Cross-borehole tomography and borehole gravity meter (BHGM) surveys allow estimations of seismic velocity and rock density to be made in the vicinity of a borehole. Two such surveys recently conducted in Oklahoma and onshore Gulf Coast sediments show some of the first applications of these combined technologies. The results show an encouraging correlation between density and P-wave velocity for these borehole surveys in shallow clastic sediments. Such a correlation is predicted by Gardner's equation.
The cross-borehole resistivity method measures the electrical potential in one well due to direct-current flowing from electrodes located in another well. This arrangement permits the sensing of regions remote from either well. The paper examines the use of the cross-borehole resistivity method in sensing electrical resistivity perturbations caused by steam-floods, water-floods, and fire-floods.Our examination consists of three parts. We first estimate the magnitude of resistivity perturbations caused by enhanced oil recovery (EOR) processes. We then calculate the theoretical voltage responses, for several theoretical sweep geometries, for a 2.5 acre well-spacing and a hypothetical shallow, heavy-oil field. For case of computation, we assume that the swept zone is two-dimensional. Finally, we contaminate the calculated voltages with Gaussian noise with a 5% standard deviation and invert them in a least-squares sense to sweep geometry estimates. The starting models for these inversions are dissimilar to the theoretical sweep geometries. After ten or so iterations the estimated sweep geometries agree well with the theoretical geometries when the models are sufficiently well discretized. This shows that interpretation of cross-borehole data can give information about sweep geometries. We conclude that the cross-borehole resistivity technique has promise in monitoring enhanced oil recovery (EOR) processes, particularly when combined with effective inversion schemes.
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.
In the Agadez region of Niger, the alluvial water reservoir of the Teloua is only sporadically recharged during periods of flooding. The town's drinking-water reservoir is contained in the ancient paleochannel meanders underlying the present drainage system. A geophysical exploration project was carried out to attempt to reconstruct these paleochannels using an electromagnetic bipole-bipole method (with a 20 m coil seperation and 2048 Hz frequency). The resulting apparent resistivity map enabled these channels to be distinguished as zones of high apparent resistivity. Nine boreholes were subsequently drilled to confirm these results and to be used as piezometers. The importance of correlating the two methods was highlighted through the comparison of the drillhole flowrates and the geophysical resulting drillhole flowrates of 10 m3/h correlated with areas of apparent resistivity values between 90–200 ω·m. One of the nine drillholes has been developed into a production well (80 m3/h, positioned on a resistivity high of 120 ω·m). The bipole-bipole geophysical method enabled the observation drillholes and production well to be positioned quickly and efficiently.
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.
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.
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 ( < 10 OMEGA.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.
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:(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.(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-OMEGA-m, whereas low resistivity lithologies have minimum values of approximately 10-OMEGA-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 approximately 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.
A series of shallow-depth hydraulic fracturing experiments was carried out in the summer of 1988 at the Elda landfill near Cincinatti, Ohio and mise-à-la-masse (MLM) borehole-to-surface resistivity measurements were obtained in an attempt to detect the fracturing. The well casing of an injection borehole was energized and potentials were measured at various points on the surface near the borehole before and after hydraulic fracturing was performed with a conductive fluid. Forward and inverse modeling algorithms based on the DC alpha center method were developed and tested with synthetic data to create a tool for interpretation of the experimental data. The alpha center forward algorithm incorporates a vertical line source of current to model an energized steel well casing. The advantages of the alpha center method are its speed and simplicity, and its ability to handle 3D geometry and indicate positions of conductive inhomogeneities. The forward solution is incorporated into an iterative least-squares inversion algorithm, with constraints applied to the alpha center parameters to facilitate modeling of fractures.
