scispace - formally typeset
Search or ask a question

Showing papers in "Geophysics in 1968"


Journal ArticleDOI
TL;DR: In this paper, a direct method is presented to determine an acoustic layered medium from the seismogram due to a time-limited plane wave incident from the lower halfspace, which transforms the problem to the acoustic reflections problem as solved by Kunetz.
Abstract: A direct (noniterative) method is presented to determine an acoustic layered medium from the seismogram due to a time-limited plane wave incident from the lower halfspace. It is shown that one side of the autocorrelation of the seismogram due to an impulsive source at depth is the seismogram due to an impulsive source on the surface. This transforms the problem to the acoustic reflections problem as solved by Kunetz. Both the deep source time function and the layering can be determined from a surface seismogram.

782 citations


Journal ArticleDOI
TL;DR: In this paper, a three-dimensional structural model can be computed automatically from gravity anomaly data by successive approximations, where the causative body is assumed to be either flat-topped, flat-bottomed, or symmetrical about a horizontal plane.
Abstract: Given gridded gravity anomaly values and certain limiting restrictions on the mass distribution, a three‐dimensional structural model can be calculated automatically from gravity anomaly data by successive approximations. The causative body is optionally assumed to be either flat‐topped, flat‐bottomed, or symmetrical about a horizontal plane. Density contrast and the position of a horizontal plane delimiting either the top, base, or midsection of the causative body must be specified. A first approximation of structure is obtained by means of the Bouguer slab relationship. The gravity field of this first model is calculated and at each grid point the ratio of observed to calculated gravity is used to modify the first structural model, thus leading to a second approximation of structure. The process is iterated until a satisfactory agreement between observed and calculated gravity is obtained.

295 citations


Journal ArticleDOI
TL;DR: In this article, the authors compared basalt and granite over a range of stress rates from 2×10 to 3×1010 psi per second and showed that although their static strengths were close, their dynamic strengths were different.
Abstract: The ultimate strengths of basalt and granite were measured over a range of stress rates from 2×10 to 3×1010 psi per second. A comparison of basalt and granite showed that, although their static strengths were close, their dynamic strengths were different. The static strengths of basalt and granite were 27.5 and 29 kpsi, respectively, at the stress rate of 2×10 psi per second while their strengths at the stress rate of 3×1010 psi per second were 59 and 70 kpsi, respectively. In order to obtain an insight into the basic mechanisms of rock fracturing, the combined effects of stress rate and temperature were studied. The strength of basalt was increased from 27.5 kpsi at room temperature to 45 kpsi at liquid nitrogen temperature at the stress rate of 2×10 psi per second. The mechanisms of fracturing were thermally activated. The activation energy for basalt at 50 kpsi equalled 450 calories per mole. The dynamic modulus of basalt measured by the pulse technique was 1.7×107 psi. The value of the dynamic modulus...

178 citations


Journal ArticleDOI
TL;DR: In this article, the velocity of compressional waves in consolidated porous media, conducted within a temperature range of 26 °C to −36 C, indicate that compressional wave velocity in water-saturated rocks increases with decreasing temperature whereas it is nearly independent of temperature in dry rocks.
Abstract: Measurements of velocity of compressional waves in consolidated porous media, conducted within a temperature range of 26 °C to −36 °C, indicate that: (1) compressional wave velocity in water‐saturated rocks increases with decreasing temperature whereas it is nearly independent of temperature in dry rocks; (2) the shapes of the velocity versus temperature curves are functions of lithology, pore structure, and the nature of the interstitial fluids. As a saturated rock sample is cooled below 0 °C, the liquid in pore spaces with smaller surface‐to‐volume ratios (larger pores) begins to freeze and the liquid salinity controls the freezing process. As the temperature is decreased further, a point is reached where the surface‐to‐volume ratio in the remaining pore spaces is large enough to affect the freezing process, which is completed at the cryohydric temperature of the salts‐water system. In the ice‐liquid‐rock matrix system, present during freezing, a three‐phase, time‐average equation may be used to estimat...

