bluuebunny/crossref_metadata_2025 huggingface dataset:
Integrated geological and geophysical interpretation case study, and Lamé rock parameter extractions using AVO analysis on the Blackfoot 3C‐3D seismic data, southern Alberta, Canada
Authors: Dufour Jocelyn , Squires Jason , Goodway William N. , Edmunds Andy , Shook Ian
Published: October 2002

Blackfoot field, southeast of Calgary, Alberta, Canada, has produced oil and gas from a Glauconitic compound incised‐valley system. The Glauconitic compound incised valley has three cycles of incision and valley fill: lower, lithic, and upper incised valleys. The upper and lower incised valleys are the main reservoirs. The geophysical interpretation of compressional PP‐seismic data resulted in the definition of the compound‐valley extent, and in the mapping of the upper and lower incised valleys. A stratigraphic well‐log template was built using the most significant lithological information and well logs. To integrate both geological and geophysical interpretations, the well log cross‐sections and corresponding depth‐converted seismic were superimposed. Furthermore, a detailed geological facies interpretation of the upper and lower incised valleys was undertaken and incorporated. A good correlation was found between the interpreted geological facies and the seismic data response.

Information about the nature of the fill within the compound valley was gained from the integration of the PP‐ and PS‐wave interpretations. However, this is limited to Vp/Vs analyses on given intervals. Amplitude‐variation‐with‐offset analysis of the PP‐data was run to discriminate lithology and pore‐fluid saturates. The products of the Lamé rock parameters, incompressibility (λ) and rigidity (μ), with density (ρ) were extracted from seismic inversions for P‐ and S‐impedances. The extraction of λ ρ and μρ showed the presence of gas‐bearing porous sandstone within the Glauconitic incised‐valley system.
Depth characterization of shallow aquifers with seismic reflection, Part II—Prestack depth migration and field examples
Authors: Bradford John H. , Sawyer D. S.
Published: October 2002

It is common in shallow seismic studies for the compressional‐wave velocity in unconsolidated sediments to increase by a factor of four or more at the transition from dry or partial water saturation to full saturation. Under these conditions, conventional NMO velocity analysis fails and leads to large depth and layer thickness estimates if the Dix equation is assumed valid. Prestack depth migration (PSDM) is a means of improving image accuracy. A comparison of PSDM with conventional NMO processing for three field examples from differing hydrogeologic environments illustrates that PSDM can significantly improve image quality and accuracy.
Acoustic measurements in unconsolidated sands at low effective pressure and overpressure detection
Authors: Prasad Manika
Published: October 2002

Shallow water flows and over‐pressured zones are a major hazard in deepwater drilling projects. Their detection prior to drilling would save millions of dollars in lost drilling costs. I have investigated the sensitivity of seismic methods for this purpose. Using P‐wave information alone can be ambiguous, because a drop in P‐wave velocity (Vp) can be caused both by overpressure and by presence of gas. The ratio of P‐wave velocity to S‐wave velocity (Vp/Vs), which increases with overpressure and decreases with gas saturation, can help differentiate between the two cases. Since P‐wave velocity in a suspension is slightly below that of the suspending fluid and Vs=0, Vp/Vsand Poisson's ratio must increase exponentially as a load‐bearing sediment approaches a state of suspension. On the other hand, presence of gas will also decrease Vpbut Vswill remain unaffected and Vp/Vswill decrease. Analyses of ultrasonic P‐ and S‐wave velocities in sands show that the Vp/Vsratio, especially at low effective pressures, decreases rapidly with pressure. At very low pressures, Vp/Vsvalues can be as large as 100 and higher. Above pressures greater than 2 MPa, it plateaus and does not change much with pressure. There is significant change in signal amplitudes and frequency of shear waves below 1 MPa. The current ultrasonic data shows that Vp/Vsvalues can be invaluable indicators of low differential pressures.
Directional induction logging for evaluating layered magnetic formations
Authors: Cheryauka Arvidas B. , Sato Motoyuki
Published: October 2002

We present a modeling study to investigate the ability of directional induction logging to evaluate magnetic properties of formations. Simultaneous measurement of in-phase and quadrature components of electromotive force, or R- and X-signals, respectively, gives complete information about both the electrical resistivity and the magnetic susceptibility of the formation. We describe an approach for designing a directional induction tool with tilted transmitting and receiving coils. We discuss spatial resolution using directional excitation–measurement and evaluation of formation parameters. The proposed method can detect magnetic susceptibility inhomogeneity in addition to electrical resistivity inhomogeneity using the X-signal. The X-signal log response reflects abrupt changes in magnetic properties, with sharp gradients located in the vicinity of magnetic susceptibility interfaces.
The use of quad–quad resistivity in helicopter electromagnetic mapping
Authors: Huang Haoping , Fraser Douglas C.
Published: October 2002

The apparent resistivity from a helicopter-borne frequency-domain electromagnetic (EM) system is typically obtained from the in-phase and quadrature responses arising from the flow of conduction currents in the earth. The most commonly used resistivity algorithms, derived from half-space models and using single-frequency data, do not account for magnetic polarization and consequently do not yield a reliable value for apparent resistivity in highly magnetic areas. This is because magnetic polarization modifies the EM response, causing the computed resistivity to be erroneously high. The impact of magnetic permeability on the EM response is much greater for the in-phase component than for the quadrature component. If magnetic polarization is to be ignored, the calculation of the apparent resistivity using the quadrature component at two frequencies (the quad–quad algorithm) is less subject to error from magnetic polarization than if the in-phase and quadrature responses at a single frequency are used (the in-phase–quad algorithm). The quad–quad algorithm, however, can display undesirable behavior for large induction numbers, i.e., when conductivities and frequencies are large. Determining which algorithm is optimum is a data-dependent choice, which, of course, is area dependent.

