Geoexploration最新文献

In the summer of 1988 a new gravity survey of the city of Milan was carried out. Within the Municipality's territory (182 km²) gravity was measured at 320 stations, utilizing the city levelling network that is checked every two years. The stations in the inner area were tied to the Agip^∗ gravity network of the Po valley, surrounding the city, in order to evaluate the regional field. In 1950 a first survey of the central urban area had been made on 220 stations; a residual, circular negative anomaly was found, occupying approximately the area of the municipality; at that time the source of this anomaly was not investigated.

The results show that the anomaly can be attributed mainly (50–60%) to the combined geological effects of the shallow formation (within a depth of about 150 m). Also the shorter wavelength anomalies can be attributed to lateral variations of the very shallow formations. Temporal variations due to the modifications of the aquifers or to subsidence were not observed on comparing the results of the two surveys, owing to the insufficient accuracy of the first measurements, but also because they are small in comparison with the effects of permanent sources.

Rocks containing the sulphide mineral pyrrhotite are often associated with pronounced aeromagnetic anomalies. The magnetic properties of these rocks have been examined in many laboratory studies. An attempt is made, based on results from SW England, to combine the magnetic anomalies and the magnetic properties in order to improve the understanding of any inter-relationship. It is shown that the pyrrhotite is characterised by high Q-values and well-developed magnetic anisotropies. The pyrrhotite-bearing rocks therefore retain a strong remanence but the direction of this has been strongly influenced by the form and nature of the magnetic bodies. The magnetic anomalies in SW England are extensive and it is suggested that this resulted from the way in which pyrrhotite is readily formed in suitable horizons within sedimentary rock sequences. Pyrrhotite is also easily destroyed by weathering of outcrops so that the availability of borehole cores plays an important part in the study of the area.

Surface and downhole mise-à-la-masse data have been collected around a massive sulphide body in Gairloch, northwest Scotland. Qualitative analysis has led to the location of sub-surface faults dividing the mineralization and to the establishment of continuity of ore between boreholes. In this paper, however, quantitative analysis of the data including the use of a simple interpretation scheme and three-dimensional modelling by the integral equation technique is presented giving rise to a detailed interpretation of the area, which includes estimates of the lateral extents of each of the mineralized blocks.

Investigations by the VLF method in mountainous and foothill regions are greatly complicated by rugged-relief effects. A brief account has been given of the available approaches to tackling this problem and a correlation technique has been suggested based on deriving a linear relation between VLF magnetic field intensities and the heights of observation points. The effectiveness of this technique has been confirmed by the results of its application on some Caucasian pyrite deposits.

Along with the introduction of full waveform sonic logging tools has come a variety of associated digital signal processing techniques designed to estimate the formation compressional and shear slownesses (Δt, inverse velocity, travel time). In this paper, we have described these techniques and applied them, for the most part, to the same set of field waveforms. We have divided our treatment into those techniques associated with traditional two-received tools, and those associated with the recent multi-receiver array tools.

The processing associated with two-receiver tools generally consists of methods which make use of time windows and coherence measures. Specifically, time windows are positioned on each trace, and the coherence of the windowed signals is computed. The window positions which result in the highest coherence can be used to derive an estimate of the wave slowness. Issues associated with two-receiver processing seem to be focused on methods for locating the arrival time of the waves at the receivers. The semblance coherence measure seems to be more popular in the literature than either cross-correlation methods or cross-spectral techniques. In implementing these methods, we have found the resulting estimates to be somewhat sensitive to issues such as the shape and duration of the time window. The estimation of the shear slowness from two-receiver data is more difficult than the estimation of compressional slowness, due to interference from other arrivals including mode conversions from bed boundaries and fractures, and due to dispersion. Some techniques address the difficulties associated with shear estimation more than others.

With the recent commercial introduction of multi-receiver sonic array tools, a number of processing techniques have appeared in the literature. Analogous to windowed coherence methods developed for two-receiver tools, multi-receiver windowed coherence methods have developed for array tools. Again, semblance processing seems to be particularly useful. Because longer array apertures can lead to a reduced ability to resolve thin beds in the formation, a new technique has been developed which extracts sub-arrays from the full arryas associated with successive firings of the source transducer. The shorter arrays result in higher resolution, and the multiplicity of sub-arrays provide added stability. Frequency domain techniques have also been developed which are able to handle dispersive wave propagation and can aid in situations where waves are overlapped in space and time due to close slownesses. These two situations can cause coherence based methods to perform poorly. An assumption common to all processing techniques is that the formation is homogeneous across the aperture of the (sub-)array. This causes the performance of these techniques to degrade when there is a bed boundary or fracture within the aperture. An area of future research is likely to be in the area of processing for