Showing papers in "Geophysics in 1976"

Synthetic seismograms: a finite ‐difference approach

Abstract: Recent interest in the extraction of fine detail from field seismograms has stimulated the search for numerical modeling procedures which can produce synthetic seismograms for complex subsurface geometries and for arbitrary source‐receiver separations. Among the various techniques available for this purpose, the replacement of the two‐dimensional wave equation by a suitable finite‐difference representation offers distinct advantages. This approach is simple and may be readily implemented. It automatically accounts for the proper relative amplitudes of the various arrivals and includes the contributions of converted waves, Rayleigh waves, diffractions from faulted zones, and head waves. Two computational schemes have been examined. For the so‐called “homogeneous formulation,” the standard boundary conditions between media of different elastic properties must be satisfied explicitly. In the case of the alternate “heterogeneous formulation”, these elastic properties may be specified at each grid point of a f...

Velocities of seismic waves in porous rocks

Abstract: Velocities of seismic compressional and shear waves in porous rocks under different saturation conditions are calculated theoretically and compared with laboratory data. For theoretical formulations, the rocks are represented by a solid matrix and pores of spherical and oblate spheroidal shapes. The effect of confining pressure on velocities is calculated by taking into account pore closing and saturant compressibilities.The theoretical calculations show that with all other parameters fixed, thin pores (small aspect ratios) have much greater effects on elastic moduli and velocities than rounded pores at the same concentration. The properties of the saturating fluid (gas, oil, or water) have greater effects on the compressional velocities than on shear velocities. The velocities of compressional waves are higher when the rock is saturated with water than when it is dry or gas-saturated. For shear waves the behavior is generally opposite, with shear velocities higher in the dry or gas-saturated case than in the water-saturated case.Compressional and shear velocities measured as a function of pressure in laboratory samples of granite, limestone, and sandstone, under dry and water-saturated states, are fitted with theoretical curves and pore shape spectra which fit the data are calculated. A spectrum of pore shapes ranging from spheres to very fine cracks (aspect ratios 1 to 10 (super -5) ) is required to fit the data. Theoretical velocities calculated using these models fit the measured velocities in water-saturated and frozen rocks, as well as the compressional velocities in partially saturated rocks.With the rock models derived on the basis of laboratory data, theoretical seismic velocities are calculated for various pressures and temperatures for reservoir rocks fully or partially saturated with gas, oil, or brine. Compressional velocities are highest for brine saturation and lowest for gas saturation. The difference decreases with increasing pressure. The presence of a small amount (5 percent) of gas in brine as an immiscible mixture reduces the compressional velocities significantly, even below those of fully gas-saturated values at some pressures.The reflection coefficients for compressional waves at a gas-brine interface in a model of a sandstone are high at pressures corresponding to shallow and moderate (less than about 8000 ft) depths. At greater confining pressures, reflection coefficients become small, except when the pore fluid pressure (gas pressure) is very high. Thus, large reflections or 'bright spots' from great depths may indicate overpressured formations. The reflection coefficients from mixed gas-brine interfaces are lower than those of pure gas interfaces. A combination of interval velocities and reflection amplitudes may help identify the mixed gas-brine reservoirs. Poisson's ratios for gas-saturated rocks are lower than those for brine-saturated. This difference persists to great depths.

Fluid saturation effects on dynamic elastic properties of sedimentary rocks

Abstract: The influence of saturation by water, oil, gas, and mixtures of these fluids on the densities, velocities, reflection coefficients, and elastic moduli of consolidated sedimentary rocks was determined in the laboratory by ultrasonic wave propagation methods. Twenty rock samples varying in age from Pliocene to early Devonian and in porosity from 4 to 41 percent were tested under uniform pressures equivalent to subsurface depths of 0 to 18,690 ft. Fluid saturation effects on compressional‐wave velocity are much larger in low‐porosity than in high‐porosity rocks; this correlation is strengthened by elevated pressures but is absent at atmospheric pressure. At a frequency of 1 MHz, the shear‐wave velocities do not always decrease when liquid pore saturants are added to rocks as theorized by Biot; agreement with theory is dependent upon pressure, porosity, fluid‐mineral chemical interactions, and the presence of microcracks in the cementing material. Experimental results support the belief that lower compression...

