Effect of Magnetotellurics Data Density in Geothermal Resource

Exploration

Mehran GharibiQuantec Geoscience

GeoPower, Turkey 2011

Outline:

• Objectives

• Introduction to Magnetotelluric (MT) Method

• Titan-24 and Spartan MT measurements

• MT in Geothermal Exploration

• 3D Inversion and Interpretation

• Summary

Objectives:Investigate resolution and reliability of the 3D MT

resistivity models in terms of the data density used

in inversion; i.e. Resolution vs. Frequency density.

Magneto-Telluric (MT) Method• The MT is a frequency-domain natural-field EM geophysical

method – frequency range 10000 Hz - 0.001 Hz

• The method uses the earth’s EM natural time variations

- Lower Frequencies ( < 1 Hz): Ionospheric; initiated by interaction of the solar wind with the earth’s magnetic field.

- Higher Frequencies ( > 1 Hz): Atmospheric; initiated by global lightning activities.

• Image the earth's electrical resistivity structure – from nearly the surface to several tens of km.

MT measurements

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛=⎟⎟

⎠

⎞⎜⎜⎝

⎛

y

x

yyyx

xyxx

y

x

HH

ZZZZ

EE Apparent

ResistivityResistivity of SubsurfaceCalculate Inversion

Ey

Ex

HyHz

Hx

Schematic of simultaneous measurement of plane-wave electromagnetic field fluctuations in five components: Ex, Ey, Hx, Hy and Hz

Electric field dipoles

Magnetic field sensors

Ground surface

Calibration Chamber for Magnetic Sensors3-layer passive shielded and active field cancellation calibration room

Frequency (Hz)

Passive Sheilding

Factor

Active Sheilding

Factor

10K 100,000 100,000

1K 50,000 50,000

100 2,000 2,000

10 100 1,000

1 50 1,000

0.1 20 1,000

0.01 10 1,000

3-layer Passive Magnetically

Shielded Room

Active-Field Cancellation

Frame

4 m

Solenoid coil

Magnetic sensor

Spartan MT acquisition

Field setup • Man portable• Coils buried• Flexible spacing• Low environmental footprint

Acquisition system • Full tensor MT• 24-bit resolution• Automatic data acquisition• Remote reference processing

8

Line Length - 2400 m24 Ex 100m dipoles12 Ey 100m dipoles25 current stations2 Bx/By magnetometer sites

current electrode (mobile)current electrode (mobile)

100m

crosscross--line potential electrode (fixed)line potential electrode (fixed)

2400m

inin--line potential electrode (fixed)line potential electrode (fixed)

2 channel AM2 channel AM

1 channel AM1 channel AM

BxBy

2 ChannelAM

Battery

TypicalStationSet-up

BxBy

RemoteRemoteMagnetometerMagnetometer

sitesite

>20 km

Base magnetometer SiteBase magnetometer SiteLAN Link to Logging Truck

50m

infinity current electrode (fixed)infinity current electrode (fixed)>10 km

100m

Titan-24 MT Acquisition• Full tensor MT• 24-bit resolution• High spatial site density• DC-IP acquisition capability

MT Method in Geothermal Explorations:

• MT data are used to produce electrical resistivity distribution of the subsurface

• Electrical resistivity is a function of;• solid matrix - geological formation and alteration• pore fluids - chemistry and salinity• porosity – geological fissure and fracture• temperature

• A geothermal system or a hydrothermal reservoir is defined and controlled by a combination of the above factors.

• Resistivity signature associated with the geothermal system is used to detect/map/characterize the reservoir

Interpretation of the MT Data

• Data

• Inversion

• ResistivityModel

⎟⎟⎠

⎞⎜⎜⎝

⎛− 00Z

Z⎟⎟⎠

⎞⎜⎜⎝

⎛0

0

yx

xy

ZZ

⎟⎟⎠

⎞⎜⎜⎝

⎛

yyyx

xyxx

ZZZZ

1-D 2-D 3-D

ρ ρxy, ρyx

Spartan MT Field Survey

• Large scale MT survey over a geothermal resource area

• More than 160 MT sites

• Site spacing between 500 m and 1000 m

• Frequency range 250 Hz – 0.001 Hz

• Objective is to identify an exploration drilling location

• Geothermal reservoir target at >1500 m

• Interpretation based on 3-D with different number of frequencies.

MT survey location Map

3D inversion area 2D cross-sections

Original Decimated

3-D InversionsThe original frequency band is decimated for the 3-D inversions

5 Frequencies (1 per decade)

3-D Inversion – 5 Frequencies• # of frequency per site: 5• # of sites: 162• # of Mesh: 25 x 41 x 18• Inversion time: 602 hours• Hardware: 2 x CPU processor with 4 GB RAM

4 km

3-D Inversion – 5 Frequencies

Interpreted Geothermal

System

ResistiveCore

ConductiveCap

Surface = 0 m

3-D Inversion – 5 Frequencies

Interpreted Geothermal

System

ResistiveCore

ConductiveCap

Surface = -1300 m

3-D Inversion – 5 Frequencies

Interpreted Geothermal

System

Anomaly orArtifact !?

Surface = -2000 m

3-D Inversion – 5 Frequencies

Interpreted Geothermal

System

Anomaly orArtifact !?

Surface = -3000 m

Selected Cross-sections

Interpreted Geothermal

System

ResistiveCore

ConductiveCap

3-D Inversion – 5 Frequencies

3-D InversionsThe original frequency band is decimated for the 3-D inversions

18 Frequencies (3 per decade)

Original Decimated

3-D Inversion – 18 Frequencies• # of frequency per site: 18• # of sites: 162• # of Mesh: 25 x 41 x 18• Inversion time: 552 hours• Hardware: 2 x 4 CPU processor with 24 GB RAM

4 km

3-D Inversion – 18 Frequencies

Interpreted Geothermal

System

ResistiveCore

ConductiveCap

Surface = 0 m

3-D Inversion – 18 Frequencies

Interpreted Geothermal

System

ResistiveCore

ConductiveCap

Surface = -1300 m

3-D Inversion – 18 Frequencies

No Artifact !Geothermal reservoir!?

Surface = -2000 m

Selected Cross-sections

Interpreted Geothermal

System

Interpreted Geothermal

System

18 Frequencies 5 Frequencies

Summary

• Geothermal fields and resources can efficiently be mapped and characterized using MT measurements.

• 3-D inversions are computationally expensive;- a subset of the dataset with 3 frequencies/decade

would be sufficient to produce the resistivity distribution of the subsurface while minimizing the artifacts

• Development in 3D inversion;- Parallel computing of the 3-D MT inversion using

cluster of high performance CPUs (e.g. 48 CPUs and 64 GB RAM)

- It speeds up the inversion process several times (5-10 times).

Thank You

Arequipa, Peru

Reno, USA

Toronto, Canada

Lobatse, BotswanaSantiago, Chile

Mendoza, Argentina

Hermosillo, Mexico

Goiania, Brazil

Mumbai, India

Brisbane, Australia

www.quantecgeoscience.com