16

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 – Equipment Manufacturers

Phoenix (Canada) Metronix (Germany) Zonge (USA) KMS (USA) Etc…….

GE

OS

YS

TE

M


MT – Onshore Contractors

WesternGeco (Italy, US, UK, Canada) CGG (Italy, US, France) Phoenix (Canada) Quantec (US/Canada) Zonge (US) Geodatos (Chile) BGP (China) EMPulse (Canada)

– Plus more…………..

GE

OS

YS

TE

M

“know about your contractor”


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


PNG Overthrustx

| 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

31

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

32

MT – Geothermal: Newberry

Waibel et all, 2012, GRC Transactions 33© Chinook Geoconsulting, Inc

MT – Geothermal: Newberry

Waibel et all, 2012, GRC Transactions


Salt Problem

Tr Salt

OligocenePliocene

Jurassic Carbonates and older

WellTop of salt 1800m

GEOSYSTEM

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

GEOSYSTEM 36

Turkey


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


Columbia Plateau WA

MILES 0 20 40 60 80

| || | | | | | | | | | | | ||| | 1 2 3 4

5 6 7

8 9

10 11

1213

141516 17

18

20000

0

50

100

500

7

DepthinFeet

West East

100000

0

2-D Model crbp

W E

W-E

|001|002

|003|004

|005 |

006

|007 |

008

|009|010

|011|

012

|013|

014

|015|016

|017|

018

120

Ohm-M

0.1

0.2

0.5

1

2

7

10

50

50

100

200

100

1000

2000

500

300

Volcanic-covered province

Covers 35000 sq miles

Where is the basin?

Flood basalts (Miocene) up to 20000 feet thick

Clastic section up to 20000 feet thick beneath basalts

Non-seismic area

Basalt

Basement

clastics


Acknowledgments

Geosystem srl

USA/Italy/UK Lisle Gravity, Inc.

40

Documents

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