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Geological exploration of an engineering site
Published in A.C. McLean, C. D. Gribble, Geology for Civil Engineers, 2017
The magnitude of a Bouguer anomaly is related to the size of structure and to the density contrast across the interface being investigated. Since structures of interest in site exploration are relatively small, the gravity method is usable only if the density contrast is relatively large, and even then only as a reconnaissance method because of poor resolution. This condition is met in two types of problem. The first is searching for large underground caverns in thick limestone, where the contrast may be as high as 2.7 g cm−3 between limestone and air. The second is tracing buried channels where an infill of unconsolidated drift deposits may differ in density by about 0.5 g cm−3 from the solid rocks, for example Carboniferous sediments. The cavities produced by mining do not give rise to gravity anomalies large enough to be recognised with certainty in a survey with a standard gravity meter. A new generation of more sensitive instruments with a sensitivity of 0.001 mgal may be of use for this purpose (Appendix D).
Geophysical Applications
Published in Stephen M. Testa, Geological Aspects of Hazardous Waste Management, 2020
Gravity surveys are not commonly employed, but can provide data on regional geologic features over hundreds of square miles and on major fractures and cavities in rocks. The Bouguer anomaly, the difference betweer observed and theoretical gravity, is measured reflecting the changes in the density of the soil and rock. The measurements are made up of deep-seated effects (regional Bouguer anomaly) and shallow effects (local Bouguer anomaly). The regional Bouguer anomaly has merit in regional studies associated with geologic repositories, while the local Bouguer anomaly may be useful for localized studies.
Application of microgravity for searching of cavities in historical sites
Published in Soňa Molčíková, Viera Hurčíková, Vladislava Zelizňaková, Peter Blišťan, Advances and Trends in Geodesy, Cartography and Geoinformatics, 2018
J. Chromčák, J. Ižvoltová, M. Grinč
where ΔgB is the Bouguer anomaly, gobs is the observed gravity, gf is the correction for latitude, δgF is the well-known free-air correction, δgB is the Bouguer correction that allows for the gravitational attraction of the rocks between the observed point and sea level and δgT is a terrain correction [6].
Gravity and magnetic data processing further constrained inversion for 3D modelling and tonnage calculation
Published in Applied Earth Science, 2020
Saâd Soulaimani, Saïd Chakiri, Ahmed Manar, Ayoub Soulaimani, Abdelhalim Miftah, Mustapha Boujamaoui
The acquired gravity data from ministry of energy and mines of morocco specifically Geological Survey of the Directorate Geology, included all the calculations details. Numeric programme has been used to calculate the Free Air and Complete Bouguer anomalies. The value used for free air correction was 0.3083 mgal m−1. Calculation of the Bouguer anomaly requires estimating the density of basement rocks and taking into account the regional topography (Figure 4) for the calculation of terrain corrections. A density value of 2.67 has been used for all terrain corrections, corresponding approximately to the average density of sedimentary rocks from the orebody zone. Terrain corrections are often larger than 8 mgal in the area. The 2.67 density value has been estimated using gravity measurements performed at different elevations on the area. Of course, the basement density is homogeneous except the mineralised zone. Concerning relief correction that takes account of altitudes variations around the measuring station, the used method was one that uses Hayford and Hammer charts (Sandberg 1959), for large distance corrections, faster and more precise numeric methods was used. For data processing, we created a database that contains the Bouguer anomaly (Ba) which is the sum of a regional field (Rg) and a residual one (Rs) (El Azzab et al. 2019), it constitutes the result of different origin effects. Indeed, the values 2.67 and 2.3 g cm−3 is the density commonly used for gravity surveys in Morocco. In contrast, the 2.3 g cm−3 density was used for HAJJAR area because the orebody is buried under a sedimentary overburden (dominant overburden density near to the mineralisation area). We first calculated the residual anomaly map by subtracting the regional effect. The first trials to eliminate this effect using low-pass filtering or upward continuation did not result in a satisfactory outcome. The best result was obtained using the polynomial regression method (Draper and Smith 1998).