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or resistivity mapping in MT-VLF and implications for data processing
226A
SITE INVESTIGATIONSGEOPHYSICS
the stability of the inversion is tested with noisy data. (from Authors) 955191 An airborne tensor VLF system: from concept to realizatiOll
L. B. Pederson,
Wei Qian, L. Dynesius 8z Ping Zhang,
Geophysical Prospecting, 42(8), 1994, pp 863-883.
The tensor VLF technique described in this paper makes use of all signals available in a predefined frequency band to construct transfer functions relating the vertical magnetic field and the two horizontal magnetic field components. These transfer functions are uniquely determined for a particular measuring site and contain information about the lateral conductivity variations in all directions. First experiences with real field data, acquired during a test survey in Sweden, show that maps of the so-called peaker, the spatial divergence of the transfer functions, give an image of the conducting structures. Most of the structures can be correlated to small valleys tilled with conducting sediments or valleys underlain by conductive fracture zones in the crystalline rocks. (from Authors) 955192 Field and/or resistivity mapping in MT-VLF and impfications for data processing R. Guerin, A. Tabbagh & P. Andrieux, Geophysics, 59(11), 1994, pp 1695-1712. A theoretical calculation of the responses of simple 2-D and 3-D prismatic bodies shows that the measurable lateral variations in both fields are independent, uncoupled, and correspond to frequency-independent, static variations. It is advantageous therefore to process and map the fields separately. Processing methods used in gravimetry and magnetics are especially appropriate in correcting the apparent anisotropy related to the horizontal polarization of the primary tield and in integrating (upward continuation) the high-frequency spatial variations. The different processes tested on a synthetic case and on data obtained at the Centre de Recherches Geophysiques test site showed how to eliminate shallow features and a current channeling conductor to favor the anomaly of a fault. A third example with a hydrogeological application shows the advantages of this method over Wenner direct-current resistivity measurements. (from Authors) 955193 An automatic technique for presentation of coincident-loop, impulse-response, transient, electromagnetic data R. S. Smith, R. N. Edwards & G. Buselli, Geophysics, 59(10), 1994, pp 1542-1550. Coincident-loop TEM sounding data are often presented by plotting the half-space apparent conductivity as a function of delay time. A new algorithm generates an improved presentation that plots the apparent conductivity as a function of depth. The resulting data may be further processed to sharpen or ‘spike’ the smoothly varying apparent-conductivity/depth curves in an attempt to better represent the rapid changes in conductivity that often exist in the earth. The algorithm described involves an approximation, but is simple, easy to use, and computationally efficient. (from Authors) 955194 The boundary integral calculations of the forward problem for DC sounding and MMR methods for a 3-D body near a vertical contact M. Hvozdara & P. Kaikkonen, Studia Geophysics et Geodnetica, 38(4), 1994, pp 375-398. Presents comprehensive theory based on the boundary integral method for calculations of the electric potential,
electric field and corresponding magnetic field due to a pair of DC source electrodes near a vertical resistitity contact in the halfspace, inducing a 3-D disturbing body in the vicinity of the contact. Special attention is paid to the case when the disturbing body touches the vertical contact. It is shown that the presence of the disturbing body at the contact is most pronounced in the electrical characteristics. (from Authors) 955195 The potential of a noise-reducing antenna for surface NMR groundwater surveys in the Earth’s magnetic field D. V. Trushkin, 0. A. Shushakov & A. V. Legchenko, Geophysical Prospecting, 42(8), 1994, pp 855-862. A method of non-invasive NMR in the earth’s field has been developed and is now used for groundwater surveys to depths of investigation of 100 m or more. A circular wire loop of diameter 100 m, laid out on the ground, is employed to excite and receive the NMR signal in the earth’s field. However, in areas with high electromagnetic noise, the NMR measurements may be inaccurate. To overcome this problem, a noisereducing