Geoexploration最新文献

This study deals with the application of the Hartley transform in the inversion of total field magnetic anomalies due to a two-dimensional dyke model. The ratio of the Hartley transform to the amplitude spectrum gives the function from which the centre, magnetisation angle and dip may be determined. The depth and width of the dyke are determined from the amplitude spectrum.

Tests using theoretical anomalies have shown that most of the estimates of the parameters are in agreement with parameters used to compute the anomalies. Proper scaling of the transform is required to better estimate depth and intensity of magnetisation.

Because of its potential to detect changes in pore-water salinity the surface electrical resistivity method can be a valuable aid in coastal groundwater exploration and investigations. It is essential, however, that the resistivity interpretation be consistent with a hydrogeological model reflecting the fresh-water-salt-water relationship of coastal aquifers. In the electrical resistivity interpretation of phreatic aquifers it should be recognized that the lower boundary of the unsaturated zone corresponds to the top of the capillary zone, not to the water table, and that the lower boundary of the fresh-water layer corresponds only approximately to the top of the fresh-water-salt-water transition zone. The existence of a fresh-water layer can be ascertained qualitatively by visual inspection of electrical sounding curves, provided there is a fresh-water/unsaturated layer thickness ratio of at least four. Good interpretative methodology using an appropriate coastal hydrogeological model can enable the extent of the fresh-water layer to be quantified, but it is not possible to quantify the thickness of the transition zone by geoelectrical interpretation because of suppression effects.

Two- and three-dimensional gravimetric models of two meteoritic craters from Clearwater Lake are presented. The gravimetric data were obtained from the Geophysics Division of the Geological Survey of Canada and a survey carried out during this investigation. A regional gravity map was extracted from the Bouguer anomaly corrected for topography. The negative residual anomalies are circular and concentric with respect to the morphologies of the two basins of the lake. The anomaly of the western basin has an intensity of − 100 μm s⁻² and is composed of two superimposed anomalies (∼-26 km and ∼-8 km in diameter respectively). The anomaly of the eastern basin has an intensity of about −95 μm s⁻²and a diameter of ∼ 16 km. The models obtained through direct modelling are characterized by a bowl-shaped geometry. The western basin is characterized by two concentric zones; the central zone has the highest density contract (−210 kg m⁻³) with respect to the bedrock outside the crater rim. The intermediate zone has a contrast of −250 kg m⁻³ with respect to the surrounding bedrock. The bodies of the model are dipping towards the centre and a thickening of the structure takes place in its centre. The eastern basin is more difficult to model because of the occurrence of layers of overburden and sedimentary rocks deposited in the crater after the meteoritic impact.

An algorithm is presented for computing the gravitational attraction of a two-dimensional polygon whose density is a polynomial function of depth. The contribution of each side and its partial derivatives with respect to the vertex coordinates are expressed analytically, and they contain only powers of the z-coordinates and the same logarithm and arctangent terms used with homogeneous polygons; thus, both direct and inverse calculations can be efficiently performed. A variant using rectangular blocks permits also to handle lateral density changes.

The case of an exponential density-depth function is covered by a series expansion, and bounds of error are given in order to select the proper number of terms.

In the application to a real case, the determination of the basement of a deep sedimentary basin serves to compare the performance of different density-depth estimates.

