bluuebunny/crossref_metadata_2025 huggingface dataset:
A STUDY OF SOME EFFECTS OF A CONDUCTING HOST ROCK WITH A NEW MODELLING APPARATUS
Authors: Sarma D. Gupta , Maru V. M.
Published: October 2002

An instrument which uses a new sampling technique has been constructed and used to study the electromagnetic induction effects in a laboratory scale model representing conductors in the presence of host rock. It measures, in a dipole‐dipole setup, the in‐phase and out‐of‐phase components of the anomalous field directly, and has provision for recording the same components on a chart recorder. Idealized ore bodies can be represented by solid conductors and the host rock, by an aqueous salt solution. The operating frequency can be smoothly varied and set at any value between 10 khz and 100 khz. The operating separation between the source and the receiver in the coplanar coil setup may be up to 21 cm at 10 khz and up to 29 cm at 30 khz or above. Measurements may be made to within ±0.1 percent of the free‐space field, though absolute accuracy is less. It is found that the presence of a conducting host rock may substantially enhance the observed EM anomaly if the introduction of the target in the host rock alters the geometry of induced current paths. If this geometry is unaltered, a simple screening effect involving a reduction and phase rotation of the anomalous field occurs.
THE EFFECT OF OVERBURDEN ON VERTICAL COMPONENT ANOMALIES IN AFMAG AND VLF EXPLORATION: A COMPUTER MODEL STUDY
Authors: Vozoff K.
Published: October 2002

New computer techniques permit the calculation of AFMAG and VLF anomalies in the vertical magnetic field component for situations which cannot be conveniently modeled with analog models. Conductivity and geometry of the “ore” zone, the overburden, and the bedrock can all be varied independently; although all must have the same strike direction. Model results accurate to about 10 percent can be obtained at negligible cost. One example shows how a decrease in overburden resistivity from 100 ohm‐m to 10 ohm‐m can obliterate a distinct AFMAG anomaly. An increase of overburden thickness from 20 m to 50 m has the same effect. This study shows the importance of obtaining some direct overburden resistivity data before ruling out areas which show no anomalies.
MORPHOLOGICAL STUDY OF GEOPHYSICAL MAPS BY VIEWING IN THREE DIMENSIONS
Authors: Gay S. Parker
Published: October 2002

Stereo pairs of contour maps may be constructed by a process analogous to, but the inverse of, the process used to make contour maps from stereo pairs of aerial photographs. This construction can be carried out either manually or with computer plotting devices. The contoured stereo pairs are then viewed in three dimensions by a variety of methods: with lens or mirror stereoscopes, with bicolor anaglyphs in drafted, printed, or projected form, or by polarized light methods. Preferred techniques at present are 1) 5‐inch wide prints viewed by the mirror stereoscope, and 2) anaglyphic rear projection on a 3×4 ft screen. The advantages of studying complex contour maps in three dimensions are striking. The entire morphology of the map may be studied rapidly by the human optical system, the latter carrying out such functions as trend filtering, wavelength filtering, form and texture recognition, and location of linears almost simultaneously. Important features that are missed or seen with difficulty on the flat contour map become obvious in a three‐dimensional view, and may be studied further in detail by manual or computer techniques. The method appears to be a significant interpretational breakthrough, bringing us one step closer to a total interpretation, wherein all observable map features are correlated with their geological causes.
VELOCITY SPECTRA AND DIFFRACTION PATTERNS
Authors: Dinstel W. L.
Published: October 2002

Analysis of velocity spectra in Tertiary sand‐shale sequences sometimes shows anomalously high velocity events, with high correlation factors, deep in the section. One source of these events may be diffraction patterns. Figure 1 shows a model where a velocity gather is centered over a diffraction source; the diffraction acts like a normal reflection common depth point. Calculations yield the expected velocity of 7500 ft/sec. In Figure 2, the velocity gather is centered 6000 ft on the ground horizontally from the vertical projection of the diffraction source. Analysis of the data of Figure 2 results in a velocity which is quite a bit too high. Various combinations of velocity, depth, and offset produce varying degrees of error. The velocity computed from a spectrum not centered over a diffraction pattern is always too high, when we assume that the diffraction source is in the plane of the section.
SEISMIC STRATIGRAPHIC EXPLORATION PART II
Authors: Marr John D.
Published: October 2002

Past seismic stratigraphic exploration capabilities, which found large amounts of oil with crude tools from 1938 to 1962, are illustrated with three additional case histories covering a contribution to the discovery of the only super‐giant reef‐type oil field in the United States, the discovery of a thin sand pinchout reservoir along a strand line, and an exploration method for the delineation of a barrier reef complex and the overlying compaction fold complex. Present and future seismic stratigraphic exploration capabilities will be presented in Part III in the August, 1971 issue of Geophysics.
EVALUATION OF TERRAIN EFFECTS IN GROUND MAGNETIC SURVEYS
Authors: Gupta V. K. , Fitzpatrick M. M.
Published: October 2002