Gravity and magnetic fields of polygonal prisms and application to magnetic terrain corrections

Abstract: Computer programs based on the exact calculations of the gravity and magnetic anomalies of polygonal prisms are faster in operation and more accurate than previous programs based on the numerical integration of polygonal laminas. The prism programs also are of more general application than existing computer programs that are based on the exact gravity and magnetic effects of rectangular prisms. There are no restrictions on the use of the exact formula for the gravitational attraction of a polygonal prism, but the formulas for the magnetic effect are restricted in that demagnetization is not considered, and a finite answer is not obtained in the unrealistic circumstance where an observation point coincides with an edge of the prism. Least‐squares methods permit calculation of the gravity or magnetic effect of models without knowledge of the density or magnetization contrasts, respectively, by comparison of the observed anomalies with theoretical dimensionless values to determine contrasts as regression coe...

Effect of brine‐gas mixture on velocity in an unconsolidated sand reservoir

Abstract: Gas in an unconsolidated sand reservoir encased in shale often results in a dramatic increase in amplitude of the seismic reflection from the shale/gas-sand interface. Unfortunately, reflection amplitude appears not to vary linearly with water (brine) saturation, and thus cannot be used to estimate gas quantity. Previously presented theoretical velocity computations, for a Tertiary sedimentary section, which demonstrate that compressional-wave velocity in an unconsolidated gas sand varies nonlinearly with brine saturation, qualitatively agree with laboratory velocity measurements on a sand specimen composed of pure quartz grains. However, significant departure of measured and theoretical velocities at high brine saturation indicates that the technique for partially saturating the sand specimen by flowing a gas-brine mixture through the specimen does not provide a sufficiently uniform distribution. The gas preferentially seeks larger pores. In a subsequent experiment on a specimen composed of spherical glass beads of nearly uniform size, the previous, as well as a modified, fluid injection technique was used. For the latter, brine only was injected into the pore space previously filled with a mixture of gas and brine in nearly equal proportions. This resulted in a more uniform distribution of the gas-brine mixture. For approximately equal brine saturations, this modified technique resulted in a measured compressional-wave velocity approximately one-half of the velocity measured for the previously used fluid injection technique. This result implies that if the gas-brine mixture is uniformly distributed in a reservoir, the fluid compressibility is the weighted-by-volume average of the constituent compressibilities.

Shear‐wave velocity versus depth in marine sediments: a review

Abstract: The objectives of this paper are to review and study selected measurements of the velocity of shear waves at various depths in some principal types of unlithified, water‐saturated sediments, and to discuss probable variations of shear velocity as a function of pressure and depth in the sea floor. Because of the lack of data for the full range of marine sediments, data from measurements on land were used, and the study was confined to the two “end‐member” sediment types (sand and silt‐clays) and turbidites. The shear velocity data in sands included 29 selected in‐situ measurements at depths to 12 m. The regression equation for these data is: Vs=128D0.28, where Vs is shear‐wave velocity in m/sec, and D is depth in meters. The data from field and laboratory studies indicate that shear‐wave velocity is proportional to the 1/3 to 1/6 power of pressure or depth in sands; that the 1/6 power is not reached until very high pressures are applied; and that in most sand bodies the velocity of shear waves is proportio...

Theoretical Modeling Of The Magnetic And Gravitational Fields Of An Arbitrarily Shaped Three–Dimensional Body

Abstract: An analytical method is developed for modeling the magnetic field or the gravitational attraction of a homogenous, arbitrarily shaped, three–dimensional body. In this method, the body to be modeled is represented by a polyhedron composed of triangular facets. A convenient method for inputting the apices and assembling the facets of a polyhedron is developed. As the solution is analytic, the polyhedron facets can be any size, and the number of input points depends only upon the number of apices required to outline the body.

