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Factor analysis of ambiquity in geophysics
72A
SITE INVESTIGATIONS:GEOPHYSICS
western haft of the Thor Lake Syenite, and the other forms a crescent-shaped feature within the Grace Lake Granite. These two anomalies are related to magnetite produced by alteration of ferromaguesian minerals. The electromagnetic patterns relate primarily to conductive lake bottom material and faults. (from Authors)
952200 Mapping l~nCs at Maralinga nuclear test site, South Australia, using conventional 4-channel airborne gammaray spectrometry B. R. S. Minty & R. C. Brodie, AGSO Journal of Australian Geology & Geophysics, 15(2), 1994, pp 217-222. A simple method has been developed for separating the gamma radiation due to 137Cs from that due to natural sources using conventional 4-channel airborne gamma-ray spectrometry. The method uses the fact that whilst the 137Cs photopeak falls within the conventional total-count window, it occurs well below the three windows used to monitor naturally present K, U and Th. The method maps the distribution of 13~Cs contamination in the vicinity of the Marafinga nuclear test site very effectively. (from Authors)
952201 Correlation filtering: a terrain correction method for aeromagnetic maps with application P. S. Naidu & M. P. Mathew, Journal of Applied Geophysics, 32(2-3), 1994, pp 269-277. Correlation filtering attempts to remove that component of the aeromaguetic field which is closely related to the topography. The magnetization vector is assumed to be spatially variable, but it can be successively estimated under the additional assumption that the magnetic component due to topography is uncorrelated with the magnetic signal of deeper origin. The filtered field compares very well with the known signal of deeper origin. This method was applied to real data from the south Indian shield. (Authors)
952202 A foidradar modszerfejlesztes mnsfei ELGI-ben
eves
tapnsztalatal az
(One and a half year experiences of ground penetrating radar applications in ELGI, Hungary) M. Pattantyus-A, B. Neducza, Z. S. Pronay & E. Toros, Magyar Geofizika, 35(1), 1994, pp 32-41. This paper gives a short view on the activity of ELGI in introducing GPR to domestic geophysical prospection and reviews the result of several field applications and of the development in data processing. (from English summary)
952204 Density in Bougner anomalies and its consequences M. Bernabini, P. Favaro & L. Orlando, Journal of Applied Geophysics, 32(2-3), 1994, pp 187-197. This paper e x a ~ n e s the density-induced errors which can result when calculating the Bonguer anomaly in areas of complex geology and morphology. In order to minimize the errors in such areas the authors suggest a procedure consisting of: 1) considering Bouguer anomalies at constant density; 2) interpreting them with 3-D models taking into account all geological bodies starting from ground surface. As an example the comparison between the usual procedures and the method proposed has been applied to a 50 x 50 km area in central Italy. (Authors)
952205 Terrain correction progrm for regional gravity surveys X. Q. Ma & D. R. Watts, Computers & Geosciences, 20(6), 1994, pp 961-972. An algorithm is presented for the computation of gravity terrain corrections from a digitized grid representing topography. Different approximations for topography at various distances from the gravity station are used to make the computation more efficient. A new method is used to compute the correction for the inner zone which is the digitized square where the gravity station is located. The gravity effect of topography in the inner zone is calculated from four triangular prisms with vertices made of the station location and the corners of the square. The computer program has been tested comprehensively with a regional gravity database from the Southern Uplands of Scotland. (from Authors)
952206 Analyses of wavefields by the common excitation array (CEA) method M. Bliznetsov & C. Juhlin, Journal of Applied Geophysics, 32(2-3), 1994, pp 245-256. There are a number of problems associated with the processing of seismic data acquired over crystalline rocks where velocities vary as much in the horizontal direction as in the vertical direction and near-surface low-velocity layers may distort the signals recorded on the surface. In addition, waves arriving at the same time but at different dips will interfere with one another thereby masking reflections of interest. By employing a series of closely spaced shotpoints in an array, quasi-planar waves emanating at different angles may be simulated through slant stacking. After slant stacking to illuminate interfaces of various dips, the major reflecting events may be picked automatically and the data quantity reduced to the time, slowness and amplitude of the picked events. These picked events can then be migrated onto a depth section image. The method is applied to seismic data from the Urals. (from Authors)
952203 Felhagyott melysegi banyamuveletek hatnsvizsgalata geo-
952207 Factor analysis of ambiquity in geophysics
flzikal modszerekkci (Prospecting for old mine workings by GPR and seismics) E. Gogh, M. Pattantyus-A, B. Neducza, E. Toros & L. Hermann, Magyar Geofizika, 35(1), 1994, pp 19-23.
