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In situ measurement of transverse isotropy in shallow-water marine sediments
292A 915210 Advances in electromagnetic methods for groundwater studies McNeill, J D Geoexploration V27, N1/2, Feb 1991, PCS-80 (paper to the International Symposium on Application of Geophysics to Water Prospecting in Arid and Semi-Arid Areas, Lausanne, 22-24 August 1990) Electromagnetic methods have been successfully used in exploration for potable groundwater, measuring salinity levels in aquifers, and mapping soil salinity. Small variations in terrain conductivity can be detected. Surveys are rapid and economic when compared to conventional DC resistivity methods, especially at shallow depth. Further applications of horizontal loop EM, ground conductivity meters, and VLF to locate fracture zones in a basement complex, detect bedrock depressions beneath saprolite, and measure overburden thickness are described. 915211 Groundwater exploration in fissured media witb electrical and vLFmethods Bernard, J; Valla, P Geoexploration V27. N1/2. Feb 1991, Pal-91 (paper to the International Symposium on Application of Geophysics to Water Prospecting in Arid and Semi-Arid Areas, Lausanne, 22-24 August 1990) DC electrical methods are widely used in groundwater exploration. VLF induction methods, although of more limited depth of investigation, are used to locate conductive zones. Automated, easy to use resistivity meters and VLF electromagnetic instruments are described. Applications are described to groundwater exploration in fissured dolomites (resistivity mapping), in crystalline basement rocks (resistivity profiling and tilt angle mode VLF), and in a granitic basement where anomalies identified by electrical profiles are compared to those from VLF profiles in the tilt angle and resistivity modes. 915212 Geophysical methods for lineameot studies in groundwater exploration. A case history from SE Botswana Zeil, P; Volk, P; Saradeth, S Geoexploration V27, NI/2, Feb 1991, P165-177 (paper to the International Symposium on Application of Geophysics to Water Prospecting in Arid and Semi-Arid Areas, Lausanne. 22-24 August 1990) Channel shaped fractured aquifers of narrow width and variable length are found in basement and consolidated sedimentary rocks in Botswana. Detection of these lineaments by satellite imaging is made difficult by the cover of Kalahari sand, although they can be found if airborne magnetic surveying is used in parallel. Ground based electromagnetic surveys, in particular VLF or multifrequency horizontal loop electromagnetic methods (HLEM), allow successful location of these aquifers. Field examples are illustrated. 915213 Application of uphole data from petroleum seismic surveys to groundwater investigations, Abu Dhabi (United Arab Emirates) Woodward, D; Menges, C M Geoexploration V27, N1/2, Feb 1991, Pl93-212 (paper to the International Symposium on Application of Geophysics to Water Prospecting in Arid and Semi-Arid Areas, Lausanne, 22-24 August 1990) Heavy groundwater withdrawals in the Al Ain district of Abu Dhabi caused depression of the water table and influx of more saline groundwater. To evaluate the shallow aquifer system, a 0
groundwater flow model is required. One data source for the production of this model is a regional set of uphole data collected between 1981 and 1983 as part of a seismic reflection survey for petroleum exploration. Direct measurements of seismic velocities and thickness of near surface sediments are available. Their application to map the water table and base dune sand/top alluvium interface is described. 915214 In situ measurement of transverse isotropy in shallow-water marine sediments Berge, PA; Mallick, S; Fryer, G J; Barstow, N; Carter, J A; Sutton, G H; Ewing, J I Geophys J VlO4, N2, Feb 1991. P241-254 Most marine sediments are anisotropic, showing transverse isotropy with a vertical axis of symmetry. Conventional marine seismic experiments do not record unconverted shear waves and cannot quantify this anisotropy. Experiments recording direct shear waves in shallow marine sediments using an ocean-bottom shear source and a multi component on-bottom receiver are described. Two intersecting refraction profiles were interpreted using an anisotropic reflectivity program to describe the anisotropy of the top 50m of the sediments. The profiles were not sufficiently long to constrain P wave velocities and anisotropy, but adequate to find S wave anisotropy. 