Geophysical Prospecting - Volume 18, Issue s1, 1970

Results of rock magnetic investigations and magnetic field measurements in the Logar Valley, Afghanistan, are presented. Rock magnetic investigations on samples of the ore and the country rock have demonstrated that the chromite is strongly magnetic, whereas the country rock has proved predominantly non‐magnetic, partly weakly magnetic. The isanomalic maps show strong anomalies of some 1000 γ above the known chromite occurrences. Anomalies of the country rock amounting to some 100 γ have been observed only above basic dikes. The measured anomalies have been interpreted quantitatively by 2‐ and 3‐dimensional models. This interpretation is demonstrated and the question of chromite detection discussed. Model graphs are used to investigate in detail whether the extension in depth of ore bodies can be estimated.

During the last decade many hypotheses were suggested to explain the phenomenon of induced electrical polarization in ionic conductive media. The most reliable of these is Fredricksberg's. Fredricksberg (1962) supposed that the pore spaces of a rock is composed of successively narrow (active zones) and wide (inactive zones). He simulated these pore spaces by a synthetic material that has an extremely high resistance. The pore spaces were generally in tube forms which exhibited some constrictions. He saturated these tubes with an electrolyte of a given concentration. An electric current was passed through this model. He observed an induced polarization voltage after current interruption. He attributed the formation of this voltage to a concentration gradient which took place due to the presence of excess charges in the active zones. Fredricksberg introduced a parameter (9) which described the relation among the lengths and cross sectional areas of the wide zones, the number of ions within each zone after current interruption with the recorded polarizability. The aim of this work is to correlate Fredricksberg's parameter with a parameter determined for natural rocks and to show experimentally the validity of this hypothesis when applying for some varieties of sandstones and volcanic rocks. The new parameter will help to evaluate a relationship between the polarizability and the water‐collecting properties of rocks. Herein, we used the tortuosity T of sandstone samples instead of the parameter φ which was used by Fredricksberg to represent the pore geometry within his model (tortuosity of the passes within the model). It was shown that both φ and T have the same relationship with the polarizability ν of the rock samples and if φ or T have very low or very high values the polarizability ν tends to its minimum value, i.e. the curve representing the relation between ν and T has a maximum point corresponding to an intermediate value of T. This result supports Fredricksberg's hypothesis and confirm his results on synthetic models.

For volcanic rocks the formation factor F was used since it was difficult to determine the porosity of the samples and consequently to calculate the tortuosity T as for sandstone samples. Experimental results confirm those obtained from sandstone.

The grain constituents of sandstone samples were represented on equilateral triangle and the magnitude of induced polarization ν of each sample was deduced and represented on this triangle. Equipolarizability values ν drawn on this triangle showed that TJ will increase as the silty fractions of the rock increase, where the center of this triangle (represents minimum porosity) has polarizability less than 0.25%.

An attempt was made to determine the coefficient of anisotropy of volcanic rock samples using the induced polarization method. For this reason the polarizability was deduced by measuring the induced polarization voltage for two perpendicular directions in a fractured cubes of andesitic basalt samples the coefficient of anisotropy was found to be equal 1.18.

The feasibility of using temperature measurements at a depth of about 2 m for locating and delineating salt domes and faults has been investigated both theoretically and in experimental field surveys.

It is shown that measurable temperature anomalies in the soil are to be expected over shallow salt domes.

In a field survey over a salt‐dome area bordering the Groningen gas field, a large number of temperature measurements were made in small holes (2 m deep, 3 cm in diameter) within a relatively short time (some weeks). The results clearly indicate several temperature anomalies with differential temperatures of about 1°C. Comparison of our thermal contour map with interpretations of available seismic or gravity data, or with direct evidence from wells, showed an excellent correlation. Seismic data even support the shape of the thermal contours.

Results in similar agreement with gravity or well data were obtained over salt ridges in a tropical area. Experiments showed that the technique worked as well in lakes and marshes as on dry land.

In addition, some experimental evidence collected so far over shallow and surface faults is presented. In several cases, strong thermal anomalies coincided with known surface faults. A thermal model for a surface‐fault zone is suggested which accounts satisfactorily for the observed thermal data. It suggests some diagnostic value for the fault's geometry.

For shallow faults, however, lack of knowledge of subsurface detail prevented any unambiguous correlation with observed thermal anomalies. Accordingly any geological use of thermal analysis over shallow faults remains debatable.

The field technique is simple, needs little correction and can, where useful, easily be included in routine gravity work to provide additional local information.

The magnetic ground survey of ΔZ across the orebody near Cortegana suggests that the direction of the magnetization of the orebody deviates from the present earth field direction in that area. Magnetic measurements of more than 500 specimens of drilling cores of several vertical and one nearly horizontal drill holes showed that the magnetization of the orebody points essentially to the north in the direction of the inclination of the orebody and the banding of the ore. In the central part of the orebody with an average magnetite content of about 50 vol% the magnetization amounts to 0.35 Gauss, the remanent and induced component having the same order of magnitude. The outer parts of the orebody have a much smaller magnetization according to both the smaller magnetite content and greater inhomogeneity of the remanent magnetization, also partly due to their reversed magnetization which is brought about by the stray field of the central part of the orebody. As all drilling cores have been chemically analyzed with respect to their Fe content a logarithmic relationship could be established between the magnetite content, ranging from 25 to 80 vol%, and the susceptibility.