S. G. C. Fraiha & J. B. C. Silva, Geophysics, 59(7), 1994, pp 1083-1091.
An integrated geophysical survey was carried out over an old mine. The site is to be built up but problems arose due to the unconsolidated zones and improperly bacidilled shafts of the mine. The method used were GPR, EM conductivity and resistivity profiling for detecting near surface features and seismics for detecting fractured or unconsolidated zones caused by old mine workings. (English summary)
The paper presents an empirical ambiguity analysis method based on a finite number of acceptable solutions that are representative of the ambiguity region. These solutions are submitted to a Q-mode factor analysis that indicates which parameters are ambiguous and their ambiguity range. A synthetic nonlinear example illustrates that the method is more effective than singular value decomposition analysis in producing an average trend of the ambiguity region. It requires less restrictive hypotheses and is more robust than
SITE INVESTIGATIONS:BOREHOLES analytical methods of ambiguity analysis, in the sense of being applicable to a broader class of problems. (Authors)
Borehole and core logging
952208 Permuent dowahole gauges used in reservoir muagement of complex North Sea oil fields T. Unneland & T. Haugland, SPE Production & Facilities, 9(3), 1994, pp 195-203. This paper presents experience with and applications of permanent downhole pressure and temperature gauges in the reservoir management of two complex North Sea oil fields, Gullfaks and Veslefrikk. In total, 40 quartz and quartz capacitance gauges from three different suppliers have been installed in platform wells over 6 years. The gauges have given invaluable realtime data from reservoir management of these two fields and contribute directly to increased daily oil production. The installations have proved to be safe and reliable, as well as good investments. (Authors)
952209 Dead time of dual detecter tools J. A. Czuhek, Nuclear Geophysics, 8(4), 1994, pp 317-326. A theory of the dead time for the dual detector nuclear tool with the analogue signal transmission is given in the paper. At least two different times exist in such tools: the dead time of detectors and the dead time of the signal transmission set-up. A method of two radioactive sources is proposed to measure these two different dead times. Two codes have been designed. The first code calculates the dead time based on the recorded countrates only, the second is doing a 'simulation job' and provides information on the statistical distribution of the observed dead times. (from Author)
952210 Combined use of radioactive and inactive tracers in a borehole experiment P. Viitanen, Nuclear Geophysics, 8(4), 1994, pp 335-341. The flow of groundwater between two boreholes has been studied tudng radioactive S2Br and inactive sodium fluorescein as tracers. The primary use of bromine was to determiine the flow rate at the injection and the primary use of sodium fluorescein was to serve as the tracer at the system output. Either of the tracers would have been adequate for both tasks but the combined use provided an increased reliability for the test. (Author)
952211 A theoretical model for the temperatme response of compensated thermal neutron tools G. L. Mathis, Nuclear Geophysics, 8(4), 1994, pp 327-334. The overall temperature response of a compensated thermal neutron tool has two major components. The first is due to the temperature of the thermal neutrons as they interact with the hefium detectors within the tool. The magnitude of this effect is governed exclusively by the pressure of the helium within the detector and the diameter of the interacting volume. The second component comes from the changes in neutron slowing down length and diffusion length with formation temperature. Over the range in which the porosity response is finear these two components can be computed independently of each other and added together to get the total temperature effect. (Author)
73A
952212 Neutron porosity logging and core porosity measurements in the Beauvoir granite, Massif Central Range, France C. Galle, Journal of Applied Geophysics, 32(2-3), 1994, pp 125-137. With its low porosity, its weak fracturing and its high homogeneity, the Beauvoir granite was chosen for the analysis of the relationship between logged data and the properties measured in the core. The Beauvoir granite has a total free water porosity of around 2% and an average neutron porosity of around I0%. The origin of this significant difference is related to the neutron matrix effect of the granite. This phenomenon is partly due to the slowingdown effect of the combined water of clays and micas but also to the nentronic capture effect linked with the relatively high lepidolite content of the granite. This experimental study shows that geophysical logging must he adapted to the type of rock and that the geological context has to be taken into account before running the tool in the borehole. (from Author) 952213 A low activity spectrometric gamma-gamma borehole logging tool for the coal industry M. Borsaru & C. Ceravolo, Nuclear Geophysics, 8(4), 1994, pp 343-350. A logging tool based on the spectrometric T-7 technique was tested in water-filled boreholes of nominal diameter 96 mm in a coal deposit. The tool was centralized and employed a T-raY source of very low activity (1.8 MBq). The tests proved that the tool could be used to delineate the coal seams in boreholes and measure the ash content of coal. The tool is sensitive to changes in the Z~q number of the matrix surrounding the borehole and could find application in the metalliferous mining industry for ore body delineation. (from Authors)
