Geoexploration最新文献

A study is presented of the genuine external specular reflections from salt plugs which manifest themselves on individual seismic sections as apparently unconnected foreswipe and backswipe reflections for seismic lines that cross salt plugs and as sideswipe reflections for lines that pass them by. It is shown that these external reflections can be combined, mapped and seismically migrated. An example demonstrates that this allows the depth and shape of the shoulders of a salt plug to be defined. It is also shown that sideswipe reflections on individual seismic sections can be used to determine the maximum possible distance from the seismic line to the flank of a salt plug. This is done by constructing “maximum-distance arrows” on a map; they help in locating salt plugs that are not crossed by a regular grid of regional seismic lines. Reflected refractions, internally reflected against the far salt flank, have been identified. The relation of sideswipe reflections to maximum convexity curves, and the relation between the acoustic impedance of the salt plug itself and that of the surrounding sediments have been investigated. Real seismic data which relate to salt plugs in the Norwegian-Danish Basin have been used in this investigation and as illustrations. The reflection process that gives rise to sideswipe reflections from salt plugs has been investigated by means of small-scale physical models. Routine seismic processing discriminates strongly against salt flank reflections such as foreswipe and backswipe reflections. Aspects of the special processing necessary to enhance these reflections, particularly the determination of the unusually high stacking velocities required, are reviewed. Routine seismic processing does not to the same extent discriminate against sideswipe reflections. The stacking velocities required to enhance them are also discussed.

Geophysical electrical methods play an important role in the study of geothermal fields. They are widely used, by virtue of large resistivity contrasts encountered in them, in almost all stages of geothermal research. The different techniques and the methodologies employed, vary according to the target sought.

In this work, initially a general review of the use of the electrical methods in geothermal exploration is given. Then a description of a typical-form geothermal field is presented and its impact to electrical parameters of the geological formations encountered in it is commented.

The basic elements of the various electrical methods are described in a short form, and their application on the specific geothermal targets is pointed out.

As an example, the Milos/Greece high-enthalpy geothermal field case history is finally presented.

In this presentation, apart from the extensive references for each topic discussed, more weight has been put on the relationbetween the various geothermal targets and the electrical methodologies applied for on one hand, while on the other hand emphasis has been put on the study of fracture zones and faults. The latest are the most important targets in geothermal research since they are considered as the most promising drilling sites for productive drillholes.

This paper illustrates the practicality and efficiency of gravimetry for aquifer prospecting in arid zones. Known for the long and tedious data-processing it requires, this method becomes expeditious when simplified as presented here. Its use is then fully justified in a survey of this kind.

During the study of the Teloua alluvial aquifer (Agadez, Niger), several ancient channels were clearly and rapidly located. Comparison of the results obtained here with those from previous studies demonstrates anew that for comprehensive prospecting, several complementary geophysical methods should always be employed.

Velocity data from uphole surveys were used to map the water table and the contact at the base dune sand/top alluvium as part of a joint National Drilling Company-United States Geological Survey Ground Water Research Project in the Emirate of Abu Dhabi. During 1981–1983, a reconnaissance seismic survey was conducted for petroleum exploration in the eastern region of Abu Dhabi. Approximately 2800 kilometers of seismic data, consisting of 92 lines, were acquired in the 2500 km² concession area near Al Ain. Uphole surveys were conducted about 2 km apart along each seismic line, and were used to calculate weathering corrections required to further process in the seismic data. Approximately 1300 uphole surveys were completed in the concession area between March 1981 and June 1983.

Reinterpretation of the velocity profiles derived from the uphole surveys provided data for determining the following subsurface layers, listed in descending order: (1) a surficial, unconsolidated weathering layer with a velocity from 300 to 450 m/s; (2) surficial dune sand, from 750 to 900 m/s; (3) unsaturated, unconsolidated alluvium, from 1000 to 1300 m/s; and (4) saturated, unconsolidated alluvium, from 1900 to 2200 m/s. Two interfaces—the water table and the base dune sand/top alluvium — were identified and mapped from boundaries between these velocity layers.

Although the regional water table can fluctuate naturally as much as 3 m per year in this area and the water-table determinations from the uphole data span a 27-month period, an extremely consistent and interpretable water-table map was derived from the uphole data throughout the entire concession area. In the northern part of the area, unconfined groundwater moves northward and northwestward toward the Arabian Gulf; and in the central and southern parts of the area, groundwater moves westward away from the Oman Mountains. In the extreme southern area east of Jabal Hafit, groundwater moves southward into Oman. The map of the base dune sand/top alluvium suggests a buried paleodrainage network trending westward to southwestward away from the Oman Mountains. These paleodrainages, now buried by dune sand, probably contain alluvial fill and are logical targets for groundwater exploration.

