The detection of cavities using the microgravity technique: case histories from mining and karstic environments

Abstract

Th.e presence of mining-related cavities (workingS, shafts and tunnels) or karstic (solution cavities and sinkholes in limestone) within the top 100m in the rock mass restricts land utilisation, and their migration to the surface may damage property or services or cause loss of life. Confirmation of features marked on existing plans prior to design and construction may be sufficient but it is often necessary to determine the detailed sub-surface structure. The standard method of site investigation is to drill a pattern of boreholes to locate the spatial extent of any cavities. However, unless the spacing is less than the cavity dimensions it is possible to miss it completely. A cavity may be filled with air, water, or collapse material resulting in a contrast in physical properties which may be detected using appropriate geophysical methods. One powerful technique is microgravity which locates areas of contrasting sub-surface density from surface measurements of the earth's gravity. Although the method is fundamentally simple, measurement of the minute variations in gravity (1 in 108) requires sensitive instruments, careful data acquisition, and data reduction and digital data analysis. Final interpretation must be performed in conjunction with independent information about the site's history and geology. This paper presents three examples in both mining and karstic environments demonstrating that microgravity is a very effective technique for detecting and delineating cavities in the sub-surface.

Wildlife and Safety of Earthen Structures: A Review

“…Gravity survey Density [82,[118][119][120][121] Resistivity methods Electrical resistivity [83,84,122,123] Seismic reflection Seismic velocity [85,124,125] Ground penetrating radar Dielectric constant [88][89][90] J Fail. Anal.…”

“…South Florida hosts hundreds of thousands of Central American green iguanas [31,118]. The highest reported green iguana density is 626.6 animals per square kilometer in Florida.…”

Long-term time-lapse microgravity and geotechnical monitoring of relict salt mines, Marston, Cheshire, U. K.

“…Noninvasive near-surface geophysical methods have successfully detected and characterized near-surface voids, relict mineshafts, and low-density ground for geotechnical investigators to then target and remediate (e.g., McCann et al, 1987;Bishop et al, 1997;Wilkinson et al, 2005;Pringle et al, 2008;Tuckwell et al, 2008;Banham and Pringle, 2011;Orfanos and Apostolopoulos, 2011).…”

الكشف عن الفجوات الصخرية باستخدام التصوير ثنائي الأبعاد للمقاومية الكهربائية في حوض الشريعة (شمال شرق الجزائر)

“…They allow large areas to be covered in short times and represent an efficient way to detect subsurface heterogeneities including voids, subsidence areas, or refilled cavities. Techniques used include seismic reflection and refraction (Cook 1965), gravimetry (Colley 1963;Butler 1984;Bishop et al 1997;Rybakov et al 2001), ground-penetrating radar (Ballard 1983Annan et al 1991), and resistivity tomography (Zhou et al 2002). More recently, surveying based on magnetic susceptibility differences has also proved useful for detecting and modeling cavities and soil heterogeneities on the outskirts of urban areas (Rybakov et al 2005;Mathé et al 2006).…”