Showing papers in "Geophysics in 1973"
TL;DR: In this article, the amplitude of a reflection from a thin bed is to the first order of approximation equal to 4πAb/λb, where b is the thickness of the bed and A is the amplitude amplitude of the reflection if the bed were to be very thick.
Abstract: Based on reflective properties, a thin bed may be conveniently defined as one whose thickness is less than about λb/8 where λb is the (predominant) wavelength computed using the velocity of the bed. The amplitude of a reflection from a thin bed is to the first order of approximation equal to 4πAb/λb, where b is the thickness of the bed and A is the amplitude of the reflection if the bed were to be very thick. The equation shows that a bed as thin as 10 ft has, for typical frequency and velocity, considerably more reflective power than is usually attributed to it.
969 citations
TL;DR: An alternative to least-squares data modeling techniques is the use of absolute value error criteria as discussed by the authors, where the inclusion of some infinite blunders along with the data will hardly affect the solution to an otherwise well-posed problem.
Abstract: An attractive alternative to least‐squares data modeling techniques is the use of absolute value error criteria. Unlike the least‐squares techniques the inclusion of some infinite blunders along with the data will hardly affect the solution to an otherwise well‐posed problem. An example of this great stability is seen when an average is, determined by using the median rather than the arithmetic mean. Algorithms for absolute error minimization are often approximately as costly as least‐squares algorithms; however, unlike least‐squares, they naturally lend themselves to inequality or bounding constraints on models.
807 citations
TL;DR: In this article, the authors apply ray theory to determine an expression for amplitude correction factors in terms of initial incidence, source-receiver offset, and reflector depth in an earth model consisting of horizontal, isotropic layers and assuming a point source.
Abstract: Of the various factors which influence reflection amplitudes in a seismic recording, divergence effects are possibly of least direct interest to the interpreter. Nevertheless, proper compensation for these effects is mandatory if reflection amplitudes are to be of diagnostic value. For an earth model consisting of horizontal, isotropic layers, and assuming a point source, we apply ray theory to determine an expression for amplitude correction factors in terms of initial incidence, source‐receiver offset, and reflector depth. The special case of zero offset yields an expression in terms of two‐way traveltime, velocity in the initial layer, and the time‐weighted rms velocity which characterizes reflections. For this model it follows that information which is needed for divergence compensation in the region of normal incidence is available from the customary analysis of normal moveout (NMO). It is hardly surprising that NMO and divergence effects are intimately related when one considers the expanding wavefr...
168 citations
TL;DR: In this article, a recursive computer algorithm giving the inverse solution automatically in terms of assigned density parameters is applied to a gravity profile over a graben structure at San Jacinto, California.
Abstract: In the analysis of gravity data over thick sedimentary basins, density contrast can sometimes be approximated by a continuous function decreasing exponentially with depth. Typical values of the exponential decrement are in the order of 0.3 to 1.5 km (super -1) . The gravity effect of an infinite (Bouguer) slab in this system tends to a finite limit as the slab becomes infinitely thick, leading to quantitative, and sometimes stringent, limitations on the resolving power of gravity analysis over deep structures. The exponential density term acts as an integrating factor in deriving the gravity effect of a prismatic building element, leading to a simple expression involving both the gravity field and its vertical derivative. A recursive computer algorithm giving the inverse solution automatically in terms of assigned density parameters is applied to a gravity profile over a graben structure at San Jacinto, California. The computed basin configuration is consistent with the seismically determined basement depth of 2.4 km and the estimated density layering of the graben fill.
165 citations
Esso1
TL;DR: In this article, normal moveout correction stretched a reflection pulse in such a way that the spectrum of the pulse is a linearly compressed version of the uncorrected pulse spectrum, and the amplitude of the spectrum is increased by the same factor that expresses the spectral compression.