Residual statics analysis as a general linear inverse problem

Abstract: To estimate near‐surface time anomalies, it is commonly assumed that apparent seismic reflection times are comprised of the sum of “surface‐consistent” source and receiver static terms, “subsurface‐consistent” structure and residual normal moveout (RNMO) terms, and indeterminate noise. The model parameters (statics, RNMO, and structural terms) that, in a least‐squares sense, best satisfy traveltime observations in multifold seismic data are solutions to a set of linear simultaneous equations. Because these equations are ill conditioned and their solutions are known to be nonunique, conventional direct methods of solution are not applicable. Problems of this type which have both overdetermined and underconstrained aspects can be analyzed using the general linear inverse methodology. In this approach, observed time deviations are decomposed into linear combinations of orthogonal eigenvectors, each of which determines a related linear combination of model parameters. A property of this decomposition is that ...

Random Networks and Mixing Laws

Abstract: Random networks are investigated as models of heterogeneous media. A general approximate structure is used where the networks are described as a system of embedded networks, and the critical behavior and averaging behavior of such networks are developed. These results are applied to a study of the electrical conductivity of porous media, with special attention to an Archie9s law behavior. It appears that the wide range of crack and pore widths in rocks makes the resulting conductivity relatively insensitive to the topology of their interconnections and allows one to make reasonable predictions of rock conductivities, given the distribution of crack and pore widths. It also appears that with low porosity rocks the conductivity is controlled by the microcrack population which only accounts for a fraction of the total porosity. It would seem, therefore, that Archie9s law is a feature of some general trend between porosity and crack and pore width distributions rather than a fundamental property of porous media. The law of the geometric mean is an accurate predictor of the physical properties of a mixture of different materials. This mixing law can result from an equal balance of series and parallel arrangements which can be produced by an appropriate distribution of shapes. A brief look is given to problems of anisotropic distributions for the conductivity problem and it is shown how the averaging process greatly dilutes the microscopic anisotropy in producing the macroscopic properties.

Scattering of Elastic Waves from Depth-Dependent Inhomogeneities

Abstract: We have studied scattered pulse shapes by modeling inhomogeneities as a sequence of infinitesimally thin homogeneous layers. With oblique incidence of plane P or SV waves, the reflected-converted-transmitted waves are obtained by taking the calculus limit for the sum of primary interactions of the incident wave with all layer boundaries. The resulting scattered waves thus present themselves naturally in the time domain. For an incident impulse, the scattered pulse shape is merely an analytic function of the depth from which scatter has taken place within the inhomogeneity. The direct application of this simple method appears to be new, and we have found it remarkably accurate when compared with methods in which higher-order boundary interactions are also retained (i.e., Haskell methods and an adaptation in the time domain which also keeps all multiples).In specific studies of P-waves incident (up to 30 degrees away from the vertical) upon a 5 km thick crust-mantle transition, between materials having impedance ratio 1:2.8, we find the scattered pulse shapes are given adequately by our theory, for the passband of short-period seismometers. Indeed, the theory remains remarkably accurate even for long periods, being in error by only 8 per cent at zero frequency.

