Showing papers in "Geophysics in 1970"
TL;DR: In this article, a mathematical basis for the application of power spectrum analysis to aeromagnetic map interpretation is developed and an ensemble of blocks of varying depth, width, thickness, and magnetization is considered as a statistical model.
Abstract: A mathematical basis for the application of power spectrum analysis to aeromagnetic map interpretation is developed. An ensemble of blocks of varying depth, width, thickness, and magnetization is considered as a statistical model. With the use of the fundamental postulate of statistical mechanics, a formula which can be used to analyze the power spectrum of an aeromagnetic map is developed. The influences of horizontal size, depth, thickness, and depth extent of the blocks on the shape of the power spectrum are assessed. Examples which include power spectra of maps from Canada and Central America demonstrate the application of the approach. In the cases studied a double ensemble of blocks appears to best explain the observed power spectrum characteristics.
1,440 citations
TL;DR: In this article, the televiewer takes an oriented acoustic picture of the inside of the well bore in the form of a continuous well log, which is then used to locate fractures 1/32 inch wide.
Abstract: The televiewer takes an oriented acoustic picture of the inside of the well bore in the form of a continuous well log. The resulting log is a representation of the well bore wall as if it were split vertically along magnetic north and laid out flat. Both induced and natural fractures are defined in remarkable detail. Well bores can be filled with any homogeneous liquid such as lease crude oil, water, or drilling mud. Resolution of the tool is sufficient to locate fractures 1/32 inch wide. Examples of natural fractures in extremely fractured formations, in shales, and also drilling‐induced fractures are discussed. Fracture orientation information useful for local and regional tectonic studies is presented.
213 citations
TL;DR: In this article, the multidimensional scalar wave equation at a single frequency is split into two equations, one controlling the downgoing transmitted wave; the other controlling the upcoming reflected wave.
Abstract: The multidimensional scalar wave equation at a single frequency is split into two equations. One controls the downgoing transmitted wave; the other controls the upcoming reflected wave. The equations are coupled, but in many reflection seismology situations the transmitted wave may be calculated without consideration of the reflected wave. The reflected wave is then calculated from the transmitted wave and the assumed velocity field. The waves are described by a modulation on up‐ or downgoing plane waves. This modulation function is calculated by difference equations on a grid. Despite complicated velocity models (steep faults, buried focus, etc.), the grid may be quite coarse if waves of interest do not propagate at large angles from the vertical. A one‐dimensional grid may be used for a two‐dimensional velocity model. With approximations, a point source emitting waves spreading in three dimensions may be included on the one‐dimensional grid. Calculation time for representative models is a few seconds. P...
195 citations
TL;DR: In this paper, the authors provide a practical understanding of diffraction behavior by making simplifying but realistic approximations which permit the diffractive response of an arbitrary subsurface with point-source excitation to be obtained in a simple closed form.
Abstract: Existing diffraction theory is often cast in such a way as to preclude a ready qualitative understanding of diffraction phenomena. This difficulty can be overcome by making simplifying but realistic approximations which permit the diffractive response of an arbitrary subsurface with point‐source excitation to be obtained in a simple closed form. The main approximations are that the subsurface behaves as an acoustic medium, that its average velocity is constant, and that its reflectivity is low. An objective of this paper is to provide the field interpreter with a practical understanding of diffraction behavior.
183 citations
TL;DR: In this paper, a far-field approximation of the retarded potential equation is used for direct time domain evaluation and synthesize the calculated sections, which can be used for enhancing field interpretations.
Abstract: Record sections from three‐dimensional acoustic models often contain diffracted events not predictable by classical raypath theory. Several observed and calculated record sections from models of typical geologic structures such as synclines, anticlines, and faults verify this diffraction phenomenon. A careful interpretation of the character and moveout of these diffracted events is required to delineate certain portions of the geologic structures. A far‐field approximation of the retarded potential equation is suitable for direct time‐domain evaluation and is used to synthesize the calculated sections. The excellent comparisons between the calculated and observed record sections suggest that the mathematical modeling technique can be a useful tool for enhancing field interpretations.
119 citations
TL;DR: In this article, the electromagnetic field components produced by a horizontal loop, carrying a current and placed on or above the surface of an n-layered half-space, are deduced in a form such that numerical integration can be performed easily.
