bluuebunny/crossref_metadata_2025 huggingface dataset:
The Spectrem airborne electromagnetic system—Further developments
Authors: Leggatt Peter B. , Klinkert Philip S. , Hage Teo B.
Published: October 2002

The Spectrem airborne electromagnetic (AEM) system has been in full production in Canada and Africa since 1989. The prototype, built by A‐cubed of Mississauga, Canada, was subject to two major upgrades by Anglo American Corp. staff to produce the Spectrem II instrument that located the Photo Lake and Konuto Lake orebodies in Manitoba, Canada. Subsequent work in tropical Africa has made it desirable to further increase the transmitter power and use greater available computer power in the aircraft to substantially improve the ability of the system to reject sferic noise, although there appears to be a limiting sferic level above which current filtering methods are ineffective. Although designed to detect massive sulfide bodies deep below conductive overburden, techniques have been developed to map the regolith—in particular, to compute resistivity‐depth sections using an approximate but fast method. A comparison is presented of such a section with the same section flown and processed using a frequency‐domain helicopter AEM system. The latest upgrade of the system, Spectrem 2000, with a 50% increase in transmitter power, was completed in October 1999 and is currently at work in northern Canada.
Higher derivatives analysis of 2-D magnetic data
Authors: Abdelrahman E. M. , Abo‐Ezz E. R.
Published: October 2002

This paper presents a new approach for determining the depth of a buried structure from numerical second‐, third‐, and fourth‐horizontal‐derivative anomalies obtained from 2-D magnetic data using filters of successive graticule spacings. The problem of depth determination has been transformed into the problem of finding a solution to a nonlinear equation of the form z = f(z). Formulas have been derived for a horizontal cylinder and a dike. The depths obtained from the second‐, third‐, and fourth‐derivative anomaly values can be used to determine simultaneously the actual depth to the buried structure and the optimum order of the regional magnetic field along the profile. This powerful technique can solve two major potential field problems: regional residual separation and depth determination. The method is applied to theoretical data with and without random errors and is tested on a field example from Arizona.
Application of the perfectly matched absorbing layer model to the linear elastodynamic problem in anisotropic heterogeneous media
Authors: Collino Francis , Tsogka Chrysoula
Published: October 2002

We present and analyze a perfectly matched, absorbing layer model for the velocity‐stress formulation of elastodynamics. The principal idea of this method consists of introducing an absorbing layer in which we decompose each component of the unknown into two auxiliary components: a component orthogonal to the boundary and a component parallel to it. A system of equations governing these new unknowns then is constructed. A damping term finally is introduced for the component orthogonal to the boundary. This layer model has the property of generating no reflection at the interface between the free medium and the artificial absorbing medium. In practice, both the boundary condition introduced at the outer boundary of the layer and the dispersion resulting from the numerical scheme produce a small reflection which can be controlled even with very thin layers. As we will show with several experiments, this model gives very satisfactory results; namely, the reflection coefficient, even in the case of heterogeneous, anisotropic media, is about 1% for a layer thickness of five space discretization steps.
3-D dc resistivity modeling using the sweeping‐seed, conjugate‐gradient method
Authors: Wang Tsili , Mezzatesta Alberto
Published: October 2002

This paper presents a sweeping‐seed, conjugate‐gradient, finite‐difference (SSCG-FD) method for modeling 3-D resistivity responses at multiple source positions. The SSCG method improves the efficiency of the conventional conjugate‐gradient linear solver by exploring the field similarity between adjacent source positions. The method performs the regular conjugate‐gradient (CG) iterations on one (seed) source position out of a group of adjacent source positions while simultaneously providing initial guesses for the other source positions in the group. Once the solution for the seed source position converges, the next source position is selected as the seed position and the regular CG iterations are started with the approximate solution derived earlier. This process continues until all the solutions converge. We have applied the SSCG method to simulate the response of an array resistivity logging device in a 60° deviated well. Our experiments show that for a logging depth interval of 1.5 inches, the SSCG method can reduce the simulation time by 60% compared to the conventional CG method without sacrificing the accuracy of the results.
Modeling of the EM inductive‐limit surface currents
Authors: King Andrew , MacNae James C.
Published: October 2002

