摘要:

AbstractAn iterative stacking algorithm (superstack) has been applied in velocity analysis as a velocity filter prior to the coherency measurements. The effect of this filtering is demonstrated on both synthetic and real data using the following three methods of velocity analysis: Constant velocity stack, constant velocity gather, and velocity spectrum. The experimental results indicate that the velocity analysis can be improved by this approach.

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1981.tb01007.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

AbstractIt is proposed that the vertical resolving power of a seismic signal is controlled by three aspects: the width of the central lobe, the side lobe ratio, and the side‐tail oscillations. A comparative study of zero‐phase signals covering the same frequency range shows that improvement of any one of these aspects inevitably leads to deterioration of one of the other aspects.An analytical simulation model is proposed of zero‐phase signals free from side‐tail oscillations, in which both the width of the central lobe and the side lobe ratio are adjustable. Analysis of the spectra of this model shows that, while the high frequency content of the spectrum is essential for obtaining a small width of the central lobe, the low frequency content of the spectrum plays an essential part in causing a low value of the side lob

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1981.tb01008.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

AbstractThirty‐eight gravity method estimates of the depth to the pre‐Cenozoic surface under Yucca Flat are used to determine the statistical accuracy associated with the gravity method. Results indicate that the gravity method tends to overestimate the depth to the pre‐Cenozoic surface, on the average, by 3.5 m with a standard deviation of 75.2 m. No significant change of this statistical estimate was observed with (1) depth, (2) location, or (3) date when the gravity‐method estimate w

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1981.tb01009.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

AbstractThe generalized integral equation for the electric potential governed by a quasi‐harmonic equation can be derived via a variational formulation. For surface current distributions it is not always a Fredholm integral equation of the second kind. Numerical solutions of the general heterogeneous problem can be obtained with the “reciprocal averaging technique”, where the solution is obtained a second time after exchange of source and field p

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1981.tb01010.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

AbstractSince its development some thirty years ago, the airborne electromagnetic (AEM) method has been primarily used as a tool for mineral prospecting. However, advanced AEM systems are capable of other tasks, such as geological mapping and groundwater exploration. Excellent correlation between maps of apparent conductivity and geological maps was observed in several regions of Brazil where AEM surveys were performed. The degree of correlation seems to depend on the local climate. In humid and subhumid tropical regions, a weathered layer develops whose thickness and conductivity depend upon bedrock lithology. Therefore certain lithological types can be recognized from their conductivity signature; e.g., granites and Precambrian coarse clastic rocks are resistive, metavolcanic (particularly mafic) and volcanic rocks are conductive, Phanerozoic sediments are generally highly conductive.Two geophysical surveys are analyzed in the paper. The first was conducted with the time‐domain, towed‐bird AEM system in the Itapicuru greenstone belt in the state of Bahia. The apparent conductivity map correlated better with the local lithology than the magnetic map. Results of the AEM survey were successfully used to improve the regional geological map. A helicopter EM system was used in the second survey, which covered a portion of the Precambrian shield of Rio Grande do Sul. Also in this region ground checks confirmed the usefulness of conductivity surveys in geological mapping.The technique outlined in the paper holds great promise for countries of humid tropical climate, where few outcrops exist and access is often difficult. The tests performed in Brazil indicate that by executing AEM/aeromagnetic surveys during initial stages of mapping and exploration programs, time and expenditure required for geological field work can be considerably reduced. The resulting geological maps are more accurate and the inventory of mineral occurrences becomes more compl

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1981.tb01011.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

AbstractThe objective of this study is to generate the separation‐distance‐domain (r‐domain) transformation of the theoretically calculated wave number domain (m‐domain) electromagnetic induction field componentBz(m,ω) of a stratified medium and to search for interpretive information which has been absent in the previously achieved numerical solutions of the problem.Ther‐domain kernelR̃(r,ω) function defining the induction field appears to adequately reflect the layering and electrical properties of the medium if it is expressed as a function of the frequency if the source‐receiver separationris small with respect to the thickness of the first layer. However, exact values of the conductivity cannot be distinguished from those of the neighboring values unless a resistive basement layer is present. This feature is the result of the truncation in series representation of the kernel functionR̃(m,ω). However, this truncation is regarded as significant in the case of a conductive first layer. Inm‐domain static‐zone studies, a conductive first layer slightly influences itsr‐domain correspondent.Although the computational cost of obtaining the kernelB(r,ω) by evaluation of the convolution in a cylindrical coordinate system is high, this semi‐analytic solution is still superior to those based on t

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1981.tb01012.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

AbstractA nomogram has been devised for situations, in which the source of a self‐potential anomaly can be approximated by an obliquely polarized sphere or horizontal cylinder embedded in a homogeneous half space. The nomogram can be used for rapid determination of three parameters of the target: (1) depth to the centre, (2) angle between the axis of polarization and the horizontal, (3) shift of the point vertically above the centre of the body from zero potential value.The nomogram has been tested and the parameters determined for SP results obtained over ore bodies Weiss and Süleymanköy in the Ergani Copper district, Turkey. The curves computed for the estimated parameters match the field curves w

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1981.tb01013.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

AbstractA method to calculate the resistivity transform of Schlumberger VES curves has been developed. It consists in approximating the field apparent resistivity data by utilizing a linear combination of simple functions, which must satisfy the following requirements: (i) they must be suitable for fitting the resistivity data; (ii) once the fitting function has been obtained they allow the kernel to be determined in an analytic way.The fitting operation is carried out by the least mean squares method, which also accomplishes a useful smoothing of the field curve (and therefore a partial noise filtering). It gives the possibility of assigning different weights to the apparent resistivity values to be approximated according to their different reliability.For several examples (theoretical resistivity curves in order to estimate the precision of the method and with field data to verify the practicality) yield good results with short execution time independent of shape the apparent resistivity curve.

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1981.tb01014.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

AbstractOne of the most troublesome problems in resistivity sounding is caused by the spurious effects of near‐surface lateral resistivity variations. It has been found that the effects can be strongly reduced by measuring earth resistances at two electrode array positions such that in one position the lateral effect is a positive contribution to the total measured signal and in the other the contribution is negative. The subsequent combination of the two measurements virtually eliminates the lateral effect.The same technique can also result in a significant reduction in the total number of necessary electrode positions. Consequently the method can be used with a multicore cable system. In this way, in addition to improving the accuracy of the measured results, time and manpower are considerably reduced.Details of the method, examples and comparisons with traditional sounding techniques are give

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1981.tb01015.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1981.tb01016.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY