Exploration Geophysics - Volume 32, Issue 1, 2001

High temperature superconducting quantum interference device (SQUID) magnetometers have been developed in a collaborative project between BHP and CSIRO specifically for application in airborne time domain electromagnetic (TEM) surveying. The objective of this development was to improve the performance of the system in detection of conductors with longer decay time constants, particularly in the presence of a conductive overburden. The sensors were incorporated into a specially designed receiver system and successfully tested as receivers for the GEOTEM system with the assistance of Fugro Airborne Surveys. Their performance was shown to be comparable with, but not superior to current induction coil TEM systems. The development of the receiver system required solutions to a range of problems for the sensor devices and for the receiver system. The principal obstacle for the receiver was in overcoming the high dynamic environment of the towed receiver bird and the consequent high level of noise associated with motion of the sensor in the Earth’s magnetic field. The high dynamic range of the magnetometer response, which arises from this motion, was addressed by a combination of modification of the sensor fluxlocked loop and periodic resetting of the sensor offset. A digital stacking filter was used to eliminate low-frequency noise associated with motion and a specially designed suspension system was developed to isolate the sensor from higher-frequency motions of the towed bird.

With the advent of wideband three-component airborne electromagnetic (AEM) systems, the primary-field components at the sensor (bird) can be determined with a high degree of accuracy. Using all three components of the primary-field information, it is possible to determine the vertical, longitudinal and transverse offsets of the bird from the transmitting loop.

At altitudes high above the ground surface, the primary-field values will be unaffected by any response from the ground, so the estimated offsets should be fairly accurate. This has been confirmed by independent measurements from a laser range finder. At survey altitudes, laser range finders are not practical, so the primary-field method has been used to dynamically estimate the bird position. In two examples presented, the estimated positions are not affected significantly by ground response, and therefore appear to be reasonable estimates. For a typical flight using a bird and tow cable with reasonably high drag coefficients, the bird position is confined to within a few metres of the mean position.

The dynamic bird position was used as one of the inputs to a conductivity/depth estimation algorithm and the conductivity/depth results are compared with the results obtained when the nominal bird position was used. In this particular example, using the estimated position rather than the nominal position resulted in minimal improvement of the conductivity/depth section.

The weathered rock and transported overburden, which covers much of Australia, reduces the effectiveness of electromagnetic (EM) surveys. This material, termed the regolith, is variably conductive, both laterally and vertically, and reduces the sensitivity and penetration of EM systems.

AMIRA (Australian Mineral Industries Research Association) project 407 was set up to improve the effectiveness of EM and especially airborne EM (AEM) in Australia. One of the specific aims was to study the electrical structure of the regolith in the expectation that a better understanding might lead to a significant reduction, or even removal, of its effect on deep penetrating AEM methods. As part of that study, several sites around Australia were selected for a detailed study of the electrical structure of the regolith and its effect upon AEM systems. This report discusses the results from a SALTMAP survey over the Lawlers area, which lies within the Eastern Goldfields region of the Yilgarn Craton in Western Australia.

To test the accuracy and resolvability of the AEM data, a number of AEM responses were followed up with ground EM surveys. Both sets of data were interpreted using layered earth inversion algorithms. It was appreciated that a stitched set of 1D layered structures would not accurately model the physics of all of the responses, however it was expected that the vast majority could be usefully treated this way. The results show that although there may be up to six or even seven geologically identified layers with distinct electrical properties in the regolith, neither the airborne or surface EM data can effectively resolve more than three layers. However, good fits to three layer models can be obtained and much can be inferred about the nature of the regolith. At Lawlers, these layers can be broadly represented as follows: (1) the (mostly) resistive surface layer of colluvium/alluvium; (2) underlying conductive saprolite, mottled clays, etc.; and (3) resistive basement.

Fixed wing airborne electromagnetic (AEM) data in Australia commonly exhibit numerous local responses, most of which have been attributed to regolith inhomogeneities rather than the isolated conductive targets of sulphide exploration. To define the regolith structures causing local AEM responses several steps are needed. The first involves selecting a geological mapping scale matched to the scale of an AEM system. Sensitivity analysis indicates that an AEM system has a limited range, and is generally insensitive to any features located more than 200 to 300m from the system. Generally, any confined conductive target cannot be detected at a distance more than a few times its lateral dimensions. With a nominal transmitter altitude of 120m, this would set (say) a 30m or 40m minimum size on any structure likely to produce an anomaly. Long narrow features can however gather current and be detectable. A mapping scale was therefore indicated in the range between 1:10,000 and 1:100,000.

In the Lawlers district of WA, the electrical structure could be resolved into at most 3 layers, specifically a thin (commonly 0 to 10m), moderately resistive layer of alluvium/colluvium overlying a thick (commonly 30 to 50m) conductive layer of saprolite/sediments, overlying a relatively resistive basement. One or both of the upper layers are absent in some areas.

Seven important structures were identified at Lawlers at the scales suggested above. Drillhole data, ground and airborne EM, and ground resistivity were used to define vertical and lateral variations of importance within this overall layering scheme. The three common electrical structures were classified as 1) Layer (locations far from lateral inhomogeneity), 2) Contact, and 3) Wedge models. Four less common structures were the 4) Variable basement topography, 5) Variable surface topography, 6) Lateral inhomogeneity, and 7) Palaeochannel models. Geological and physical reasoning constrain the permissible geometry and dimensions of each of these structures.