We have studied the behavior of the quad–quad (apparent) resistivity and its comparison to in-phase–quad resistivity to determine the conditions under which the use of quad–quad resistivity is appropriate. For a two-layer earth, the behavior of the quad–quad resistivity depends mainly upon the ratio of the lower frequency fL to the upper-layer resistivity ρ1. If this ratio is low, the quad–quad resistivity will behave well. In areas yielding a high value of the ratio fL/ρ1, the quad–quad resistivity may lie outside of the range of the true resistivities of the earth and therefore provide misleading information. Our studies therefore suggest that the quad–quad resistivity algorithm should be avoided in areas where the ratio is large, i.e., when using high frequencies in conductive areas. The term large is relative. For a two-layer case, for example, the use of quad–quad resistivity is only recommended for magnetic areas where fL/ρ1 < 500 Hz/ohm-m, when conductive cover exists, and where fL/ρ1 < 50 Hz/ohm-m when a conductive basement underlies resistive cover. In spite of these limitations, quad–quad resistivity is often preferable to in-phase–quad resistivity in highly magnetic areas.
Resistive limit modeling of airborne electromagnetic data
Authors: Reid James E. , Macnae James C.
Published: October 2002

When a confined conductive target embedded in a conductive host is energized by an electromagnetic (EM) source, current flow in the target comes from both direct induction of vortex currents and current channeling. At the resistive limit, a modified magnetometric resistivity integral equation method can be used to rapidly model the current channeling component of the response of a thin-plate target energized by an airborne EM transmitter. For towed-bird transmitter–receiver geometries, the airborne EM anomalies of near-surface, weakly conductive features of large strike extent may be almost entirely attributable to current channeling. However, many targets in contact with a conductive host respond both inductively and galvanically to an airborne EM system. In such cases, the total resistive-limit response of the target is complicated and is not the superposition of the purely inductive and purely galvanic resistive-limit profiles. Numerical model experiments demonstrate that while current channeling increases the width of the resistive-limit airborne EM anomaly of a wide horizontal plate target, it does not necessarily increase the peak anomaly amplitude.
High-frequency electromagnetic impedance method for subsurface imaging
Authors: Song Yoonho , Kim Hee Joon , Lee Ki Ha
Published: October 2002

This paper presents a high-frequency electromagnetic (EM) impedance method that extends the utility of conventional controlled-source audio-frequency magnetotelluric (CSAMT) method to the frequency range from 100 kHz to 100 MHz. In this frequency range diffusion and wave propagation must be considered together. In principle, both the electrical conductivity and the permittivity of the shallow subsurface can be imaged using impedance data gathered on the surface of the earth. The impedance approach has a distinct advantage in that coupling with the source is unnecessary, provided that the source can be positioned far enough away to yield plane waves at the receiver positions.

At high frequencies the EM impedance is a function of the angle of incidence or the horizontal wavenumber, so the electrical properties cannot be readily extracted without eliminating the effect of horizontal wavenumber on the impedance. For this purpose, this paper considers two independent methods for accurately determining the horizontal wavenumber, which in turn is used to correct the impedance data. The apparent electrical properties derived from the corrected impedance data correlate poorly to the real structure, especially for the model with a resistive overburden. However, the impedance data along with the incidence angles thus estimated can be successfully inverted to yield an accurate subsurface layering through the simulated annealing inversion scheme.
Calculation of electrical potentials along a longitudinal section of a 2-D terrain
Authors: Xu Shi-zhe , Zhang Dahai , Ruan Baiyao , Dai Shikun , Li Yuguo
Published: October 2002

The problem of 2-D terrain corrections for point-source electric resistivity data is considered. The total electric potential is divided into normal and anomalous terms. An integral equation is derived for the Fourier transform of the anomalous potential and is solved using a boundary element method. An inverse Fourier transform is applied to recover the anomalous potential along a “longitudinal” profile passing through the point source and oriented perpendicular to the vertical plane containing the 2-D terrain variations. The sum of the normal and anomalous potentials are then used to calculate an apparent resistivity. A sample calculation demonstrates that the longitudinal apparent resistivity calculated in this manner is less sensitive to terrain variations than the traditional “transverse” apparent resistivity that is computed from potential measurements made parallel to the vertical plane containing the 2-D terrain variations.
Multiple-source Euler deconvolution
Authors: Hansen R. O. , Suciu Laura
Published: October 2002