149 citations


Journal ArticleDOI
TL;DR: An aeromagnetic survey extending from the Gulf of Maine to the tip of Florida was conducted by the U. S. Naval Oceanographic Office between 1964 and 1966 as mentioned in this paper, where flight lines were approximately 800 km long and had an 8 km separation.
Abstract: An aeromagnetic survey extending from the Gulf of Maine to the tip of Florida was conducted by the U. S. Naval Oceanographic Office between 1964 and 1966. Flight traverses were flown in a northwesterly direction at right angles to the geologic grain. The flight lines were approximately 800 km long and had an 8‐km separation. The survey traversed part of the New England, Piedmont, and Coastal Plain provinces and extended some 320 km beyond the continental shelf into the Atlantic Ocean. Despite the wide flight‐line spacing, numerous geological and structural features became apparent from this survey. Interpretation of these features was aided by using the available gravity and seismic data in addition to the State and Provincial geologic maps. The residual aeromagnetic map shows a continuous magnetic high on or near the continental slope as far south as the 31st parallel. At about the 36th parallel, this east‐coast magnetic anomaly splits into two branches, and both of them parallel the 850‐fathom contour. ...

128 citations


Journal ArticleDOI
TL;DR: In this article, a relation connecting the spectrum of the random density or the intensity of magnetization and spectrum of a random field has been established, where the random sources are assumed to be confined to a thin sheet or a thick, infinite sheet, or to a semi-infinite medium.
Abstract: Covariance and spectral density functions of all three components of random potential fields are mutually related. The spectra of any one component on two different levels are related through an exponential function of the separation of the levels. If, starting from the earth’s surface, we continue the observed random field downward, we find that at a certain level the field becomes unstable. Such a limiting depth can be estimated from the spectrum of the observed field. The spectrum of the random field is related to the spectrum of random density or intensity of magnetization. The random sources are assumed to be confined to a thin sheet or a thick, infinite sheet, or to a semi‐infinite medium. In all these cases, a relation connecting the spectrum of the random density or the intensity of magnetization and the spectrum of the random field has been established.

74 citations


Journal ArticleDOI
TL;DR: In this article, it was shown that the spurious precursor introduced by maximum likelihood processing, and shown to be caused primarily by the signal amplitude scatter within a subarray, can be effectively reduced by using amplitude equalization.
Abstract: Processing of seismic data from LASA can be accomplished by simple delay and sum or beam forming, weighted delay and sum or by maximum‐likelihood filtering. It is found that band‐pass prefiltering the data has a considerable effect on the results. The extra gain produced by maximum likelihood filtering does not justify use of this process on‐line, but could justify off‐line processing in special cases. The spurious precursor introduced by maximum‐likehood processing, and shown to be caused primarily by the signal amplitude scatter within a subarray, can be effectively reduced by using amplitude equalization. The signal amplitude scatter within a subarray, and between subarrays, does not degrade seriously the performance of the signal processing. The effectiveness of the processing depends not only on the array spatial filtering ability, but on differences in absolute level of the noises in the various sensors. Our experience in processing a weak event whose apparent magnitude, as averaged over the LASA wa...

73 citations


Journal ArticleDOI
TL;DR: In this article, a series of field and laboratory tests in wells with diameters of 4.8 cm and 10.2 cm showed that water columns are unstable at temperature gradients as low as 0.003°C/m.
Abstract: Critical thermal gradients for the onset of convection in small‐diameter wells are computed by means of an expression given by Krige (1939) and are shown graphically for water of several salinities and for glycerol. A series of field and laboratory tests in wells with diameters of 4.8 cm and 10.2 cm showed that water columns are unstable at temperature gradients as low as 0.003 °C/m. Magnitudes of thermal oscillations at higher gradients were as large as 0.49 °C, and apparent sizes of convection cells ranged up to 210 cm in height in a 10.2 cm cylinder and to 48 cm in wells 4.8 cm in diameter. Comparison of temperatures in adjacent wells showed temperature differences as great as 3 °C when thermal gradients were greater than the theoretical critical gradients. It is concluded that temperatures in the upper zone of deep wells may depart widely from ambient temperatures in the formations penetrated.