The very-low-frequency electromagnetic (VLF-EM) method has been used extensively for the detection and delineation of weak conductors such as those formed by water and/or clay-filled fracture and shear zones in the Precambrian rocks of the Canadian Shields as part of the Canadian Nuclear Fuel Waste Management Program. However, no satisfactory method of interpreting VLF anomalies over such conductors was available when the field data became available in the early 1980s. A study was undertaken to develop a method for quantitative interpretation of ground VLF-EM data over two-dimensional (2D) sheet-like conductors of low conductance, finite depth and depth extent, embedded in a resistive host rock of finite resistivity, using characteristic curves. Numerical modelling of the VLF response of such conductors revealed that the axes-ratio (inappropriately termed ellipticity in VLF literature) variations are much smaller than the corresponding tilt angle variations, and much less sensitive to the depths of the conductors. Extensive ground VLF-EM surveys over weakly conductive fractures and shear zones at several test areas in Ontario also indicated that the axes-ratio response is not only much smaller than the corresponding tilt angle response, it is often difficult to identify in the field data. An interpretation scheme was devised to determine the conductance, depth and depth extent of such weak vertical conductors located in a host rock of finite resistivity which depends on the peak-to-peak tilt angle response and the horizontal separation between the peaks. A complete interpretation of such conductors requires knowledge of the host rock resistivity and the approximate depth, if the depth extent of the conductor is required. The interpretation method was applied to interpret a VLF anomaly over a complex conductivity structure at East Bull Lake, Ontario which illustrated the limitations of the technique. A ground VLF anomaly from Chalk River, Ontario, was interpreted using the interpretation scheme described, which agrees well with the information from geological mapping and with the numerical forward modelling response.

Owing to the band limited nature of seismic data, ridge regression plays an important role in the processing and particularly in the deconvolution of seismic sections. To improve the condition number of the autocovariance matrix a ride regression parameter (RRP) is added which is a small fraction of the zero-lag autocovariance value. As pointed out by Berkhout, the size of the RRP is important since it affects the phase of the residual wavelet. In a previous work, Matsuoka and Ulrych explored this property of the RRP by considering the effect of very large values of this parameter on the deconvolution of Vibroseis data. In this paper we briefly summarise our previous findings and extend the method by considering the effect of band limited ridge regression on the deconvolution of Vibroseis data. We call this approach BLRR deconvolution and we develop a theoretical justification for its use. We show that the use of BLRR is effective in the removal of not only short- and long-period multiples, but also in the deconvolution of phase shifted and attenuated wavelets. The latter result is of particular importance, since, as shown by Gibson and Larner, the conventional phase corrected deconvolution is not effective in low-Q environments. The determination of the value of the BLRR parameter in this work is accomplished by monitoring a modified form of the Varimax of the deconvolved trace. The use of the Varimax norm was suggested by Levy and Oldenburg as a measure of phase distortion, and we have found it to be of particular value in deconvolution studies.

A simple algorithm to solve inverse geophysical problems is presented. It uses a statistical modelling of the forward problem in the presence of an unknown disturbing parameter vector with nonlinear dependence between their components and the model field. The nonlinear dependence is investigated directly in the parameter space and a particular linear approximation is made. Thus, a nearly optimal solution is obtained. The quality of the estimates is statistically predicted, this allows a rational selection of the most informative observation points. The method is especially suitable for many typical inverse problems in geophysical prospecting. A gravity exploration example is presented.

The key to obtaining improved knowledge of the subsurface seismic structure lies in the use of multicomponent recording (multi-directional sources and triaxial detection) and vector (elastic) wavefield imaging. To this end we have assembled a two-dimensional, biaxial physical seismic model system.

The laboratory facility has been used to acquire vector seismic data over a very simple reflecting structure for both VSP and MSP (inverted VSP) array configurations. Controlled direction reception (CDR) filtering of the data yields four sections for interpretation: upgoing P, downgoing P, upgoing S and downgoing S.

The model experiment has revealed an interesting mode conversion from Rayleigh wave (or tube wave, when extended to the field situation) to scattered S wave, which dominates the seismic section. This wave could be a troublesome form of noise on scalar seismograms, but it can be eliminated by means of CDR to pass the P waves only. Alternatively, it can be viewed as a useful multicomponent diagnostic for detecting fractures or other impedance boundaries intersecting the borehole.

Intermediate layers are frequently hidden in seismic refraction surveying, in the sense that their presence cannot be directly detected from a travel time graph of first arrivals. Consequently, large errors in seismic interpretation may occur. Not only are the layers missed, but the calculated depths to deeper refractors may be overestimated (velocity inversion problem) or underestimated (masked layer problem).

A rigorous solution to the problem of hidden layers is presented here based on anomaly offset. The procedure preserves consistency between observed and calculated offsets, thus yielding corrected depths and limits on hidden layer velocity values. The method is illustrated by means of a field example involving a buried river channel.