Magnetotelluric (MT) surveys

SBA11
MT surveys conducted within Western Australia on a background of the Bouguer Anomaly.

Magnetotelluric (MT) is a passive geophysical technique that measures the natural time-varying electromagnetic fields that pass through the Earth associated with world-wide lightning storms and the interaction of the Earth’s magnetic field with the sun’s solar winds. By measuring these naturally occurring electromagnetic fields the conductivity structure of the subsurface can be understood to 100’s of kilometers depth.

The relationship between these horizontal and mutually perpendicular fields recorded at each station provides apparent resistivity and phase lag information as a function of frequency, commonly referred to as MT response curves. With increasing depth there is an exponential decrease in the amplitude of electromagnetic fields, the so-called skin-depth phenomenon. The depth of penetration (or skin depth) of these fields is directly related to frequency (the lower the frequency, the greater the depth) and the resistivity of the material (the greater the resistivity, the greater the depth). This means estimates of resistivity vs depth can be made beneath each site based on the MT apparent resistivity response curves. However, since the conductivity of the Earth varies from location to location, so the conversion factor from frequency to depth will also vary.

A variety of different passive MT surveying techniques are available for different depths of investigation:

  • Audio magnetotellurics (AMT)
  • Broadband mangnetotellurics (BBMT)
  • Long–period magnetotellurics (LPMT)

AMT methods are typically used for investigating the conductivity variations of the near surface to several kilometers depth, while BBMT methods have the added advantage of also detecting responses down to 10s of kilometers depth. In contrast, LPMT surveys measure responses from depths of several 100 kilometers and provide conductivity information of the lower crust and upper mantle.

RPI_electromagnetic_graph
Range of frequencies/periods typically detected by different electromagnetic surveying methods.

Equipment

LPMT surveys utilize buried potential field sensors (electrodes) and a magnetic induction sensor (magnetometer) connected to a centralized data logging unit to record natural time-varying electromagnetic fields. A typical site setup will include 4 electrodes pots each buried ~30cm into the ground, spaced 100m apart, and located in a crosshair pattern aligned with magnetic N-S and E-W. A magnetometer is buried within one of the 4 electrode quadrants to a depth of ~50cm, and a grounding electrode is buried close to the center point of the site. All the sensors are connected to a data logger, powered by a battery and solar panel. Equipment is deployed for 3-6 weeks to record data at periods greater than 104s.

Schematic diagram illustrating the typical site setup for LPMT equipment.
Schematic diagram illustrating the typical site setup for LPMT equipment
DJI_0037
Aerial photograph of an AusLAMP LPMT station deployed in the Eucla, WA

Specifically, the electromagnetic field information recorded by LPMT surveys come from the interaction of the Earth’s magnetic field with the sun’s solar winds. The suns solar activity is predicted to increase over the next 3 years which is ideal for the collection of LPMT data used for understanding the Earth’s deep crust and upper mantle.

Current solar cycle
Current solar cycle prediction from the Space Weather Prediction Center

WA-MT

SBA9
Plan for the acquisition of LPMT across WA as part of the WA-MT program.

The Geological Survey of Western Australia (GSWA) is acquiring MT surveys in collaboration with the Centre for Exploration Targeting at The University of Western Australia, The University of Adelaide, AuScope and with Geoscience Australia. GSWA has also commenced WA-MT, a program that will aim to cover the West Australian state with MT data by expanding on the Australian Lithospheric Architecture Magnetotelluric Project (AusLAMP), a collaborative national survey which acquires long-period MT data. The WA-MT program involves the deployment of LPMT equipment arranged in a 40km x 40km grid pattern that will coincide with the WA Array passive seismic data acquisition sites. The entire state will be mapped by relocating instruments monthly to progressively cover the nine WA Array regional project areas. The acquired MT data will be made available for geological interpretation of the subsurface by explorers, researchers, and the community.

Western Australia remains under-imaged in this technique. WA-MT aims to provide significant additional information about Australia’s geodynamic framework as well as valuable public data for resource exploration.

Outcomes of this project include:

a) increased pre-competitive geoscience data for reducing exploration risk

b) a world-leading dataset covering the WA state to attract resource investment

c) a lithospheric architecture model complimenting the WA-Array passive seismic project

d) complimentary statewide conductivity data products to the recently acquired statewide airborne electromagnetic survey data across the state.

Three dimensional (3D) resistivity model
Three dimensional (3D) resistivity model with Traverses 1 and 2 combined. Overlain are constraints derived from previous geophysical studies.

 

 

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Survey

 

MAGIX R#

2023-24

AusLAMP Eucla Long–period MT Survey

Planned Geoscience Australia MT collection for 2023-2024

2023-24

AusLAMP South–west Long–period MT Survey

Planned Geoscience Australia MT collection for 2023-2024

2020

Kalgoorlie EIS Funded Broadband MT Survey

72154

2019

Eastern Goldfields Long–period MT Survey

72164

2017

Coompana Pre-drill Boradband Magnetotelluric Data bordering WA

Download from Geoscience Australia 

2017 NEW: Rhea Broadband MT Survey 72579

2016-18

AusLAMP Musgraves Long–period MT Survey

72444 

2016

AusLAMP Capricorn Long–period MT  

Data being processed by Geoscience Australia

2016

Yamarna Broadband MT Survey  71590 
2016 NEW: Plumridge Lakes Broadband MT Survey 72583

2015-16

AusLAMP South Australia Long–period MT bordering WA 

Download from SARIG 

2014

Eucla–Gawler Deep Seismic Reflection and Broadband MT Surveys 

71124 

2014 NEW: Abra Broadband MT Survey 72580

2013-15

Central and East Capricorn Orogen Broadband MT Surveys 

71507 

2012-13

Albany-Fraser Orogen Broadband MT Survey 

70989 

2012

Kimberley Craton Broadband MT Survey 

71007 

2012

Tropicana Broadband MT Survey 

71292 

2011 NEW: Mt Monger Broadband MT Survey 72575

2011

Yilgarn Craton–Officer Basin–Musgrave Province Deep Seismic Reflection and MT Surveys (Long–period and Broadband) 

71009 

2011

East Capricorn Orogen (Marymia) Broadband MT Survey 

71022 

2011

West Musgrave Province Broadband MT survey 

71024 

2011 NEW: DeGrussa Broadband MT Survey 72582
2011 NEW: Magnus Broadband MT Survey 72581

2010

West Capricorn Orogen Deep Seismic Reflection and Broadband MT Surveys  

70968 

2010

Youanmi Deep Seismic Reflection and MT Surveys (Long–period and broadband) 

70992 

2009

Southern Cross Broadband MT survey 

71023 

2009

Byro Broadband MT Survey 

71181 

2006

Kalgoorlie Deep Crustal Reflection Seismic Survey and Long–Period MT Survey 

72001 

2003

Gascoyne Long–period MT survey 

72006 

2000

Kimberley–Halls Creek Long–period MT Survey 

72000 

 

Contact
For more information contact:
wa.mt@dmirs.gov.au