Abstract: Integral expressions for the electromagnetic field components produced by a horizontal loop, carrying a current Ieiωt and placed on or above the surface of an n‐layered half‐space, are deduced in a form such that numerical integration can be performed easily. The expressions are free of approximations and completely general for all frequencies. They are constrained only to the uniformity of current around the transmitting loop. The resulting computed electromagnetic fields are valid for arbitrary values of the electrical parameters σ, μ, and e. The quasi‐static approximation for the region above the half‐space, wherein the wave equation is replaced by the Laplace equation, can be avoided. Measurements outside the loop constitute induction depth sounding. Induction depth sounding curves of field components and magnetic polarization parameters show good resolution of subsurface layering. In particular, it is suggested that the measurements of tilt angle and/or ellipticity of the magnetic polarization ellips...
102 citations
TL;DR: In this paper, the forward problem of calculating the waves in a medium of plane, homogeneous, isotropic layers is extended to P and SV body waves at nonnormal incidence, where the horizontal phase velocity of each wave is greater than the shear and compressional velocities of each layer.
Abstract: For plane waves at normal incidence to a layered elastic medium, both the forward and inverse discrete time problems have been previously solved. In this paper the forward problem of calculating the waves in a medium of plane, homogeneous, isotropic layers is extended to P and SV body waves at nonnormal incidence, where the horizontal phase velocity of each wave is greater than the shear and compressional velocities of each layer. Vertical traveltimes for P and SV waves through each layer are rounded off to unequal integer multiples of a small time increment Δτ. This gives a 4×4 layer matrix analogous to the 2×2 layer matrix for normal incidence obtained by previous authors. Reflection and transmission responses recorded at the free surface of a layered half space are derived as matrix series in integer powers of the Fourier transform variable z=e-iωΔτ. These responses are generated recursively by polynomial division and include all multiply reflected P and SV waves with mode conversions. It is shown that...
101 citations
TL;DR: In this article, a self-adjusting curve-matching algorithm for the interpretation of magnetic anomalies produced by dike-like bodies is described. But the method yields an optimum set of dike parameter values by selecting a best fit model anomaly curve for a given set of discrete observed data along a profile across a two-dimensional anomaly.
Abstract: A self‐adjusting curve‐matching computer program for the interpretation of magnetic anomalies produced by dike‐like bodies is described. The method yields an optimum set of dike parameter values by selecting a best‐fit model anomaly curve for a given set of discrete observed data along a profile across a two‐dimensional anomaly. The criterion used by the computer for the selection of a best‐fit curve may be either the linear or least squares condition. Convergence to either of these conditions requires the solution of a nonlinear problem. In the present case, solution is achieved by automatic trial and error modifications of an initial model. Selection of the initial model is based upon the shape of the observed anomaly; the proper choice of an initial model assures convergence of the error of fit to an absolute rather than to a relative minimum. The repeatability of the convergence of the iterative process can be tested by changing the initial model within the allowable ranges of variation of the dike pa...
79 citations
TL;DR: In this paper, the waveform distortion of an impulse-excited plane wave as it decays and spreads on passing through large distances of a solid has been studied and theoretical expressions for waveform distortions have been obtained.
Abstract: Under the assumption of almost constant‐Q behavior of solids over a wide range of frequencies, together with some meaningful assumptions about the linear frequency behavior of the attenuation function, we have been able to obtain theoretical expressions describing the waveform distortion of an impulse‐excited plane wave as it decays and spreads on passing through large distances of the solid. When typical values for the parameters (as obtained from laboratory model or short range field experiments) for a solid having a mechanical Q of about 50 are used, the resulting waveforms at first glance appear to have a simple and not unexpected behavior. The peak amplitude of the waveform in the time domain varies roughly with the inverse of the square of the travel distance (this includes an inverse first power due to geometrical spreading). Also, a spreading of the waveform occurs that varies roughly linearly with the travel distance. This spreading is such that a positive impulse simply broadens as it travels wi...
77 citations
TL;DR: In this article, Rayleigh reflection coefficients and bottom losses of compressional waves at normal incidence on the water-sediment interface are computed with values of density and velocity measured in sea-floor sediment samples; main sediment types in three major environments of the Pacific and adjacent areas are included.
