Ground Gravity Surveys Research Articles

Geophysical characterisation of the Challenger gold deposit, Gawler Craton, South Australia

The Challenger Project is located in Archaean rocks of the Gawler Craton and is the most significant gold-only discovery made in South Australia. The deposit was discovered through geochemical sampling of calcrete. It has a measured and indicated resource of 1.7M t at a grade of 8.5 g/t for 465,000 oz of gold. The resource is open down-plunge and has the potential to be significantly greater. Mineralisation occurs as discrete shallowly (30°) plunging shoots hosted by granulite facies paragneiss, which lacks marker horizons to aid in geological and structural mapping. High-grade moderate to steeply dipping auriferous quartz veins occur within the shoots. No major structures are observable in the area in regional (400 m line spacing) aeromagnetic data. However, the deposit lies on the edge of a regional-scale gravity high, as do the majority of significant Archaean lode gold sites in the region. The gravity high is interpreted to indicate a buried intrusive, or crustal underplating. A high-resolution aeromagnetic survey has mapped a subtle, but easily discernible, magnetic high (5-6 nT) that is spatially coincident with the mineralisation. Modelling, constrained by core-magnetisation studies and known mineralised geometries, shows pyrrhotite within the mineralising system is the probable source of the anomaly. However, interpretation is complicated by lateral interference from lamprophyre dikes/sills. During exploration in the Challenger area, radiometrics, digital elevation models and enhanced Landsat data provided a basis for mapping the regolith. The deposit is coincident with a slight topographic high and a pronounced increase in the depth of weathering. The former is mapped by digital terrain data gathered during airborne geophysical surveys. The latter has been mapped by ground electrical resistivity, electromagnetic and gravity surveys, and drilling. A TEMPEST AEM survey has mapped the conductivity structure of the regolith in good agreement with that derived from deposit-scale ground electrical geophysics. The increase in weathering over the Challenger deposit is mapped as a second- or third-order effect. However, the survey was extremely useful because it identified and delineated hitherto unknown 2 km wide, 200 m deep, north-south striking, palaeochannels. Drilling of one of these has shown they are excellent sources of water for the future mine development. A weak bedrock IP response is coincident with the mineralisation, but is not diagnostic. Extreme difficulties were experienced in collecting high-quality IP data due to the highly resistive nature of the surface calcrete and silcrete, and the highly conductive nature of the underlying clays and saprolite material.

Rapid melting of the terminal section of the Hooker Glacier (Mt Cook National Park, New Zealand)

A 3 km long section of the Hooker Glacier near its terminus was studied in 1996 using GPS, tacheometric, and bathymetric surveys, as well as ground penetrating radar and gravity surveys. With reference to sparse surface levels and oblique photos dating back to 1889, the studies indicate that between c. 1915 and 1964 downwasting of an axial strip along the terminal section occurred at a rate of c. 0.7 m/yr. Between 1964 and 1986 the rate increased to 1.0 m/yr. Marginal segments of the glacier near the terminus experienced positive buoyancy from 1982 onwards, which promoted rapid melting. Apparent subaqueous melting rates of c. 9 m/yr occurred between 1986 and 1996 over large stretches of the downmelting terminal area. By 1996, a 1 4 km long sector of the glacier had melted down forming a melt lake (Hooker Lake) with a volume of c. 40 × 106 m3 covering an area of 0.78 × 106 m2. A maximum water depth of 135 m was measured near the retreating glacier front where the ice wall descends as a vertical cliff to the lake bottom and temperatures of 0.5°C prevail. Ice thickness measurements by radar surveys along profiles 1.7 and 3.0 km upstream from the terminus indicate a maximum thickness of 165 and 260 m, respectively; the results have been confirmed by the interpretation of residual gravity anomalies. The lake level is controlled by the outlet level (875 m a.s.l.) of the Hooker River, which has remained almost constant since 1889. Melting of the glacier front at a rate of c. 40 m/yr will cause the glacier to retreat at a rather uniform rate. The lake will continue to grow until it reaches the glacier bed c. 5 km upstream from the terminus.