The seismic velocity of sedimentary rocks is determined by the porosity, the velocity in the matrix and that in the fluid. The last two are known experimentally. The velocity can be measured from the surface. By applying the time average equation we can find the porosity. If the only difference at two points is the nature of the fluid in one layer, the ratio of the reflectivities of the layer at these points determines the nature of the fluid; this ratio is equal to the ratio of the amplitudes at these points. The size of the reservoir can then be found.

The method can be applied to both oil exploration and production. It is more accurate in the latter case because more information is available than in the former case. The method is also more effective in developing discovered fields than the conventional method of relying only on the geological structural configurations.

In order to locate relatively optimum sites for drilling exploratory holes for fresh water, an electrical resistivity survey was conducted along the new Mahukona‐Kawaihae Road on the west flank of the Kohala Mountain. Two resistivity soundings made at the same stations, using the Schlumberger electrode configuration, determined an a spacing of 275 feet for horizontal profiling with the Wenner array.

The correlation coefficient of the elevation to profile data was 0.41. A procedure for removing elevation effect from observed apparent resistivity was developed. Based on the reduced resistivity profile, four relatively optimum sites for additional exploration, such as by drilling, are specified. There is no specific interpretation of the data that can definitely indicate the occurrence of large underground reservoirs of fresh water anywhere along the profile. This is because the interpretation of horizontal profiling data is essentially relative and not absolute.

This work deals with a theoretical investigation of the magnetotelluric sounding where the horizontal layers of a geological section do not have a sharp but a transitional boundary. The ratio of the horizontal electric to the horizontal magnetic components gives a measure of “Cagniard impedance”, which together with the phase relationship between them, enables one to determine the subsurface geometry and the true resistivities.

Utilising the principle of similitude a set of Resistivity Master Curves and the corresponding phase relationships are presented here for the case where the lower half‐space is infinitely resistive. The relation between the relative transition thickness and the cut‐off period is illustrated.

Earth dams are constructed on a wide scale for hydrotechnical and reclamative purposes. However, the great water permeability of soils and their bad gradation are frequently responsible for the development of mechanical suffusion and other phenomena resulting in dam destruction.

Geophysical observations allow under certain conditions to forecast the development of adverse factors. They thus are, as practice shows, an important tool to observe the technical status of dams.

The difference in propagation velocities of elastic waves in the zone of aeration and full saturation determines the applicability of the reflected wave method for determining the depth of seepage flow in the dam supporting mass. This can be also achieved by electrical sounding and induced polarization methods since the resistivity and polarizability of air‐dry and water‐saturated grounds differ considerably.

Maps of equal potentials measured on the dam slope reflect the configuration of the contours of water table of the water flow passing through the dam. This makes it possible to obtain data on the direction and intensity of flow as well as on the presence of heterogeneous grounds filling the dam supporting mass. Besides, regime observations conducted according to the SP method allow to forecast the development of suffusion and observe the redeposition of grounds. If the SP anomalies do not increase with time, the suffusion processes can be regarded as fading out. Enhanced amplitude and increasing areas of anomalies show the intensification of these processes.

Combined application of some of the above mentioned methods allowed to obtain many practical results on a number of dams in Transcaucasia and Central Asia. The true position of the depression surface determined in the dam of one of the high‐mountain water reservoirs, for instance, considerably differed from the designed one. The configuration of the seepage flow surface and its fluctuation caused by the rise and drop of the water level in the reservoir have been studied in detail on another dam situated on a plain. The sites of the most intensive outwash of fine material have also been located and the processes of soil redeposition in the dam supporting mass characterized. The data of the geophysical investigations were used to plan hydroinsulation and repair work.

A point source or a dipole source of electric current, placed on a randomly layered semi‐infinite medium, produces an inhomogeneous random potential field on the surface. The variance of the random potential, normalized with reference to the normal field (that is, a field due to a point source or a dipole source on a homogeneous medium), falls off inversely as the distance from the source. The conductivity of the layers is assumed to vary randomly about a mean value (unity) such that the variations can be represented by a zero mean homogeneous random function. Further the variations are assumed to be small compared to the mean so that the first order perturbation is adequate. The analysis shows that the dipole field is more sensitive than the single pole field to the conductivity variations.

The equation which determines the distribution of the stationary potential ϕ in isotropic heterogeneous conductive media with continuously varying local conductivity σ, takes the symmetrical form

if we choose as new variables

For certain grounds (half‐spaces) in which α is a harmonic function (Δα=ΔΨ= 0), it is possible to obtain by means of simple calculations the lines of equal apparent resistivity and the geoelectrical apparent cross‐sections for the usual devices of d.c. prospecting methods. Graphical examples are also given.

One of the most important aspects in interpretation of gravity and magnetic anomalies, which generally represent the sum of many effects from the subsoil, is that these effects cannot be adequately separated into individual effective anomalies by means of known procedures. Moreover, the concept itself of anomaly separation must be expanded in a certain sense, because the addition of these effects in the mapped anomalies can arise from many structures situated in the subsoil in different relative positions to one another. A combined procedure is proposed, based on Fourier analysis utilising both periodic complex functions and nonperiodic ones, by use of which the number of individual effective anomalies cumulated in the mapped anomalies is obtained. In order to illustrate the use of this procedure and the advantages of obtaining more valuable informations on geological causes of anomalies, one example from Romania is given.

The apparent resistivity is considered as a linear transformation of the potential by an operator in the form of an infinite matrix A. The inverse transformation expresses the potential as a function of the apparent resistivity or of the difference of potentials. It is found by calculation of the inverse matrix^‐1A. The relation between apparent resistivities for different arrays is expressed as a product of transformations.