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C program for automatic contouring of spherical orientation data using a modified Kamb method
388A
SITE INVESTIGATIONS:DATA
A computer program for inversion of potential field data using the damped approximate technique is presented. The underlying principles for the optimized damped approximate inversion scheme are discussed. The damping factors and threshold level are selected optimally so that maximum information contained in the data is retained. The facility for constraining or allowing the parameters to range freely is incorporated in GMINV. The analysis of the resolution matrix, eigenvalues, damping factors, V-matrix, and correlation matrix simultaneously facilitates the study of degrec of uniqueness of the retrieved parameters and their interdependence. (from Authors) 958227 XCORR: a cross-over technique to adjust track data Shu-Kun Hsu, Computers & Geosciences, 21(2), 1995, pp 259-271. Cross-over errors (XOEs) from track data may give rise to pseudostructures in a contoured map. Such pitfalls can be reduced by adjusting all the measured values. FORTRAN programs are presented to determine automatically cross-over points (XOPs) and XOEs, to adjust XOEs to zero and to correct the data proportionally between every two XOPs. (Author) 958228 Two-dimensional, regularised inversion of VLF data D. Beamish, Journal of Applied Geophysics, 32(4), 1994, pp 357-374. Given a profile of VLF-R measurements, this study demonstrates an automatic method for the constriction of a resistivity cross-section. The technique used is one of a new generation of reguiarised inversion methods. These techniques attempt to overcome the problem of equivalence/nonuniqueness in EM sounding data by constructing the resistivity distribution with the minimum amount of structure that fits the data. In order to provide insight into the practical application of regularised inversion techniques to VLF data, both synthetic and field examples are considered. Both sets of examples are primarily concerned with VLF data applied to near-surface fault mapping where the main aim is to assess the location, dip and depth extent of conductive subsurface features. (from Author) 958229 Invariants for correcting field polarisation effect in MTVLF resistivity mapping R. Guerin, A. Tabbagh, Y. Benderitter & P. Andrieux, Journal of Applied Geophysics, 32(4), 1994, pp 375-383. MT-VLF resistivity mapping is well suited to perform hydrology and environment studies. However, the apparent anistropy generated by the polarisation of the primary field requires the use of two transmitters at a right angle to each other in order to prevent errors in interpretation. A processing technique that uses approximate invariants derived from classical developments in tensor maguetotellurics is proposed. Both synthetic and field cases show that they give identical results and correct perfectly for the apparent anisotropy generated by the polarisation of the transmitted field. (Authors) 958230 Analysis of GPR data: wave propagation velocity determination S. Tillard & J.-C. Dubois, Journal of Applied Geophysics, 33(1-3), 1995, pp 77-91. In a granitic quarry, with the help of a horehole cored 40 m deep, the limits of velocity analysis based on normal moveout corrections were appraised. The degree of error caused by slightly dipping reflectors was characterised and instability of interval velocity calculations caused by uncertainties in
reflection time and RMS velocity assessment was shown. At a limestone quarry site it was shown that, by working on several paths of electromagnetic waves, direct velocity determinations could help in differentiating rocks in the same formation, based on quality of the field data. Finally, velocities calculated in an anisotropic environment and in geological formations covered by a road or a concrete surface were analysed. (from Authors) 958231 C program for automatic contouring of spherical orientation data using a modified Kamb method F. W. Vollmer, Computers & Geosciences, 21(1), 1995, pp 3149. Spherical orientation data are plotted typically as scatter diagrams on equal area projections of a unit hemisphere and contoured to produce density diagrams. Although widely used, the Sehmidt and other 1% area methods are not well suited for comparing data sets because the contours are dependent strongly on sample size. Kamb's method is based on the departure from a uniform distribution, and, unlike the Sehmidt method, the dependence of contours on sample size is limited. Several improvements can be made to Kamb's method particularly with regard to the implementation of automatic contouring. To reduce smoothing, the expected count for a random sample drawn from a uniform distribution can be decreased. Density estimates are done directly on the sphere for accuracy. This also permits contouring on stereographic and other nonequal area projections, and accommodates vectorial data. Weighting functions provide better density estimates and increase the smoothness of contour lines. These concepts are implemented in the C program Sphere Contour. Options include selection of data rotation, linear or planar data, equal area of stereographic projection, upper or lower hemisphere, and scatter diagrams. (from Author) 958232 DTM-based surface and volume approximation. Geophysical applications J. Kalmar, G. Papp & T. Szabo, Computers & Geosciences, 21(2), 1995, pp 245-257. The DTM (Digital Terrain Model) as a practical realization of a single valued and continuous function can be used for many purposes in the field of science and engineering. It has recently been an important input to GIS software. Because the input data for surface interpolation usually are measured quantities, the point distribution is irregularly spaced, whereas regular spacing would make the simplest manipulation and presentation of the data possible. That is why the first step of DTM appheations is griding and this process has considerable influence on the accuracy of further graphical and numerical applications. Based on earlier investigations, two new methods for local surface interpolation were applied to derive regular grids, from the digitized contour lines of two surfaces. These surfaces represent two main structural boundaries of the Earth's crust, the pre-Tertiary basement and the Moho. The grid representation provides an evident and elementary way for modeling a body by right rectangular prisms of Ax x Ay x z, where Ax and Ay are distances between grid knots in the Xand Ydirections respectively, and z represents a grid value at a grid knot. However, because of the nature of forward gravity modeling, the elementary prisms might be drawn together in a simple volume density distribution and in such a way that the computation time can be reduced significantly. (from Authors) 958233 A geostatistical ore reserve estimation of the Itakpe iron ore deposit, Okene, Kogi State E. O. Akinrinsola & J. I. D. Adekeye, Journal of Mining & Geology, 29(1), 1993(1995), pp 19-25.