Abstract: Rayleigh reflection coefficients and bottom losses of compressional waves at normal incidence on the water‐sediment interface are computed with values of density and velocity measured in sea‐floor sediment samples; main sediment types in three major environments of the Pacific and adjacent areas are included. Some typical average computed values of acoustic bottom loss at normal incidence in db are (1) continental shelf: sands, 8; silty sand, 10; sandy silt, 14; silty clay, 16; (2) abyssal plain: clayey silt, 17; silty clay and clay, 21; and (3) abyssal hill: silty clay and clay, 17. Comparisons with actual measurements at sea by several investigators demonstrate the validity of the approach.
74 citations
TL;DR: In this article, it was shown analytically that for kth harmonic distortion, the zero-to-peak amplitude of the distortion tail relative to the autocorrelation peak when the seismogram is crosscorrelated with, the undistorted outgoing signal is given approximately by √ 1/(k−1)TW times the amplitude ratio of harmonic to fundamental.
Abstract: Harmonic distortion apparently arising from nonlinear processes, especially coupling of the vibrator to the ground, has often been observed in land seismic operations utilizing a frequency‐modulated mechanical vibrator as a surface source. The effect of harmonic distortion is to add a long oscillatory tail to the correlogram of a single sweep for the case in which the sweep frequency decreases with time. For up‐sweeps the distortion effect appears as a forerunner. It is shown analytically that for kth harmonic distortion, the zero‐to‐peak amplitude of the distortion tail (or forerunner) relative to the autocorrelation peak when the seismogram is crosscorrelated with, the undistorted outgoing signal is given approximately by √1/(k−1)TW times the amplitude ratio of harmonic to fundamental. W is the frequency range of the fundamental sweep and T is its length. Some examples computed by crosscorrelating a fundamental sweep (generated artificially) with its harmonic components are shown to demonstrate the effe...
TL;DR: In this paper, a party from the Scott Polar Research Institute, Cambridge, England, carried out a radio echo sounding survey of the Antarctic ice sheet; their data have been used in the analyses discussed discussed.
Abstract: In 1967 a party from the Scott Polar Research Institute, Cambridge, England, carried out a radio echo sounding survey of the Antarctic ice sheet; their data have been used in the analyses discussed The radio echo sounding apparatus, basically a pulse‐modulated radar operating at 35 Mhz, was installed in a US Navy Constellation aircraft with the aerials attached to the tail, giving a fan‐shaped beam The beam is broadest along the line of flight, causing problems of interpretation analogous with those encountered in marine echo sounding and seismic surveying Also, bottom echoes are affected by refraction at the ice surface Examples of hyperbolic variations of echo delay with horizontal movement may be seen at the edges of ice shelves, crevasses, or cracks at the bottom of the ice Sometimes more complex surfaces can give roughly hyperbolic echo profiles It is possible to distinguish specular from nonspecular reflections; we use a method of computing the position of specular reflecting points from the
TL;DR: In this paper, a theory for weighting seismic records in the stacking process has been developed from a statistical seismic model, which applies to common depth point seismic records which have been statically and dynamically corrected; the same model applies to an ordinary stacking procedure.
Abstract: A theory for weighting seismic records in the stacking process has been developed from a statistical seismic model. The model applies to common‐depth‐point seismic records which have been statically and dynamically corrected; the same model applies to an ordinary stacking procedure. The model stipulates for the signal and noise components, respectively, of a seismic record that (1) the signal is coincident with and similarly shaped to the signal on other records, and (2) the noise is statistically independent of that on any other record and of the signal and has zero mean value. In accord with the model, a seismic record is completely described for the purpose of weighting by its signal scale and its signal‐to‐noise energy ratio. Several statistical procedures for evaluating these parameters for seismic field data are presented. The most favorable procedure is demonstrated with both synthetic and field seismic records.
TL;DR: In this article, the response function of a single-channel digital filter can be specified in terms of scalar-valued weighting coefficients, while the corresponding response function for a multichannel filter is more conveniently described by matrix-valued weighted coefficients.
