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Apparent resistivity curves in controlled-source electromagnetic sounding directly reflecting true resistivities in a layered Earth
332A
SITE INVESTIGATIONS:GEOPHYSICS
seismic method for exploring near-surface structures is the use of dispersive surface waves. Geoclectric exploration of the structure underground may be carded out with the wellknown methods of DC resistivity sounding. The joint inversion algorithm is tested by means of synthetic data. It is demonstrated that the geoelectric joint inversion of Schlumberger, radial-dipole and two-electrode sounding data yields more reliable results than the single inversion of a single set of these data. The same holds for the seismic joint inversion of Love and Rayleigh group slowness data. The best inversion result is achieved by performing a joint inversion of both geoelectrie and surface-wave data. The effect of noise on the accuracy of the solution for both Gaussian and nonGanssian errors is analysed. (from Authors)
957210 Boundary integral solutions of geoelectric potential problems for the perturbing body of extreme conductivity M. Hvozdara, Studia Geophysica et Geodaetica, 38(3), 1994, pp 293-303. Theoretical formulae for boundary integral solutions of the geoeiectric potential fields for a perfectly conducting perturbing 3D body, as well as for a perfectly insulating body are presented. It is shown that the boundary integral equation for the double-layer density, can be solved for all physically possible eonduetivities of the perturbing body. (Author)
957211 Detection of some cavities in NE Iraq using the electrical resistivity method T. A. Fattah, Zeitschrift fur Angewandte Geologie, 40(2), 1994, pp 103-107. Electrical surveys using Wenner and Sehiumberger configurations were conducted at two localities in NE Iraq. Two cavities were detected by the survey. The first cavity, which is located in the first area (Sarchinar) near Sulaimaniyah city, is in the Kometan limestone of Upper Cretaceous age and has maximum height of about 2.2 m. The cavity extends laterally about 73 m. The second cavity, which is situated in the second area (Chamachamal), formed as a result of dissolution of a huge gypsum block over a long time. The cavity has a height of about 6 m and extends laterally about 25 m. (from Author)
957212 Apparent resistivity curves in controlled-source electromagnetic sounding directly reflecting true resistivities in a layered Earth U. C. Das, Geophysics, 60(1), 1995, pp 53-60. Conversion of the measured voltages in direct current resistivity sounding methods into apparent resistivity po is a useful step since Pa data provide information about the subsurface resistivity variations with depth. This resistivity information then helps select a model for inverting the sounding data. In the controlled source electromagnetic method, two measured components - the vertical magnetic field Hz and the tangential electric field E,~ as a result of a vertical magnetic dipole source - are combined to derive an exact apparent resistivity in a way similar to that used in direct current resistivity methods. Conversion of the measured Hz and E~, field components into apparent resistivity is found to be simple and can be carried out on a programmable pocket calculator. Theoretical apparent resistivity curves for frequency-domain electromagnetic sounding show features similar to magnetoteliurie (MT) and direct current dipoledipole apparent resistivity curves. (Author)
957213 A parametric study of the vertical electric source L. Pellerin & G. W. Hohmann, Geophysics, 60(1), 1995, pp 43-52.