Magnetic effects due to terrain can be as much as 1000 gammas in areas of even moderate relief where bedrock contains only a few percent magnetite. These effects may be evaluated simply by subdividing the topography into a series of sloping steps of infinite extent; computation of a theoretical terrain contribution then permits separation of the magnetic effects due to terrain from those due to structural or stratigraphic causes. In areas where terrain effects occur, we can estimate average magnetic susceptibility by comparing observed and computed magnetic profiles.
On: “Analysis of the Relationship between Acoustic Reflectivity and Sediment Porosity” by R. W. Faas (GEOPHYSICS, August 1969, p. 546–553)
Authors: Blackman Abner
Published: October 2002

Faas states that a linear regression equation relating sediment porosity and acoustic reflectivity was derived empirically from data previously published by several investigators. The distinction between empirically and theoretically derived relationships is important because empirically derived relationships can be used for predictive purposes with far greater confidence than theoretical ones. The weight of observational data stands behind empirical relationships, while theoretical relationships are untested by observation.
SEISMIC STRATIGRAPHIC EXPLORATION—PART III
Authors: Marr John D.
Published: October 2002

A review of seismic research and development shows there is available in 1971 an important new, mature, and sophisticated seismic technology. This technology, when added to the solid foundation of basic science and experience of the pre‐1963 model seismic stratigraphic exploration capability, produces a capability which can, in the future, discover stratigraphic oil. We suggest there are many ways of achieving seismic high resolution which are not now in widespread use. Applications of our high resolution concepts to stratigraphic exploration are illustrated, including the direct detection of hydrocarbon saturated sands. To illustrate types of situations where it seems reasonable to search for major undiscovered stratigraphic reserves, we analyze briefly the Gulf Coastal province. This province contains a huge carbonate basin and a colossal clastic basin which may have tremendous potential from a modern stratigraphic exploration viewpoint. It is again emphasized that the stratigraphic exploration process will have to be performed by fully integrated exploration teams composed of management, geologists, and geophysicists.
GEOPHYSICAL ANALYSIS IN CENTRAL INDIANA USING POTENTIAL FIELD CONTINUATION
Authors: Rudman Albert J. , Mead Judson , Whaley Joseph F. , Blakely Robert F.
Published: October 2002

Seismic, gravity, and total, vertical, and horizontal magnetic surveys were conducted over a prominent basement anomaly in Hamilton County, Indiana, as part of a statewide effort to understand the nature of the basement complex. Application of Poisson’s relationship between the gravity and horizontal magnetic fields demonstrates that the source is not a single, vertically sided prism, but is a body whose outline (shape) varies with depth. Digital model studies support the suggestion that continuation of the fields observed at the surface to levels below the upper surface of the source can be useful in defining the shape of certain sources. Application of this technique was successful in defining a model whose fields closely match those observed in Hamilton County. Geologically, the final model resembles a vertical pipe associated with a succession of overlying flows, a model that is consistent with our present understanding of basement geology in midwestern United States.
DIAPIRIC SALT VOLUME OFFSHORE LOUISIANA
Authors: LaFehr T. R. , Herring Alan T.
Published: October 2002

This short note reports that approximately 25,600 cubic miles of diapiric salt exists in offshore Louisiana to a water depth of 100 fathoms (Figure 1).
EXAMPLES OF THE EFFECT OF SOURCE AND RECORDING SITE PARAMETERS ON THE SEISMIC RESPONSE OBSERVED FROM PLOWSHARE PROJECTS, GASBUGGY AND RULISON
Authors: Hays Walter W.
Published: October 2002

The amplitude and frequency composition of the seismic motions observed over a wide geographic area from Gasbuggy and Rulison, two Plowshare detonations, illustrates the effect which source and recording site parameters have on the resultant ground motion. Gasbuggy (29 kt) and Rulison (40 kt) were detonated, respectively, in the San Juan and Piceance Creek sedimentary basins of New Mexico and Colorado. Because both detonations were emplaced in sedimentary formations to stimulate the flow of natural gas, these detonations were placed at a greater depth of burial than typically required for containment. The effect of source parameters (device depth of burial and energy release) was simulated by seismic scaling theory. This theory (based on an extension of Sharpe’s problem) predicted a smaller elastic radius and, consequently, higher dominant frequency of generation for the elastic waves than would be expected on the basis of Nevada Test Site experience with typically contained events of 29 and 40 kt. Observed effects of the source variables were displayed in the Gasbuggy and Rulison seismic data: (1) a shift of the frequency of maximum spectral response to the high frequency end of the spectrum and (2) enhancement of the peak vector surface particle accelerations and velocities and a decrease in the peak vector surface particle displacements. Some of the Rulison recording sites were located on thin (50 ft thick or less) layers of alluvium which, on the basis of refraction surveys, exhibited a fairly significant contrast in acoustic impedance relative to that of the underlying sedimentary rocks. The effect of the lowvelocity alluvium layers at recording sites of interest was simulated by amplitude amplification modeling (based on the Haskell‐Thompson matrix formulation) using interpreted data from refraction surveys to define the layer physical parameters. These calculations indicated that significant local frequency‐dependent amplification would occur at some locations in the Piceance Creek Basin, a prediction which was verified by the seismic data.
DESIGN OF SMALL OPERATORS FOR THE CONTINUATION OF POTENTIAL FIELD DATA
Authors: Mufti Irshad R.
Published: October 2002