Abstract: The transition from single‐channel to multichannel data processing systems requires substantial modifications of the simpler single‐channel model. While the response function of a single‐channel digital filter can be specified in terms of scalar‐valued weighting coefficients, the corresponding response function of a multichannel filter is more conveniently described by matrix‐valued weighting coefficients. Correlation coefficients, which are scalars in the single‐channel case, now become matrices. Multichannel sampled data are manipulated with greater ease by recourse to multichannel z‐transform theory. Exact inverse filters are calculable by a matrix inversion technique which is the counterpart to the computation of exact single‐channel inverse operators by polynomial division. The delay properties of the original filter govern the stability of its inverse. This inverse is expressible in the form of a two‐stage cascaded system, whose first stage is a single‐channel recursive filter. Optimum multichannel ...
TL;DR: In this article, the field profile is decomposed into symmetrical and antisymmetrical components by a new method that uses arbitrarily chosen conjugate points, which does not require any previous knowledge or assumption of the zero datum level or of the center of the body.
Abstract: Most anomalies encountered in ground and airborne magnetic surveys are caused by rock and/or ore masses whose shapes are either dike-like and sheet-like structures of infinite length or limited parallelepipedoidal prisms of varying dips, widths, lengths, and depths of burial of the top and bottom faces. The quantitative interpretation problem consists essentially in determining all of the above parameters through a complete analysis of the field profile. Our research group has discovered new methods with which to perform this interpretation. These techniques are both analytically correct and very fast.First, the field profile is decomposed into its symmetrical and antisymmetrical components by a new method that uses arbitrarily chosen conjugate points. Our procedure does not require any previous knowledge or assumption of the zero datum level or of the center of the body.Second, the symmetrical and antisymmetrical components of the field profile are analyzed separately. In the case of dike or bed-like structures, the entire procedure takes but a few minutes and requires a single set of six master curves.The case of the interpretation of parallelepipedoidal prisms is still under study and will be presented shortly in a second paper.
TL;DR: In this paper, a detailed investigation of the dielectric properties of powder and solid dry rocks in the frequency range of 50 hz to 2 mhz has revealed the following general characteristics: 1) All dry rocks, powdered and solid, show an increase in both the Dielectric constant and the loss tangent as frequency decreases and as temperature increases.
Abstract: A detailed investigation of the dielectric properties of powdered and solid dry rocks in the frequency range of 50 hz to 2 mhz has revealed the following general characteristics: 1) All dry rocks, powdered and solid, show an increase in both the dielectric constant and the loss tangent as frequency decreases and as temperature increases. This dispersion is believed to be due to polarization associated with charge buildup at grain boundaries or at grain imperfections. 2) Dry powdered rocks often show a thermally‐activated relaxation peak with a typical Debye‐relaxation character. This is due to the presence of pyroxene and biotite and may be associated with other minerals. The relaxation peak is not seen in solid rocks, where it is hidden by the low‐frequency dispersion. 3) At high frequencies, the loss tangent approaches a constant value which is frequency independent. This behavior is observed in many dielectrics and may be the result of a distribution of relaxation times.
TL;DR: The derivation given of the Kalman filter is based on the representation of systems by state variables and the modeling of random processes as the output of linear systems excited by white noise.
Abstract: The Kalman filter is applied to the inverse filtering or deconvolution problem. The derivation given of the Kalman filter emphasizes the relationship between the Kalman and Wiener filter. This derivation is based on the representation of systems by state variables and the modeling of random processes as the output of linear systems excited by white noise. Illustrative results indicate the applicability of these techniques to a variety of geophysical data processing problems. The Kalman filter offers exploration geophysicists additional insight into processing-problem modeling and solution.
TL;DR: In this paper, the authors apply analytic techniques and computer simulation to linear and nonlinear subsurface models in order to obtain better definitions of the limitations of the reflection seismic method and clarify a variety of commonly occurring but puzzling situations such as phantom faults, incomplete contacts and segmented reflectors at unconformities.
Abstract: Piece-wise local linearity of the subsurface reflectors and uniqueness of primary reflection travel path between particular source-receiver pairs are two of the fundamental assumptions of multiple ground coverage reflection seismic techniques as currently employed. Some recent developments in seismic exploration which spring from the progress made in velocity determination techniques violate these fundamental assumptions with potentially serious consequences such as spurious geometries after migration and unrealistic interval velocities.In this paper we apply analytic techniques and computer simulation to linear and nonlinear subsurface models in order to obtain better definitions of the limitations of the reflection seismic method. The resulting lessons are of immediate practical value in seismic interpretation and clarify a variety of commonly occurring but puzzling situations such as phantom faults, incomplete contacts and segmented reflectors at unconformities, and apparent reversal of dip.In its basic approach this work has much in common with the generation and study of synthetic seismograms. Examination of the groundwork of the seismic method provides a firm base for extending its use.