Applications of the VES, as in mineral exploration, seafloor exploration, and process monitoring such as enhanced oil recovery, are varied but are limited in this study to a classic mining problem - the location of a confined, conductive target at depth in the vicinity of a borehole. By analyzing the electromagnetic responses of a thin, vertical prism, a horizontal slab and an equidimensional body, the resolving capabilities are investigated, survey design problems are identified and interpretational insight to vertical magnetic field responses arising from a VES is provided. (from Authors)
957214 Using fractal crustal magnetization models in magnetic interpretation M. Pilkington, M. E. Gregotski & J. P. Todoesehuck, Geophysical Prospecting, 42(6), 1994, pp 677-692. Evidence from borehole susceptibility logs and the spectral analysis of aeromagnetic data suggests that the three-dimensional distribution of magnetization within the crust can be described as fractal. This property can be exploited in magnetic interpretation methods which explicitly require statistical information on the spatial variation of magnetization. This paper addresses the problem of magnetic source depth estimation through downward continuation and gridding aeromagnetic survey data using the method of kriging. (from Authors)
957215 Airborne gravity tests over Lake Ontario J. F. Halpenny & D. M. Darbha, Geophysics, 60(1), 1995, pp 61-65. In this paper, a trial gravity survey over Lake Ontario, which was carried out to demonstrate that measurement of gravity from a small aircraft can be a practical and economical survey technique is described. Airborne gravity measurements were made over Lake Ontario in March 1991 to establish the feasibility of using a small survey aircraft for rapid and economical gravity surveys. (from Authors)
957216 Assessment of bridge pier scour using impulse radar S. G. Millard & C. Thomas, Proceedings - ICE: Structures & Buildings, 1 I0(2), 1995, pp 216-227. In recent years, three bridges in the UK, Glanrhyd (1987), Ness (1989) and Fortevoit (1993) have collapsed during flood conditions. In each case, failure has been attributed to the presence of scour holes at the base of one, or more, of the supporting piers. This paper describes the use of impulse subsurface radar in detecting the presence of scour holes in river beds. The basic theory of radar signal emission is briefly described to show the theoretical limits of a system. This is followed by a review of surveys carded out in lakes and rivers, with the values of the limits actually attained. Transmission line equipment developed at Liverpool University to measure the electrical properties of river water, mud and silt suspensions is described. The variation of these properties along the length of a river is shown for River Dee water and silt samples. Computer modelling of radar signals has also been developed, and the results obtained are compared with measured signals. Finally, the field results obtained during on-site testing on the River Severn at Gloucester are presented. (Authors)
957217 Applying nondestructive remote sensing techniques to hazardous waste management G. J. Weft & R. J. Graf, Bulletin - Association of Engineering Geologists, 31(4), 1994, pp 533-540. The recent incorporation of two testing techniques has led to a viable solution for dependable, cost effective location and
SITE INVESTIGATIONS:GEOPHYSICS
seismic method for exploring near-surface structures is the use of dispersive surface waves. Geoclectric exploration of the structure underground may be carded out with the wellknown methods of DC resistivity sounding. The joint inversion algorithm is tested by means of synthetic data. It is demonstrated that the geoelectric joint inversion of Schlumberger, radial-dipole and two-electrode sounding data yields more reliable results than the single inversion of a single set of these data. The same holds for the seismic joint inversion of Love and Rayleigh group slowness data. The best inversion result is achieved by performing a joint inversion of both geoelectrie and surface-wave data. The effect of noise on the accuracy of the solution for both Gaussian and nonGanssian errors is analysed. (from Authors)
957210 Boundary integral solutions of geoelectric potential problems for the perturbing body of extreme conductivity M. Hvozdara, Studia Geophysica et Geodaetica, 38(3), 1994, pp 293-303. Theoretical formulae for boundary integral solutions of the geoeiectric potential fields for a perfectly conducting perturbing 3D body, as well as for a perfectly insulating body are presented. It is shown that the boundary integral equation for the double-layer density, can be solved for all physically possible eonduetivities of the perturbing body. (Author)
957211 Detection of some cavities in NE Iraq using the electrical resistivity method T. A. Fattah, Zeitschrift fur Angewandte Geologie, 40(2), 1994, pp 103-107. Electrical surveys using Wenner and Sehiumberger configurations were conducted at two localities in NE Iraq. Two cavities were detected by the survey. The first cavity, which is located in the first area (Sarchinar) near Sulaimaniyah city, is in the Kometan limestone of Upper Cretaceous age and has maximum height of about 2.2 m. The cavity extends laterally about 73 m. The second cavity, which is situated in the second area (Chamachamal), formed as a result of dissolution of a huge gypsum block over a long time. The cavity has a height of about 6 m and extends laterally about 25 m. (from Author)
957212 Apparent resistivity curves in controlled-source electromagnetic sounding directly reflecting true resistivities in a layered Earth U. C. Das, Geophysics, 60(1), 1995, pp 53-60. Conversion of the measured voltages in direct current resistivity sounding methods into apparent resistivity po is a useful step since Pa data provide information about the subsurface resistivity variations with depth. This resistivity information then helps select a model for inverting the sounding data. In the controlled source electromagnetic method, two measured components - the vertical magnetic field Hz and the tangential electric field E,~ as a result of a vertical magnetic dipole source - are combined to derive an exact apparent resistivity in a way similar to that used in direct current resistivity methods. Conversion of the measured Hz and E~, field components into apparent resistivity is found to be simple and can be carried out on a programmable pocket calculator. Theoretical apparent resistivity curves for frequency-domain electromagnetic sounding show features similar to magnetoteliurie (MT) and direct current dipoledipole apparent resistivity curves. (Author)
957213 A parametric study of the vertical electric source L. Pellerin & G. W. Hohmann, Geophysics, 60(1), 1995, pp 43-52.