Two‐dimensional continuation of potential fields is commonly achieved by employing a continuation operator which consists of a number of coefficients operating upon uniformly gridded field data. To obtain accurate results, the size of the operator has to be quite large. This not only requires a lot of computational work, but also causes a considerable loss of information due to the reduced size of the field obtained after continuation. Small‐size “equivalent” operators were designed which are free from these drawbacks but yield accurate results. In order to demonstrate the efficiency of these operators, a potential field was continued upward by using 31×31 Tsuboi coefficients. This required 961 multiplications for computing the continued field at each grid point. When procedure was repeated using the equivalent operator, the number of multiplications required for each grid point was reduced to 15, the size of the resulting map was much larger, but the results in both cases were practically identical in accuracy. Frequency characteristics of the equivalent operators and the continuation of data very close to the boundary of the field map are discussed.
A NEW MULTICOIL AERIAL ELECTROMAGNETIC PROSPECTING SYSTEM
Authors: Fraser D. C.
Published: October 2002

A helicopter‐towed multicoil electromagnetic system was developed using a 30‐ft bird and a transmitted frequency of 918 Hz. Its primary features of interest are (1) low aerodynamic and geological noise, (2) three‐dimensional vector recording of anomalies, and (3) quantity of geological information which is extracted by a computer‐oriented interpretive procedure. The multicoil system‐was developed to obtain more diagnostic geologic information about a conductor than was possible with other aerial electromagnetic equipment. This led to two approaches: (1) three‐dimensional recording of anomalies was developed to provide information on the electrical current distribution within the conductor and, hence, on the geometry of the conductor and (2) high‐resolution, low‐frequency design was chosen to bias the response of the system toward high‐conductivity targets to minimize interference from the usual geologic noise sources, thereby enhancing the response of discrete massive sulfide conductors. A consequence of these design criteria is a system yielding an advance in the state‐of‐the‐art of aerial electromagnetic exploration.
On: “Radius of Investigation in dc Resistivity Well Logging,” by A. Roy and R. L. Dhar (GEOPHYSICS, August 1971, p. 754–760)
Authors: Moran J. H.
Published: October 2002

This recent article continues the approach take by the authors in an earlier article (Roy and Dhax, 1970) on induction systems. As in that earlier article, the authors confine their attention to homogeneous media. Limiting the analysis to homogeneous media yields results of very questionable value. The approach taken in the paper is to determining a “polarization” of the conducting medium resulting from source currents and to then calculate the sonde response by summing up the effects of these distributed sources. The weighting factor ascribed to each cylindrical shell is then identified with a “geometrical factor,” presumably giving the relative weight of the particular shell.
On: “Isochrons for a Three‐Dimensional Seismic System,” by J. W. Dunkin and F. K. Levin (GEOPHYSICS, December 1971, p. 1099–1137)
Authors: Savit Carl H.
Published: October 2002

In the introduction to this paper, the impression is given that the two‐dimensional imaging of isochrons, as a method of detailing subsurface geometry, was first described by Walton in 1970. I would call the attention of the authors to U. S. patent No. 2,925,138 (reissue 25,204) originally issued on February 16, 1960 to C. H. Becker. So far as I have been able to determine, this is the earliest published description of the basic method described by the authors. Attention should also be called to U. S. patent No. 3,327,287 issued to J. D. Ball and W. D. Mounce, June 20, 1967, which describes a technique whereby crossed linear arrays may be used in place of areal arrays.
EQUIRIPPLE APPROXIMATION OF FAN FILTERS
Authors: McClellan J. H. , Parks T. W.
Published: October 2002

An efficient procedure for the approximation of two‐dimensional (2‐D) fan filters is presented. The Chebychev (or min‐max) error criterion is used to measure the optimality of the approximation as opposed to previous designs which use the least‐squares norm. Since the direct solution of a 2‐D Chebychev approximation problem is difficult, a change of variables is introduced to convert the 2‐D approximation problem into an equivalent 1‐D low‐pass filter approximation problem. The response is opptimized in the 1‐D domain, and then the change of variables is applied to obtain the 2‐D filter. Thus the techniques of 1‐D Chebychev approximation are applied to yield a 2‐D fan filter.
REVIEW OF DATA PROCESSING AND INTERPRETATION METHODS IN GRAVITY AND MAGNETICS, 1964–71
Authors: Grant F. S.
Published: October 2002