Rapid three-dimensional (3-D) source location methods can be extremely useful in framing a subsurface structural model from gravity or magnetic data. However, existing implementations of Euler deconvolution are limited to a single source in each window. This can be a significant limitation in areas of complex structure. We have generalized the method to the multiple-source case, and implemented the 3-D algorithm. Results from synthetic data and from the Gold Acres mining district in Nevada suggest that the new algorithm can be a useful interpretive tool.
Full wavefield inversion to estimate impact-source orientation from multicomponent land seismic data
Authors: Ramos-Martínez Jaime , McMechan George A.
Published: October 2002

Orientation of an impact source is estimated from multicomponent land data. The seismic wavefield produced by an arbitrarily oriented impact source is represented by the linear combination of the nine independent wavefield components (Green's functions) for a full vector source. Coefficients of this linear combination are expressed in terms of the parameters that define source orientation. The parameters are an azimuthal angle, a vertical angle, and source strength, which are the unknowns of the inverse problem. Once the Green's functions are computed, source parameters are estimated by full wavefield inversion using a grid search over the source parameters. This inverse procedure also provides information about the uniqueness and resolution of the solution. Information on the source orientation is dominated by the relative amplitudes between components, so the inversion can be performed even when the source-time wavelet is not well known. The approach is tested using a synthetic common-source gather. Results for a field data example from northeast Texas show deviations between 11° and 16° from the vertical for an expected effective vertical impact, and between 19° and 23° from the horizontal for an expected horizontal cross-line impact, depending on whether a geophone rotation is performed to reduce the uncertainty produced by geophone misalignment. Results show the potential to quantify and isolate the effects of source misorientation in multicomponent land data.
Lithology discrimination from physical rock properties
Authors: Bosch Miguel , Zamora Maaria , Utama Widya
Published: October 2002

The estimation of lithology from multiple geophysical survey methods needs to be addressed to develop advanced tomographic methods. An initial requirement for lithology discrimination is that lithology should be discriminable from the media properties physically related to the geophysical observations. To test this condition for different combinations of the most common crustal rocks, we performed several lithology discrimination exercises on rock samples under laboratory conditions. The physical properties included mass density, compressional velocity, shear velocity, electric conductivity, thermal conductivity, and magnetic susceptibility. A categorical description of the sample lithology was followed; hence, the inference consisted of predicting the sample rock category (lithotype) membership. The joint information provided by the physical properties of the rocks allowed us to discriminate the sample lithotype correctly, with an overall success rate of 100% in the most favorable situation and over 85% in the least favorable situation. We obtained successful classification results for a variety of common lithotypes (granite, gabbro, limestone, tuff, marble, basalt, and gneiss) using three common classification methods: clustering analysis, Gaussian classification, and discriminant analysis. Although discrimination was positive with each of these multivariate classification techniques, discriminant analysis showed some advantages for the classification and graphic analysis of the data. These results support our postulate that lithology can be estimated reliably if multiple geophysical observations are considered jointly.
A stable and efficient approach of inverse Q filtering
Authors: Wang Yanghua
Published: October 2002

Stability and efficiency are two issues of general concern in inverse Q filtering. This paper presents a stable, efficient approach to inverse Q filtering, based on the theory of wavefield downward continuation. It is implemented in a layered manner, assuming a depth‐dependent, layered‐earth Q model. For each individual constant Q layer, the seismic wavefield recorded at the surface is first extrapolated down to the top of the current layer and a constant Q inverse filter is then applied to the current layer. When extrapolating within the overburden, instead of applying wavefield downward continuation directly, a reversed, upward continuation system is solved to obtain a stabilized solution. Within the current constant Q layer, the amplitude compensation operator, which is a 2‐D function of traveltime and frequency, is approximated optimally as the product of two 1‐D functions depending, respectively, on time and frequency. The constant Q inverse filter that compensates simultaneously for phase and amplitude effects is then implemented efficiently in the Fourier domain.
On: “A PROPOSED GEOPHYSICAL METHOD FOR ORIENTING CORES”
Authors: Lynton E. D.
Published: October 2002

We have read with interest the author’s proposed ingenious method for orienting cores in a drilling well, but certain practical difficulties would have to be overcome in applying it.
AUTHOR’S REPLY
Authors: Vacquier V.
Published: October 2002

The questions raised by E. D. Lynton can be answered as follows: 1. The matter of drilling a pilot hole 1 in. in diameter at the bottom of an oil well has been discussed with production engineers and has been declared feasible, but it is admitted that there are difficulties to surmount.
On:“ADDITIONAL EVIDENCE ON THE RELATION OF TEMPERATURE TO STRUCTURE IN THE SALT CREEK OIL FIELD, NATRONA COUNTY, WYOMING”
Authors: Weaver Paul
Published: October 2002

Mr. Van Orstrand states in his abstract; “it was found from temperature surveys in 21 wells that the temperatures over a considerable portion of the field were definitely related to the structure”; and in commenting on Fig. 2, the author states “the results shown in Fig. 2 prove without question that the temperatures at a given depth diminish rather rapidly in all directions from the crest to the flanks of the dome.”
An Analysis of Abnormal Reflexions
Authors: Johnson Curtis H.
Published: October 2002