73 citations


Journal ArticleDOI
TL;DR: In this paper, a regional polynomial surface is fitted by least squares to the original and interpolated points, and the surface of step (2) is deformed smoothly to pass through the original observations.
Abstract: Machine contouring of irregularly spaced observations can be performed in three basic steps: (1) In large areas with no data points, control values are interpolated by a specified mathematical rule. These values keep the next step “well behaved.” (2) A regional polynomial surface is fitted by least squares to the original and interpolated points. (3) The surface of step (2) is deformed smoothly to pass through the original observations. The final product is similar in appearance to hand‐drawn maps. The complete mathematical theory is developed in an appendix.

67 citations


Journal ArticleDOI
TL;DR: In this paper, a spiking filter to deconvolve a seismogram must be time-varying and a zero-delay filter must at all times be minimum phase for the signal waveform and the law governing the change of waveshape with time are both minimum phase.
Abstract: Deconvolution or spiking filters are frequently employed to sharpen the character of seismograms and hence improve resolution of the earth’s layering. In media having significant attenuation or scattering, the shape of an input swaveform will change as it propagates. On a reflection seismogram this results in a change with time in the character and frequency content of the seismic signal. Since a statistical description of this situation would be time‐dependent, the seismogram should be treated as a nonstationary random process. A spiking filter to deconvolve such a seismogram must be time‐variable. Optimum filters for nonstationary inputs can be designed using the least‐mean‐square error criterion. For it zero‐delay filter to give satisfactory results, the signal waveform must at all times be minimum phase. This will be true if the input waveform and the law governing the change of waveshape with time are both minimum phase. If these conditions are not met, a filter which has a time delay should be used....

62 citations


Journal ArticleDOI
TL;DR: In this paper, a system of geophone and hole patterns was designed to form an effective filter against long period surface waves, which were recorded on FM analog magnetic tape and the results were digitized in order to apply digital processing techniques.
Abstract: Seismic reflections from discontinuities deep within the crust (reflection times of 8 to 16 sec) have been recorded along four different lines over a widespread area in southern Alberta, resulting in a total of 90 km of near‐vertical‐incidence profiling. Systems of geophone and hole patterns were designed to form an effective filter against long period surface waves. The data were recorded on FM analog magnetic tape and the results were digitized in order to apply digital processing techniques. Power‐spectra calculations indicate that the energy of the reflected wavelets is concentrated in the range 5 to 15 Hz. Synthetic seismograms were made for comparison with field recordings and they suggest that velocity transition zones within the deep crust are less than one kilometer in vertical extent. Along one profile an expanding spread was utilized and a strong reflection at 11.6 sec was continuously correlated over nearly 25 km. A least‐squares analysis of the X2, T2 plot gives an average vertical velocity o...

Journal ArticleDOI
TL;DR: In this article, a family of source and receiver types and for geometry corresponding to plausible acoustic logging tools in oil wells are computed for the purely elastic case, which is necessary to cope with the singularities of the integrands in performing numerical integrations.
Abstract: Transient waveforms have been computed for a family of source and receiver types and for geometry corresponding to plausible acoustic logging tools in oil wells. The acoustic tool now in commercial use is one member of the family. The computations also treat flexural waves and torsional waves. All quantities are expressed in terms of appropriate vector and scalar potentials, and output waveforms are obtained by numerical evaluation of triple Fourier transforms. When loss parameters are included in the stress‐strain relations, the Fourier inversion formulas are free of singularities and numerical integration is straightforward. For the purely elastic case, it is necessary to cope with the singularities of the integrands in performing the numerical integrations. Dispersion curves and other features agree with earlier publications on wave propagation along concentric cylinders.