This paper reviews the literature in data processing and interpretation in gravity and magnetic methods for the period 1964–71. The topics that are covered include gravimetry, reduction of data, automatic contouring, map analysis and filtering, interpretation technique, and applications. It is concluded from the literature that a very strong swing toward automatic processing and semi‐automatic methods of interpretation using computers has taken place in both methods during the time period under review. The key to further development in this direction appears now to be with making more imaginative use of graphic display devices.
AUTOMATIC CONTOURING USING BICUBIC FUNCTIONS
Authors: Hessing R. C. , Lee Henry K. , Pierce Alan , Powers Eldon N.
Published: October 2002

A method is described for using a digital computer to construct contour maps automatically. Contour lines produced by this method have correct relations to given discrete data points regardless of the spatial distribution of these points. The computer‐generated maps are comparable to those drawn manually. The region to be contoured is divided into quadrilaterals whose vertices include the data points. After supplying values at each of the remaining vertices by using a surface‐fitting technique, bicubic functions are constructed on each quadrilateral to form a smooth surface through the data points. Points on a contour line are obtained from these surfaces by solving the resulting cubic equations. The bicubic functions may be used for other calculations consistent with the contour maps, such as interpolation of equally spaced values, calculation of cross‐sections, and volume calculations.
On: “Diapiric Salt Volume Offshore Louisiana,” by T. R. LaFehr and Alan T. Herring (GEOPHYSICS, December 1971, p. 1205–1251)
Authors: Tucker Lewis R.
Published: October 2002

In the interest of brevity, I will summarize the points I raised and the conclusions I drew in my somewhat extended correspondence with LaFehr and Herring concerning my objections to their paper. 1) Any geologic interpretation of gravity data includes a number of assumptions. 2) Of all these assumptions, the most tenuous one is that concerning a “regional.” 3) Once assumptions have been made, the answer is only the end result of an arithmetical exercise; the conclusion has been established already. 4) In the specific case of the calculated quantity of salt in the study area in the Gulf of Mexico, many of the “facts” that had to be included in the reduction of the data are actually assumptions that, through constant repetition, are now thought of as actual knowledge. 5) The conclusion in their paper as to the quantity of salt is only one of a large family of results, though it may be in the correct magnitude.
PROCESSING AND INTERPRETATION OF MAGNETOTELLURIC SOUNDINGS
Authors: Kunetz G.
Published: October 2002

A few methods in the processing and interpretation of magnetotelluric soundings over a stratified earth are investigated, with emphasis on the less commonly used time‐domain procedures. Analytical expressions of the theoretical transfer function between the magnetic‐ and electric‐field variations, both in frequency and time domain, are derived. Their properties are studied, and recursive algorithms are given for their numerical computation. On the other hand, a procedure is outlined which leads directly in the time domain to the experimental values of this transfer function. It is similar to the methods used in seismic analysis for signal determination and makes use of the auto‐ and crosscorrelation functions of the measured field variations. Finally, methods of interpretation, based either on a visual or on an automatic comparison of these theoretical and experimental transfer functions, are proposed. For the case of automatic interpretation, complementary geologic data should be used where possible to take care of the lack of uniqueness of the solution.
THE DIELECTRIC PROPERTIES OF SELECTED BASALTS
Authors: Hansen W. , Sill W. R. , Ward S. H.
Published: October 2002

Basaltic samples of known chemical composition were measured for dielectric constant and loss tangent in the frequency range 100 hz to 50 Mhz. All samples were prepared and measured by a contact substitution method in a dry nitrogen atmosphere at room temperature. The measurements indicated a general spectrum of dielectric constant with a range of 10–65 at 100 hz to 7–15 at 50 Mhz with frequency dispersion observed to be minimal above 1 Mhz. The loss tangent decreases from a range .2–.9 at 100 hz to a range .02–.15 at 100 khz. A finer‐grained sample was observed to have higher dielectric properties below 1 Mhz than a coarser‐grained sample of identical composition. A qualitative and subsequent quantitative study of possible compositional effects indicated that iron‐titanium compounds may be responsible for observed variations in the dielectric spectra of typical basalts.
AN AUTOMATED METHOD FOR THE MEASUREMENT OF CATION EXCHANGE CAPACITY OF ROCKS
Authors: Worthington A. E.
Published: October 2002

An automated method of determination of the cation exchange capacity of rocks is presented. The principal novel feature is the containment of the sample within a dialysis membrane bag during ion exchange with barium chloride and during washing to remove the excess salt. This procedure prevents sample loss, permits control of the washing step to prevent hydrolysis, and permits auto mation of these steps. Exchange capacity is determined by an automated conductometric titration of the barium ions with magnesium sulfate solution. The method requires one to ten grams of sample and is reproducible to five percent.
ELECTRICAL CONDUCTIVITY OF SEVERAL SAMPLES OF OLIVINITES, PERIDOTITES, AND DUNITES, AS A FUNCTION OF PRESSURE AND TEMPERATURE
Authors: Dvořák Z.
Published: October 2002