Mr. Deacon’s paper seems to me to be an example of the type of computational research which will become more common and more in demand as the reflection seismograph is called upon to solve more and more complicated structural and stratigraphic problems. Probably most of us in reflection seismograph work have noticed and have plotted by routine methods, abnormal wave fronts appearing on our records such as those illustrated in Mr. Deacon’s paper. Unfortunately it is seldom that individual study is given to such “reflection” and still more seldom that such individual study is published. Mr. Deacon has collected a rather convincing set of data from which to draw conclusions, although a California operator would wish for more information regarding velocity.
Reply to the Discussion by Dr. Kunz
Authors: Clark A. R.
Published: October 2002

It is quite true that in order to prevent the shape of curves depending on the absolute values of the resistivities and the electrode spacings, the quantities to be plotted must be expressed as ratios. Once this has been done the scale of plotting is chosen to illustrate best a particular characteristic of these ratios.
Reply by Dr. Maeda to preceding discussion
Authors: Maeda Katsura
Published: October 2002

It was a pleasure for me to receive a copy of the discussion of my paper by Drs. Van Nostrand and Cook. Through their detailed description, the previous studies on dipping beds and the relation between them have been greatly clarified.
Proposed Geological Institute Of America
Authors: Denison A. Rodger
Published: October 2002

At the Annual Meeting of The American Association of Petroleum Geologists at Denver in April, 1942, Dr. Carey Croneis gave an address on “Geology in War and Peace.” In this address Croneis “ … contended that the profession of Geology does not occupy as high a position in National esteem, and thus in fundamental national utility, as its potential importance warrants.” To correct this condition he made “ … several suggestions for the good of the science.” The first of these was—“Organize an American Geological Association to which each geologist, each person fundamentally interested in the subject may belong, regardless of the field of specialization, or prime professional application.” Three other suggestions were made, the last of which has a very close relationship to the first and is as follows—“Inaugurate and consistently maintain a unified, vigorous, and yet dignified public relations program.”
Laboratory of Applied Geophysics and Geochemistry
Authors:   The Pennsylvania State College
Published: October 2002

A laboratory of applied geophysics and geochemistry has recently been established at the Pennsylvania State College under the direction of Dr. Sylvain J. Pirson. The program of work includes both resident instruction and fundamental as well as practical researches in the development of new mineral resources.
Reply by authors to discussion by Kenneth B. S. Burke and J. E. Wyder
Authors: Lennox D. H. , Carlson V.
Published: October 2002

We appreciate the comments on our paper by Dr. Burke and Mr. Wyder. They are quite correct in remarking that our calculated “Wenner configuration results” are not necessarily identical with those which would have been obtained by direct use of the expanding Wenner configuration in the field. The calculated results were derived from the actual field results on the assumption of horizontal layering, an assumption which should have been explicitly stated in our paper. “Complex subsurface geology and numerous lateral changes in resistivity” can certainly cause the two sets of results to differ to the extent that depth and thickness determinations using the calculated Wenner results may be, at best, unreliable. We feel, nevertheless, that the differences will not be so great that the calculated Wenner results may not be used to advantage in a qualitative interpretation of the subsurface.
Reply to Mr. Brooks
Authors: Hicks Warren G.
Published: October 2002

I agree that we have too long considered geophone surveys sacred. However, if a true two‐receiver log is run it should neither be sensitive to spacings nor to elapsed time since drilling. If, as in Figure 3, three logs are run yielding three different answers, at least two of them are incorrect. Inconsistencies among logs necessitate a standard check. The geophone survey is useful as a standard in measuring times over long intervals and has been of value in pointing out deficiencies in our logs. If logs are run which are insensitive to spacing, a valid argument exists for ignoring computed shot times which disagree with the log.
On: “Three-Dimensional Filtering with an Array of Seismometers” by John P. Burg (GEOPHYSICS, October 1964, p. 693–713)
Authors: Schoenberger Michael
Published: October 2002

In Section IV of the above paper, Burg writes the three-dimensional impulse response of a seismometer as [Formula: see text] where [Formula: see text] is the time-impulse response of a seismometer, and [Formula: see text] is the location of the seismometer in an [Formula: see text]coordinate system. Although at first glance this is intuitively appealing, it is not correct.
Reply by author to discussion by N. F. Uren
Authors: Paul M. K.
Published: October 2002

I thank Mr. N. F. Uren for his interest in my paper. Mr. Uren’s discussion mainly concerns the appropriateness of the difference / differential equation that the regional gravity field, [Formula: see text], has been implied in my paper to satisfy. I think that this point has been discussed in considerable detail in my reply to an earlier discussion on this paper (Geophysics, June 1969, p. 483–485), which can, however, be summarized as follows.
Reply by author to discussion by J. A. Colligan
Authors: Foster Manus
Published: October 2002

As Mr. Colligan points out, there is a typographical error in equation (11) (p. 604) in which [Formula: see text] is incorrectly written for [Formula: see text].
Reply by author to discussion by Lyle V. A. Sendlein
Authors: Duguid James Otto
Published: October 2002

Mr. Sendlein’s discussion suggests two errors in the author’s paper. These were errors in the plotting of the data and not in the data itself. In reply to Mr. Sendlein, the following three items will be discussed: 1. The method of determination of velocities from the time distance curve; 2. Reliability of velocities determined; and 3. The interpretation of the time‐distance curve for station 87+00.
Reply by author to discussion by P. A. Paul
Authors: Negi Janardan G.
Published: October 2002