952214 Electrical resistivity measurement through metal casing C. J. Schenkel & H. F. Morrison, Geophysics, 59(7), 1994, pp 1072-1082. Methods using dc electrical arrays to measure formation resistivity through casing have relied on approximate forms for the current and potential distributions to derive a ~-nple relationship between the formation resistivity and the transverse resistance calculated from measurements of the potential and its second derivative inside the casing. A numerical solution has been derived for the potentials and their derivatives to examine the accuracy of the approximate forms for casing of finite-length, annualar zones of varying radius, and for vertical discontinuities such as layers or abrupt changes in annular zone radius. (from Authors)
952215 Borehole transmission tomography for velocity plus statics L. J. Squires, P. L. Stoffa & G. Cambois, Geophysics, 59(7), 1994, pp 1028-1036. The accuracy of velocity tomograms reconstructed from borehole transmissioin traveltime data is highly sensitive to traveltime statics. The paper presents a least-squares tomography algorithm that includes a traveltime static term. The algorithm solves for both the velocity field and the traveltime statics simultaneously. This enables us to separate traveltime signal from traveltime noise, reducing the tomographic velocity artifacts caused by the statics. Application of the algorithm to numerical crosswell data results in velocity and statics' estimates that are accurate to within 1%. Appfieation of the algorithm to Exxon's Friendwood tomography data results in velocity and statics' estimates that correlate with independent data. (from Authors)
SITE INVESTIGATIONS:GEOPHYSICS
western haft of the Thor Lake Syenite, and the other forms a crescent-shaped feature within the Grace Lake Granite. These two anomalies are related to magnetite produced by alteration of ferromaguesian minerals. The electromagnetic patterns relate primarily to conductive lake bottom material and faults. (from Authors)
952200 Mapping l~nCs at Maralinga nuclear test site, South Australia, using conventional 4-channel airborne gammaray spectrometry B. R. S. Minty & R. C. Brodie, AGSO Journal of Australian Geology & Geophysics, 15(2), 1994, pp 217-222. A simple method has been developed for separating the gamma radiation due to 137Cs from that due to natural sources using conventional 4-channel airborne gamma-ray spectrometry. The method uses the fact that whilst the 137Cs photopeak falls within the conventional total-count window, it occurs well below the three windows used to monitor naturally present K, U and Th. The method maps the distribution of 13~Cs contamination in the vicinity of the Marafinga nuclear test site very effectively. (from Authors)
952201 Correlation filtering: a terrain correction method for aeromagnetic maps with application P. S. Naidu & M. P. Mathew, Journal of Applied Geophysics, 32(2-3), 1994, pp 269-277. Correlation filtering attempts to remove that component of the aeromaguetic field which is closely related to the topography. The magnetization vector is assumed to be spatially variable, but it can be successively estimated under the additional assumption that the magnetic component due to topography is uncorrelated with the magnetic signal of deeper origin. The filtered field compares very well with the known signal of deeper origin. This method was applied to real data from the south Indian shield. (Authors)
952202 A foidradar modszerfejlesztes mnsfei ELGI-ben
eves
tapnsztalatal az
(One and a half year experiences of ground penetrating radar applications in ELGI, Hungary) M. Pattantyus-A, B. Neducza, Z. S. Pronay & E. Toros, Magyar Geofizika, 35(1), 1994, pp 32-41. This paper gives a short view on the activity of ELGI in introducing GPR to domestic geophysical prospection and reviews the result of several field applications and of the development in data processing. (from English summary)