Journal ArticleDOI
TL;DR: In this paper, a computer analysis technique is described for use in conjunction with reflection seismic data shot for conventional common depth point (CDP) stacking, with the objective of providing a space-time variant measure of the energy relationships among CDP traces.
Abstract: A computer analysis technique is described for use in conjunction with reflection seismic data shot for conventional common‐depth‐point (CDP) stacking, with the objective of providing a space‐time variant measure of the energy relationships among CDP traces This energy analysis may be interpreted to yield an estimate of the relative contribution of primary and multiple reflections versus record time, their velocity functions, and the fraction of uncorrelated energy which is present These data, in addition to their interpretive value, provide a space‐time variant model of the signal and noise fields for use in the design of optimum CDP stacking filters The heart of the method is based upon crosscorrelations computed between CDP traces from numerous sequential short data gates along the trace These are averaged over an ensemble of like members obtained along the line of profiles and manipulated to yield a reliable estimate of the cross correlation peak amplitude and delay upon which the primary and mult

Journal ArticleDOI
TL;DR: In this article, a nonlinear, least square method is used to invert magnetotelluric sounding curves to obtain the layer resistivities and thicknesses, and the convergence of the method to the correct values seems to be ensured.
Abstract: Based on the model of a flat layered earth, a nonlinear, least‐squares method is used to invert magnetotelluric sounding curves to obtain the layer resistivities and thicknesses. Partial derivatives of the apparent resistivity with respect to layer parameters show the manner in which the layer parameters are contrbuting to the apparent resistivities. Uniqueness of the inversion is not guaranteed, but when the partial derivatives are linearly independent and the relative magnitudes of the layer resistivities of the initial guess are not too far from the correct ones, the convergence of the method to the correct values seems to be ensured.

Journal ArticleDOI
TL;DR: In this article, the signal estimate S is produced by filtering each trace with its corresponding filter Gl and summing the outputs S⁁(t)=ΣlGl(t)*Il(t).
Abstract: Optimum systems have been developed to correspond to the sub‐optimum moveout discrimination systems presented previously by several authors. The seismic data on the lth trace is assumed to be additive signal S with moveout τl, coherent noise N with moveout τ˜l, and incoherent noise Ul, expressed Il(t)=S(t-τl)+N(t-τ˜l)+Ul(t) where S, N, and Ul are independent, second order stationary random processes and τl and τ˜l are random variables with prescribed probability density functions. The signal estimate S⁁ is produced by filtering each trace with its corresponding filter Gl and summing the outputs S⁁(t)=ΣlGl(t)*Il(t). We choose the system of filters {Gl} to make the signal estimate optimum in the Wiener sense (minimum mean‐square error of the signal ensemble). For the special cases discussed, the moveouts are linear functions of the trace number l determined by the moveout/trace τ for signal and τ˜ for noise. Thus, the optimum system is determined by the probability densities of τ and τ˜ together with the no...

Journal ArticleDOI
TL;DR: In this paper, a multichannel linear filter is designed with only a knowledge of the relative times of the arrivals for the various signal and noise events but with no knowledge of actual event shapes.
Abstract: A large class of seismic processing operations involve the linear combination of several traces in such a way that certain “signal” events are emphasized over other “noise” events. If the noise events are randomly distributed between traces, then simple addition of the traces represents the best possible technique for suppressing the noise. However, in many cases a noise event on one trace will also occur at relatively well‐determined points on other nearby traces. For these cases, multichannel linear filters offer better discrimination against organized noise than the straight addition of traces does. Such multichannel filters can be designed with only a knowledge of the relative times of the arrivals for the various signal and noise events but with no knowledge of the actual event shapes. In addition, the filter may be specified to allow for such contingencies as the presence of random noise, variable gain between traces, and random variations of the relative time of arrival between traces. Once these p...

Journal ArticleDOI
TL;DR: In this paper, the demagnetization effect of a body of some other shape, such as a rectangular prism, was taken into account, and the normal practice was to approximate it by an ellipsoid of known demagnetic factors.
Abstract: Demagnetization factors have so far been introduced and calculated only for bodies bounded by a second degree surface. For taking into account the demagnetization effect of a body of some other shape, such as a rectangular prism, the normal practice hitherto has been to approximate it by an ellipsoid of known demagnetization factor. As shown later, the demagnetization factors so obtained are, in general, subject to considerable errors excepting for some particular dimensional ratios.