The electrical conductivity σ of several samples of olivinites, peridotites, and dunites was measured in the temperature range between 250 and 700°C under quasi‐hydrostatic pressures from 1 to 20 kb. Using a straight line extrapolation of logσ values taken at pressures greater than 6–8 kb, graphs of logσ versus [Formula: see text] were plotted for 0 and 20 kb. These graphs exhibit similar general features to those obtained under room pressure by other authors, but they appear to be shifted towards the low temperature side. This shift may be explained plausibly by assuming much higher values of the preexponential term but only minor variations of the exponential term in the expression [Formula: see text] exp ([Formula: see text]). The electrical conductivity of the samples with high percentage of serpentinized olivine (more than 35 percent) is characterized by a decrease of σ in the temperature interval between 470 and 625°C and is related to the content of serpentine in the samples and its dehydration in this temperature range. Some estimates of temperature within the earth based on conductivity data inferred from magnetotelluric and geomagnetic variation methods are discussed in terms of the present results which suggest substantially lower values of T to be attained at a particular depth. Also, an attempt is made to reexamine assumptions involving the theoretically deduced equation of the electrical conductivity within the earth.
ON THE PHENOMENOLOGY OF ELECTRICAL RELAXATION IN ROCKS
Authors: Shuey R. T. , Johnson M.
Published: October 2002

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 conductivity decay times are shorter than the corresponding resistivity decay times. There are several relations between the resistivity decay spectrum and the usual IP measures. The initial value of the time‐domain voltage curve is proportional to the integral over the spectrum of resistivity decay times. The area under the time‐domain voltage curve, divided by its initial value, is the mean of the distributed resistivity decay times. Finally, PFE and phase shift are roughly proportional to the strength of the resistivity decay in the relevant spectral region.
LOW‐FREQUENCY IMPEDANCE PARAMETERS OF BASALT, GRANITE, AND QUARTZITE
Authors: Khalafalla A. S. , Maegley W. J.
Published: October 2002

An analog circuit digital computer setup was used to evaluate rock electrical impedance and phase angle in the frequency range 0.05 to 2 khz. Room‐temperature measurements were made on several samples of basalt, granite, and quartz. Argand diagram presentation of rock reactance as a function of its resistance at a series of frequencies described a semicircular arc. Rock impedance circular arcs can be used to define the rock resistivity at infinite frequency and the dc resistivity at the limit of zero frequency. Variation of rock resistance with the logarithm of frequency indicated finite rock dielectric dispersions at characteristic frequency ranges. Also, variation of rock reactance with the logarithm of frequency exhibited finite relaxation peaks, which can be used to difine the rock characteristics or turnover frequency as well as a characteristic or relaxation time.
RAY TRACING IN THREE DIMENSIONS
Authors: Shah Pravin M.
Published: October 2002

Special methods for tracing rays through restricted earth models are reported in the literature. For example, Sattlegger (1965) has dealt with a two‐dimensional model consisting of a sequence of constant‐velocity layers separated by interfaces that may be dipping. More recently, Sorrells et al. (1971) have considered three‐dimensional (3D) models with plane interfaces of arbitrary strike and dip and with spherical surfaces of a special kind. In the earlier part of 1970, we were using a more general algorithm which allowed us to trace rays through a 3D model consisting of plane or curved surfaces of arbitrary nature. This algorithm was presented by the author at the 41st Annual SEG convention in November, 1971. A number of inquiries received by us since that time has encouraged us to summarize our results in this brief note.
SOSIE: A NEW TOOL FOR MARINE SEISMOLOGY
Authors: Barbier M. G. , Viallix J. R.
Published: October 2002

The development of marine reflection methods over the past few years has shown the technical and economic advantages that can be derived from highly repetitive nonexplosive sources. Pursuing the idea to its limits the Geophyiscal Division of SNPA’s Research Center has designed and implemented a new seismic method. The source energy is released into the water in the form of discrete pulses of the same sign and essentially the same amplitude, according to a long pseudo‐random sequence. This process is not restricted to any particular source; it can be used with a number of different sources, provided they can reach a sufficient rate of repetition. The basic principle has been checked with a sparker, chosen for its flexibility; however, conventional sources such as air and steam guns were also tested. Comparison section indicate improved quality for the new method due to multiplication of seismic ray paths and a more adequate frequency spectrum. Additional benefits are maximum flexibility in recording and processing and reduction of costs.
COMPARISON OF AIRBORNE EM COIL SYSTEMS PLACED OVER A MULTILAYER CONDUCTING EARTH
Authors: Sinha Ajit K.
Published: October 2002