SITE INVESTIGATIONS:DATA
A computer program for inversion of potential field data using the damped approximate technique is presented. The underlying principles for the optimized damped approximate inversion scheme are discussed. The damping factors and threshold level are selected optimally so that maximum information contained in the data is retained. The facility for constraining or allowing the parameters to range freely is incorporated in GMINV. The analysis of the resolution matrix, eigenvalues, damping factors, V-matrix, and correlation matrix simultaneously facilitates the study of degrec of uniqueness of the retrieved parameters and their interdependence. (from Authors) 958227 XCORR: a cross-over technique to adjust track data Shu-Kun Hsu, Computers & Geosciences, 21(2), 1995, pp 259-271. Cross-over errors (XOEs) from track data may give rise to pseudostructures in a contoured map. Such pitfalls can be reduced by adjusting all the measured values. FORTRAN programs are presented to determine automatically cross-over points (XOPs) and XOEs, to adjust XOEs to zero and to correct the data proportionally between every two XOPs. (Author) 958228 Two-dimensional, regularised inversion of VLF data D. Beamish, Journal of Applied Geophysics, 32(4), 1994, pp 357-374. Given a profile of VLF-R measurements, this study demonstrates an automatic method for the constriction of a resistivity cross-section. The technique used is one of a new generation of reguiarised inversion methods. These techniques attempt to overcome the problem of equivalence/nonuniqueness in EM sounding data by constructing the resistivity distribution with the minimum amount of structure that fits the data. In order to provide insight into the practical application of regularised inversion techniques to VLF data, both synthetic and field examples are considered. Both sets of examples are primarily concerned with VLF data applied to near-surface fault mapping where the main aim is to assess the location, dip and depth extent of conductive subsurface features. (from Author) 958229 Invariants for correcting field polarisation effect in MTVLF resistivity mapping R. Guerin, A. Tabbagh, Y. Benderitter & P. Andrieux, Journal of Applied Geophysics, 32(4), 1994, pp 375-383. MT-VLF resistivity mapping is well suited to perform hydrology and environment studies. However, the apparent anistropy generated by the polarisation of the primary field requires the use of two transmitters at a right angle to each other in order to prevent errors in interpretation. A processing technique that uses approximate invariants derived from classical developments in tensor maguetotellurics is proposed. Both synthetic and field cases show that they give identical results and correct perfectly for the apparent anisotropy generated by the polarisation of the transmitted field. (Authors) 958230 Analysis of GPR data: wave propagation velocity determination S. Tillard & J.-C. Dubois, Journal of Applied Geophysics, 33(1-3), 1995, pp 77-91. In a granitic quarry, with the help of a horehole cored 40 m deep, the limits of velocity analysis based on normal moveout corrections were appraised. The degree of error caused by slightly dipping reflectors was characterised and instability of interval velocity calculations caused by uncertainties in
reflection time and RMS velocity assessment was shown. At a limestone quarry site it was shown that, by working on several paths of electromagnetic waves, direct velocity determinations could help in differentiating rocks in the same formation, based on quality of the field data. Finally, velocities calculated in an anisotropic environment and in geological formations covered by a road or a concrete surface were analysed. (from Authors) 958231 C program for automatic contouring of spherical orientation data using a modified Kamb method F. W. Vollmer, Computers & Geosciences, 21(1), 1995, pp 3149. Spherical orientation data are plotted typically as scatter diagrams on equal area projections of a unit hemisphere and contoured to produce density diagrams. Although widely used, the Sehmidt and other 1% area methods are not well suited for comparing data sets because the contours are dependent strongly on sample size. Kamb's method is based on the departure from a uniform distribution, and, unlike the Sehmidt method, the dependence of contours on sample size is limited. Several improvements can be made to Kamb's method particularly with regard to the implementation of automatic contouring. To reduce smoothing, the expected count for a random sample drawn from a uniform distribution can be decreased. Density estimates are done directly on the sphere for accuracy. This also permits contouring on stereographic and other nonequal area projections, and accommodates vectorial data. Weighting functions provide better density estimates and increase the smoothness of contour lines. These concepts are implemented in the C program Sphere Contour. Options include selection of data rotation, linear or planar data, equal area of stereographic projection, upper or lower hemisphere, and scatter diagrams. (from Author) 958232 DTM-based surface and volume approximation. Geophysical applications J. Kalmar, G. Papp & T. Szabo, Computers & Geosciences, 21(2), 1995, pp 245-257. The DTM (Digital Terrain Model) as a practical realization of a single valued and continuous function can be used for many purposes in the field of science and engineering. It has recently been an important input to GIS software. Because the input data for surface interpolation usually are measured quantities, the point distribution is irregularly spaced, whereas regular spacing would make the simplest manipulation and presentation of the data possible. That is why the first step of DTM appheations is griding and this process has considerable influence on the accuracy of further graphical and numerical applications. Based on earlier investigations, two new methods for local surface interpolation were applied to derive regular grids, from the digitized contour lines of two surfaces. These surfaces represent two main structural boundaries of the Earth's crust, the pre-Tertiary basement and the Moho. The grid representation provides an evident and elementary way for modeling a body by right rectangular prisms of Ax x Ay x z, where Ax and Ay are distances between grid knots in the Xand Ydirections respectively, and z represents a grid value at a grid knot. However, because of the nature of forward gravity modeling, the elementary prisms might be drawn together in a simple volume density distribution and in such a way that the computation time can be reduced significantly. (from Authors) 958233 A geostatistical ore reserve estimation of the Itakpe iron ore deposit, Okene, Kogi State E. O. Akinrinsola & J. I. D. Adekeye, Journal of Mining & Geology, 29(1), 1993(1995), pp 19-25.