Abstract: Using a synthetic seismogram as input, normal moveout correction stretches a reflection pulse in such a way that the spectrum of the pulse is a linearly compressed version of the uncorrected pulse spectrum. The amount of compression depends on t0, the source‐detector separation, velocity, and the rate at which velocity varies with t0. The amplitude of the spectrum is increased by the same factor that expresses the spectral compression. As a result, the summed pulse from a CDP stack is richer in low frequencies than one might anticipate and has a smaller signal‐to‐noise ratio than the square root of the number of traces in the stack.
133 citations
TL;DR: In this paper, 16 vertical electrical soundings of the Schlumberger type were made along a 3.7 mile traverse to evaluate the electrical resistivity distribution within this geothermal field.
Abstract: The Mud Volcano area in Yellowstone National Park provides an example of a vapor‐dominated geothermal system. A test well drilled to a depth of about 347 ft penetrated the vapor‐dominated reservoir at a depth of less than 300 ft. Subsequently, 16 vertical electrical soundings (VES) of the Schlumberger type were made along a 3.7‐mile traverse to evaluate the electrical resistivity distribution within this geothermal field. Interpretation of the VES curves by computer modeling indicates that the vapor‐dominated layer has a resistivity of about 75–130 ohm‐m and that its lateral extent is about 1 mile. It is characteristically overlain by a low‐resistivity layer of about 2–6.5 ohm‐m, and it is laterally confined by a layer of about 30 ohm‐m. This 30‐ohm‐m layer, which probably represents hot water circulating in low‐porosity rocks, also underlies most of the survey at an average depth of about 1000 ft. Horizontal resistivity profiles, measured with two electrode spacings of an AMN array, qualitatively corrobo...
130 citations
TL;DR: In this article, the theoretical wave nature of the electromagnetic fields about various types of dipole sources placed on the surface of a low-loss dielectric half-space and two-layer earth was examined.
Abstract: Radio interferometry is a technique for measuring in‐situ electrical properties and for detecting subsurface changes in electrical properties of geologic regions with very low electrical conductivity. Ice‐covered terrestrial regions and the lunar surface are typical environments where this method can be applied. The field strengths about a transmitting antenna placed on the surface of such an environment exhibit interference maxima and minima which are characteristic of the subsurface electrical properties. This paper (Part I) examines the theoretical wave nature of the electromagnetic fields about various types of dipole sources placed on the surface of a low‐loss dielectric half‐space and two‐layer earth. Approximate expressions for the fields have been found using both normal mode analysis and the saddle‐point method of integration. The solutions yield a number of important results for the radio interferometry depth‐sounding method. The half‐space solutions show that the interface modifies the directio...
127 citations
TL;DR: In this paper, the complex resistivity spectra, in the frequency range 0.001 to 10 hz, have been obtained through computer analysis of waveforms tape recorded at porphyry copper deposits.
Abstract: Complex resistivity spectra, in the frequency range 0.001 to 10 hz, have been obtained through computer analysis of waveforms tape recorded at porphyry copper deposits. When care is taken to avoid distortions caused by the geometric effects of electrical inhomogeneities, the spectra of typical porphyry copper mineralization are remarkably uniform in character. The geometric effects of veins, which generally complicate the response of hand samples, can be reduced through in-situ measurements using electrode separations of a few meters.In the frequency range of interest for induced-polarization (IP) exploration, the observed spectra are accurately described byEquationwhere K is a constant and b is a positive fraction.The fraction b has a value less than 0.1 and is a complete measure of IP. In the frequency range of interest, Laplace transformation gives the step response to be approximated byEquationTypical current waveforms used in IP prospecting can be synthesized by superposition of steps. Using these frequency and time response functions, percent frequency effect, phase, and pulse-transient parameters are compared as measures of IP.By using a volume distribution function, it is shown that a distribution of lossy capacitors will explain the observed response. Physically, this might correspond to the double-layer capacitance of metallic particles in mineralized rock.
122 citations
TL;DR: The magnetotelluric method can determine subsurface electrical conductivity structure at depths appropriate for mineral exploration as discussed by the authors, which can be useful in defining sharp lateral contrasts in resistivity and in'seeing' through high resistivity cover.