A detailed analysis has been made on the relative effectiveness of the four types of coil arrangements used in airborne electromagnetic surveying for the mapping of subsurface inhomogeneities. By comparing the theoretical responses for the four systems over known geologic sections, it has been established that the horizontal coplanar system is the most effective for detecting inhomogeneities in single‐frequency airborne surveys and produces the largest anomaly in multifrequency work. The next largest anomaly in both the cases is obtained in the vertical coplanar arrangement. The multilayer computer program used to derive the theoretical master curves for comparing the performance of the four systems is completely general except for our assumption of the quasi‐static conditions, which is justified in all geophysical induction problems. A study of the “hidden‐layer problem” to determine the best system and conditions for detection of a thin layer sandwiched between two thicker beds has been made with the help of the program. Horizontal coplanar and vertical coplanar systems are again found to be the best and second best systems, respectively, for their detection. It is also clear that a conductive sandwiched bed is much easier to detect than a resistive one.
THE APPLICATION OF AUDIO‐FREQUENCY MAGNETOTELLURICS (AMT) TO MINERAL EXPLORATION
Authors: Strangway D. W. , Swift C. M. , Holmer R. C.
Published: October 2002

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.
A PARADOX IN APPARENT RESISTIVITY MEASUREMENTS OVER A GROUND SECTION WITH CONDUCTIVE SUBSTRATUM
Authors: Satpathy B. N.
Published: October 2002

It is generally accepted that the resistivity of the top layer largely influences the apparent resistivity measured through conventional surface electrode arrays. Thus, intuitively it would be assumed that the overlaying of a conductive layer would give an apparent resistivity which is less than the value obtained without the conducting cover and an opposite result would be obtained by introducing a resistive top layer. Though this belief is valid for many geoelectric sections, it is here shown that for a two‐layer section with [Formula: see text], if the top portion of the first layer is replaced with a layer of higher or lower resistivity, the apparent resistivity value changes in an opposite manner after a critical value of the electrode separation: the apparent resistivity for large separations is decreased due to the introduction of the resistive top layer. It is intended in this note to elucidate this paradox through a suitable example.
SEISMIC DATA COMPRESSION METHODS
Authors: Wood Lawrence C.
Published: October 2002

This paper discusses two ways of compressing seismic data prior to long‐distance transmission for display. A Walsh transform technique and an analogous time‐domain method eliminate redundant seismic information allowing data sets to be compressed with little visual degradation. The basic approach consists of using an average 3-bit code to describe data in such a way as to minimize information loss; the method also uses the Walsh transform to achieve further compaction through sequency bandlimiting. A second technique is entirely a time‐domain operation and does not use transforms. The Walsh method, however, produces larger compression ratios than the time technique before serious image degradation occurs. Both schemes have six basic parts: bandlimiting, quantization, encoding, decoding, interpolation, and band‐pass filtering; they differ only in band limiting and interpolation. Band limiting sequencies in the Walsh domain is very similar to, but not the same as, alias filtering and resampling in time. Reducing Walsh bandwidths by some power of two has a time‐domain implementation consisting of an averaging procedure with subsequent resampling, while the inverse Walsh transform step can be viewed as a means of interpolating in the time domain. The convergence properties of three Rademacher derived transforms—Hadamard, Paley, and Walsh—are studied with regard to exploration seismic data. Hadamard energy has been found to be uniformly distributed over its entire spectrum, whereas Walsh and Paley transforms concentrate about 80 percent of the total energy into a major lobe occupying about 15 percent of the total bandwidth (2 msec sampling). Smaller minor lobes containing the remaining 20 percent are discarded while bandlimiting. The major lobe energy suffices for many seismic applications such as VA/VD plot displays. Optimum quantization and encoding of major lobe energy results in an overall 28:1 compression factor for 12 bit data sampled every 2 msec. Analogous time domain compression, on the other hand, only achieves a 16:1 reduction because of the power of two restriction imposed by the resampling and averaging process.
A COMPUTER ORIENTED VELOCITY ANALYSIS INTERPRETATION TECHNIQUE
Authors: Beitzel John E. , Davis James M.
Published: October 2002

Extensive velocity analysis interpretation can impose a substantial man‐hour cost. A computer‐implemented technique is described which edits the velocity analysis data, thereby reducing the interpreter’s task. The technique uses graph theory to simulate the complex decision‐making inherent in the interpreter’s method of velocity analysis interpretation. The heart of the technique lies in defining a distance measure between candidate time‐velocity points from the velocity analysis. This metric is a function of the specific and potentially complex constraints imposed by the interpreter and of the weighted separation of the points. The graph theoretic techniques employed use the metric to join and, hence, select appropriate points from the candidate points while rejecting those which are invalid. Additional editing, based in part on implied interval velocities, further reduces the bulk of data presented to the interpreter. The method makes efficient use of computer time and has yielded encouraging results, as demonstrated by examples.
AN AUTOMATIC EVENT DETECTOR AT THE TONTO FOREST SEISMIC OBSERVATORY
Authors: Blandford R. R.
Published: October 2002

