1887
Volume 24 Number 5
  • ISSN: 0263-5046
  • E-ISSN: 1365-2397

Abstract

The presence of mining-related cavities or karstic features in the rock mass and their actual or potential collapse pose a severe geohazard and a range of subsidence-related problems for both current and future users of that land. Cavities constitute a hazard to both development and redevelopment as their migration to the surface, as sinkholes or fractured and disturbed ground, may seriously damage property and services, and in severe and catastrophic failure, cause potential significant loss of life. The most common natural targets in karst environments are solution-related features such as voids, extended cavern systems, and the collapse/drainage features associated with swallow holes (or sinkholes). Manmade cavities, including mine workings, shafts and tunnels, are just as hazardous and can be even more prevalent than natural features, particularly in industrialized environments. Prior to the development (or redevelopment) of a site, the most common method of site investigation has been to drill an extensive pattern of boreholes over the target area in an attempt to locate and then define the spatial extent of any cavities. Indirect techniques such as geophysics can give a cost-effective, non-invasive method of cavity delineation with targeted drilling used as a verification tool rather than a primary search technique. The existence of a cavity alters the physical state of the strata and results in a contrast between the cavity and the host stratum that can be detected using suitable geophysical methods if the contrasts are large enough and the features are of a sufficient size (McDowell, 2002). Microgravity involves measuring minute changes in the gravitational pull of the Earth and interpreting the presence of subsurface density variations, such as those produced by voids and cavities, from an analysis of these readings. A cavity usually has a lower density than the surrounding material and may be filled with water, sediment, collapse material, or a mixture of all of these. A void therefore represents a mass deficiency in the subsurface and a very a small reduction in the pull of the Earth’s gravity is observed, which is called a negative gravity anomaly. Although the method is simple in principle, measurement of the minute variations in the gravity field of the Earth to a few parts per billion requires the use of highly sensitive instruments, strict data acquisition procedures, stringent quality controls, careful data reduction, and sophisticated digital data analysis techniques in order to evaluate and interpret the data. These gravity anomalies are superimposed onto much larger variations produced by elevation, topography, latitude earth tides, and regional geological variations and are, usually, almost undetectable by conventional gravity investigations. Microgravity surveying has developed considerably over the last 10 years with the development of modern, high resolution instruments, careful field acquisition procedures, sophisticated data reduction methods, and advanced analysis techniques. Qianshen (1996) presents a thorough review of the fundamentals of the microgravity technique although interpretation in particular has developed significantly since then. It is now possible to detect and interpret anomalies as small as 10 microgal with a repeatability of a few microgals. Not only can the isolated anomalies reveal the location of mines, caverns and voids, either natural or man-made, but they also provide information on their depths, shapes and morphology. Through the use of Euler deconvolution and Gauss’s theorem, the topology and the ‘missing mass’ associated with the void can be calculated in order to provide vital information for the development of remediation strategies and, ultimately, the costs associated with cavity filling. Through the targeted use of repeated post-remediation microgravity surveys, assessments can be made on the success, or not, of the remediation process and help verify the location and distribution of materials used to fill the void space. These attributes have led to the method becoming widely used in hydrogeological, engineering and geotechnical investigations with the significant advantage of leaving the ground completely undisturbed. Conventional site investigation techniques, nowadays sometimes guided by laser cavity scanning, are then employed as directed by the microgravity results to verify the areas deficient in mass. Emsley et al. (1992) and Bishop et al. (1997) describe the application of the microgravity method in the detection of both karstic and man-made cavities and also describe how the resulting data can be enhanced by image processing to better define the anomalies associated with the targets. This paper describes two detailed applications of the microgravity technique for the delineation of mining-related geohazards, the first in a currently operational open-cut gold mine at Kalgoorlie in Western Australia, and the second for the detection of historic chalk mining in the United Kingdom which caused the collapse of the main A2 trunk road into central London in 2002. Both required detailed terrain corrections to be made, in the first case for the effects of the main open cut workings and, in the second, for the influence of surrounding buildings as well as topography. The methods by which these are calculated are very different for the two different environments but are essential if interpretation of small-amplitude subtle anomalies is to be made.

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/content/journals/10.3997/1365-2397.2006013
2006-05-01
2021-01-25
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  • Article Type: Research Article
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