Abstract: With the use of frequencies in the audio range, the magnetotelluric method can determine subsurface electrical conductivity structure at depths appropriate for mineral exploration. In 1963, Kennecott initiated a program to determine the feasibility of this technique as a geophysical tool.As opposed to the broad-band recording and subsequent Fourier analysis commonly utilized in low-frequency magnetotelluric studies, Kennecott's AMT instrumentation is a multifrequency, narrow-band, analog system which yields scalar apparent resistivities. Since the natural source fields at frequencies from 10 hz to about 20 khz are due to thunderstorm energy, the AMT technique is most useful in summertime operation, as is Afmag.Considerable experience in the field has led to useful applications in several problems: (a) uniform sedimentary columns, (b) high-resistivity cover, and (c) massive, layered sulfides. Although of predictably little assistance in problems relating to disseminated mineralization exploration, deep targets, or areas with low-resistivity cover, the AMT technique can be useful in defining sharp lateral contrasts in resistivity and in 'seeing' through high-resistivity cover.
119 citations
Esso1
TL;DR: In this paper, a Dix-type formula relating the normal moveout velocity (VNMO) to the parameters of a two-dimensional, dipping bed, multilayered model of the earth is discussed.
Abstract: A Dix-type formula relating the normal moveout velocity (VNMO) to the parameters of a two-dimensional, dipping bed, multilayered model of the earth is discussed. The formula results from (1) showing that the VNMO is related to the radius of curvature of the wavefront emerging at the surface after reflection from some interior horizon, and (2) expressing this radius in terms of the parameters of the earth model. An example with severely dipping layers is used to compute the VNMO from the formula, which is then compared to the value of moveout velocity obtained from a synthetic velocity gather generated at the same point in the model.
88 citations
TL;DR: In this paper, a nonlinear multichannel filter was developed for enhancing seismic refraction and teleseismic array data, where the basic filter involves the extraction of the Nth root of each element in the matrix forming the data set, where N is any positive integer, and Nth power of the summation over the channels.
Abstract: A nonlinear multichannel filter is developed which appears to be particularly useful for enhancement of seismic refraction and teleseismic array data The basic filter involves the extraction of the Nth root of each element in the matrix forming the data set, where N is any positive integer, and the Nth power of the summation over the channels The filter is effective in reducing random noise, whereas identical signals which are in-phase on all channels are retained at the expense of some distortion The output from this nonlinear filter has far greater resolution in specifying phase velocity than any multichannel linear filter we have employed Examples of theoretical and actual field seismograms are presented after various forms of filtering to illustrate their effectiveness
TL;DR: In this paper, a multilayered earth was evaluated for frequency and time-domain induced polarization measurements over a dissipative layered subsurface, and the percent frequency effects of electromagnetic coupling were seen to be as high as 60 percent for large L2(σ1f) values, where L is the length of the receiving dipole, σ 1 is the conductivity of the top layer of the half-space, and f is the higher frequency of excitation used.
Abstract: Electromagnetic coupling responses in frequency and time‐domain induced‐polarization measurements over a multilayered earth are evaluated. For collinear dipole‐dipole and pole‐dipole configurations over a dissipative layered subsurface, the percent frequency effects of electromagnetic coupling are seen to be as high as 60 percent for large L2(σ1f) values, where L is the length of the receiving dipole, σ1 is the conductivity of the top layer of the half‐space, and f is the higher frequency of excitation used. In both frequency and time‐domain analyses, the distinctive effects of layering compared to that of a homogeneous half‐space response are shown for different electrode configurations, layer geometry, and electrical parameters of the subsurface. The pole‐dipole configuration of electrodes, in general, exhibits higher coupling compared to the dipole‐dipole configuration. In time‐domain measurements, the late off‐time transient decays reflect almost entirely the normal polarizability of the layered subsu...
TL;DR: In this article, simple expressions for the vertical and total field magnetic anomalies due to a polygonal body of finite strike length and arbitrary magnetization are presented for the two-dimensional case.