The inductive limit of an electromagnetic (EM) response, which is the early time or infinite‐frequency response, may be quickly and efficiently calculated when compared to the effort and time required to calculate a full 3-D EM model. This can be achieved through matrix solution of a simple set of simultaneous conditions that any primary magnetic field must not penetrate the target body. In airborne EM (AEM) data acquisition, hundreds of local anomalies may need to be interpreted each day of flying, and the inductive limit modeling algorithm provides a useful and very fast model for realistic target geometries. In particular, it allows typical single‐peaked field anomalies in the vertical component of fixed‐wing AEM systems to be fitted, using a multi‐faceted solid body. Thin‐plate models, which are commonly used, produce “M-shaped” responses rarely seen in field data.
Potential‐field inversion: Choosing the appropriate technique to solve a geologic problem
Authors: Silva João B. C. , Medeiros Walter E. , Barbosa Valéria C. F.
Published: October 2002

To produce a unique and stable solution in potential‐field interpretation, an inversion method must introduce particular constraints. These constraints will inevitably restrict the type of geological setting where the method may be applied. We present a nonmathematical overview of most stabilizing constraints used in inversion methods. Our purpose is to demonstrate that the inversion results are valuable only if the mathematical stabilizing constraints are translated from the geological setting. We identify five basic types of constraints: (1) lower and upper bounds of parameter estimates; (2) proximity of a parameter estimate to a specified value; (3) proximity between pairs of parameter estimates; (4) concentration of the anomalous source about a geometrical element such as an axis; and (5) source compactness. In practice, if used in isolation, constraints (1), (2), (4), and (5) will not produce geologically meaningful results, regardless of the geological setting of the interpretation area. Constraint (3) may produce geologically meaningful results if the anomalous source has a spatially smooth attribute such as the physical property. We illustrate that constraints 1–4, if used in isolation, cannot delineate the geometry of a simulated sill intruded into a sedimentary basin. The basic constraints may (and should) be combined in inversion to produce geologically meaningful results. We present two examples of effective constraint combination: (1) proximity to a specific value and mass concentration about an axis (used to delineate the thickness variation of a sill intruded in a sedimentary basin) and (2) inequality, proximity of a parameter estimate to a specified value, and proximity between pairs of parameter estimates (used to map a discontinuous basement relief). Usually, the stabilizing constraints are too restrictive to hold at all points of a given geological environment. In this case, we use different constraints in different sub‐areas. Each constraint is based on its compatibility with the actual geology of the subarea.
On the singularities of the gravity field of a homogeneous polyhedral body
Authors: Tsoulis Dimitrios , Petrović Sveto
Published: October 2002

The study of the gravity field of arbitrary polyhedral bodies of homogeneous density has provoked a series of publications over the last decades. Some of the researchers represented an arbitrary three dimensional body in terms of contours obtained by the intersection of horizontal planes with the body.
Stable reduction to the pole at the magnetic equator
Authors: Li Yaoguo , Oldenburg Douglas W.
Published: October 2002

The solution of reduction to the pole (RTP) of magnetic data in the wavenumber domain faces a long standing difficulty of instability when the observed data are acquired at low magnetic latitudes or at the equator. We develop a solution to this problem that allows stable reconstruction of the RTP field with a high fidelity even at the magnetic equator. The solution is obtained by inverting the Fourier transform of the observed magnetic data in the wavenumber domain with explicit regularization. The degree of regularization is chosen according to the estimated error level in the data. The Fourier transform of the RTP field is constructed as a model that is maximally smooth and, at the same time, has a power‐spectral decay common to all fields produced by the same source. The applied regularization alleviates the singularity associated with the wavenumber‐domain RTP operator, and the imposed power spectral decay ensures that the constructed RTP field has the correct spectral content. As a result, the algorithm can perform the reduction to the pole stably at any magnetic latitude, and the constructed RTP field yields a good representation of the true field at the pole even when the reduction is carried out at the equator.
Seismic imaging and velocity analysis for an Alberta Foothills seismic survey
Authors: Yan Lanlan , Lines Larry R.
Published: October 2002