A subsurface electromagnetic pulse radar

Abstract: An electromagnetic pulse radar has been developed for investigation of subsurface geology and man-made targets. The radar uses separate broadband dipole-type antennas for transmission and reception. The antennas are well matched to the soil or rock surface. An orthogonal orientation of the antennas on the medium surface effectively decouples them and also prevents reflections from the air-medium interface or any horizontal stratification from being seen. Two versions of the radar are used: small 6-ft dipoles with a shock-type (250 picosec) pulse for shallow soundings and 24-ft dipoles with a 45 nanosec pulse for deeper soundings. Signatures of faults, joints, cavities, and lithologic contrasts in soft rock have been obtained with the radar, and these results are presented. A sampling oscilloscope acts as a receiver for the radar, and the target signatures are isolated portions of the time record whose time delays agree with the physical geometry and measured pulse velocities for the medium. For a large void at a depth of 20 ft in limestone, a frequency domain signature is also given to illustrate the potential of using both temporal and spectral signatures. Signatures of an exposed fault in a dolomite quarry are used via mapping measurements to delineate themore » direction of a minor fault. Signatures of two lithologic contrasts at depths of 40 ft in the dolomite are given. The signatures of a drift coal mine tunnel as measured from a hill 11 to 26 ft above the tunnel are shown. Unique features of the radar are enumerated and present capabilities are summarized.« less

Finite‐difference resistivity modeling for arbitrarily shaped two‐dimensional structures

Abstract: Resistivity surveying is commonly done by using a point‐source dipole. Consequently, a finite‐difference evaluation of apparent resistivity curves implies the use of three‐dimensional simulation models which necessitate prohibitive computer costs. However, if we assume variation of resistivity only in two dimensions and use a line‐source dipole for setting up the finite‐difference model of a given structure, the potential field can be evaluated easily. A discrete version of the resistivity problem in two dimensions, which takes into account nonuniform grid spacing, is presented as a system of self‐adjoint difference equations. Since the iterative solution of such a system does not require grid spacing to be less than a certain critical value, it was successfully used for the development of fast‐convergence finite‐difference models. By examining in detail the characteristics of the matrix associated with the evaluation of the potential field, it is demonstrated that the proposed modeling procedure will rem...

An improved method of computing the thermal conductivity of fluid-filled sedimentary rocks

Abstract: In the absence of suitable material on which to make thermal conductivity measurements, it is still possible to compute a value on the basis of an idealized model. The Maxwell model for the thermal conductivity of a two-component material, a model which has frequently been successfully used for relatively small values of porosity (<0.1) and moderate conductivity ratios (<10), may be extended to much wider ranges of these parameters (0.3 and 300, respectively) by the use of appropriate correction factors obtainable from data already in the literature. Significant errors due to overlooking a thermal contact resistance have been detected in some experimental work. The errors can be as high as 50 percent in the measured value for an air-filled sandstone of 0.2 porosity. Unless corrected, such errors can contribute a great deal to the lack of agreement between computed and experimental values. Correction for these errors also leads to much better consistency among published experimental values measured with various techniques. Conductivity values accurate to better than 15 percent may therefore be predicted for specimens saturated with low conductivity fluids such as air, steam, or oil.

The Electromagnetic Response of a Conductive Inhomogeneity in a Layered Earth

Abstract: A numerical model has been constructed to determine the three‐dimensional electromagnetic fields in the vicinity of a finite, thin, conductive plate buried in a horizontally stratified, conductive environment. The EM source is a rectangular loop. The problem is formulated as an integral equation for the electric field in the plate. However, to avoid certain numerical difficulties, the actual working variables are a pair of scalar potentials which represent divergence‐free and curl‐free current flows in the plate, and whose values are known at the nodes of a rectangular grid. The basic integral equation is then reduced to a set of linear equations which can be solved numerically.The cases modeled are a simulation of the Turam method. The models were a shallow plate and a deep plate in a conductive half‐space, a deep plate in an insulating host rock under a conductive layer, and a deep plate in a conductive host rock under a conductive layer. In all cases, the top of the plate was separated from the overbur...