Abstract: Simple expressions are presented for the vertical and total field magnetic anomalies due to a polygonal body of finite strike length and arbitrary magnetization. These formulas incorporate end corrections into the well‐known Talwani‐Heirtzler (1964) formulas for two‐dimensional polygonal bodies and reduce to the latter for large strike length. Because of their simplicity, the formulas with end corrections lend themselves to rapid use in digital interpretation. Analysis of the formulas shows that interpretation with end corrections gives a body which is deeper and has a larger magnetization and different shape than the body inferred without end corrections.
TL;DR: A line-of-dipoles distribution, obtained by solving the linear inverse problem, provides an analytic base for computing the following quantities from an observed field: first and second vertical derivative fields, upward-and downward-continued fields, field reduced to the pole, amplitude spectrum of the field, and band-passed field.
Abstract: Equivalent sources are useful in processing total magnetic field profiles. A lines-of-dipoles distribution, obtained by solving the linear inverse problem, provides an analytic base for computing the following quantities from an observed field: first and second vertical derivative fields, upward- and downward-continued fields, field reduced to the pole, amplitude spectrum of the field, and band-passed field. A theoretical example demonstrates the validity of the approach, and a field example shows that reasonable results are readily obtained.
TL;DR: In this paper, the authors analyzed the interfacial effects arising in electrical measurements of rocks, when a two-electrode system is used, via the Maxwell-Wagner effect, and concluded that it is necessary to have volume charge accumulations at interfaces within the sample and at the electrode-sample interface in order to have electrical steadystate conditions in rock samples.
Abstract: The interfacial effects arising in electrical measurements of rocks, when a two‐electrode system is used, are analyzed via the Maxwell‐Wagner effect. The similarity in electrical behavior between rock samples and heterogeneous dielectrics with nonnegligible ohmic conductivities leads to a general analysis of the Maxwell‐Wagner effect. We conclude that, in general, it is necessary to have volume charge accumulations at interfaces within the sample and at the electrode‐sample interface in order to have electrical steady‐state conditions in rock samples. The expression for the charge accumulation at the interface as a function of frequency is obtained as well as expressions for the effective dielectric permittivity in a composite material; the relaxation time associated with the charge accumulation and that of the effective dielectric permittivity are the same. Given a sample consisting of two different materials, it is shown that whenever both components have nonnegligible ohmic conductivities, the imaginar...
TL;DR: In this paper, the double-Fourier series coefficients of the measured magnetic anomaly are computed by a filter operator, and the magnetic components are evaluated by taking the inverse Fourier transform.
Abstract: Three‐component magnetic data are derivable from measurements of one single component of the magnetic field over a plane. The technique involves computation of the double‐Fourier‐series coefficients of the measured magnetic anomaly, multiplication of the coefficients by a filter operator, and, finally, evaluation of the magnetic components by taking the inverse Fourier transform. The desired filter operator is obtained from a simple relationship between the components of a potential field. The scheme has been tested with excellent results on the fields of a vertical prismatic model.
TL;DR: In this paper, an automatic computer method has been developed at the Geological Survey of Canada for the interpretation of magnetic data using models generated in this manner, such as the finite strike length thick dike, the vertical prism, the loping step, the parallelepiped body, etc.
Abstract: The magnetic anomalies caused by such diverse model shapes as the finite strike length thick dike, the vertical prism, thes loping step, the parallelepiped body, etc., may be obtained through an appropriate numerical integration of the expression for the magnetic effect produced by a finite thin plate. Using models generated in this manner, an automatic computer method has been developed at the Geological Survey of Canada for the interpretation of magnetic data. Because the magnetic anomalies produced by the various model shapes are nonlinear in parameters of shape and position, it is necessary to use an iterative procedure to obtain the values for the various model parameters which yield a least‐squares best‐fit anomaly curve to a set of discrete observed data. The interpretation method described in this paper uses the Powell algorithm for this purpose. The procedure can sometimes be made more efficient using a Marquardt modification to the Powell algorithm. Examples of the use of the method are presente...
TL;DR: An electrical rock measurement program has been initiated at the Geological Survey of Canada as mentioned in this paper, where the theory of the parameters that describe the complex electrical characteristics (conductivity, permittivity, and loss tangent) of rocks are reviewed.