Seismic imaging of complex structures from the western Canadian Foothills can be achieved by applying the closely coupled processes of velocity analysis and depth migration. For the purposes of defining these structures in the Shaw Basing area of western Alberta, we performed a series of tests on both synthetic and real data to find optimum imaging procedures for handling large topographic relief, near‐surface velocity variations, and the complex structural geology of steeply dipping formations. To better understand the seismic processing problems, we constructed a typical foothills geological model that included thrust faults and duplex structures, computed the model responses, and then compared the performance of different migration algorithms, including the explicit finite difference (f-x) and Kirchhoff integral methods. When the correct velocity was used in the migration tests, the f-x method was the most effective in migration from topography. In cases where the velocity model was not assumed known, we determined a macrovelocity model by performing migration/velocity analysis by using smiles and frowns in common image gathers and by using depth‐focusing analysis. In applying depth imaging to the seismic survey from the Shaw Basing area, we found that imaging problems were caused partly by near‐surface velocity problems, which were not anticipated in the modeling study. Several comparisons of different migration approaches for these data indicated that prestack depth migration from topography provided the best imaging results when near‐surface velocity information was incorporated. Through iterative and interpretive migration/velocity analysis, we built a macrovelocity model for the final prestack depth migration.
Groundwater flow after heavy rain in landslide‐slope area from 2-D inversion of resistivity monitoring data
Authors: Suzuki Koichi , Higashi Sadanari
Published: October 2002

In the interest of monitoring the flow of groundwater from heavy rains, we conducted surface resistivity tomography through the vadose zone over several days. The measured data were modeled by 2-D inversion. A laboratory experiment was carried out to estimate the difference in geological characteristics. We measured the effects of groundwater fluctuation using two tank models: one using sand and one using bricks, which represent unconsolidated sediments and jointed rocks, respectively. In the sand model, we succeeded in monitoring water‐level fluctuation as the change in resistivity over time. In the brick model, the water level did not appear as a distinct boundary. Field experiments were carried out in an area in which the near‐surface conditions consisted of heavily weathered rocks with an extensive shear zone several tens of meters thick. Measurements were performed over 42 days, during which we observed changes in resistivity associated with heavy rainfall. It became clear that resistivity decreases of up to 8% were present in highly porous zones in heavily weathered layers. This result suggests a connection between a decrease in resistivity and the infiltration of rainwater. In particular, a pronounced zone of minimum resistivity may indicate infiltration of rainwater into the vadose zone. The relationship between resistivity changes and rainfall was inferred to depend on the permeability of geological structure.
Finite‐element analysis of controlled‐source electromagnetic induction using Coulomb‐gauged potentials
Authors: Badea Eugene A. , Everett Mark E. , Newman Gregory A. , Biro Oszkar
Published: October 2002

A 3-D finite‐element solution has been used to solve controlled‐source electromagnetic (EM) induction problems in heterogeneous electrically conducting media. The solution is based on a weak formulation of the governing Maxwell equations using Coulomb‐gauged EM potentials. The resulting sparse system of linear algebraic equations is solved efficiently using the quasi‐minimal residual method with simple Jacobi scaling as a preconditioner. The main aspects of this work include the implementation of a 3-D cylindrical mesh generator with high‐quality local mesh refinement and a formulation in terms of secondary EM potentials that eliminates singularities introduced by the source. These new aspects provide quantitative induction‐log interpretation for petroleum exploration applications. Examples are given for 1-D, 2-D, and 3-D problems, and favorable comparisons are presented against other, previously published multidimensional EM induction codes. The method is general and can also be adapted for controlled‐source EM modeling in mining, groundwater, and environmental geophysics in addition to fundamental studies of EM induction in heterogeneous media.
HOW CAN GEOPHYSICISTS BEST SERVE?
Authors: Peacock H. B.
Published: October 2002

By this time we should all realize that we are at war—not just our nation or our armed forces but we as individuals are at war. It is only by full realization of this fact that we can hope for a favorable outcome of this struggle for humanity. It behooves each of us, therefore, to consider seriously the question, not am I doing my part, for that allows our efforts to be comparative, but am I doing my best toward the common cause. This is not a question of individual loyalty or patriotism, for loyalty and patriotism must be assumed. Neither is it a question of effort, entirely. It is a question as to how our special experience and training can be most efficient in the war effort.
GEOPHYSICS IN THE MID‐CONTINENT
Authors: Faust L. Y.
Published: October 2002

Exploration of oil, before the advent of scientific methods, was based on the principle: “Oil is where you find it.” The high concentration of geophysical crews in old producing areas suggests we may now be operating on the principle: “Oil is where you FOUND it.”
AN APPLICATION OF SEISMIC SURVEYING TO THE LOCATION OF BAUXITE IN ARKANSAS
Authors: Gillin J. A. , Shock Lorenz , Alcock E. D.
Published: October 2002