952204 Density in Bougner anomalies and its consequences M. Bernabini, P. Favaro & L. Orlando, Journal of Applied Geophysics, 32(2-3), 1994, pp 187-197. This paper e x a ~ n e s the density-induced errors which can result when calculating the Bonguer anomaly in areas of complex geology and morphology. In order to minimize the errors in such areas the authors suggest a procedure consisting of: 1) considering Bouguer anomalies at constant density; 2) interpreting them with 3-D models taking into account all geological bodies starting from ground surface. As an example the comparison between the usual procedures and the method proposed has been applied to a 50 x 50 km area in central Italy. (Authors)
952205 Terrain correction progrm for regional gravity surveys X. Q. Ma & D. R. Watts, Computers & Geosciences, 20(6), 1994, pp 961-972. An algorithm is presented for the computation of gravity terrain corrections from a digitized grid representing topography. Different approximations for topography at various distances from the gravity station are used to make the computation more efficient. A new method is used to compute the correction for the inner zone which is the digitized square where the gravity station is located. The gravity effect of topography in the inner zone is calculated from four triangular prisms with vertices made of the station location and the corners of the square. The computer program has been tested comprehensively with a regional gravity database from the Southern Uplands of Scotland. (from Authors)
952206 Analyses of wavefields by the common excitation array (CEA) method M. Bliznetsov & C. Juhlin, Journal of Applied Geophysics, 32(2-3), 1994, pp 245-256. There are a number of problems associated with the processing of seismic data acquired over crystalline rocks where velocities vary as much in the horizontal direction as in the vertical direction and near-surface low-velocity layers may distort the signals recorded on the surface. In addition, waves arriving at the same time but at different dips will interfere with one another thereby masking reflections of interest. By employing a series of closely spaced shotpoints in an array, quasi-planar waves emanating at different angles may be simulated through slant stacking. After slant stacking to illuminate interfaces of various dips, the major reflecting events may be picked automatically and the data quantity reduced to the time, slowness and amplitude of the picked events. These picked events can then be migrated onto a depth section image. The method is applied to seismic data from the Urals. (from Authors)
952203 Felhagyott melysegi banyamuveletek hatnsvizsgalata geo-
952207 Factor analysis of ambiquity in geophysics
flzikal modszerekkci (Prospecting for old mine workings by GPR and seismics) E. Gogh, M. Pattantyus-A, B. Neducza, E. Toros & L. Hermann, Magyar Geofizika, 35(1), 1994, pp 19-23.
S. G. C. Fraiha & J. B. C. Silva, Geophysics, 59(7), 1994, pp 1083-1091.
An integrated geophysical survey was carried out over an old mine. The site is to be built up but problems arose due to the unconsolidated zones and improperly bacidilled shafts of the mine. The method used were GPR, EM conductivity and resistivity profiling for detecting near surface features and seismics for detecting fractured or unconsolidated zones caused by old mine workings. (English summary)
The paper presents an empirical ambiguity analysis method based on a finite number of acceptable solutions that are representative of the ambiguity region. These solutions are submitted to a Q-mode factor analysis that indicates which parameters are ambiguous and their ambiguity range. A synthetic nonlinear example illustrates that the method is more effective than singular value decomposition analysis in producing an average trend of the ambiguity region. It requires less restrictive hypotheses and is more robust than
SITE INVESTIGATIONS:BOREHOLES analytical methods of ambiguity analysis, in the sense of being applicable to a broader class of problems. (Authors)
Borehole and core logging
952208 Permuent dowahole gauges used in reservoir muagement of complex North Sea oil fields T. Unneland & T. Haugland, SPE Production & Facilities, 9(3), 1994, pp 195-203. This paper presents experience with and applications of permanent downhole pressure and temperature gauges in the reservoir management of two complex North Sea oil fields, Gullfaks and Veslefrikk. In total, 40 quartz and quartz capacitance gauges from three different suppliers have been installed in platform wells over 6 years. The gauges have given invaluable realtime data from reservoir management of these two fields and contribute directly to increased daily oil production. The installations have proved to be safe and reliable, as well as good investments. (Authors)
952209 Dead time of dual detecter tools J. A. Czuhek, Nuclear Geophysics, 8(4), 1994, pp 317-326. A theory of the dead time for the dual detector nuclear tool with the analogue signal transmission is given in the paper. At least two different times exist in such tools: the dead time of detectors and the dead time of the signal transmission set-up. A method of two radioactive sources is proposed to measure these two different dead times. Two codes have been designed. The first code calculates the dead time based on the recorded countrates only, the second is doing a 'simulation job' and provides information on the statistical distribution of the observed dead times. (from Author)