Abstract: An electrical rock measurement program has been initiated at the Geological Survey of Canada. Although there has been a considerable amount of work done in the past on electrical properties of rocks, the basic concept of the electrical theory applied to rocks and the definition of electrical parameters seem to differ between each group of scientists. This paper reviews the theory of the parameters that describe the complex electrical characteristics (conductivity, permittivity, and loss tangent) of rocks. A review is made of the measuring systems and classified by technique over the frequency range from 10-2 to 108 hz. For measurements on dry rocks and massive sulfide minerals, a 2‐electrode configuration is used. Measurement accuracy and reproducibility are within ±2 percent. The accuracy is checked against international standard samples. The research aspect of the laboratory is concerned with problems encountered in field surveys. Besides measurements on dry rocks, investigations on ultramafic rocks tha...
TL;DR: In this paper, the authors introduced the postulate that the response of the complex variable ω is non-resonant, i.e., the frequency response, continued off the real frequency axis, is an analytic function of ω except at purely imaginary frequencies.
Abstract: The work of Keller (1959) with decay‐time distributions is extended by incorporating ideas developed by Gross (1953) for viscoelasticity. In addition to the usual premises of linear systems theory, we introduce the postulate that the response is nonresonant. Mathematically, this postulate is that the frequency response, continued off the real frequency axis, is an analytic function of the complex variable ω, except, possibly, at purely imaginary frequencies. The singularities in the response at purely imaginary frequencies are identified with decay times. There are two decay‐time distribution functions, one associated with resistivity and one associated with conductivity. The former is the inverse Laplace transform of the voltage response to a current step, and the latter is the inverse Laplace transform of the current response to a voltage step. These two decay spectra are not the same; in the normal situation where resistivity decreases and conductivity increases with increasing (real) frequency, the co...
TL;DR: In this article, a computer program was developed by the U.S. Bureau of Mines for designing a two-dimensional, layered earth model as an aid in interpreting seismic refraction field measurements.
Abstract: A computer program has been developed by the U.S. Bureau of Mines for designing a two‐dimensional, layered earth model as an aid in interpreting seismic refraction field measurements. The program requires input data consisting of shot‐point and geophone locations, refraction traveltimes, and identification of the refraction layer associated with each traveltime. In the program, a first approximation model is generated by a computer adaptation of the delay‐time method, followed by a series of improved approximations that are made by use of a ray‐tracing procedure. The final result of the program is a model designed to minimize the discrepancy between field‐measured traveltimes and computed traveltimes of rays traced through the model. Test applications indicate that the accuracy of interpretation is improved, and that time and effort of the professional interpreter are greatly reduced by use of the computer technique.
TL;DR: In this article, the resolving power of a seismic array is defined in terms of the array response function and via the classical uncertainty principle, and it is shown that arbitrarily high resolution is achievable in the limit as the background white noise tends to zero.
Abstract: The resolving power of a seismic array is defined in terms of the array response function and via the classical uncertainty principle. Using the theory of maximum likelihood wavenumber spectra (Capon, 1969), we show for the case of two correlated plane waves that arbitrarily high resolution is achievable in the limit as the background white noise tends to zero. This extends Barnard’s (1969) result to the case of correlated plane waves. The increased resolution arises from the additional assumption that the data are plane waves over all space, and not zero off the array as the classical result assumes. It is found that a sample rate (in time) large compared to the Nyquist rate, is needed in the case of a short time gate at a small array. Cross‐power spectral matrices are estimated at 4 hz from 1 sec of computer generated data consisting of two correlated plane waves in white noise. These spectral matrices are then used to generate maximum likelihood wavenumber spectra. The two plane waves are resolved at v...
TL;DR: In this paper, the responses of dipole-dipole, pole-dipsole, and gradient arrays to a set of ten structures have been computed using the finite element method.