The exploration for bauxite has been greatly stimulated by the increased demand for aluminum. The geology of the formation of bauxite in Arkansas is closely related to the syenite exposures on the old land surface at the end of Midway time. A refraction seismic survey mapped the attitude of the high velocity formations identified as syenite or Paleozoic rocks. By combining this map with a map of the Midway from scattered core holes, a map of the old land surface at the end of Midway time was obtained. By interpreting this map, future explorations by core drills can be guided to the most favorable locations.
NOTE ON THE VARIATION FROM EQUATOR TO POLE OF THE EARTH’S GRAVITY
Authors: Hammer Sigmund
Published: October 2002

The various physical factors constituting the increase in the normal acceleration of gravity of the earth from equator to pole are analyzed mathematically. The magnitudes of the essential factors are (1) centripetal acceleration +3.39 gals, (2) decrease in distance from the center of the earth—the so‐called “Free Air” effect +6.63 gals, and (3) mass‐shape effect due to polar flattening −4.85 gals.
Seismic Effects of Quarry Blasting. J. R. Thoenen and S. L. Windes. Bureau of Mines Bulletin No.442; Damage From Air Blast. Progress Report I. S. L. Windes. Bureau of Mines Report of Investigations No. 3622; Design and Calibration of Air Blast Meters. A. T. Ireland. Bureau of Mines Technical Paper No. 635.
Authors: Widess M. B.
Published: October 2002

Bulletin No. 442 summarizes the results of investigations performed by the Bureau of Mines since 1930 relative to damage to buildings from quarry blasts, a study which was undertaken to supply basic data in settlement of damage claims.
A RAPID METHOD FOR MEASURING THE PROFILE COMPONENTS OF HORIZONTAL AND VERTICAL GRAVITY GRADIENTS
Authors: Heiland C. A.
Published: October 2002

The trend in gravity exploration in the past years indicates the rather remarkable fact that a method of low resolving power (the gravity meter) has replaced one of higher resolving power (the torsion balance). This is entirely due to the superior speed of the former and suggests an instrument and procedure in which observation time is reduced by (1) reduction in number of quantities measured; (2) use of a reference direction near that of the maximum effect; (3) elimination of the torsionless position as unknown; (4) reduction in period, with compensating increase in optical sensitivity; (5) stabilization of thermal conditions. These objectives are attained by (1) measuring the profile components of gradients and curvature values, preferably at right angles to the assumed strike; whereby, for an ideal two‐dimensional feature, also the vertical gravity gradient is obtained, and the vertical and horizontal gravity components may be calculated by integration; (2) by holding the torsionless position constant with temperature control; (3) by decreasing the period and observation time to 3–4 minutes, and (4) by using a beam arrangement which will give the gradient in only one azimuth, and the profile gradient of the horizontal gravity component in a second azimuth if desired. Latitude and terrain corrections are also somewhat simplified by the proposed procedure.
NOMOGRAMS FOR COMPUTING TIDAL GRAVITY
Authors: Elkins Thomas A.
Published: October 2002

It is possible, by the use of a nomogram devised by Dr. James B. Friauf (Journal of the Franklin Institute, Vol. 232, pp. 151–174, 1941) for the solution of spherical triangles, to make simple nomograms for the computation of tidal gravity. Two types of such nomograms are explained and illustrated. The first is for computations at a fixed latitude on the earth; the second is for computations at a fixed point. With the second type the computation of tidal gravity is reduced to a simple routine matter, especially if The American Air Almanac is used as the source of the astronomical data needed. The accuracy of these nomograms is studied and a comparison is made of this new method with that recently devised by Dr. Adler (Geophysics, Vol. 7, pp. 35–44, 1942).
UTILITY OF THE ELECTRIC METHODS IN GEOPHYSICAL EXPLORATION
Authors: Evjen H. M.
Published: October 2002