In analytical continuation, the implicit convergence range parameter in the Taylor series formalism remains incomprehensible and paradoxical when perceived through only the real values of the functions and the variables. Puzzling, erroneous, and sometimes conflicting statements from papers mentioned in Geophysics, Nature, and Dr. Paul’s present discussion of the paper by Negi (1967), hereafter referred as paper (A), originate from the same limited perception.
Discussion of V. Baranov’s paper, “A New Method of Interpretation of Aeromagnetic Maps, Pseudo‐gravimetric Anomalies”
Authors: Affleck James
Published: October 2002

Baranov’s pseudo‐gravimetric anomaly method is based on well‐known relationships and his development appears to be sound. Applications of his methods should be of very appreciable use in evaluating aeromagnetic surveys. The comments which follow should not be interpreted as criticisms. They are intended to emphasize the conditions and limitations under which Baranov’s techniques might be utilized.
On: “Statistical Models for Interpreting Aeromagnetic Data” by A. Spector and F. S. Grant (GEOPHYSICS, v. 35, p. 293–302)
Authors: Gudmundsson Gudmundur
Published: October 2002

In their paper “Statistical Models for Interpreting Aeromagnetic Data,” Spector and Grant define the “power spectrum” of the magnetic anomalies over a single rectangular block as the squared Fourier amplitude spectrum. For their expression of the “power spectrum” of a single block, Spector and Grant quote Bhattacharyya (GEOPHYSICS, v. 31, p. 97–121).
On: “HRVA—a Velocity Analysis Technique Applied to Seismic Data” by J. C. Robinson (GEOPHYSICS, June 1969, p. 330–356)
Authors: Pan Cheh
Published: October 2002

Recently I read Robinson’s paper, and it seems to me that the fundamental mathematical derivations in that article are somewhat erroneous. In this discussion I would like to present some comments on that paper.
Reply by author to Dr. Naidu’s discussion
Authors: Gendzwill D. J.
Published: October 2002

I appreciate Dr. Naidu’s interest in my paper. He has omitted some detail in the equations (such as the factor G) but the sense of his argument is perfectly clear, and I agree that the method of Fourier transformation is more general than my particular solution. In fact, Novosolitskii (1965) has presented the inverse solution for any horizontal density distribution in a slab. Nevertheless, it seems to me that the existence of a closed exact expression for the model I discussed is of some interest in that it presents a unique formula for an elementary gravity model. The closed expression may be manipulated to derive characteristic interpretation curves such as those presented. The gradational density contrast models may be superimposed to produce almost any arbitrary one‐dimensional density pattern in a slab.
Reply by authors to discussion by J. G. Saha
Authors: Odegard M. E. , Berg J. W.
Published: October 2002

As Saha has pointed out, the Fourier transform for the vertical fault given in our paper (Odegard and Berg, 1965) is incorrect and the correct transform is given by Davis (1971). In our derivation of the Fourier transform the transformation [Formula: see text] was used. This transformation is correct only for [Formula: see text] with the transformation for [Formula: see text] given by [Formula: see text].
Reply to E. Szaraniec by S. H. Ward
Authors: Ward S. H.
Published: October 2002

We are grateful to Mr. Szaraniec for expressing interest in our work and for bringing one of his publications to our attention. However, his comments have little bearing on the main theme of the paper nor on the results we presented. We confined our discussion to the Schlumberger array, and we checked our forward solutions with some of those of La Compagnie Generale de Geophysique; correct convergence was assured in all cases tested, and illustrations of correct convergence are contained in the paper. We agree thoroughly with Mr. Szaraniec that convergence to the correct asymptotic value requires careful attention and, hence, since we did not use a perfectly insulating subsurface, we also did not use the asymptote pertinent to that model.
Reply by authors to Dr. Thyssen‐Bornemisza’s discussion
Authors: Kuo John T. , Ottaviani Mario , Singh Shri K.
Published: October 2002

Our paper is concerned principally with surface measurements of vertical gradient of gravity, and we purposely have reserved any discussions on borehole measurements.
Reply by authors to discussion by Charles M. Swift, Jr. and Gerald W. Hohmann
Authors: Zonge K. L. , Wynn J. C.
Published: October 2002

Dr. Hohmann and Dr. Swift have actually answered the main thrust of their query by stating that our exact coupling removal technique is proprietary. However, the intent of this paper was not to show how we perform decoupling, but rather to provide examples or various types of coupling and coupling‐corrected data. Any apparent obfuscation is unfortunate and unintentional, and we will try to answer some of the responders’ objections by presenting a general outline of ZERO’s decoupling technique without becoming involved in the proprietary aspects.
Reply to J. W. Adams by R. N. McDonough.
Authors: McDonough R. N.
Published: October 2002

Adams correctly points out a blunder of mine, in that the spatial correlations [Formula: see text] in my equation (75) should be identified with the correlations [Formula: see text] in equation (32), and not with the data samples [Formula: see text] in equation (11). In effect, the averaging in equation (67) replaces the averaging in equation (12). In the case of nonuniform array spacing, the necessary interpolation can still be done in [Formula: see text], as I suggest, but one might also interpolate directly in the Fourier coefficients [Formula: see text], or even in the data [Formula: see text]. These latter two possibilities would require more computation, however. It is in the details of the data averaging and interpolation that experimentation must show the way.
Reply to S. J. Pirson by L. L. Nettleton
Authors: Nettleton L. L.
Published: October 2002

