Geoexploration最新文献

Transient electromagnetic soundings and terrain conductivity meter measurements were used to map paleochannel geometry in the Al Jaww Plain of eastern Abu Dhabi Emirate, U.A.E. as part of an integrated hydrogeologic study of the Quaternary alluvial aquifer system. Initial interpretation of the data without benefit of well log information was able to map the depth to a conductive clay layer of Tertiary age that forms the base of the aquifer. Comparison of the results with induction logs reveals that a resistive zone exists that was incorporated into the interpretation and its lateral extent mapped with the transient electromagnetic sounding data.

In the past decade, both inductive electromagnetic survey instrumentation and associated interpretive techniques have become refined to the point that electromagnetic techniques are widely used for geological mapping as well for the direct detection of conductive ore bodies. Electromagnetic survey techniques have been particularly successful in exploration for potable groundwater, for measuring salinity levels in aquifers and monitoring coastal saline intrusion, and for mapping soil salinity in connection with crop growth.

Regardless of the techniques employed, it is the terrain conductivity that is measured, and it is a particular advantage of electromagnetic techniques that small variations in the bulk conductivity of the terrain can often be detected. A further advantage is that most electromagnetic techniques allow measurements to be made rapidly, and survey costs are generally less than those associated with conventional DC resistivity surveys or, conversely, larger areas can be surveyed in greater detaul for comparable cost. A disadvantage of electromagnetic instrumentation is that although the shallower units cost about the same as resistivity equipment, the deeper penetration systems are relatively expensive. In general, electromagnetic systems are most effective in looking for the better conductors and are ineffective in searching for resistive material. In all cases some knowledge of electromagnetic theory is desirable for a successful interpretation.

In this paper we present several case history selected from the literature in which a variety of electromagnetic systems (horizontal loop EM, ground conductivity meters and VLF) are used either alone or in conjunction with conventional resistivity to explore for groundwater.

Exploration studies for village water supply from basement and consolidated sedimentary rocks in Botswana show that groundwater occurrence is often restricted to linear structural features associated with faulting. The fractured aquifer which develops in this situation is characteristically channel-shaped, of narrow width and variable length.

The prevalent cover of Kalahari sands hampers considerably the detection of lineaments from satellite imagery.

By presenting aeromagnetic data as raster images, subtle changes in the magnetic field pattern can be resolved. Breaks or level changes due to structural features, such as faults or fracture zones, are displayed indirectly in airborne magnetic surveys. If satellite imagery and aeromagnetic data are processed in the same format (raster) and geographically referenced by the coordinates of grid points (pixels) they can be inspected simultaneously. The overlay of the two data sets by the use of a Geographic Information System helps to define linear features more accurately than in one data set alone.

The location of fracture zones associated with major lineaments can be mapped successfully on the ground with electromagnetic methods (VLF, HLEM). Multi-frequency horizontal loop systems proved to supply the best guidance to optimal borehole locations if the operating frequencies and the coil separations are properly adjusted to the local geological environment.

Even though an inclined borehole, sited according to the results of this investigation, did not intersect a major fault, the high degree of fracturing in a depth range of 50 to 100 m, together with the circulation losses encountered, correlate with our interpretation.

Many coastal plains in the semi-arid regions of Mexico become fertile lands when properly irrigated. In the last thirty years extensive drilling in several places has disturbed the natural equilibrium of the aquifers; this is partly due to poor knowledge of their distribution and properties, as well as lack of adequate exploitation strategies. This study constitutes a case history of the valley of Guaymas in northwestern Mexico, in which three sets of data are considered: (a) a set of 262 wells, (b) four telluric lines of approximate total length of 90 km, and a set of 326 randomly distributed gravimetric stations. The valley dimensions are 20 km by 50 km; two aquifers have been located, one above 160 m and the other below 320 m. Models have been computed for the four telluric lines and four gravimetric sections. They suggest that sediments on the south-central portion of the valley have a thickness of 800 m. The basement becomes shallow toward the north and south portions of the valley, reaching depths ranging from 200 to 300 m. The valley is flanked by two buried depressions oriented in NNE-SSW direction; these regions reach depths of over 1000 m in some places and apparently constitute reservoirs in which the surface recharge waters are maintained relatively free of contamination from hydrothermal fluids. Such fluids are extracted from shallow wells (under 200 m) in some areas in which the basement approaches the surface. It is concluded that performing geophysical studies on the aquifer's location, in order to determine its regional geological characteristics, is a cost-effective procedure, that allows the establishment of timely extraction strategies.

A typical khondalitic terrain covering an area of 450 km² in Chipurupalli taluk of the Vizianagaram District, Andhra Pradesh, India, has been chosen for well planned and systematic investigations for locating potential aquifers in a typical semi-arid region. A methodology has been developed by assigning numerical weights and ratings to hydrogeological and geophysical parameters depending upon their physical and electrical characteristics which led to proposing a groundwater potential index (gwpi). The efficiency of the method is shown by the improvement in the success rate from 64 to 80%. The proposed methodology further serves to identify the relative groundwater potential of different locations within similar hydrogeologic settings.

Between 1977 and 1982 a series of short term geophysical surveys for groundwater exploration was conducted within a joint integrated development project. The Dhamar and Rada districts are situated at an elevation between 2000 and 2400 m. The geology of the area is complicated and predominantly volcanic. The main formations are Precambrian metamorphic rocks, Tertiary volcanics, Cretaceous sandstone and Quaternary volcanics.

Groundwater occurs in the metamorphic formations mostly in large faults and to a lesser degree in relatively thin alluvial overburden. The Tertiary volcanics consist of alternating layers of more or less permeable lavas, weathered or fractured volcanics and gravel, sand and clay layers. The Quaternary volcanics consist mostly of tuffs (ashes) and basic lavas with a vesicular nature, resulting in a good porosity and permeability. The Tawilah sandstones have a considerable variation in a quifer properties, but form generally an acceptable aquifer.

Site location surveys were carried out in a large number of villages using resistivity sounding and at a later stage EM profiling. The main practical problem during surveying was often caused by limitations in space for electrode expansion.

A limited number of cases is presented as examples of the results obtained in the various geological formations.

In the Precambrian metamorphic formations resistivity sounding was successfully used to locate groundwater in a major faulting system. Yet it must be concluded that EM profiling should be used in advance of resistivity sounding to obtain good results.

The surveys in the Tertiary volcanics were also successful, although a good correlation could not always be made between interpretation and borehole logs. The main reason must be found in the generally low formation resistivities of even dry formations, which makes it difficult to indicate water bearing strata. Water bearing formations occur at various depths, presenting an inconsistent groundwater occurrence.

A survey near the northwestern edge of the sandstone formations near Rada led to the discovery of a major local aquifer, possibly related to a large fault. The question is raised whether recent volcanic activity may have removed (part of) the sandstones in this area.

Resistivity soundings at the foot of the caldera Jebel Isbil indicated groundwater potentials at a depth greater than 200 m. Drilling in this location was successful, while previous boreholes were abandoned as dry, proving that geophysics can improve the rate of success considerably. These results, combined with those obtained near the boundary between the Young Volcanics and the sandstones southeast of the caldera, give good hopes of finding groundwater for villages situated on the Young Volcanics in between.

The successful results of the surveys over this period has led to a firm establishment of this activity within the project, where it now plays a permanent and pr