Geophysical method of electrical resistivity was used in the detection of saline water intrusion in the aquifers within the coastal belt of Mtwara region in southeast Tanzania. A formation saturated with saline water has a low value of the electrical resistivity.

The data from the electrical resistivity surveys were interpreted by the classical method of curve-fitting. Formations with low electrical resistivity ($\text{< 10 Ω·}\text{m}$) were inferred at depths of within 50 m, in the sedimentary coastal zone. Such formations were suspected to be saturated with saline water. Electrical conductivity tests, and drilling logs where feasible, were used to confirm the existence of saline water. The saltwater-freshwater interface is expected to follow the Ghybe-Herzberg lens model.

Formations of low electrical resistivity are observed to continue inland up to 20–30 km, from the shoreline. The top surface of these formations is relatively horizontal and hence does not follow the Ghybe-Herzberg lens model. Presently there is very limited abstraction of groundwater from the coastal aquifers. A moderate amount of annual recharge from rainfall is indicated. Hence saline water intrusion from the sea under prevailing conditions is very unlikely.

However, the relative elevation of the top surface of formations saturated with saline water suggests that it is possible to induce saline water intrusion even with moderate pumping rate. Development of groundwater resources in the coastal zone of Mtwara region in Tanzania ought therefore to be pursued cautiously.

Resistivities above 60,000 Ω·m have bee measured alongside the desert border, under an over-burden of approximately 20 m. Such unusually high resistivities had been previously attributed to the presence of dolerite domes. Considering that there are only two permanent wells in the area surveyed (80 × 100 km²), that dolerites had been found in one well and that their presence is suspected in the other, it was hoped to find perched aquifers in the depressed areas at the top of the domes.

In fact, wide-mesh (10–20 km) electrical soundings have shown that these high resistivities are due to dry sedimentary formations consisting of coarse sand and sandstone. The impermeable bedrock consists of argillites of 200 Ω·m. The dip of such formations (of the order 2 per thousand) is sufficient to produce a “dry wedge” formation in the topographically elevated parts of a 80 km-wide belt along the desert border.

The two wells are located on low-resistivity anomalies within that wedge. Other narrow anomalies were found, some of them on lineaments that are visible on satellite imagery. This suggests that water is trapped by dykes and not in the top of the domes. A borehole drilled to test one of the conductive anomalies encountered weathered dolerites clay, and water was found in fractures of the bedrock.

A resistivity map (AB = 200 m) shows the overall characteristics of the sedimentary basin surveyed, including the conductive anomalies within the dry wedge that may bear water.

A transient electromagnetic (TEM) survey was conducted in northern Nevada (USA), as a part of a groundwater exploration project. The study area consists of alluvium-covered volcanic flows with brecciated fault zones. The objective of the survey was twofold: (1) to identify locations of faults and alluvium with large amounts of clay, both of which are likely to affect groundwater flow; and (2) to identify formations that may be good aquifers.

An optimal range of formation resistivity values was estimated using Archie's law, to serve as a preliminary model for identifying good aquifers. The field survey was conducted along profiles with closely spaced central loop sounding, providing almost continuous coverage. Color modulated pseudosection of logarithmic apparent resistivity versus time were plotted for all survey lines. A one-dimensional numerical model was used to determine depths of investigation and formation resistivities in areas where pseudo-sections did not indicate rapid lateral changes in electrical properties.

The pseudo-sections show two low-resistivity features at depth which are consistent with two faults which have been mapped in the volcanic unit south of the study area. Formation resistivity values from the soundings in the vicinity of four wells were compared with hydraulic tests and electrical well logs where available. For three of the wells, formation resistivity values were in the range of formation resistivity values associated with a good aquifer. Production rates and hydraulic conductivity values at these wells are satisfactory. At the fourth well, which was drilled after the TEM survey to intercept a prominent lineament feature, low-resistivity TEM data had suggested the possibility of poor hydraulic characteristics due to the presence of clay-sized fault gouge material. The presence of fault gouge material subsequently was confirmed by drilling and, as predicted by the TEM survey, the hydraulic properties of the fourth well were considerably less desirable than the other three locations.