Abstract: The responses of dipole‐dipole, pole‐dipole, and gradient arrays to a set of ten structures have been computed using the finite element method. Comparison of the responses indicates that: (1) The dipole‐dipole array usually gives the largest anomalies, but the anomalies rarely give information on dip of a structure and are strongly influenced by overburden irregularities. This array gives best overall resolution. (2) Pole‐dipole anomalies are almost as large as dipole‐dipole ones and have the same general form but show less resolution. The lower resolution and the asymmetry of this array make results more difficult to interpret. (3) The gradient array provides dip information and good horizontal resolution. Responses to thin vertical structures are weak, anomalies are strongly affected by overburden irregularities, and there is no effective depth discrimination.
TL;DR: In this article, the generalized linear inverse theory was applied to vertical magnetic dipole sounding problems and it was shown that the information density matrix may be used to optimize the data information distribution by choosing only data that contributes information above some minimal level.
Abstract: It is demonstrated that the generalized linear inverse theory may be applied to vertical magnetic dipole sounding problems. An analysis of inversion of theoretical data for a two‐layer model illustrates the method and indicates certain features not inherent in the commonly practiced curve‐matching method of interpretation. In particular, the standard deviations of the layered model parameters may be estimated. Also the data may contain varying degrees of information about individual model parameters. Indeed, the information density matrix may be used to optimize the data information distribution by choosing only data that contributes information above some minimal level. The relative importance of the information distribution to the determination of individual model parameters may be assessed using both the structure of the information density matrix and the size of the estimated parameter standard deviations. Data may be removed until the estimated standard deviations of the parameters exceed some critic...
TL;DR: In this paper, four quantitative models were investigated, vertical halfplane, vertical ribbon, dipping half-plane, and homogeneous half-space, which make it possible to determine conductor depth, and dip for sheet-like conductors.
Abstract: Recent improvements of the INPUT airborne electromagnetic system have made possible a more quantitative approach to interpretation. The necessary interpretational aids can be obtained in two ways: either by correlating the system and ground EM measurements, or by devising computational or analog quantitative models. Both approaches have been explored. In the former, the system decay rate can be correlated with the apparent conductivity‐thickness (σt) estimated by ground surveys. In the latter, four quantitative models were investigated, vertical half‐plane, vertical ribbon, dipping half‐plane, and homogeneous half‐space. Nomograms have been constructed which make it possible to determine σt, conductor depth, and dip for sheet‐like conductors, and conductivity for a homogeneous half‐space. Field examples show that this procedure can be used satisfactorily in the routine interpretation of records obtained by this system.
TL;DR: In this paper, the effects of electromagnetic coupling in induced polarization surveys, when the earth can be approximated by a two-layer model, were investigated using a dipole-dipole array.
Abstract: A technique has been developed for computing the effects of electromagnetic coupling in induced polarization surveys, when the earth can be approximated by a two‐layer model. Results are given for the dipole‐dipole array, but the numerical solution described can be applied to any electrode configuration. No unusual effects were observed for the models computed. As the thickness of the upper layer is increased, there is a smooth transition between the coupling response of a homogeneous earth having the resistivity of the upper layer and that of a homogeneous earth having a resistivity equal to that of the lower layer.
TL;DR: In this paper, active resistivity and magnetotelluric experiments in southwest Iceland suggest typical resistivities for the crust of 10 to 100 ohm−m, compared with calculations on the expected resistivity of fluid-saturated crustal rocks for plausible ranges of temperature, pore pressure, and water chemistry.
Abstract: Active resistivity and magnetotelluric experiments in southwest Iceland suggest typical resistivities for the crust of 10 to 100 ohm‐m. These low crustal resistivities are compared with calculations on the expected resistivity of fluid‐saturated crustal rocks for plausible ranges of temperature, pore‐pressure, and water chemistry. The comparison of the synthetic models with actual field data suggests: (1) The suppression of resistivity at shallow depth is caused by regional hydrothermal activity. (2) Appreciable effects from water are obtained from depths to 8 or 10 km. (3) Below 10 km the effects from conduction along electrolytic paths are probably dominated by conduction in the solid rock itself.
