- Home
- A-Z Publications
- Geophysical Prospecting
- Previous Issues
- Volume 42, Issue 8, 1994
Geophysical Prospecting - Volume 42, Issue 8, 1994
Volume 42, Issue 8, 1994
-
-
Location of secondary faults from cross‐correlation of the second vertical derivative of gravity anomalies1
Authors Hualin Zeng, Qinghe Zhang and Jun LiuAbstractA 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. One‐dimensional model computations show that the magnitude of the extremum of the cross‐correlation is related to the depth to the top of the hanging wall and the throw of the fault. Application of the method to the Bouguer gravity map of the former mouth of the Yellow River in the Shengli Oilfield area near the Bo Hai Sea shows the effectiveness of the method.
-
-
-
The potential of a noise‐reducing antenna for surface NMR groundwater surveys in the earth's magnetic field1
Authors D.V. Trushkin, O.A. Shushakov and A.V. LegchenkoAbstractA 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 noise‐reducing figure‐of‐eight‐shaped antenna, consisting of two touching coils each of diameter 50 m, has been utilized.
Using this antenna, the NMR signal has been calculated for different depths of water‐saturated layers with various inclinations of the geomagnetic field. The model calculations and experimental data have been compared and found to be mutually consistent. The two‐coil antenna is shown to be suitable for studies at depths of up to 30–40 m, which is of practical importance for engineering geology.
-
-
-
An airborne tensor VLF system. From concept to realization1
Authors Laust B. Pedersen, Wei Qian, Lars Dynesius and Ping ZhangAbstractRadio signals from very low frequency (VLF) transmitters distributed world‐wide have been used for several decades to study the lateral variations of the electrical conductivity in the upper few hundred metres of the earth's crust. Traditionally, in airborne applications, the total magnetic fields from one or two transmitters are measured to form the basis for construction of maps that primarily show those conductive structures that are parallel or subparallel to the direction to the transmitters. 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 filled with conducting sediments or valleys underlain by conductive fracture zones in the crystalline rocks.
-
-
-
Transversely isotropic media equivalent to thin isotropic layers1
More LessAbstractAny set of isotropic layers is equivalent, in the long wavelength limit, to a unique transversely isotropic (TI) layer; to find the elastic moduli of that layer is a solved problem. The converse problem is to find a set of isotropic layers equivalent to a given TI media. Here, explicit necessary and sufficient conditions on the TI stiffness moduli for the existence of an equivalent set of isotropic layers are found by construction of a minimal decomposition consisting of either two or three isotropic constituent layers. When only two constituents are required, their elastic properties are uniquely determined. When three constituents are required, two have the same Poisson's ratio and the same thickness fraction, and even then there is a one‐parameter family of satisfactory minimal decompositions. The linear slip model for fractured rock (aligned fractures in an isotropic background) yields a restricted range of transverse isotropy dependent on only four independent parameters. If the ratio of the normal to tangential fracture compliance is small enough, the medium is equivalent to thin isotropic layering and in general its minimal decomposition consists of three constituents.
-
-
-
Statistical analysis of non‐stationary voltage recordings in geoelectrical prospecting1
Authors V. Lapenna, M. Macchiato, D. Patella, C. Satriano, C. Serio and V. TramutoliAbstractWe study the geoelectrical problem of picking out the useful signal from voltage time series, monitored under conditions of a low signal‐to‐noise ratio and non‐stationary noise. Statistical tests performed at different sites show that geoelectrical noise often belongs to the class of non‐stationary phenomena with non‐Gaussian probability distributions. In such cases, the application of conventional methods of geoelectrical useful signal extraction, based on the stationary white‐noise assumption, gives biased estimates. For the on‐line processing of geoelectrical recordings, we recommend the use of the periodogram technique combined with the Kolmogorov–Smirnov test, a suitable algorithm of which is described in detail. The suggested procedure allows data acquisition to stop as soon as the useful signal power is estimated with a relative error smaller than a predetermined value. Finally, we compare the suggested procedure with the autoregressive approach. The previously used and simpler periodogram method, applied to the solution of problems of this kind, appears to give better performances than the autoregressive analysis.
