The Mise-a-la-Masse (MALM) method is a cost-effective and quick method of energising conductive mineralisation, such that the data garnered will yield information on the structure and geometry of the target mineralisation. The method is adept at brownfields exploration of known mineralisation, as it can record the subsurface electrical interconnections between mineralisation in the area. Interpretation methods for MALM are primarily qualitative, involving a plot of electrical potential data acquired from the instrument with little or no processing (Ketola 1972). Whilst this is a valid approach, it yields little depth information of the anomalous targets, and simply indicates areal extent of them. The purpose of this study is to investigate alternate means of readily interpreting the data, through threedimensional (3D) forward modelling, simple processing, and image approximation to acquire further information, in particular, depth from the MALM dataset.
In a previous paper (Heinson & Constable 1992) we discussed the effect of coastlines on sea-floor magnetotelluric (MT) data collected deep within the ocean basins. In order to quantify the geomagnetic coast effect we developed a model for oceanic upper mantle electrical conductivity independent of previous sea-floor MT interpretations, based on sea-floor controlled-source electromagnetic (CSEM) soundings, laboratory studies of mantle materials, and global geomagnetic-sounding estimates for lower mantle conductivity. We demonstrated that the coast effect of our model was large, was not manifest as severe anisotropy in the MT response if the ocean basins were bounded by coastlines on three or more sides, and that a qualitative agreement existed between the MT response of our simple coastline model and published sea-floor MT data. The implication of the latter is that by including the effect of the coastlines additional mechanisms for enhanced upper mantle conductivity, such as volatiles, carbon, melt, hydrogen and other factors, may not be required by the MT data. Tarits, Chave & Schultz (1993) question the validity of a large number of aspects of our work. Their principal conclusions are that (a) horizontal or vertical leakage paths between the ocean and other oceans or the mantle will substantially reduce the magnitude of the ocean-wide coast effect; (b) the principle of extrapolating laboratory electrical-conductivity measurements to mantle conditions is unreliable from uncertainties in the understanding of pressure, temperature and petrological states of the upper mantle; (c) published sea-floor MT data sets examined by us were of such vintage that any conclusions drawn were unreliable or incorrect; and (d) the conductivity profile compiled by Chave, Flosadottir & Cox (1990), based on 1-D interpretations of sea-floor MT and CSEM data that took no account of the coast effect, is consequently a more reliable estimator of mantle-conductivity structure than our model. While all aspects of Tarits et d’s comments were addressed to some extent in our original paper, we did not investigate all details fully either in the interests of conciseness, or because the impact on our conclusions was minimal. We will take this opportunity to delve further into some of these issues, but before considering the complications it will be useful to reiterate the motivation for our work. First, Oldenburg’s (1981) and Oldenburg, Whittal & Parker’s (1984) 1-D interpretations of three sea-floor MT data sets included a high-conductivity zone (HCZ) in the oceanic upper mantle that decreased in magnitude and deepened with increasing age. The existence of a HCZ has become a paradigm not only amongst the electromagnetic community but also with seismologists, petrologists and mineral physicists. Many entire papers are written on the subject, attempting to explain the disparities between laboratory measurements of olivine conductivity at upper mantle temperatures and the HCZ, correlating the HCZ with seismic low-velocity zones (LVZ), examining the temporal evolution of the HCZ, etc. The impact of Oldenburg’s work has been so large that it seemed appropriate to re-examine the basis of the HCZ and question whether it is indeed required by the data. Note that
AbstractThe Newer Volcanic Province (NVP), western Victoria, Australia, represents the most extensive and youngest volcanism of the entire intraplate volcanic field of eastern Australia. The nature of, and mechanism(s) for, melting of the source magma of the NVP is still unclear. Previous teleseismic studies associate the magma genesis for the NVP to conduits of a mantle plume. Here we present data from a long-period MT array conducted over the same grid as the teleseismic survey, across the southern end of the Lachlan and the Delamerian Orogenies, western Victoria in a rectangular grid with nominal 270 km x 150 km dimensions. Forward modelling of MT data suggests that the lithosphere beneath the Lachlan orogeny is more conductive than the Delamerian counterpart by several orders of magnitude, perhaps associated with thinning of the lithosphere beneath the Lachlan orogeny. The phase tensor analysis illustrates that there is an increasing conductivity trend beneath the Central Highlands, observed up to 500 s, that is perhaps associated with NVP magma source region. Furthermore, the geoelectric strike direction beneath the Central Highlands is aligned parallel to the NW-SE Mesozoic-Cenozoic fracture zones, which coincides with the highest density of eruptions of the volcano field.KeywordsMagnetotelluricPhase tensorLachlanVolcanoAustralia
SummaryMagnetotelluric (MT) techniques measure natural time variations of the Earth’s magnetic and electric fields to infer subsurface electrical conductivity structure. Data are collected over a range of frequencies, providing insights into how this structure varies with depth. Depending on the Earth conductivity and frequencies used, information can be obtained from the near surface to depths of hundreds of kilometres.MT surveying has been used in a wide variety of geological scenarios, from investigations of continentalscale structures to mineral and geothermal exploration, and in the search for ground-water, and many such surveys have now been undertaken in South Australia. Recently, surveys have been conducted by Geoscience Australia (GA) under the Australian Government’s Onshore Energy Security Program (OESP) along deep crustal seismic reflection transects, in part in collaboration with the University of Adelaide (UA), the Geological Survey of South Australia, Primary Industry and Resources South Australia (GSSA, PIRSA) and the Australian National Seismic Imaging Resource (ANSIR) across the Gawler Craton and Curnamona Province. Given the wide range of applications for MT data, it is proposed to deliver these data online as industry-standard electrical data interchange (EDI) files, starting with the most modern datasets.This paper presents an overview of the MT data and reports presently available for South Australia. All MT data are available for download online from the South Australian Resources Information Geoserver (SARIG), and both seismic and MT data acquired by GA and collaborators under the OESP are available for download from the GA web site.
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