I have definitely established that the townships were marked wrong on the copy of the map as printed. This has been done through old records at the Gulf Oil Co.
Reply to M. J. Kuhn by A. W. Trorey
Authors: Trorey A. W.
Published: October 2002

M. J. Kuhn has raised two well‐taken points which, in the interests of perhaps excessive brevity, were not examined in my recent paper. In his reply I shall invoke some qualitative arguments and illustrate them by means of examples. As in the subject paper, all responses will be corrected for spherical spreading and will be viewed through the same mild filter (3 dB down at 10 and 60 Hz with low and high cut slopes of 6 and 18 dB/octave, respectively). All plots are normalized to their maxima (hence maximum amplitudes are imprinted thereon), and all times are referenced to the onset of the arrivals (thus gross normal‐moveout effects are suppressed in the displays).
Reply by author to Dr. LaFehr
Authors: Singh Shri Krishna
Published: October 2002

It is difficult to include all references when dealing with a subject so well studied as the gravitational attraction of a circular disc. Although the practical usefulness of Nettleton’s paper can not be denied by anyone, it nevertheless gives no details (except for some references) of the computation of solid angles subtended by a disc from which his graphs (Geophysics, 1942, Figure 4) result. My short note deals with (in what I consider an easy way of) obtaining a closed form expression for the solid angle. For applications of the result the reader would do well to look up Nettleton’s classic paper.
Reply to K. Helbig by James G. Berryman
Authors: Berryman James G.
Published: October 2002

The subject of my paper is limited to layered, anisotropic materials. The fact that layered materials are to be discussed is mentioned in the first sentence of the abstract and at various places throughout the introduction where the scope of the paper is defined. No other materials are mentioned and the results of the paper obviously do not apply to other types of materials.
Reply by the authors to discussion by H. Pascal and D. Rankin
Authors: Freedman R. , Vogiatzis J. P.
Published: October 2002

The interesting comment by Pascal and Rankin discusses, inter alia, the physical basis of a equation proposed by Schlumberger. This equation expresses formation traveltimes (e.g., inverse phase velocities) derived from measurements made by Schlumberger’s electromagnetic propagation tool (EPT) as the volume weighted sum of the traveltimes of the formation constituents. Pascal and Rankin interpret this equation as implying a layered medium consisting of layers of different dielectric properties. There is no doubt that the Schlumberger traveltime equation is correct for such a layered medium if the direction of wave propagation is normal to the direction of the layering planes. This does not preclude the possibility, however, that the Schlumberger equation may provide an adequate description, at microwave frequencies, of the composite traveltimes of more complex heterogeneous systems.
Reply by authors to Discussion by W. Hernandez and J. Jacobs.
Authors: Rankin David , Kao Dominic
Published: October 2002

The discussion by Hernandez and Jacobs of our paper deals with a specific example which we did not discuss, since the result falls out naturally from the more general case. Their remarks, however, apply not only to a one‐dimensional (1‐D) model but also to a two‐dimensional (2‐D) model if one measures in the principal directions. Both cases are, in fact, covered in their set of equations (3).
On: “Densities determined from surface and subsurface gravity measurements,” by B. A. Sissons (Geophysics, v. 46, p. 1568–1571).
Authors: Garland G. D.
Published: October 2002

There is one point of general interest not mentioned by the author of this careful investigation. The final result was that the mean density determined by gravity measurements was 4 percent less than the mean yielded by sample measurements, and the author reasonably attributes this to voids not sampled by laboratory specimens. On the other hand, Stacey and Tuck (1981), in proposing a possible scale effect on the value of the gravitational constant G, pointed out that all previous experiments of this type (which were recorded in sufficient detail to permit G to be calculated) gave the opposite result, suggesting that G on the geophysical scale has a value greater by approximately 1 percent than that determined in the laboratory. The new results discussed by Sissons therefore present the first evidence against such a hypothesis.
Reply by the author to S. D. Stainsby and M. H. Worthington
Authors: Horowicz Leon
Published: October 2002

S. D. Stainsby and M. H. Worthington present a very elegant solution to upgoing and downgoing wave separation in the presence of random noise, which effectively overcomes the problem of spectral singularities.
Reply by the authors to H. Soininen
Authors: Pelton W. H.
Published: October 2002

We appreciate the discussion by Heikki Soininen and welcome the opportunity to qualify use of the dilution factor derived by Seigel (1959).
Reply by the authors to R. Nagendra and H. V. Ram Babu
Authors: Mohan N. L. , Sundararajan N. , Rao S. V. Seshagiri
Published: October 2002

The comments of Nagendra and Ram Babu mainly center around the sign of the imaginary component in the integral [Formula: see text]. We have defined [Formula: see text] and maintained the consequential algebra throughout. The Hilbert transform we considered is a modified version of Thomas (1969) without any theoretical deviations. Since the modification is elementary, we thought more elaboration was not necessary.
Reply by the authors to R. Green
Authors: Rao T. K. S. Prakasa , Subrahmanyam M.
Published: October 2002