TL;DR: In this paper, a complete solution of the boundary value problem of a horizontal magnetic dipole over homogeneous and n-layered half-spaces is outlined and a comparison of the complete solution with the quasi-static approximation in practical frequency ranges is made.
Abstract: A complete solution of the boundary value problem of a horizontal magnetic dipole over homogeneous and n‐layered half‐spaces is outlined. Quasi‐static expressions for the electric and magnetic fields have been obtained and a comparison of the complete solution with the quasi‐static approximation in practical frequency ranges is made. An analysis of the phases and amplitudes of the magnetic field components and of the polarization parameters of the magnetic field reveals that the phase of the vertical component of the magnetic field and the ellipticity of the magnetic field polarization ellipse are the most sensitive indicators of layering. Amplitude measurements are, in general, less effective than phase measurements for resolution of layered earth structures. Results from both parametric and geometric modes of sounding have been studied in detail for a number of two‐ and three‐layered models of varying thicknesses and conductivity contrasts. Deduction of layering for different thicknesses of the top laye...
TL;DR: In this article, the amplitude measurements of the electric and magnetic fields about a horizontal loop of wire carrying current at frequencies ranging from 15 hz to 1500 hz are used to detect IP anomalies.
Abstract: The induced polarization (IP) technique is based on the observation that variations of earth conductivity with frequency may be indicative of buried metallic mineralization. Conventional IP involves the use of grounded electrodes, the installation of which can be quite time consuming. However, the electromagnetic fields about an oscillating magnetic dipole also depend on ground conductivity. Thus, it is conceivable that we might be able to detect IP anomalies with an inductive system, thereby eliminating the need for grounded electrodes. An airborne induced polarization method is a theoretical possibility.Theoretical calculations based on a conductivity model determined experimentally at frequencies less than 30 hz suggest that the effect of polarizable material on electromagnetic response is quite small.In order to check the theory and to determine experimentally whether inductive IP is feasible, field tests were conducted in two areas in Nevada which exhibit strong conventional IP anomalies. The field tests consisted of measurements of the amplitudes of the electric and magnetic fields about a horizontal loop of wire carrying current at frequencies ranging from 15 hz to 1500 hz. The presence of the polarizable material is not evident in the inductive data; in fact, the observations can be fitted to theoretical curves for nonpolarizable models.Hence, on the basis of both theory and field tests, it is concluded that inductive IP based on amplitude measurements is not a practical exploration tool for environments such as that of the southwestern United States.
TL;DR: A study of the statistical structure of an aeromagnetic field over approximately four and a half thousand square miles of the Parry Sound and Huntsville area of southern Ontario has revealed the following: From the point of first-order statistics, that is, mean, variance, skewness, and probability distribution function, the right half of the area which is occupied by hornblende or biotite migmatite is homogeneous and Gaussian, whereas the left half, which is mainly occupied by amphibolite gneiss, is generally inhomogeneous and non-
Abstract: A study of the statistical structure of an aeromagnetic field over approximately four and a half thousand square miles of the Parry Sound and Huntsville area of southern Ontario has revealed the following: From the point of first‐order statistics, that is, mean, variance, skewness, and probability distribution function, the right half of the area which is occupied by hornblende or biotite migmatite is homogeneous and Gaussian, whereas the left half, which is mainly occupied by amphibolite gneiss, is generally inhomogeneous and non‐Gaussian. From the point of view of second‐order statistics, that is, the spectrum, the entire field appears to be inhomogeneous. This difference between statistical pictures is attributed to the fact that there are at least two preferred directions of strike, which influence the shape of the spectrum significantly.
TL;DR: In this article, the upward continuation integral is used to solve the Dirichlet problem for a plane, known as the upward continuation integral to exploration geophysicists working with magnetic and gravity fields.