The on‐line operation of an automatic event detector has been evaluated at the Tonto Forest Observatory short‐period seismic array. For 31 seismometers and one fixed threshold, the 90 percent incremental detection threshold on the Kuril Island beam, centered at Δ=70 degrees, is [Formula: see text] with an experimentally determined false alarm rate of 0.17 per day. This compares favorably with the capabilities of a human operator. Storms in the Kurils significantly affect the distribution of amplitudes of the F-statistic detection trace, and we estimate that most of the false alarms observed at the operating threshold can be traced to the statistical bias introduced by this storm‐generated energy. If the threshold were adjusted to maintain a constant false alarm rate, the maximum effect on the threshold magnitude would be [Formula: see text].
MOVEOUT DETECTION BY AN AUTOCORRELATION MATRIX METHOD
Authors: Dash Bibhu P. , Hains Brian L. A.
Published: October 2002

An investigation is made of the application of the autocorrelation matrix method to the analysis of moveout. A number of multichannel Wiener filters are computed to pass/reject correlated events. Experiments are performed on a variety of synthetic data in order to ascertain how such filters will perform in a given situation. It is shown that very low levels of signal can be successfully recovered and a very close match must exist between the moveouts present in the data and those specified for the particular filter design. The “average” or second order autocorrelation matrix is developed, and a multichannel filter scan of a column from this matrix is incorporated in the former method. Experiments are performed on synthetic data for different noise conditions in order to ascertain the ability of these methods to detect low‐power correlated events. It is shown that such detection can be successful, provided the random noise is not too severe. Results from the analyses of real data records are consistent with those of the synthetic case.
ELEMENTS OF MIGRATION AND VELOCITY ANALYSIS
Authors: Gardner G. H. F. , French W. S. , Matzuk T.
Published: October 2002

This paper describes a theory for combining migration and velocity analysis of seismic data and uses experimental model data to illustrate some of the practical problems. It is shown that adaptive weighting of the data, based on a measure of coherence, can be effective in suppressing false images and noisy backgrounds caused by finite migration apertures, finite sampling in space and time, and random noise in the data. Velocity analysis is presented as a “range finding” method in which images for two different constant offset profiles are brought into coincidence by using the correct velocity. The method is valid even in the presence of diffraction, dipping interfaces, or curved reflectors.
IMPROVEMENTS IN SEISMIC PROSPECTING IN 1938
Authors: Adler Joseph L.
Published: October 2002

Improvements in reflection seismographs have been made during the past two or three years which have made it possible to use the method in areas previously considered unworkable. But these are not developments of the current year. The developments were worked out in regular seismograph country in the recent past, and the note‐worthy thing for this year is that they have now been applied successfully for the first time to areas which seismologists abandoned four or five years ago as unworkable.
REPORT OF THE YEAR’S ACTIVITIES IN ELECTRICAL, GEOTHERMAL, RADIOACTIVE, AND SOIL ANALYSIS METHODS
Authors: Heiland C. A.
Published: October 2002

At the outset I want to acknowledge the splendid cooperation which I have received in the preparation of this report by the members of my committee. Their support has been most gratifying, and the information received from them on progress in our field in the last year is so complete that I doubt whether I can do justice to their contributions in the space allotted to the report. The year’s activities in electrical, geothermal, radioactive, and soil analysis methods will be covered in this report in the order given. Each of these will be subdivided into applications in mining, oil exploration, engineering geology, and activities along scientific or theoretical lines.
REFRACTION AND REFLECTION OF SEISMIC WAVES—II DISCUSSION OF THE PHYSICS OF REFRACTION PROSPECTING
Authors: Dix C. H.
Published: October 2002

We observe that there are five principal factors governing the intensity of the refracted wave of “refraction prospecting.” It is shown that a clear understanding of this process can lead to decided improvements in the geophysicist’s interpretation of refraction data.
THE BLOCK MOVE SUM NORMAL MOVEOUT CORRECTION
Authors: Rupert G. B. , Chun J. H.
Published: October 2002

A new normal moveout technique designated Block Move Sum (BMS) is described. In theory it better approximates the ideal inverse NMO process than do currently utilized techniques. Data blocks are corrected as units thus eliminating trace stretching and reducing trace distortion. The proposed correction and two common production methods are applied to both synthetic and real data and compared. Crosscorrelations calculated from synthetic seismogram data suggest the BMS method creates the least signal distortion. For field CDP data, the BMS does improve signal resolution for events at early times.
MIGRATION VELOCITY DETERMINATION: PART I. PHILOSOPHY
Authors: Sattlegger Johann W.
Published: October 2002

Classical stacking velocity determination consists of applying NMO corrections, with a number of constant velocities, to the traces of a CDP gather; computing coherent energy for the NMO‐corrected traces; and mapping the coherency obtained as a function of traveltime and velocity in a form suitable for interpretation. Migration Velocity Determination may be performed by generating migrated traces with a number of constant velocities; computing coherency of the energy migrating into identical positions of the migrated traces; and mapping the coherency as a function of (in this case) vertical traveltime and velocity in a form suitable for interpretation. A typical result of Migration Velocity Determination is a contoured display of coherency as a function of time and velocity. Because of the migration process, the velocities obtained are velocities to points situated along vertical column of the subsurface and are suited to provide an optimum migrated section (migration before stack).
TOPOGRAPHIC DISTORTION OF TELLURIC CURRENTS: A SIMPLE CALCULATION
Authors: Thayer Richard E.
Published: October 2002