952210 Combined use of radioactive and inactive tracers in a borehole experiment P. Viitanen, Nuclear Geophysics, 8(4), 1994, pp 335-341. The flow of groundwater between two boreholes has been studied tudng radioactive S2Br and inactive sodium fluorescein as tracers. The primary use of bromine was to determiine the flow rate at the injection and the primary use of sodium fluorescein was to serve as the tracer at the system output. Either of the tracers would have been adequate for both tasks but the combined use provided an increased reliability for the test. (Author)
952211 A theoretical model for the temperatme response of compensated thermal neutron tools G. L. Mathis, Nuclear Geophysics, 8(4), 1994, pp 327-334. The overall temperature response of a compensated thermal neutron tool has two major components. The first is due to the temperature of the thermal neutrons as they interact with the hefium detectors within the tool. The magnitude of this effect is governed exclusively by the pressure of the helium within the detector and the diameter of the interacting volume. The second component comes from the changes in neutron slowing down length and diffusion length with formation temperature. Over the range in which the porosity response is finear these two components can be computed independently of each other and added together to get the total temperature effect. (Author)
73A
952212 Neutron porosity logging and core porosity measurements in the Beauvoir granite, Massif Central Range, France C. Galle, Journal of Applied Geophysics, 32(2-3), 1994, pp 125-137. With its low porosity, its weak fracturing and its high homogeneity, the Beauvoir granite was chosen for the analysis of the relationship between logged data and the properties measured in the core. The Beauvoir granite has a total free water porosity of around 2% and an average neutron porosity of around I0%. The origin of this significant difference is related to the neutron matrix effect of the granite. This phenomenon is partly due to the slowingdown effect of the combined water of clays and micas but also to the nentronic capture effect linked with the relatively high lepidolite content of the granite. This experimental study shows that geophysical logging must he adapted to the type of rock and that the geological context has to be taken into account before running the tool in the borehole. (from Author) 952213 A low activity spectrometric gamma-gamma borehole logging tool for the coal industry M. Borsaru & C. Ceravolo, Nuclear Geophysics, 8(4), 1994, pp 343-350. A logging tool based on the spectrometric T-7 technique was tested in water-filled boreholes of nominal diameter 96 mm in a coal deposit. The tool was centralized and employed a T-raY source of very low activity (1.8 MBq). The tests proved that the tool could be used to delineate the coal seams in boreholes and measure the ash content of coal. The tool is sensitive to changes in the Z~q number of the matrix surrounding the borehole and could find application in the metalliferous mining industry for ore body delineation. (from Authors)
952214 Electrical resistivity measurement through metal casing C. J. Schenkel & H. F. Morrison, Geophysics, 59(7), 1994, pp 1072-1082. Methods using dc electrical arrays to measure formation resistivity through casing have relied on approximate forms for the current and potential distributions to derive a ~-nple relationship between the formation resistivity and the transverse resistance calculated from measurements of the potential and its second derivative inside the casing. A numerical solution has been derived for the potentials and their derivatives to examine the accuracy of the approximate forms for casing of finite-length, annualar zones of varying radius, and for vertical discontinuities such as layers or abrupt changes in annular zone radius. (from Authors)
952215 Borehole transmission tomography for velocity plus statics L. J. Squires, P. L. Stoffa & G. Cambois, Geophysics, 59(7), 1994, pp 1028-1036. The accuracy of velocity tomograms reconstructed from borehole transmissioin traveltime data is highly sensitive to traveltime statics. The paper presents a least-squares tomography algorithm that includes a traveltime static term. The algorithm solves for both the velocity field and the traveltime statics simultaneously. This enables us to separate traveltime signal from traveltime noise, reducing the tomographic velocity artifacts caused by the statics. Application of the algorithm to numerical crosswell data results in velocity and statics' estimates that are accurate to within 1%. Appfieation of the algorithm to Exxon's Friendwood tomography data results in velocity and statics' estimates that correlate with independent data. (from Authors)