SALTMAP airborne electromagnetic (AEM) data for the Lawlers District, W.A., show that areas of complex regolith cover are characterised by marked variations in electrical conductivity. Their interpretation against ground electromagnetic, petrophysical and drill hole data indicates that conductive zones lie within the regolith, usually between a thin, relatively resistive, surface layer and a resistive basement. The conductive layer is commonly associated with the saprolite and to a lesser extent with alluvium in palaeovalleys and drainage sumps. Interpretation of the AEM with aeromagnetic and field data indicates that there is a strong lithodependence in the observed conductivity structure. Structure is also important. One-dimensional layered earth inversions and conductivity depth sections showed that the conductivity and thickness of regolith materials (predominantly saprolite) varied with lithology. For example, the felsic volcanics located in the NE margin of the survey area are characterised by a thick, poorly conductive saprolite. This contrasts with a thinner, more conductive saprolite developed over adjacent mafic lithologies. Somewhat surprising were the electrical characteristics of the ultramafic units, which appeared to be similar to those of the felsic volcanic units, suggesting thick, relatively resistive, materials. This behaviour is at odds with that reported in other studies concerning the electrical properties of weathered ultramafics in other environments. The Lawlers study suggests that differences in the electrical properties of in-situ regolith materials can be attributed to the complex interplay between the manner in which particular lithologies weather, the character of the resulting regolith in terms of porosity and permeability, and to variations in the soluble salt content and quantity of the saturant waters. The "EM response map" of the Lawlers area is further complicated by transported cover which appears to impose a cross cutting conductivity structure over that of the underlying saprolites.

The Green's function of the Poisson equation, and its spatial derivatives, lead to a family of wavelets specifically tailored to potential fields. Upward continuation of the field is seen to be identical to these wavelets' scale change operation. The maxima at all heights of the field's horizontal gradients are termed the field'smultiscale edges. A multiscale edge's field strength variation and geometry contain information about the geometry and type of discontinuity in the source. The assumptions that “rocks have edges” and that these discontinuities are represented in the field'smultiscale edges appears to collapse much of the ambiguity inherent in the inversion of potential field data. One approach to inversion is purely visual, relying upon the way that multiscale edges for dipping fault blocks sometimes “mirror” the fault geometry. A second approach recovers the density contrast, the depths to top and bottom, and the dip angle of an isolated synthetic dipping fault block by performing a search for parameters that best recreate the observed multiscale edges. A third approach relies upon naïve downward continuation. When a field is downward continued below its actual source, a common assumption about the downward continuation operator is violated, introducing wellknown oscillatory components to the previously smooth result. Such oscillatory components tend to arise first on multiscale edges. By following multiscale edges as we downward continue, we can pick the maximum depth to a source (provided our “rocks have edges” assumption is true). Finally, since the gravity field wavelet is (proportional to) a Green's function for mass dipoles, we can directly interpret the wavelet transform itself as being (proportional to) a distribution of sources composed entirely of horizontal mass dipoles. Thus, multiscale edges can be interpreted as the modes of the probability density of edges in the source distribution generated by the wavelet transform.

Detailed orebody delineation on a mine scale is usually carried out predominantly by the interpretation of information from drillhole core and cuttings. Inaccurate interpolations between drillholes can result in dilution and ore loss. This paper presents examples of the use of borehole radar to provide more continuous maps of nickel sulphide orebody outlines in Western Australia. These examples show that these maps can substantially reduce the uncertainty of orebody models resulting in improved mine plans, reduced dilution and improved ore recovery.

The data presented were collected with the GeoMole borehole radar system. This system is not directional. However, by combining information from other sources, the 3D locations of the reflectors that are imaged can often be accurately estimated and the potential errors quantified. Difficulties have been encountered when the system has been operated in salt water filled holes but good results have been obtained in these holes after they have been flushed with fresh water. The system has also been used in updipping holes up to 120 m long.

The results obtained to date indicate that substantial cost benefits can be obtained at some mine sites by incorporating borehole radar surveys into the ore delineation process.

A modified marine heat-flow probe has been proposed to conduct simultaneous measurements of thermal and electric properties of seafloor sediments. A major concern in the electric aspect is the shielding effect caused by an extremely conductive solid steel strength-member of the probe.

To appraise the feasibility of the proposed probe in electric measurements, we develop a novel numerical algorithm to calculate potential and hence apparent resistivities arising from a point-source of current near a cylinder situated in layered earth structures. The scheme is validated against an analytical solution with respect to a uniform whole-space in the presence of the cylinder. Image theory is employed to transform the layered earth structures into the whole-space models. Model parameters are taken from the design blueprint. Numerical computations for varieties of electrode configurations reveal that the dipole-dipole array suffers the smallest disturbance, and can be employed to directly measure the true electric properties of seafloor sediments as long as its electrode spacing is less than 10 cm. The electric properties are also measurable when the pole-dipole array or the Wenner array is used with the electrode spacing less than 5 cm. The pole-pole array is however not applicable.

Geophysicists commonly have a simplistic view of the water table as a sharp interface between the vadose zone (unsaturated region) and the phreatic zone (saturated region). In reality, this boundary is a transition zone where moisture content varies continuously with depth. Since geophysical methods respond to depth-related variations in water content, the use of this simplistic model could lead to significant errors in the interpretation of surface geophysical data. An improved model for the moisture content profile that incorporates different soil structures, soil type and water qualities would allow better interpretation of nearsurface geophysical surveys. In this study, an analysis of the relationship between time-varying moisture content and the response of commonly used near-surface geophysical methods has been performed using the soil-moisture simulation program LEACHM.

Model studies reveal that the interpreted depth to water table will be less than the true depth to 100% saturation, and that the non-uniqueness of interpretation generates a wide range of possible vadose models that fit data to within 5%. Heavy summer rainfall events significantly change surface geophysical responses over a two-week period; lighter winter rain will introduce smaller changes, but with greater frequency. Additionally, temperature variations in the top one metre are also important for pore-water electrical conductivity. Such variability has important consequences for the repeatability of measurements over time, and in comparing data from different areas.