Vp/Vs—A POTENTIAL HYDROCARBON INDICATOR

Abstract: Theoretically and experimentally, the shear‐wave velocity of a porous rock has been shown to be less sensitive to fluid saturants than the compressional wave velocity. Thus, observation of the ratio of the seismic velocities for waves which traverse a changing or laterally varying zone of undersaturation or gas saturation could produce an observable anomaly which is independent of the regional variation in compressional wave velocity. One source of shear‐wave data in reflection seismic prospecting is mode conversion of P waves to shear waves in marine areas of high water bottom P-wave velocity. A relatively simple interpretative technique, based on amplitude variation as a function of the angle of incidence, is a possible discriminant between shear and multiple compressional arrivals, and data for a real case are shown. A normal moveout velocity analysis, carefully coupled with this offset discriminant, leads to the construction of a shear‐wave reflection section which can then be correlated with the usua...

Numerical Solutions of the Response of a Two-Dimensional Earth to AN Oscillating Magnetic Dipole Source

Abstract: A finite difference formulation is developed for computing the frequency domain electromagnetic fields due to a point source in the presence of two‐dimensional conductivity structures. Computing costs are minimized by reducing the full three‐dimensional problem to a series of two‐dimensional problems. This is accomplished by Fourier transforming the problem into the x-wavenumber (kx) domain; here the x-direction is parallel to the structural strike. In the kx domain, two coupled partial differential equations for H⁁x(kx,y,z) and E⁁x(kx,y,z) are obtained. These equations resemble those of two coupled transmission sheets. For a requisite number of kx values these equations are solved by the finite difference method on a rectangular grid on the y-z plane. Application of the inverse Fourier transform to the solutions thus obtained gives the electric and magnetic fields in the space domain. The formulation is general; complex two‐dimensional structures containing either magnetic or electric dipole sources can ...

Modern Standards for Gravity Surveys

Abstract: The modern gravity meter is capable of determining the difference in gravity between measurement sites with very high accuracy. However, the gravity values established by one survey can be related to those established by another survey only when both are tied to gravity base stations whose gravity values are in a uniform system.

An automatic method of direct interpretation of magnetic profiles

Abstract: The well‐known equations for the total magnetic field due to thin sheets and the edges of a thick body (Werner, 1953) can be programmed to compute automatically the depth to the top, susceptibility contrast, and the dip of these features from a given total magnetic field profile. The synthetic anomalies show that in ideal cases the depths can be determined to the accuracy of 10 percent or better, provided the source of the anomaly can be identified as a bounding edge or the thin sheet. It has also been found that the anomalies due to edges approximately one depth unit apart in horizontal direction can be resolved. Vertically, the interpretation of shallow bodies is not affected by the presence of deeper bodies. However, the deeper bodies can be located only when they cause anomalies much stronger than those associated with shallow bodies, or when the shallow bodies are displaced from the deep edges horizontally by a distance equal to or greater than the depth to the top of deep edges. The shallow high‐fre...

2-D multiple reflections

Abstract: Starting with a 1-D subsurface model, a method is developed for modeling and inverting the class of multiple reflections involving the near‐perfect reflector at the free surface. A solution to the practical problem of estimating the source waveform is discussed, and application of the 1-D algorithm to field data illustrates the successful elimination of seafloor and peg‐leg multiples. Extending the analysis to waves in two dimensions, we make the approximation that the subsurface behaves as an acoustic medium. Based on several numerical and theoretical considerations, the scalar wave equation is split into two separate partial differential equations: one governing propagation of upcoming waves and a second describing downgoing waves. The result is a pair of propagation equations which are coupled where reflectors exist. Finite difference approximations to the initial boundary value problem are developed to integrate numerically the surface reflection seismogram. Use of the 2‐D algorithm for modeling free‐...

Autoregression Order Selection

Abstract: In the recent paper by Ulrych and Bishop (1975), Akaike’s (1969) final prediction error criterion (FPE) was used to select the order of autoregressions calculated using Burg’s (1967) maximum entropy algorithm, and compared to the Yule‐Walker estimates for which this criterion was developed (Jones, 1974). For the Burg estimates, the FPE criterion tended to choose too large an order.