Electrical conductivity is an important petrophysical property used to predict lithology and fluid content in petroleum reservoirs, Conductivity distribution between wells can, in principle, be mapped with electrical or electromagnetic (EM) techniques when sources or receivers (or both) are located in the wells. Unfortunately, the resolution of these methods is relatively poor. Resolution is improved, however, in monitoring applications where the response of a dynamic reservoir process is recorded at different times and compared. Examples of such dynamic processes are fluid replacement during primary production, secondary recovery, and enhanced oil recovery (EOR) techniques as fireflooding, steamflooding, and CO2 flooding. Such measurements can be made with technology currently available within the geophysical industry and at relatively low expense.A numerical model study of the Holt Sand in situ combustion EOR experiment was conducted to lest the feasibility of electromagnetically monitoring the progress of the advancing fire flood. The resistivity and geometry of the bum zone was obtained from pre-burn and post-burn well log data. The study shows that the surface-to-borehole electromagnetic method detects a clear signature from changes in resistivity of the burned reservoir horizon from distances as great as 100 m. Similar conclusions hold for steam flood processes. An important phenomenon which complicates interpretation of EM monitoring of thermal EOR processes is a zone of decreased resistivity adjacent to the reservoir horizon caused by conduction of heat into the bounding shales.
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.
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 (DELTA-t, 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 arrays 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 arrays in inhomogeneous media.
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 placeo-drainage network trending westward to southwestward away from the Oman Mountains. These placeo-drainages, now buried by dune sand, probably contain alluvial fill and are logical targets for groundwater exploration.
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 soundings, providing almost continuous coverage. Color modulated pseudo-sections 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.
Resistivities above 60,000 OMEGA.m have been measured alongside the desert border, under an overburden 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 X 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 OMEGA.m. The dip of such formations (of the order of 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 computer algorithm was developed to model time-domain electromagnetic (TEM) fields for conductive structures radially symmetric about a horizontal loop transmitter. This algorithm improves on previous finite-difference algorithms by calculating the product of the radius times the electric field, which is more accurate than solving for the electric field directly, by using the Crank-Nicholson method of stepping through time which allows for coarser time steps and by using simplified boundary conditions which require less computational effort. These improvements allow models to be calculated on an IBM PC instead of main frame computers.The finite-difference algorithm was used to calculate results for simplified hot-water flood and streamflood simulations at four different stages of the flood front advance, and for thin layers. All the simulations used small horizontal coil sources and receivers and assumed the oil reservoir to be a resistive, horizontal layer (50-OMEGA.m) in a conductive (5-OMEGA.m) background. The TEM responses were dominated by the resistive reservoir layer. In model results, beds as thin as 1/24 the distance between the transmitting and receiving wells are detectable.For both the steam flood and the hot-water flood the greatest change in TEM responses between the flooded and the unflooded reservoirs occurs at early times, from 10 to 100-mu-s. The early-time responses can be qualitatively described as being influenced by the resistivity changes along a relatively narrow signal path which is a straight line in homogeneous regions and refracts along the high velocity reservoir boundary. As a conductive water flood front moves outward from the source borehole, the TEM response is delayed and attenuated for signal paths which pass through the flood zone but little change is seen for signal paths that do not cross the flood zone.
A new method determines the top and bottom of the hydraulic fracturing in a cased treatment well from the microseismicity induced within the fracturing. The method uses a wall-locking sonde to passively record three-component data over a range of depths within and outside the fracturing immediately following either the fracture treatment or subsequent fluid injection into the fractured formation. The processed data (i.e., the background motion after removing the obvious events) show an anomalous inversion as a function of depth that delineates the fracture height. Specifically, the ratio of the horizontal motion component, H, to the vertical component, Z, inverts subdividing the recording traverse into three regions: those above and below the affected zone defined by H/Z < 1 and the affected zone with H/Z > 1. The hydraulic fracture treatment creates the horizontally elongated, in situ stress-aligned affected zone that is comprised of a dilatant communicating network of new and preexisting fractures, joints, pores, and weaknesses, nucleates from the casing perforations. The zone has anomalously reduced elastic properties (i.e., seismic velocities) and acts as an embedded seismic and hydraulic waveguide with the borehole running through it. Pressure gradients, temperature gradients, and stress recovery within the low-velocity zone induce a pervasive microseismic cloud for several hours after the pressurization. The microsource cloud and the low-velocity zone create the data anomaly and its alignment with the extent of the affected zone. Computer simulations of the in situ setting and recording corroborate the H/Z inversion and its interpretation.
In the past decade, both inductive electromagnetic survey instrumentation and associated interpretive techniques have become refined to the point that electromagnetic techniques are now 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 detail 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 histories 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.
Intense aeromagnetic anomalies with amplitudes ranging from several thousands of nanotesla (nT) are observed over the Cuddapah-Kurnool group of sediments and also over parts 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.
