Geoexploration最新文献

EM conductivity surveys have been carried out over an air-filled drainage conduit, near Alcala de Henares, Spain. The surveys failed to detect a measurable response from the gallery, but successfully delineated a desaturated drainage channel above the conduit. Target diagnosis was assisted by theoretical one-dimensional modelling, which showed that relative response magnitudes can provide a limited basis for source discrimination. The results confirm that conductive features are more likely to be detected than resistive voids. More versatile 2D modelling schemes are required, however, to provide greater interpretative insight and to exploit the full potential of the $\text{EM31}\text{34}$ systems.

Surface collapse over abandoned subsurface coal mines is a problem in many parts of the world. High-resolution P-wave reflection seismology was successfully used to evaluate the risk of an active sinkhole to a main north-south railroad line in an undermined area of southeastern Kansas, USA. Water-filled cavities responsible for sinkholes in this area are in a 0.6 m thick coal seam, 7 m deep. Dominant reflection frequencies in excess of 200 Hz enabled reflections from the coal seam to be discerned from the direct wave, refractions, air wave, and ground roll on unprocessed field files. Repetitive void sequences within competent coal on three seismic profiles are consistent with the “room and pillar” mining technique practiced in this area near the turn of the century. The seismic survey showed that the apparent active sinkhole was not the result of reactivated subsidence but probably erosion.

An extensive TDEM survey, which covered almost the whole Mediterranean coastal strip of Israel, was carried out during the summer of 1989 in order to make a quantitative evaluation of the TDEM method in detecting seawater intrusion into the coastal aquifer. To achieve statistical significance of results, a total of approximately 100 TDEM stations was established, some in close proximity to existing observation wells. In order to realize the most objective evaluation of the TDEM results, respective data interpretation was carried out prior to the use of, or even acquaintance with, any geological or hydrological information from the wells. The two main objects of the survey were:

• 1.

  (a) Statistical corroboration of an extremely important feature of seawater-bearing aquifers, namely their consistent resistivity values which are completely different from those of any known lithology in the area.

• 2.

  (b) Quantitative evaluation of the accuracy level of the TDEM method and assessment of its applicability to solving various hydrological problems.

It was found that the resistivity values of the seawater-intruded aquifers vary within a range of 1.1 to 2.9 Ω-m, whereas low resistivity lithologies have minimum values of approximately 10 Ω-m.

Of the 31 TDEM measurements which detected seawater intrusion and which could be quantitatively compared with borehole data, 21 showed good agreement (the discrepancy in the depth to the interface is less than 5 m), five measurements showed fair agreement (discrepancy between 6 and 10 m) and three measurements showed poor agreement (discrepancy between 11 and 16 m). In one case there was complete disagreement between the TDEM and borehole data (discrepancy ~ 70 m) and in another case, although the TDEM results indicate seawater, brackish water was actually found in the well.

Twelve additional TDEM measurements which detected seawater intrusion and which were carried out close to the observation wells, could not be quantitatively correlated with borehole data owing to technical limitations in the wells. However, qualitative and sometimes semiquantitative correlation was possible and showed an agreement between the TDEM and borehole data in all measurements.

Seven other TDEM measurements carried out near the boreholes did not detect seawater intrusion; five of them were compared with observation wells and in four cases agreement was found, while seawater was found in one of the wells.

Intense aeromagnetic anomalies with amplitudes ranging from several thousands of nanotesla (nT) are observed over the Cuddapah-Kurnool group of sediments and also over pans of the outcropping Archaean basement in the Vijayapuri-Macherla region of the Proterozoic Cuddapah basin in South India. The anomaly zone has a NW-SE elongation and extends over a strike length of about 30 km with varying widths of 5 to 10 km. From the nature and amplitude of the magnetic anomalies it is inferred that the source of these intense magnetic anomalies could be banded iron formations (BIF's) that are commonly associated with greenstone belts of Archaean age. This inference is corroborated by Landsat, photogeological studies and field checks. Continuation of the magnetic anomalies further south and southeast suggests the possible continuation of the iron formations and the associated schistose rocks beneath the Kurnool sediments.

Results of in situ and laboratory susceptibility measurements agree with the observed variation in the amplitudes of the magnetic anomalies. NRM measurements and Koenigsberger ratios (Q_n) of the order of 10 to 30 suggest the presence of a large component of remanent magnetization in certain parts of the iron formations. The study has demonstrated the complementary role of different remote sensing tools in identification and delineation of greenstone belts of Archaean age.

Four examples of very low frequency (VLF) electromagnetic (VLF-EM) anomalies within the Basement Complex in the Igbeti district, southwestern Nigeria, revealed that the method can be successfully used to map non-conductive mineral bodies like marble and quartzite as well as geological boundaries and faults. Rocks of extremely low conductivity were marked by very distinct VLF anomalies with varying magnitudes. The VLF anomalies are believed to be due to faults and water bearing fracture zones, possibly at moderate depth, at the contact of the rocks. A poorly known N-S trending fault traversing the western margin of the Igbeti marble was mapped and confirmed by the VLF survey. The results obtained over the marble indicate that the technique is cost-effective and particularly suitable for mapping such lithologies, especially where the geology is relatively simple.

The present study reports on a telluric profile, carried out over the Campidano graben, which is one of the most interesting structures in Sardinia. The telluric data, which were interpreted together with magnetotelluric, gravity, electrical and magnetic data, depict the form of the graben with a remarkable degree of mutual consistency and reliability. The results confirm the value of a combined interpretation in solving complex problems, where the application of a single method may give only an ambiguous or incomplete solution.

This paper describes a procedure for porosity interpretation from seismic data (POISE) by exploiting merits of several recent advances in seismic data processing and interpretation.

Briefly, the procedure is as follows: starting from NMO corrected CDP gathers of P-wave data and applying amplitude versus offset (AVO) analysis, one can separate out zero offset “P-wave” and “Swave” stacks both as a function of P-wave time. Here the term “S-wave” refers to a stack that we would have obtained if we had carried out an S-wave survey and displayed the data in P-wave traveltime. Application of maximum likelihood deconvolution/modelling technique on stacks leads to models for P-wave and S-wave reflection coefficients. Finally, inverting the reflection coefficients for interval transit time, a section can be prepared to display ΔT_s-ΔT_p, the difference in slowness of the S-wave and P-wave as a function of the P-wave traveltime.

The final output of POISE for ΔT_s-ΔT_p is particularly useful for studying variations in lithology and porosity within a formation. The interpretation is fouSnded on an empirical observation by Ikwuakor that ΔT_s-ΔT_p from character logs for a large number of carbonate and sandstone samples, is proportional to the core porosity (for a given rock type).

The utility of POISE is illustrated with a field example in a carbonate reservoir.