Abstract: The integral solving the Dirichlet problem for a plane, known as the “upward continuation integral” to exploration geophysicists working with magnetic and gravity fields, is sometimes misunderstood. Recently, some geophysicists have commented that its use in continuing ΔT, the component of the total intensity anomaly in the direction of the earth’s normal field, is suspect, the thought being that only components normal to the surface can be so continued. The integral in question is ΔT(x,y,z)=-z2π⋅∫-∞∞∫-∞∞ΔT(α,β)dαdβ[(x-α)2+(y-β)2+z2]32,where ΔT(α, β) represents measured total intensity values on the plane of observations z=0, in a right handed rectangular coordinate system in which the z axis is positive vertically downward.
TL;DR: In this paper, it was shown that when the ratio k = (sea-bed conductivity)/(seawater conductivity) is greater than about 0.03, both horizontal and vertical magnetic fields are useful for measurement of bottom conductivity at sea induction numbers less than 30.
Abstract: Numerical evaluation of integral expressions for the fields about a vertical magnetic dipole in the sea allows analysis of the electromagnetic response over wide ranges of sea induction number and sea floor conductivity.Our analysis indicates that a marine electromagnetic system for measurement of bottom conductivity variations could readily be designed, with such applications as oceanographic and geologic studies, and mineral exploration.For a source-receiver system on a homogeneous sea bottom, it is found that: (i) when the ratio k = (sea-bed conductivity)/(seawater conductivity) is greater than about 0.03, both horizontal and vertical magnetic fields are useful for measurement of bottom conductivity at sea induction numbers less than 30 [induction number = 2 (horizontal transmitter-receiver separation/skin depth)]. A separation of 30 m and frequencies in the range 300-3500 hz appear suitable for investigation of the upper few meters of unconsolidated bottom sediments.(ii) When the ratio k is less than 0.03, sea induction numbers from 10 to a few hundred are required for detection of seabed conductivity variations. In this case, the horizontal magnetic field, resulting from energy transmission mainly through the seafloor, is the suitable field to use. Electromagnetic sounding of indurated rocks may thus call for frequencies of 100 to 20,000 hz at a separation of 200 m.Field strengths vary strongly with relative sea depth D/R (D = sea depth, R = horizontal source-receiver separation) when D/R is small; but sensitivity to bottom conductivity is little affected by D/R. Elevation of source and receiver above a seafloor less conductive than seawater reduces field strengths and sensitivity to seabed properties.
TL;DR: In this paper, the authors presented a theoretical and numerical computation of the quasi-static transient response of the multipoles induced in a conducting permeable sphere by a dipolar field.
Abstract: The paper presents a theoretical and numerical computation of the quasi‐static transient response of the multipoles induced in a conducting permeable sphere by a dipolar field.
TL;DR: In this article, the authors used well logs to locate and evaluate deposits of various commercially important minerals, such as potash, sulfur, gold, and coal, using a combination of Neutron, sonic, and density logs.
Abstract: Well logs can be used to locate and evaluate deposits of various commercially important minerals. It is only necessary that the mineral of interest represent a significant fraction of the formation bulk volume, and that it exhibit characterizing properties measurable by logs. Because modern logging methods measure electrical, density, acoustic, radioactivity, and certain nuclear characteristics of formations, they may be used to identify many minerals. For evaluation of sulfur deposits, either density or sonic logs provide good resolution when compared with porosity computed from neutron or resistivity logs. Trona beds are identified by a sonic reading of approximately 65 μsec/ft, neutron porosity index of about 40 percent, low natural radioactivity, and pronounced hole enlargement. Gamma‐ray logs provide important information in the location, identification, and evaluation of potash mineral deposits. Neutron, sonic, and density logs, in various combinations, augment the gamma‐ray data in such studies. Co...
TL;DR: In this article, the electroacoustic characteristics of the streamer were examined as a basis for an industry-wide standard for specifying streamer tow noise level, and the theoretical bases for various means of reducing the components of tow noise were analyzed and compared with experimental results.