-
-
-
Inversion for seismic anisotropy using genetic algorithms1
Authors Steve Horne and Colin MacBethAbstractA general inversion scheme based on a genetic algorithm is developed to invert seismic observations for anisotropic parameters. The technique is applied to the inversion of shear‐wave observations from two azimuthal VSP data sets from the Conoco test site in Oklahoma. Horizontal polarizations and time‐delays are inverted for hexagonal and orthorhombic symmetries. The model solutions are consistent with previous studies using trial and error matching of full waveform synthetics. The shear‐wave splitting observations suggest the presence of a shear‐wave line singularity and are consistent with a dipping fracture system which is known to exist at the test site. Application of the inversion scheme prior to full waveform modelling demonstrates that a considerable saving in time is possible whilst retaining the same degree of accuracy.
-
-
-
Velocity model updating using image gathers1
More LessAbstractFor successful prestack depth migration an accurate velocity model is needed. One method for model updating is based on image gather analysis. In an image gather all reflectors line up horizontally if the correct velocities are used for the depth migration. This is also true for dipping reflectors, as all traces of an image gather belong to the same surface coordinate. The images of the reflector in an image gather curve upwards if the velocity used for the migration is too low, or downwards if the velocity is too high. This deviation can be used for model updating. Curves which depend on depth, offset and a parameter which relates the estimated to the true model are fitted to the image. By calculating the coherence along the deviation curves, this parameter can be estimated and hence an update can be calculated.
Formulae are derived for the deviation curves and the update of the velocity depth model for a multilayered model for both shot and common‐offset migrated data, with and without gradients. The method is tested on synthetic data with satisfactory results.
-
Volumes & issues
-
Volume 72 (2023 - 2024)
-
Volume 71 (2022 - 2023)
-
Volume 70 (2021 - 2022)
-
Volume 69 (2021)
-
Volume 68 (2020)
-
Volume 67 (2019)
-
Volume 66 (2018)
-
Volume 65 (2017)
-
Volume 64 (2015 - 2016)
-
Volume 63 (2015)
-
Volume 62 (2014)
-
Volume 61 (2013)
-
Volume 60 (2012)
-
Volume 59 (2011)
-
Volume 58 (2010)
-
Volume 57 (2009)
-
Volume 56 (2008)
-
Volume 55 (2007)
-
Volume 54 (2006)
-
Volume 53 (2005)
-
Volume 52 (2004)
-
Volume 51 (2003)
-
Volume 50 (2002)
-
Volume 49 (2001)
-
Volume 48 (2000)
-
Volume 47 (1999)
-
Volume 46 (1998)
-
Volume 45 (1997)
-
Volume 44 (1996)
-
Volume 43 (1995)
-
Volume 42 (1994)
-
Volume 41 (1993)
-
Volume 40 (1992)
-
Volume 39 (1991)
-
Volume 38 (1990)
-
Volume 37 (1989)
-
Volume 36 (1988)
-
Volume 35 (1987)
-
Volume 34 (1986)
-
Volume 33 (1985)
-
Volume 32 (1984)
-
Volume 31 (1983)
-
Volume 30 (1982)
-
Volume 29 (1981)
-
Volume 28 (1980)
-
Volume 27 (1979)
-
Volume 26 (1978)
-
Volume 25 (1977)
-
Volume 24 (1976)
-
Volume 23 (1975)
-
Volume 22 (1974)
-
Volume 21 (1973)
-
Volume 20 (1972)
-
Volume 19 (1971)
-
Volume 18 (1970)
-
Volume 17 (1969)
-
Volume 16 (1968)
-
Volume 15 (1967)
-
Volume 14 (1966)
-
Volume 13 (1965)
-
Volume 12 (1964)
-
Volume 11 (1963)
-
Volume 10 (1962)
-
Volume 9 (1961)
-
Volume 8 (1960)
-
Volume 7 (1959)
-
Volume 6 (1958)
-
Volume 5 (1957)
-
Volume 4 (1956)
-
Volume 3 (1955)
-
Volume 2 (1954)
-
Volume 1 (1953)