An analytic solution of the electric field near a two‐dimensional offset in the earth’s surface is presented as a means of estimating the effects of nonplanar topography on telluric measurements. Results indicate that, for an offset height d, the field at the earth’s surface is within ten percent of its undisturbed value for distances, greater than 2.5 d from the offset on the lower side and for distances greater than 4 d on the upper side. The disturbance due to the offset extends even less far out in situations where the telluric currents are channeled close to the surface.
DEPTH PREDICTION FROM VELOCITY PROFILES ON THE TEXAS GULF COAST
Authors: Woeber A. F. , Penhollow J. O.
Published: October 2002

Velocity profile accuracy in depth prediction in a Texas Gulf Coast area has been statistically estimated by two methods: 1) a comparison of profiles to well surveys; 2) a study of errors in marker horizon depth estimates made in wildcats prior to drilling and in wells that had already been drilled. Both methods point to an accuracy of 1 to 2 percent in 80 percent of the samples at depths ranging from 5000 ft to 12,000 ft. Accuracy fails off fairly rapidly below 12,000 ft, but 50 percent of the samples still exhibit an accuracy of 2 percent or better at 15,000 ft. This accuracy is attributed to the two‐mile spread lengths used, the overall good quality and density of the data, and to the moderate velocity range, 7000 ft/sec to 11,000 ft/sec.
EXPERIMENTAL DETERMINATION OF EFFECTIVE DEMAGNETIZING FACTORS FOR A SHORT CYLINDER
Authors: Åm Knut , Stemland Rune Otto
Published: October 2002

Comparison between measurements on spheres and cylinders shows that a short cylinder with axial ratio (length/diameter) ≃0.9 and susceptibility ≃0.8 (cgs) has an effective demagnetizing factor N ≃ 3.7 both in the axial and radial directions. Hence, for a short cylinder with high susceptibility, the sum of the effective demagnetizing factors in three mutually perpendicular directions is less than 4π. This is important to know e.g., when determining the magnetic properties of highly magnetic magnetite ore samples.
DETECTION OF BODIES WITH SPECIFIED DENSITY‐MAGNETIZATION RATIOS BY THE USE OF MULTICHANNEL WIENER FILTERS
Authors: Gunn P. J.
Published: October 2002

It is possible to design multichannel Wiener filters to simultaneously process gravity and magnetic data in profile form so as to produce diagnostic rectangular wave outputs only if anomalies due to bodies with a specified density‐magnetization ratio are input. This method has application to detecting rocks with known density‐magnetization ratios, of interest where the rocks are known to contain economic mineralization.
THE TELLURIC‐MAGNETOTELLURIC METHOD
Authors: Hermance John F. , Thayer Richard E.
Published: October 2002

The telluric‐magnetotelluric method uses magnetotelluric measurements at the base site, but only telluric measurements at remote sites. It thus combines the economy, simplicity, and speed of the traditional telluric method with the quantitative advantages of the traditional magnetotelluric method. The dominant features of the combined method are the following: First, the time required to set up a telluric site is less by a factor of at least 5 than the time for a complete magnetotelluric site. Second, one does not need to record magnetic field data at the base site simultaneously with the electric field recorded at each remote site. One needs only enough magnetic data to adequately determine the base tensor. A telluric transfer tensor coupling electric field measurements at the base site and each remote site can be used to transfer the base impedance tensor to an impedance tensor at each remote site. By being much more selective of the magnetic data used in the analysis, one can significantly improve the signal‐to‐noise ratio. Third, the data are analyzed to determine each element of the complex impedance tensor so that important phase information as well as amplitude information is available for interpretations which are more sophisticated than those currently attempted in conventional telluric surveys. Finally, in making the ultimate interpretation in terms of the impedance tensor rather than the telluric tensor used in conventional telluric surveys, one essentially refers the interpretation of remote electric field observations to the magnetic field at the base site rather than to the electric field. Both experience and model studies demonstrate that the magnetic field is much more homogeneous than the electricfield in the vicinity of lateral heterogeneities; thus the selection of a proper base site is not as critical in the combined method as it is in the conventional telluric method.
SPECIAL REPORT OF THE SUBCOMMITTEE ON POLARITY STANDARDS
Authors: Thigpen Ben B. , Dalby A. E. , Landrum Ralph
Published: October 2002

This is a report of the SEG Subcommittee on Polarity Standards in exploration seismic recording. A Polarity Standard is recommended for analog and digital representations of seismic impulse signals. A four‐bit polarity code is proposed to define: (1) the polarity convention for impulse‐source systems; and (2) the relative polarity between a swept‐frequency pilot signal and the recorded seismic data for vibratory‐source systems. Polarity conventions are to be demonstrated by field tests and confirmed by computer analysis.