figure-of-eight-shaped antenna, consisting of two touching coils each of diameter 50 m, has been utilized. The two-coil antenna is shown to be suitable for studies at depths of up to 30-40 m. (Authors) 955196 A torque magnetometer for measurements of the high-field anisotropy of rocks and crystals F. Bergmuller, C. Barlocher, B. Geyer, M. Grieder, F. Heller & P. Zweifel, Measurement Science & Technology, 5(12), 1994, pp 1466-1470. A highly automated torque magnetometer has been designed, mainly for measuring standard sized rock samples. By employing two hinges of crossed leaf-springs it was possible to integrate a stepper motor in the sample holder for the rotation of the specimen in the external magnetic field. The dynamic range of the instrument covers the torque signals of most rock types. Additionally the instrument is able to measure the magnetocrystalline anisotropy of monocrystals. (Authors) 955197 Magnetometer profting in the evaluation of potential waste disposal sites P. Lyle & P. J. Gibson, Proceedings - ICE: Geotechnical Engineering, 107(4), 1994, pp 217-221. This paper gives details of the use of a portable magnetometer in the investigation of disused quarries as potential landfill sites. The technique is non-destructive and can be applied across a range of bedrock types including basalt, granite, schist, sandstone and limestone, irrespective of the degree of exposure of the rock. It is especially valuable in the recognition of fracture zones which are potential pollutant pathways into the groundwater regime of the proposed site. (Authors) 955198 Location of secondary faults from cross-correlation of the second vertical derivative of gravity anomalies Hualin Zeng, Qinghe Zhang & Jun Liu, Geophysical Prospecting, 42(8), 1994, pp 841-854. A method is described to locate secondary faults, which can be difficult to identify on the Bouguer gravity map. The method is based on cross-correlation between the theoretical anomaly due to a vertical step and the second vertical derivative of the Bouguer anomaly. Faults are located from the closed maxima and minima on the cross-correlation contour map calculated for two perpendicular directions. Application of the method to the Bouguer gravity map of the former mouth of the Yellow River in the Shengli Oiltield area
SITE INVESTIGATIONSGEOPHYSICS
the stability of the inversion is tested with noisy data. (from Authors) 955191 An airborne tensor VLF system: from concept to realizatiOll
L. B. Pederson,
Wei Qian, L. Dynesius 8z Ping Zhang,
Geophysical Prospecting, 42(8), 1994, pp 863-883.
The tensor VLF technique described in this paper makes use of all signals available in a predefined frequency band to construct transfer functions relating the vertical magnetic field and the two horizontal magnetic field components. These transfer functions are uniquely determined for a particular measuring site and contain information about the lateral conductivity variations in all directions. First experiences with real field data, acquired during a test survey in Sweden, show that maps of the so-called peaker, the spatial divergence of the transfer functions, give an image of the conducting structures. Most of the structures can be correlated to small valleys tilled with conducting sediments or valleys underlain by conductive fracture zones in the crystalline rocks. (from Authors) 955192 Field and/or resistivity mapping in MT-VLF and impfications for data processing R. Guerin, A. Tabbagh & P. Andrieux, Geophysics, 59(11), 1994, pp 1695-1712. A theoretical calculation of the responses of simple 2-D and 3-D prismatic bodies shows that the measurable lateral variations in both fields are independent, uncoupled, and correspond to frequency-independent, static variations. It is advantageous therefore to process and map the fields separately. Processing methods used in gravimetry and magnetics are especially appropriate in correcting the apparent anisotropy related to the horizontal polarization of the primary tield and in integrating (upward continuation) the high-frequency spatial variations. The different processes tested on a synthetic case and on data obtained at the Centre de Recherches Geophysiques test site showed how to eliminate shallow features and a current channeling conductor to favor the anomaly of a fault. A third example with a hydrogeological application shows the advantages of this method over Wenner direct-current resistivity measurements. (from Authors) 