Esso1
TL;DR: In this paper, the authors examined the nature of time-distance curves and the velocities found from them, using as their model of the subsurface beds separated by plane horizontal interfaces or a sub-surface with an interval velocity that is a continuous function of depth.
Abstract: As the spread length increases, rms velocities determined from seismic data differ from those given by the Dix relation. We have examined the nature of time‐distance curves and the velocities found from them, using as our model of the subsurface beds separated by plane horizontal interfaces or a subsurface with an interval velocity that is a continuous function of depth. Our conclusions are as follows: (1) For a given reflection, the slope of the t2-x2 time‐distance curve at any spread distance x is equal to (Vrms)-2 where Vrms(x) is the rms velocity computed along an appropriate raypath. Vrms(0) is the velocity given by the Dix relation. As x increases, Vrms(x) approaches a limiting value equal to the maximum value of the interval velocity in the section of the earth transversed by the raypath. (2) The t2-x2 curve is such that Vrms(x) is a monotonically increasing function of x. Conclusions (1) and (2) permit us to place bounds on the moveout velocity. (3) If the interval velocity is assumed to increase ...
TL;DR: In this article, the IP response and the apparent resistivity resulting from a buried current pole in the presence of a stratigraphic target and a three-dimensional target were studied and the results indicated that a survey around a drill hole containing a current electrode can be used to outline mineralization in the immediate vicinity of the drill hole.
Abstract: The IP response and the apparent resistivity resulting from a buried current pole in the presence of a stratigraphic target and a three‐dimensional target have been studied. The targets were modeled using a layered model to simulate the stratigraphic target and a buried sphere model to simulate the three‐dimensional target. The results show that there is a substantial increase in the response of the target measured at the surface for current electrode depths of greater than half the depth to the top of the target. A larger anomalous response is of particular importance when dealing with deeply buried targets from which little or no response is measured using conventional surface electrode methods. Furthermore, the results indicate that a survey around a drill hole containing a current electrode can be used to outline mineralization in the immediate vicinity of the drill hole. Some empirical observations resulting from our study are presented which relate the lateral offset of the target from the drill hol...
TL;DR: In this paper, a simple stochastic model of primary signal, coherent noise and non-coherent noise can be generated from which auto and mutual correlation functions can be derived, from which the set of linear operators corresponding to the multichannel filter for optimal rejection of coherent and noncoherent noises can be obtained by solving the proper WienerHopf equations.
Abstract: In a CDP family after normal moveout correction, multiple reflections generally have a residual moveout hyperbolically increasing with the distance from the shotpoint. Simple stochastic models of primary signal, coherent noise and noncoherent noise can be generated, from which auto and mutual‐correlation functions can be derived. The set of linear operators corresponding to the multichannel filter for optimal rejection of coherent and noncoherent noise can be obtained by solving the proper Wiener‐Hopf equations. In the solutions already published some difficulties may arise, due either to the precision of the computations necessary to avoid numerical instabilities, or to the restrictive hypotheses on which the model is founded (multiples having precisely known moveout). In this case the difficulties are overcome with a proper model and by solving the Wiener‐Hopf equations in the frequency domain at different frequencies; the time operators are obtained by means of a discrete Fourier transformation. With n...
TL;DR: In this article, an automated technique for computerized interpretation of magnetic anomalies using the three-dimensional vertical prism model was applied to model data and two magnetic anomalies for intrusive bodies, and the results indicate the necessity for a computerized prism model interpretation technique which can adequately consider the range of possibilities for interpretation.
Abstract: The vertical prism model is frequently used for interpretation of magnetic anomalies. Such interpretations have been carried out previously by manual comparison of observed anomalies to atlases of prism model anomalies. An automated technique for computerized interpretation of magnetic anomalies using the three-dimensional vertical prism model was applied to model data and two magnetic anomalies for intrusive bodies. The vertical-sided, three-dimensional prism model characteristically yields a range of models of near equally good fit to an anomaly; these prisms may have considerable variation in physical parameters. The results indicate the necessity for a computerized prism model interpretation technique which can adequately consider the range of possibilities for interpretation.