Journal ArticleDOI
TL;DR: In this article, a computed main magnetic field model was used to reduce the observed data from these surveys to a common magnetic residual chart and two anomaly trends, each composed of linear magnetic features, dominated the residual chart.
Abstract: Seven individual shipboard and airborne magnetic surveys have been conducted north of the Bahama‐Antilles Islands by the Naval Oceanographic Office during a 5.3 year period since 1962. A computed main magnetic field model was used to reduce the observed data from these surveys to a common magnetic residual chart. Two anomaly trends, each composed of linear magnetic features, dominate the residual chart. One trend strikes northeast‐southwest, the other at an angle of 80 degrees to it. By relating the residual magnetic anomalies to structural features observed on an experimental model developed by Billings and to actual structural features in the nothern Antilles, it is shown that the anomalies could be engendered by structural features resulting from an east‐west horizontal couple. The line of action of this couple is consistent with prevalent theories on continental drift.

Journal ArticleDOI
TL;DR: In this paper, the electric and magnetic field components produced by horizontal dipole antennas located within the upper layer of a two-layer conducting earth are derived for the quasi-near range, defined as that in which the measurement distance is much greater than an earth skin depth but much less than a free-space wavelength.
Abstract: The electric and magnetic field components produced by horizontal dipole antennas (both electric and magnetic) located within the upper layer of a two‐layer conducting earth are derived for the quasi‐near range. This range is defined as that in which the measurement distance is much greater than an earth skin depth but much less than a free‐space wavelength. Ionospheric effects are neglected. It is assumed that the transmitting and receiving anterenna depths are much less than their horizontal separation, and that the fields in the horizontal direction vary only slightly in a distance of one skin depth. It is well known that if the conductivity and thickness of the first layer (sea water) are known, the conductivity of the bottom layer (the sea bed) may be determined from magnetic field measurements alone. However, when extremely low‐frequency magnetic field measurements are performed at sea, the movement of the magnetic field sensors in the static magnetic field of the earth (which is many times stronger...

Journal ArticleDOI
TL;DR: In this paper, resistivity depth probes were made over water wells in an alluvial interior valley of Jamaica, known as the Queen of Spain's Valley, and the results, interpreted by standard curve matching methods, compare favorably with available well logs.
Abstract: Resistivity depth probes were made over water wells in an alluvial interior valley of Jamaica, known as the Queen of Spain’s Valley. Several type of electrode configuration were used and the observations analyzed by a technique designed to select those suitable for interpretation. The results, interpreted by standard curve matching methods, compare favorably with available well logs. They suggest that the yield is a linear function of the resistivity of the water‐bearing strata. A straight line fitted to the data by the method of least squares provides a nomogram which can be used for predicting the probable yield at points where no wells exist. It is concluded that resistivity surveys can be of material assistance in a water‐supply drilling program in the area. It is found that interpretation by multilayer curve‐matching procedures gives more realistic results than analysis by classical two‐layer master curves used in conjunction with Hummel’s rule. The results suggest also that the Wenner array gives a ...

Journal ArticleDOI
TL;DR: In this article, the effect of wavelength filtering on the shape of residual anomalies is discussed and compared with least squares polynomial fitting methods and the pseudo anomalies introduced by the latter method.
Abstract: Recently Skeels (1967) has considerably clarified the concept of the term “residual gravity” and has proposed a modified polynomial fitting method as a computational scheme. In the article Skeels refers to wavelength (or wavenumber) filtering discussed by Byerly (1965). Wavelength filtering has been recently applied by Zurflueh (1967) to the determination of residual aeromagnetic anomalies. Although Zurflueh compares wavelength filtering and least‐squares polynomial fitting methods and mentions the pseudo anomalies introduced by the latter method, he does not discuss the effect of wavelength filtering on the shape of the residual anomaly.