Gravity vertical gradient measurements for the detection of small geologic and anthropogenic forms

Abstract: The gravity tower vertical gradient has been applied to the solution of a number of important geologic, mining, and engineering problems, particularly to the search for and investigation of geologic structures and the detection of caverns and old mine workings. The effective application of the method depends upon recognizing the difference between the theoretical gravity vertical gradient Vzz and the gradient Wzz measured by means of a tower and gravimeter. The former is a derivative of the function g, the latter its differential quotient. Consequently, the differences between Vzz and Wzz in the same point may attain high values. Thus, e.g., for a sphere with a radius of 1 m, a density of 2.0 gm/cc and the depth of occurrence of its center equaling 1.2 m, the difference of the theoretical absolute amplitudes Avzz-Awzz=465.4 Eotvos units. Application of the method of the gravity tower vertical gradient on an industrial scale was possible due to the introduction of (1) a new design of the measuring tower, (...

Signal design in the "vibroseis"@ technique

Abstract: After more than twenty years of field practice, the most widely used signal in the Vibroseis technique remains the original so‐called linear sweep. To many, it seems that substantial improvement should be expected from taking advantage of more sophisticated signal design techniques. The purpose of this paper is to present a tutorial survey of the basic concepts involved in signal design, tying together the known features of the linear sweep with those of the more general type. Several particular types of nonlinear sweeps are analyzed and the results of a field test seem to confirm the conclusions drawn from the conceptual analysis. After a brief consideration of still another type of nonlinear signals, a particular variety of pseudorandom sweeps, the author concludes by giving his views of what might be the role of nonlinear signal design in the future development of the Vibroseis technique.

A method for modeling the resistivity and IP response of two-dimensional bodies

Abstract: A method has been developed for the solution of the resistivity and IP modeling problem for one or more two‐dimensional inhomogeneities buried in a space for which the Dirichlet Green’s function is known. The boundary‐value problem reduces to a Fredholm integral equation of the second kind which is parametrically a function of a spatial wavenumber. Using the method of moments, the integral equation is solved for a number of values of the wavenumber. An inverse Fourier transform is then performed in order to obtain the electric potential at any point of interest. The method agrees well with both experimental results and other numerical techniques.

Gravity gradients and the interpretation of the truncated plate

Abstract: The truncated plate and geologic contact are commercially important structures which can be located by the gravity method. The interpretation can be improved if both the horizontal and vertical gradients are known. Vertical gradients are difficult to measure precisely, but with modern gravimeters the horizontal gradient can be measured conveniently and accurately. This paper shows how the vertical gradient can be obtained from the horizontal gradient by the use of a Hilbert transform. A procedure is then presented which easily enables the position, dip angle, depth, thickness, and density contrast of a postulated plate to be precisely and unambiguously derived from a plot of the horizontal gradient against the vertical gradient at each point measured. The procedure is demonstrated using theoretical data.

Negative induced-polarization effects from layered media

Abstract: Negative induced–polarization responses from lateral resistivity distributions such as near–surface vertical contacts and dikes, three–dimensional buried bodies, etc., have been well known for years. Recent comprehensive computations of resistivity and induced–polarization response functions over a three horizontally layered earth have revealed that negative induced–polarization effects can also occur whenever the geoelectric section is of K type (ρ1, ρ3) or Q type (ρ1 > ρ2 > ρ3). For such sections, the induced–polarization function of the first layer (as defined by Seigel) can be negative. As a result, if the first layer is polarizable the response from deeper layers can be severely masked solely as a function of the resistivity distribution, thus limiting the depth of detection of buried sources. It is shown that for an m –layered earth, negative induced–polarization functions can be expected for those layers lying above a K or Q type sequence in the resistivity section. Any application of induced–polarization surveys must thus be preceded by a thorough evaluation of the resistivity data. The same applies for the quantitative interpretation of IP data.