Abstract: To specify intelligently an unambiguous tow‐noise level in marine seismic exploration, the electroacoustic characteristics of the streamer must be understood. In this paper, these characteristics are examined as a basis for an industry‐wide standard for specifying streamer tow‐noise level. Sources of tow noise—including electrical, ambient, flow, radiated, and mechanically induced—are examined and the important parameters that control their amplitude spectrum are presented. The theoretical bases for various means of reducing the components of tow noise are analyzed and compared with experimental results. With this background of tow‐noise sources and noise‐reduction schemes, the noise signal is traced from the hydrophone/seawater interface to the recording system. The hydrophone array’s amplitude response and phase response are determined from the transfer characteristics of the coupling circuit and the recording system. Finally, a method for specifying tow‐noise level is proposed. The specified tow‐noise ...
TL;DR: In this article, it is shown that within the time duration of the seismic wavelet, the estimates of signal and noise statistics are reasonable for low signal-to-noise ratio.
Abstract: A seismic trace may be represented as the sum of a signal and noise series. Each of the series may further be represented by convolution of a finite wavelet and a random series. With this representation, and provided that the signal and noise are uncorrelated, it is possible, in theory, to extract signal and noise statistics from two adjacent traces of a reflection seismogram. Some experiments are shown on model seismic traces, and it is shown that within the time‐duration of the seismic wavelet, the estimates of signal and noise statistics are reasonable for low signal‐to‐noise ratio. There remains, however, the problem of determining the optimum time lengths of the estimates.
TL;DR: In this paper, the basic equations for the gravity effect of a horizontal slab with density varying linearly between limits are presented and a series of type curves and interpretation nomograms are presented showing the characteristics of the gradational model.
Abstract: The basic equations are presented for the gravity effect of a horizontal slab with density varying linearly between limits. A series of type curves and interpretation nomograms are presented showing the characteristics of the gradational model. A notable characteristic is the horizontal gravity gradient which is nearly constant over most of the width of the variable density zone. Maximum slope and curvature of the gravity effect vary directly with the density change, depth, and thickness of the slab and inversely with the width of the variable density zone. Practically identical gravity effects are sometimes produced by step models with different dimensions and density contrasts. However, if the variable density zone is sufficiently wide, the gravity effect cannot be approximated by a simple step model.
TL;DR: In this article, a new statistical method is described for the interpretation of seismic refraction data, which is then applied to interpret the 15,220 m long shot by the Oil and Natural Gas Commission of India along the Hoshiarpur-Tanda road in Punjab State.
Abstract: A new statistical method is described for the interpretation of seismic refraction data. This method is then applied to the interpretation of a seismic refraction profile 15,220 m long shot by the Oil and Natural Gas Commission of India along the Hoshiarpur‐Tanda road in Punjab State. The 14th iteration least squares straight line fit made to the traveltimes of first refracted arrivals gives for the Hoshiarpur area five layers 144, 322, 726, 769, and 1711 m thick with velocities of 1667, 1906, 2209, 2778, and 3505 m/sec respectively above the basement at a depth of 3672±11 m. The basement velocity is found to be 6514 m/sec. Analysis of later refracted arrivals indicates the existence of a hidden layer with a velocity 4280 m/sec in the Hoshiarpur area. Due to the presence of the hidden layer, the fifth layer with a thickness of 1711 m computed from first arrival analysis is split into two layers with thicknesses of 1160±10 and 752±18 m; the 752‐m‐thick layer is the hidden layer. As a result of the hidden l...
TL;DR: An airlifted gravity survey was conducted in 1968 in the Beaufort Sea between Barter Island and Banks Island, south into the Mackenzie River Delta area and northward to about 74° latitude.
Abstract: An airlifted gravity survey was conducted in 1968 in the Beaufort Sea between Barter Island and Banks Island, south into the Mackenzie River Delta area and northward to about 74° latitude. The 1968 gravity data were combined with data from previous airlifted surveys and ice island T‐3. The major feature of the free‐air anomaly gravity map of this area is a more or less continuous 100 mgal high paralleling the coast from Barrow, Alaska, to the edge of the survey area north of Banks Island. The gravity high is explained by a thinning of the crust and a ridge in the basement rocks at about the 200 m isobath. This linear anomaly is broken by saddles off the Colville, Mackenzie, and Bernard Rivers, which are interpreted to reflect sedimentary fans built by the discharge of these rivers. Two‐dimensional crustal models constructed from gravity profiles indicate a narrow transition zone from ocean to continental crustal thickness, 55 km to 100 km shoreward of the 2000 m isobath. In a review of continental margin ...