Journal ArticleDOI
TL;DR: In this article, the relationship between electrical and thermal conductivities of consolidated sandstone rocks and sediments was investigated, and the derived equations can be used to determine thermal conductivity from electrical conductivities, providing the solid constituents can be approximated.
Abstract: Relationships between electrical and thermal conductivities of consolidated sandstone rocks and sediments were investigated. Equations given by Archie (1942) and Woodside and Messmer (1961) were combined to give an equation which relates the two conductivities for the rocks. Equations relating conductivity and porosity, given by Maxwell (1881), Bruggeman (1935), and Woodside and Messmer (1961) were combined to give two equations which relate thermal and electrical conductivities for sediments. Data from 65 rock samples and 105 sediment samples (including data for 37 rock samples published by Zierfuss and Van der Vliet, 1956) were use establishing the reliability and parameter values of the equations. The derived equations can be used to determine thermal conductivities from electrical conductivities providing thermal conductivities of the solid constituents can be approximated. For samples of sandstone rock, deviations of the data from values predicted by the equations are shown to be related to the varia...

Journal ArticleDOI
TL;DR: In this paper, the induced polarization method was used for in-situ classification of unconsolidated sediments encountered in hydrological projects, and the results indicated that the anomalous frequency effects may be related to clayey gravels.
Abstract: Clay horizons and other clay‐bearing unconsolidated sediments are potential sources of induced‐polarization anomalies. If such anomalies may be detected above system noise, the induced‐polarization method may be of value for in‐situ classification of unconsolidated sediments encountered in hydrological projects. One such project exists in Santa Clara County where near‐surface unconsolidated sediments are frequently considered as potential recharge areas. Of four areas surveyed with induced‐polarization apparatus in Santa Clara County, only two yielded significant frequency‐effect anomalies, and in each of these two the frequency effects were of the order of 3 percent. These anomalous frequency effects may be related to clayey gravels. The dipole‐dipole array, with spreads of 10 ft and 20 ft, was typically used in the study.

Journal ArticleDOI
TL;DR: The material in this paper was presented at the Annual Meeting in Houston in November 1966 and was enthusiastically received as mentioned in this paper, however, some errors and subjective material which detract from the application of this new tool.
Abstract: The material in this paper was presented at the Annual Meeting in Houston in November 1966 and was enthusiastically received. Certainly many have looked forward to its publication. However, your attention is directed to some errors and subjective material which detract from the application of this new tool.

Journal ArticleDOI
TL;DR: In this paper, the problem of electromagnetic wave propagation from an oscillating magnetic dipole placed over a uniaxially anisotropic earth has been considered, and formal expressions for the vector potentials inside the earth have been derived.
Abstract: The problem of electromagnetic wave propagation from an oscillating magnetic dipole placed over a uniaxially anisotropic earth has been considered. Formal expressions for the vector potentials inside the earth have been derived. It has been shown that for a vertical magnetic dipole, the field components are identical to those in the case of an isotropic medium in which the conductivity is the “horizontal or longitudinal conductivity.” For a horizontal dipole, directed along the x axis, it has been shown that the vector potential inside the earth will have a y component as well as x and z components. Formal expressions for the vector potentials in air have been obtained for the case of a horizontal magnetic dipole. However when the conductivity of air is considered to be negligibly small, the field components are not affected by the anisotropy.

Journal ArticleDOI
TL;DR: A density model of the upper crust of Wyoming, computed on structural and petrographic evidence without prior reference to gravity data, when compared with the observed gravity field, demonstrates that existing measurements and theory of change of sedimentary rock density with depth of burial are satisfactory for gravity interpretation for local rock types and the slower rates of sedimentation in the area as discussed by the authors.
Abstract: A density model of the upper crust of Wyoming, computed on structural and petrographic evidence without prior reference to gravity data, when compared with the observed gravity field, demonstrates that: (1) Existing measurements and theory of change of sedimentary rock density with depth of burial are satisfactory for gravity interpretation for local rock types and the slower rates of sedimentation in the area. (2) Gravity stations established over sedimentary basins are generally as satisfactory as those on basement rock outcrops for depicting regional Bouguer gravity trends. (3) The Laramide deformation was mainly germanotype. The regional gradient in Bouguer gravity across Wyoming has increasingly negative values to the southwest. Local variation in the Bouguer anomaly field is related mainly to the low density Cretaceous and Cenozoic sediments of the basins, and to a lesser degree to the Laramide structural blocks. Interpretation of the gravity field indicates that the blocks are bounded by near‐verti...