We appreciate very much the interest shown by Ronald Green in our short note. We would also like to make the following comments.
Reply by the authors to L. Eskola and H. Hongisto
Authors: Ram Babu H. V. , Atchuta Rao D.
Published: October 2002

We thank Dr. Eskola and Dr. Hongisto for their keen interest in our paper and for their useful suggestions/comments on the quantitative interpretation of SP anomalies in general, keeping in view the future methodic development of SP interpretation.
Reply to F. M. Lyakhovitskiy by J. G. Berryman
Authors: Berryman James G.
Published: October 2002

I regret that I do not read Russian. If I did, there would have been a much greater chance of my being familiar with the previous work of Lyakhovitskiy and Nevskiy on elastic anisotropy.
Reply by authors to Discussion by P. A. A. Laura
Authors: Pann K. , Eisner E. , Shin Y.
Published: October 2002

In writing this paper, we did not find it easy to locate standard terminology which is agreed upon. We adopted the term “generalized Galerkin’s method” from page 335 of Pearson (1974), but it seems to be completely equivalent to the “method of weighted residuals” of Ames (1967), or the “orthogonality method” of Collatz (1960, p. 30).
Reply to D. H. Eckhardt by D. Loewenthal
Authors: Loewenthal Dan
Published: October 2002

First, I wish to thank Dr. Eckhardt for referring me to his very interesting paper (Eckhardt, 1968) and for illuminating the magnetotelluric phase constraint from another point of view.
Reply to S. H. Ward and W. H. Pelton by the Authors
Authors: Nabighian Misac N. , Elliot Charles L.
Published: October 2002

The data presented in the author’s Table 1, based on generalized linear inverse theory and experimental design methods for the electrical model of Figure 9 of our paper, support the inferences that can be drawn from the companion Figure 10 (Nabighian and Elliot, 1976). For the typical section used in our model, both the data from Ward and Pelton and ours readily indicate that proper geometric parameters must be employed in an IP survey. Moreover, a reasonable appreciation of the geoelectric section is paramount in any quantitative interpretation of IP data.
Reply to K. Helbig by F. K. Levin
Authors: Levin Franklyn K.
Published: October 2002

In view of Dr. Helbig’s beautiful discussion, I only regret I had not read his 1965 paper. The use of the slowness surface clearly offers advantages of which I was unaware. Dr. Helbig would do a great service to exploration seismologists by writing a paper or monograph that included all his considerable work on anisotropy.
Reply by authors to Discussion by Yoich Ohta and Masanori Saito.
Authors: Gangi A. F. , Shapiro J. N.
Published: October 2002

We are pleased to hear that Ohta and Saito found our “Propagating‐least‐Squares” algorithm (PROLSQ) for fitting polynomials, [Formula: see text], (1) simple to use and efficient in execution. We appreciate their pointing out that there can be a difficulty with the algorithm under very special (but easily determined) circumstances; that is, when the independent‐variable values [Formula: see text] at the data points are so distributed that the odd‐order moments [Formula: see text] are zero. We did not experience this difficulty because we never treated a case in which all these odd moments were zero.
Reply by authors to B. B. Bhattacharya and P. K. Hazra
Authors: Palacky G. J. , Kadekaru K.
Published: October 2002

We were pleased by the interest in our work shown by Bhattacharya and Hazra. Their discussion focuses on our interpretation of horizontal‐loop EM data (our Figure 20). We are aware that interpretation diagrams obtained from scale modeling in free air are not adequate in complex geologic situations. We will briefly summarize the conditions described in case history 4; a conductor embedded in a poorly conductive host rock (amphibolite, [Formula: see text]), covered by a conductive weathered layer [Formula: see text].
Reply by the authors (D. V. Thiel and I. J. Chant) to J. R. Wait
Authors: Thiel D. V. , Chant I. J.
Published: October 2002

In his discussion of our paper Wait refers to the effect of an elliptically polarized radiation source on both electric field wavetilt and surface impedance measurements. If such measurements are made at the surface of a uniform isotropic half‐space, then both the horizontal electric and horizontal magnetic field strengths measured will depend upon the characteristics of the polarization ellipse and the orientation of the antennas whereas the vertical electric field strength is independent.
Reply by the author to C. V. Rao
Authors: Won I. J.
Published: October 2002

First, I would like to point out that the paper by Rao et al. (1973) was cited and acknowledged in the first paragraph of the subject paper. Rereading both papers again, I find the following differences in the approaches.
Reply to L. Szarka by the authors
Authors: Bartel L. C. , Jacobson R. D.
Published: October 2002

We welcome the opportunity to respond to comments by Szarka on our recent paper. The main points he raised on our near‐field correction scheme for controlled‐source audio‐frequency magnetotelluric (CSAMT) data are the application of the correction scheme and the near‐field/far‐field demarcation in the presence of layers and the application in the presence of electrical structure beneath the transmitter location. In our paper, we addressed the application for three‐dimensional electrical structure beneath the receiver location with the transmitter over a homogeneous half‐space. In this reply we wish to clarify these points and point out possible limitations of our correction scheme.
Reply by the authors to D. Winterstein
Authors: Liu Enru , Crampin Stuart , Booth David C.
Published: October 2002

