Deep crustal seismic reflection and magnetotelluric (MT) surveys

The Geological Survey of Western Australia (GSWA) undertakes collaborative projects with Geoscience Australia, neighbouring State Geological Surveys and Australian and international universities to investigate lithospheric architecture. Techniques which have been used to image to the Moho and beyond include active source seismic refection, passive seismic techniques and MT. Often these are done across the same profile/area with additional gravity surveys.

Deep crustal seismic reflection surveys

Deep crustal seismic reflection surveys
Deep crustal seismic reflection surveys conducted within Western Australia on a background of the Bouguer Anomaly

Deep seismic reflection profiling has been part of the GSWA campaign to explore how the different components of the Western Australian landmass are configured and how they came together. By listening for the reflections of a source on the surface for length of up to 20 seconds, the deeper levels of the Earth’s crust can be explored. Several transects have been completed across various craton margins enabling the seismic characteristics of the basins, craton margins and other features to be imaged.

Since 2010, Exploration Incentive Scheme (EIS) funding has enabled GSWA — in collaboration with Geoscience Australia (GA) — to substantially extend the coverage of deep crustal 2D reflection seismic transects across key geological regions of the State.

In 2019, GSWA also conducted a series of more closely spaced, higher resolution shallow crustal traverses (10 s two-way-time) in the highly prospective Eastern Goldfields area.

GSWA has reprocessed data from a 1991 deep crustal seismic reflection survey in the Eastern Goldfields. The original data were acquired by Geoscience Australia’s predecessor agency, the Bureau of Mineral Resources (BMR) along an aggregate 260 km along three lines: BMR91-EGF01 (~212 km), BMR91-EGF02 (~28 km) and BMR91-EGF03 (21 km). The reprocessed data can be downloaded from GSWA and from Geoscience Australia, who also provide the originally processed version.
Reprocessing of the data using modern processing techniques, which were unavailable at the time of the original data acquisition and initial processing, has led to improved imaging of the subsurface, with better resolution of structural features. The overall improvement in quality will allow more confident interpretation of shallow structures and lower crustal architecture. The improved seismic data complements other geoscience datasets in GSWA’s Eastern Goldfields high-resolution reflection seismic survey and of the Far East Yilgarn Program of GSWA’s Accelerated Geoscience Program.


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Magnetotelluric (MT) surveys

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

MT is a geophysical method that involves measuring and relating natural time-varying electrical and magnetic fields that have been induced within the Earth by the Earth’s geomagnetic field and solar winds and by large thunderstorms. The objective is to resolve the conductivity structure of the subsurface.

The relationship between these horizontal and mutually perpendicular fields recorded at each station provides amplitude (apparent resistivity) and phase lags as a function of frequency, commonly referred to as MT response curves. With increasing depth there is an exponential decrease in the amplitudes of the 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 versus depth can be made beneath each site based on the MT 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.

GSWA is acquiring MT surveys in collaboration with the Centre for Exploration Targeting at The University of Western Australia, and with the University of Adelaide.

GSWA is also gearing up to host the Australian Lithospheric Architecture Magnetotelluric Project (AusLAMP), a collaborative national survey which acquires long-period MT data. It aims to provide significant additional information about Australia’s geodynamic framework as well as valuable public data for resource exploration. Significant progress has been made under GA's Exploring for the Future program. This is a long-term project expected to start in 2021 and take several years to complete.



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