Journal ArticleDOI
TL;DR: In this paper, normal moveout is used to distinguish primary from multiple reflections when the average velocity changes with depth, and the error appears to be controlled by the ratio of the standard deviation of the velocities within the section to the averaged velocity.
Abstract: Normal moveout may be used to distinguish primary from multiple reflections when the average velocity changes with depth. Approximate values of normal moveout may be found using an average velocity formula, but such values are always too large. The error appears to be controlled by the ratio of the standard deviation of the velocities within the section to the average velocity. Ray tracing provides normal moveout values which agree much better with observations. As a consequence of large dip, the reflections added by the CDP method do not represent identical areas of the dipping bed. However, if the various reflection zones lie on an inclined plane, they are added in‐phase and assume the time of a reflection from a point on the plane.

Journal ArticleDOI
TL;DR: In this article, Borehole data are presented to show that the SH source of vibrations generates a downward-propagating, horizontally polarized shear wave, which is consistent with the main objective of getting information from the shear-wave data which could not be obtained from the corresponding P wave data.
Abstract: The emphasis in this shear‐wave research was placed on determining the general quality of data which could be obtained in different areas and whether such quality was consistent with the main objective of getting information from the shear‐wave data which could not be obtained from the corresponding P‐wave data. Borehole data are presented to show that the SH source of vibrations generates a downward‐propagating, horizontally polarized shear wave. Shear velocities were determined for depth intervals of two to three hundred feet, but no absolute correlation between Vs/Vp and lithology could be established. In the deeper sedimentary section, Vs/Vp averages about one‐half; but in the low velocity, or weathering layer, the ratio may be as small as one sixth. All the reflection record problems which arise from wave propagation phenomena in the LVL are generally much worse for SH waves than for P waves because of the very small SH velocities in the LVL. Nevertheless, by using large source and receiver patterns ...

Journal ArticleDOI
TL;DR: The results of the application of filters of this type to field and synthetic data show little if any advantage over single‐channel deconvolution, and this failure appears to be connected with the low cross coherence of both noise and reflection signal on field‐recorded, common‐depth‐point traces.
Abstract: Several writers have proposed the use of multichannel filters for the elimination of coherent noise on seismic records One filter of this type which can be constructed is a multichannel Wiener filter which has a multichannel input and a single channel output In this form, it is applicable to data collected for vertical or horizontal common‐depth‐point stack processing The choice of desired output characteristics for this Wiener filter is flexible and, for example, can be tuned to correspond to multichannel deconvolution The results of the application of filters of this type to field and synthetic data, in general, show little if any advantage over single‐channel deconvolution This failure appears to be connected with the low cross coherence of both noise and reflection signal on field‐recorded, common‐depth‐point traces

Journal ArticleDOI
TL;DR: A fully portable transistorized gamma-ray spectrometer has been constructed, and used to investigate the nature of the gamma ray activity at the surface of rock outcrops.
Abstract: A fully portable transistorized gamma-ray spectrometer has been constructed, and used to investigate the nature of the gamma ray activity at the surface of rock outcrops. Gamma-ray photopeaks of K 40 and members of the U and Th series have been identified, along with strong fallout activity dominated by the 0.75 Mev activity of Zr 95 . A method has been devised for measuring, in situ, the K, U, and Th contents of rocks. Calibration accounts for the interference between the radioelements, and for background radiation. The following estimates of accuracy and sensitivity are for five-minute counting intervals: 5 percent plus 0.1 percent K, 10 percent plus 0.2 ppm U, 10 percent plus 0.5 ppm Th. The main advantages of the method are its speed and versatility, and the very large sample analyzed. A number of surveys have been performed to demonstrate some of the applications of the instrument. The major project of this series is the detailed mapping of the K, U, and Th distribution in the Preissac granite of northwestern Quebec. In addition to its use as a petrologic tool, the technique is eminently suited to prospecting for U and Th, and the quantitative evaluations of occurrence of these elements.