915215 Reflection/transmission coefficients and azimuthal anisotropy in marine seismic studies Mallick, S; Frazer, L N Geophys J VlO5, NI, April 1991, P241-252 Two methods of inferring the directions of vertical fractures from marine seismic data are presented. The necessary reflection and transmission coefficients for an interface between a liquid and an azimuthally anisotropic solid are first derived. It is next demonstrated that sea bed seismometer data from surface airgun sources along two perpendicular plot lines can be rotated into the principal directions of azimuthal anisotropy to determine the orientation of vertical fractures. It is finally shown that this orientation can also be determined from P wave amplitude versus offset data. 915216 Mapping of Quatemary sediments in uortheastern Ontario using ground electromagnetic methods Palacky, G J; Stephens, L E Geophysics V55, NI2, Dee 1990, Pl596-1604 Ground EM methods for geological mapping have been investigated in Canada over a number of years. Results of trials with horizontal loop multifrequency equipment are presented. After interpretation of EM results, 70 holes were drilled in the area. Correlation of EM data and drillhole logs shows sand, clay, and till have distinctive EM signatures. Resistivity values estimated from EM/drilling information and laboratory measurements indicate that for the Quatemary sediments the resistivity is sufficiently stable to allow use of EM methods for their identification. Inversion of EM data can provide overburden thickness and qualitative mapping of bedrock topography. 915217 Experimental studies on downhole seismic sources. Short note Chen, S T; Eriksen, E A; Miller, M A Geophysics V55, N12, Dee 1990, P1645-1651 Reversed vertical seismic profiling and crosshole methods can produce more detailed images than surface seismics, but some doubt remains whether sources can be developed to generate sufficient energy downhole without damaging the borehole.
1991 Pergamon Press plc. Reproduction
not permitted
groundwater flow model is required. One data source for the production of this model is a regional set of uphole data collected between 1981 and 1983 as part of a seismic reflection survey for petroleum exploration. Direct measurements of seismic velocities and thickness of near surface sediments are available. Their application to map the water table and base dune sand/top alluvium interface is described. 915214 In situ measurement of transverse isotropy in shallow-water marine sediments Berge, PA; Mallick, S; Fryer, G J; Barstow, N; Carter, J A; Sutton, G H; Ewing, J I Geophys J VlO4, N2, Feb 1991. P241-254 Most marine sediments are anisotropic, showing transverse isotropy with a vertical axis of symmetry. Conventional marine seismic experiments do not record unconverted shear waves and cannot quantify this anisotropy. Experiments recording direct shear waves in shallow marine sediments using an ocean-bottom shear source and a multi component on-bottom receiver are described. Two intersecting refraction profiles were interpreted using an anisotropic reflectivity program to describe the anisotropy of the top 50m of the sediments. The profiles were not sufficiently long to constrain P wave velocities and anisotropy, but adequate to find S wave anisotropy. 915215 Reflection/transmission coefficients and azimuthal anisotropy in marine seismic studies Mallick, S; Frazer, L N Geophys J VlO5, NI, April 1991, P241-252 Two methods of inferring the directions of vertical fractures from marine seismic data are presented. The necessary reflection and transmission coefficients for an interface between a liquid and an azimuthally anisotropic solid are first derived. It is next demonstrated that sea bed seismometer data from surface airgun sources along two perpendicular plot lines can be rotated into the principal directions of azimuthal anisotropy to determine the orientation of vertical fractures. It is finally shown that this orientation can also be determined from P wave amplitude versus offset data. 915216 Mapping of Quatemary sediments in uortheastern Ontario using ground electromagnetic methods Palacky, G J; Stephens, L E Geophysics V55, NI2, Dee 1990, Pl596-1604 Ground EM methods for geological mapping have been investigated in Canada over a number of years. Results of trials with horizontal loop multifrequency equipment are presented. After interpretation of EM results, 70 holes were drilled in the area. Correlation of EM data and drillhole logs shows sand, clay, and till have distinctive EM signatures. Resistivity values estimated from EM/drilling information and laboratory measurements indicate that for the Quatemary sediments the resistivity is sufficiently stable to allow use of EM methods for their identification. Inversion of EM data can provide overburden thickness and qualitative mapping of bedrock topography. 915217 Experimental studies on downhole seismic sources. Short note Chen, S T; Eriksen, E A; Miller, M A Geophysics V55, N12, Dee 1990, P1645-1651 Reversed vertical seismic profiling and crosshole methods can produce more detailed images than surface seismics, but some doubt remains whether sources can be developed to generate sufficient energy downhole without damaging the borehole.
1991 Pergamon Press plc. Reproduction
not permitted