Applications of the VES, as in mineral exploration, seafloor exploration, and process monitoring such as enhanced oil recovery, are varied but are limited in this study to a classic mining problem - the location of a confined, conductive target at depth in the vicinity of a borehole. By analyzing the electromagnetic responses of a thin, vertical prism, a horizontal slab and an equidimensional body, the resolving capabilities are investigated, survey design problems are identified and interpretational insight to vertical magnetic field responses arising from a VES is provided. (from Authors)
957214 Using fractal crustal magnetization models in magnetic interpretation M. Pilkington, M. E. Gregotski & J. P. Todoesehuck, Geophysical Prospecting, 42(6), 1994, pp 677-692. Evidence from borehole susceptibility logs and the spectral analysis of aeromagnetic data suggests that the three-dimensional distribution of magnetization within the crust can be described as fractal. This property can be exploited in magnetic interpretation methods which explicitly require statistical information on the spatial variation of magnetization. This paper addresses the problem of magnetic source depth estimation through downward continuation and gridding aeromagnetic survey data using the method of kriging. (from Authors)
957215 Airborne gravity tests over Lake Ontario J. F. Halpenny & D. M. Darbha, Geophysics, 60(1), 1995, pp 61-65. In this paper, a trial gravity survey over Lake Ontario, which was carried out to demonstrate that measurement of gravity from a small aircraft can be a practical and economical survey technique is described. Airborne gravity measurements were made over Lake Ontario in March 1991 to establish the feasibility of using a small survey aircraft for rapid and economical gravity surveys. (from Authors)
957216 Assessment of bridge pier scour using impulse radar S. G. Millard & C. Thomas, Proceedings - ICE: Structures & Buildings, 1 I0(2), 1995, pp 216-227. In recent years, three bridges in the UK, Glanrhyd (1987), Ness (1989) and Fortevoit (1993) have collapsed during flood conditions. In each case, failure has been attributed to the presence of scour holes at the base of one, or more, of the supporting piers. This paper describes the use of impulse subsurface radar in detecting the presence of scour holes in river beds. The basic theory of radar signal emission is briefly described to show the theoretical limits of a system. This is followed by a review of surveys carded out in lakes and rivers, with the values of the limits actually attained. Transmission line equipment developed at Liverpool University to measure the electrical properties of river water, mud and silt suspensions is described. The variation of these properties along the length of a river is shown for River Dee water and silt samples. Computer modelling of radar signals has also been developed, and the results obtained are compared with measured signals. Finally, the field results obtained during on-site testing on the River Severn at Gloucester are presented. (Authors)
957217 Applying nondestructive remote sensing techniques to hazardous waste management G. J. Weft & R. J. Graf, Bulletin - Association of Engineering Geologists, 31(4), 1994, pp 533-540. The recent incorporation of two testing techniques has led to a viable solution for dependable, cost effective location and