Don Winterstein points out that in a uniform, transversely isotropic (TI) material (of whatever orientation), one of the two shear waves split by the anisotropy is strictly orthogonal to the ray; and the polarizations of the two shear waves traveling along a ray are mutually orthogonal. He suggests that Liu et al. (1989) should have specified these properties.
Discussion on the Paper by A. Ziolkowski
Authors: Lindsey J. P.
Published: October 2002

[Editor’s note: The paper by A. Ziolkowski which appeared in the February issue of Geophysics has generated a great deal of interest. For our readers’ convenience, we are reprinting here two letters that appeared in The Leading Edge (July 1991 TLE, p. 8), followed by three discussions on the paper and a reply to each by the author.] Dear Editors: Every once in a great while, a paper appears in Geophysics which is more significant than first meets the eye and mind. These papers, like too many others, are barely scanned and forgotten or brushed aside, or even criticized for incidental reasons (disagrees with our own ideas, isn’t written in an acceptable “tone,” etc.). I think Ziolkowski’s paper in the February 1991 issue (Why don’t we measure seismic signatures?) is possibly one of these.
On: “Why don’t we measure seismic signatures?” by A. Ziolkowski (GEOPHYSICS, 56, 190–201, February 1991)
Authors: Schoenberger M.
Published: October 2002

In his iconoelastic attack on statistical deconvolution, Ziolkowski left few idols standing. I take no issue with the tenor of his article, but I am uncomfortable with its tone and its extreme view. Certainly, signature deconvolution is underutilized and ought to become a more common practice. However, it is possible to arrive at that conclusion without discarding a number of extremely useful concepts.
Reply the author to Mark Graebner
Authors: Hale Dave
Published: October 2002

Dr. Graebner has correctly identified an error in equation (1) of the paper, “Imaging salt with turning seismic waves,” by D. Hale, N. R. Hill, and J. Stefani (November 1992 GEOPHYSICS, p. 1453–1462). I regret the error and appreciate his correction.
Reply by the authors to Arthur E. Barnes
Authors: Tygel M. , Schleicher J. , Hubral P.
Published: October 2002

We highly appreciate the useful remarks of Dr. Barnes relating our work to well‐known practical seismic processing effects. This is of particular interest as normal‐moveout (NMO) correction and post‐stack time migration are still two very important processing steps. Most exploration geophysicists know about the significance of pulse distortions known as “NM0 stretch” and “frequency shifting due to zero‐offset time migration.” As a result of the discussion of Dr. Barnes, it should now be possible to better appreciate the importance of our very general formulas (27) describing the pulse distortion of seismic reflections from an arbitrarily curved subsurface reflector when subjected to a prestack depth migration in 3‐D laterally inhomogeneous media. This discussion thus relates in particular to such important questions as how to correctly sample signals in the time or depth domain in order to avoid spatial aliasing, or how to stack seismic data without loss of information due to destructive interference of wavelets of different lengths.
Reply by the authors to the discussion by M. M. Roksandić
Authors: Hardage Bob A. , Simmons James L.
Published: October 2002

As in his Discussion of our previous seismic interpretation paper, (Roksandić, 1995), Roksandić offers readers of Geophysics several valuable interpretation concepts in this present comprehensive Discussion. We will address his issues in order and keep our responses as brief as possible.
Reply by the authors to the discussion by A. E. Barnes
Authors: Hardage Bob A.
Published: October 2002

Barnes’s discussion is an excellent summary of the story that we wanted to convey to the seismic interpretation community. His description of additional seismic attributes that react to distortions in seismic reflection waveshape should be a valuable reference for seismic stratigraphers and may indeed have greater impact on the seismic interpretation profession than will our longer paper that focused on only one of the attributes “instantaneous frequency” that can be used to detect stratigraphic and structural discontinuities.
Integrated wide‐angle and near‐vertical subbasalt study using large‐aperture seismic data from the Faeroe—Shetland region
Authors: Fruehn Juergen , Fliedner Moritz M. , White Robert S.
Published: October 2002

Acquiring large‐aperture seismic data (38 km maximum offset) along a profile crossing the Faeroe—Shetland basin in the North Atlantic enables us to use wide‐angle reflections and refractions, in addition to conventional streamer data (0–6 km), for subbasalt imaging. The wide‐angle results are complemented and confirmed by images obtained from the conventional near‐vertical‐offset range. Traveltime tomography applied to the wide‐angle data shows a low‐velocity layer (3.5–4.5 km/s) underneath southeastward‐thinning lava flows, suggesting a 2.5–3.0‐km‐thick sedimentary layer. The velocity model obtained from traveltime tomography is used to migrate wide‐angle reflections from large offsets that arrive ahead of the water‐wave cone. The migrated image shows base‐basalt and sub—basalt reflections that are locally coincident with the tomographic boundaries. Application of a new multiple suppression technique and controlled stacking of the conventional streamer data produces seismic sections consistent with the wide‐angle results. Prestack depth migration of the near‐vertical offsets shows a continuous base‐basalt reflection and a clearly defined termination of the basalt flows.