955193 An automatic technique for presentation of coincident-loop, impulse-response, transient, electromagnetic data R. S. Smith, R. N. Edwards & G. Buselli, Geophysics, 59(10), 1994, pp 1542-1550. Coincident-loop TEM sounding data are often presented by plotting the half-space apparent conductivity as a function of delay time. A new algorithm generates an improved presentation that plots the apparent conductivity as a function of depth. The resulting data may be further processed to sharpen or ‘spike’ the smoothly varying apparent-conductivity/depth curves in an attempt to better represent the rapid changes in conductivity that often exist in the earth. The algorithm described involves an approximation, but is simple, easy to use, and computationally efficient. (from Authors) 955194 The boundary integral calculations of the forward problem for DC sounding and MMR methods for a 3-D body near a vertical contact M. Hvozdara & P. Kaikkonen, Studia Geophysics et Geodnetica, 38(4), 1994, pp 375-398. Presents comprehensive theory based on the boundary integral method for calculations of the electric potential,
electric field and corresponding magnetic field due to a pair of DC source electrodes near a vertical resistitity contact in the halfspace, inducing a 3-D disturbing body in the vicinity of the contact. Special attention is paid to the case when the disturbing body touches the vertical contact. It is shown that the presence of the disturbing body at the contact is most pronounced in the electrical characteristics. (from Authors) 955195 The potential of a noise-reducing antenna for surface NMR groundwater surveys in the Earth’s magnetic field D. V. Trushkin, 0. A. Shushakov & A. V. Legchenko, Geophysical Prospecting, 42(8), 1994, pp 855-862. A method of non-invasive NMR in the earth’s field has been developed and is now used for groundwater surveys to depths of investigation of 100 m or more. A circular wire loop of diameter 100 m, laid out on the ground, is employed to excite and receive the NMR signal in the earth’s field. However, in areas with high electromagnetic noise, the NMR measurements may be inaccurate. To overcome this problem, a noisereducing figure-of-eight-shaped antenna, consisting of two touching coils each of diameter 50 m, has been utilized. The two-coil antenna is shown to be suitable for studies at depths of up to 30-40 m. (Authors) 955196 A torque magnetometer for measurements of the high-field anisotropy of rocks and crystals F. Bergmuller, C. Barlocher, B. Geyer, M. Grieder, F. Heller & P. Zweifel, Measurement Science & Technology, 5(12), 1994, pp 1466-1470. A highly automated torque magnetometer has been designed, mainly for measuring standard sized rock samples. By employing two hinges of crossed leaf-springs it was possible to integrate a stepper motor in the sample holder for the rotation of the specimen in the external magnetic field. The dynamic range of the instrument covers the torque signals of most rock types. Additionally the instrument is able to measure the magnetocrystalline anisotropy of monocrystals. (Authors) 955197 Magnetometer profting in the evaluation of potential waste disposal sites P. Lyle & P. J. Gibson, Proceedings - ICE: Geotechnical Engineering, 107(4), 1994, pp 217-221. This paper gives details of the use of a portable magnetometer in the investigation of disused quarries as potential landfill sites. The technique is non-destructive and can be applied across a range of bedrock types including basalt, granite, schist, sandstone and limestone, irrespective of the degree of exposure of the rock. It is especially valuable in the recognition of fracture zones which are potential pollutant pathways into the groundwater regime of the proposed site. (Authors) 955198 Location of secondary faults from cross-correlation of the second vertical derivative of gravity anomalies Hualin Zeng, Qinghe Zhang & Jun Liu, Geophysical Prospecting, 42(8), 1994, pp 841-854. A method is described to locate secondary faults, which can be difficult to identify on the Bouguer gravity map. The method is based on cross-correlation between the theoretical anomaly due to a vertical step and the second vertical derivative of the Bouguer anomaly. Faults are located from the closed maxima and minima on the cross-correlation contour map calculated for two perpendicular directions. Application of the method to the Bouguer gravity map of the former mouth of the Yellow River in the Shengli Oiltield area