A field test of the magnetometric resistivity (mmr) method

Abstract: The electrical prospecting method, known as the Magnetometric Resistivity (MMR) method, is based on the measurement of the low level (about 100 milligamma), low‐frequency (1–5 Hz) magnetic fields associated with noninductive current flow in the ground. The horizontal component of the magnetic field is measured along profiles which are at right angles to a baseline joining two widely separated current electrodes.The field test was conducted on a plateau in the western cordillera, where the topography is characterized by steep hills, bold ridges, gullies and narrow canyons. A steep faulted contact between basement rocks of differing resistivity is exposed on one flank of the plateau, beneath over 500 m of Tertiary volcanics and sediments.The object of the test was to determine if the basement contact could be mapped by the MMR method, working entirely on top of the plateau. The plan position of the contact could be inferred approximately from measurements at the outcrop.The object was achieved with a minimu...

Computer-assisted interpretation of electromagnetic soundings over a permafrost section

Abstract: In two-loop electromagnetic sounding, the electromagnetic coupling between two vertical-axis loops of wire is measured as a function of frequency, for frequencies ranging from 20 Hz to 20 kHz. If the electrical structure of the earth beneath the loops is horizontally stratified, these data may then be interpreted in terms of a sequence of layer resistivities and thicknesses. This interpretation is accomplished by computing a series of curves for various resistivity profiles and comparing them with the field data to determine which matches best. Calculation of the theoretical models is carried out by applying a linear filter to solve the appropriate integral expression. Interpretation is aided by using an interactive nonlinear least-squares algorithm iteratively to adjust the model parameters. This procedure was used to interpret two-loop induction soundings made along the Arctic Slope of Alaska during 1969 to determine permafrost thickness and character. The results indicate that two-loop induction sounding is an effective method for mapping permafrost characteristics. 19 figures.

The vertical array in reflection seismology—some experimental studies

Abstract: There are several advantages in using vertical arrays for recording reflected signal. Signal-to-noise ratio can be controlled to any desired level when the noise is due to scattering from layers shallower than the depth to the array. By the use of vertical arrays, the band width of useable seismic energy can be increased, events can be properly identified, the signal that eventually produces near surface induced multiples can be measured, as well as the direct pulse radiated from the source and its accompanying ghosts. A field test documents these predictions.

Seeing through rock salt with radar

Abstract: A VHF radar has been used in a salt mine in the Cote Blanche Salt Dome to probe horizontally for the dome Rank and vertically for the top of the salt. Discontinuities in the salt’s complex electric permittivity caused by anhydrite stringers, shale, sandstone, water, or fractures will cause radar reflections. A narrow beam, f 9 degrees in salt, helps to determine the dire&on to the discontinuity. The time of the radar reflection determines the range to the discontinuity. The maximum one-way penetration was 2040 ft. The radar speed in Cote Blanche salt was measured by timing radar reflections through known distances of salt. The measured radar speed was 188 ft/usec, slightly less than the radar speed in pure salt (203 ft/wsec). A shear zone in the salt was found to have many discontinuities or radar targets. A discontinuity was detected in a pillar which contained an oil well casing, and a discontinuity about 400 ft above the mine level, but still inside the salt, was also found. The top of the salt dome was detected from several locations in the mine.

The effect of finite data length in the spectral analysis of ideal gravity anomalies

Abstract: The interpretation of gravity data which has been transformed from the space domain to the frequency domain is subject to limitations imposed by the transformation process and the nature of the data. One potential source of error in the spectral analysis of gravity anomalies is the incomplete definition of the anomaly due to insufficient data length. Comparison of theoretical Fourier transforms and the transforms of finite‐length segments of gravity anomalies due to two‐dimensional vertical slabs and horizontal cylinders indicates that the interpretation error can be held to less than ten percent if the profile length is at least six times the maximum depth to the source of the anomaly. This result was achieved with a rectangular data window in the transformation process. Greater error resulted from the use of the Bartlett, Parzen, and Tukey data windows.