Terrain Corrections are Critical for Airborne Gravity Gradiometer Data

Several developments aim to produce airborne gravity gradiometers with sufficient sensitivity to detect mineral deposits of several millions of tonnes at shallow depths. In a conventional ground gravity survey, a Bouguer slab correction is always necessary, with the terrain component of the full Bouguer correction often not needed unless the topography is severe. For airborne gravity gradiometers, however, the slab correction is exactly zero but the terrain corrections prove to be critical even with subdued or moderate topography.This paper reports on research into airborne gravity gradiometer corrections, especially aspects that assessed the importance of regolith and its constraints of surface and bedrock topography, together with predicted target anomalies. Using an example of topography typical of the Western Australian shield, calculations show that densities of both regolith and bedrock need to be determined to better than 0.1 t/m3 for adequate correction to be made to raw gravity gradiometer data. Topography also needs to be defined to about ±3 m on 10 m pixels for adequate gravity corrections.

Application of Geophysics to the White Devil Gold Deposit, N.T.

The White Devil gold deposit is developed within a steeply-dipping altered shear zone (predominantly chlorite schists containing stringers of magnetite) which is hosted by inter-bedded turbiditic greywackes and siltstones of the Proterozoic Warramunga Group. Ground magnetic, gravity, and triaxial drillhole magnetic surveys have been undertaken at the deposit to assist in the mapping of the size and extent of magnetite accumulations, ranging from magnetite stringers (2?5% magnetite) to massive magnetite ironstones (>60% magnetite), which occur within the steeply-dipping shear zone. The extensive database of three component magnetic data (based on triaxial drillhole magnetometer readings from over 200 drillholes) has enabled a three dimensional view of the magnetic field within a complex, highly magnetic ironstone system.

AUTOMATIC INTERPRETATION OF GRAVITY GRADIOMETRIC DATA IN TWO DIMENSIONS: VERTICAL GRADIENT1

AbstractThe magnetic and gravity field produced by a given homogeneous source are related through Poisson's equation. Starting from this consideration, it is shown that some 2D interpretation tools, widely applied in the analysis of aeromagnetic data, can also be used for the interpretation of gravity gradiometric data (vertical gradient). This paper deals specifically with the Werner deconvolution, analytic signal and Euler's equation methods. After a short outline of the mathematical development, synthesized examples have been used to discuss the efficiency and limits of these interpretation methods. These tools could be applied directly to airborne gravity gradiometric data as well as ground gravity surveys after transformation of the Bouguer anomalies into vertical gradient anomalies. An example is given of the application of the Werner deconvolution and Euler's equation methods to a microgravity survey.

The Use of Airborne Geophysics and Ground Gravity Surveys in Understanding the Geology of the Eastern Goldfields of Western Australia

Deep weathering and extensive tertiary and quaternary cover over some 350 000 sq. km of Archaean greenstone and granitic rocks of the Eastern Goldfields province of Western Australia have rendered much of the area inaccessible to direct methods of exploration. This paper shows how combined airborne magnetic and radiometric surveys with high quality resolution flown by Aerodata Holdings Ltd comprising 140 000 line km in the Wiluna Norseman sub-province (Fig. 1) have successfully penetrated the extensive cover and assisted in the geological interpretation of some 25 000 sq. km of the sub-province.

Reply by the author to T. R. LaFehr and K. C. Chan (Discussion on “The normal vertical gradient of gravity”GEOPHYSICS, 48, 1011–1031, July 1983; discussion GEOPHYSICS, 51, 1505–1508, July 1986).

Frequently, geophysical methods can be useful to those without detailed knowledge of physics: buried conducting objects can be found by a beachcomber using only a crude metal detector, conducting ore bodies can be located with an EM instrument, distinct basement units can be mapped on an aeromagnetic map as magnetic terrains, and salt domes are apparent on gravity maps of the Gulf Coast. In all of these cases, anomalous values of some measurement stand out distinctly against their background values. However, this is not the case with ground gravity surveys in areas of high topographic relief where elevation corrections can be far greater than the target anomalies.