The theoretical possibilities and the physical background of electric methods are considered in a qualitative manner. A rigorous mathematical treatment of the problem is at present a practical impossibility because the electric properties of the ground are not sufficiently well known. By analogy with the seismic method of exploration, however, where the rigorous physical optics governing the transient generated by the exploding dynamite, and the propagation of said transient in general, have been sadly neglected, this lack of mathematical rigor should not be a serious stumbling block. It is concluded that the possible utility of electric methods can be best appraised in terms of the known utility of electric well logging. This possibility, according to the author’s experience, can be attained only by a departure from the traditional electric methods. One such departure is described in general terms.
MULTIPLE BRANCHES IN SEISMIC REFLECTION‐TIME SURFACES
Authors: Widess M. B.
Published: October 2002

When for a given shot‐point and receptor more than one reflection is recorded from a given reflecting bed, the corresponding reflection‐time surface may be said to be multiply branched. Although multiple branches are difficult to evaluate, serious errors in mapping may result when their presence is overlooked. A method is therefore proposed for the interpretation of field data to account for this phenomenon. The conditions for branching and its properties are considered, showing that branches must occur in odd‐numbered sets and in certain type forms, which are illustrated.
SELECTION OF COMPUTATIONAL METHODS FOR SEISMIC PATHS
Authors: Alcock E. D.
Published: October 2002

Due to the decrease in magnitude of structures being sought in seismic exploration, the selection of the most accurate method of computation assumes increasing significance. The underlying assumptions and characteristics of several common computational methods are discussed and an analysis is made for a specific case.
GEOPHYSICAL EXPLORATION FOR STRATIGRAPHIC OIL TRAPS
Authors: Adler Joseph L.
Published: October 2002

Stratigraphic traps are defined and classified and the applicability of various geophysical methods for the discovery of each of the different types of stratigraphic traps is discussed. Some undeveloped techniques are suggested in this connection as subjects for research.
ANALYSIS OF GASES BY THE SCATTERING OF ELECTRONS
Authors: West S. S.
Published: October 2002

Methods are described for the analysis of mixtures of hydrocarbon gases by measurement of the probability of scattering of electrons as a function of energy of impinging electron, angle of scattering, or loss of energy during collision. By such means one can determine carbon‐hydrogen ratio, proportions of single and multiple carbon‐carbon bonds, and concentrations of gases in a mixture.
FREQUENCY DISCRIMINATION IN THE LOW VELOCITY ZONE
Authors: Shugart T. R.
Published: October 2002

The various factors controlling the “character” of reflection seismograph records are so numerous and so complex that it is very difficult to isolate and investigate each of them experimentally. One of these factors is the frequency discrimination due to multiple reflections in the low velocity zone (“weathered layer”). With a few simplifying assumptions this problem reduces to one already treated in the classical theory of acoustics. It is shown that for a given weathering thickness the transmission is a periodic function of frequency. The ratio of the maximum to the minimum energy transmission is nearly equal to the square of the ratio of the specific acoustic impedance of the high velocity to that of the low velocity material. The phase lag due to the presence of the weathered layer is shown to be a non‐linear function of the weathering thickness for any given frequency. Graphs of transmission as a function of frequency are shown for various thicknesses of the low velocity zone.
A NOTE ON THE READING OF SEISMOGRAMS
Authors: Horton C. W.
Published: October 2002

It is shown that a quick appraisal of the error in reading seismograms can be made by a statistical count of the occurrence of the third decimal. A method of interpretation is shown by analyses of five samples.
Quantifying the uncertainty in an AVO interpretation
Authors: Houck Richard T.
Published: October 2002

Lithologic interpretations of amplitude variation with offset (AVO) information are ambiguous both because different lithologies occupy overlapping ranges of elastic properties, and because angle‐dependent reflection coefficients estimated from seismic data are uncertain. This paper presents a method for quantifying and combining these two components of uncertainty to get a full characterization of the uncertainty associated with an AVO‐based lithologic interpretation. The result of this approach is a compilation of all the lithologies that are consistent with the observed AVO behavior, along with a probability of occurrence for each lithology. A 2‐D line from the North Sea illustrates how the method might be applied in practice. For any data set, the interaction between the geologic and measurement components of uncertainty may significantly affect the overall uncertainty in a lithologic interpretation.
High‐resolution images of shallow aquifers—A challenge in near‐surface seismology
Authors: Francese Roberto G. , Hajnal Zoltan , Prugger Arnfinn
Published: October 2002

A near‐surface multifold high‐resolution seismic reflection experiment was conducted in the vicinity of the waste management area of a potash mine in western Canada. A buried channel was identified in the data, and the stratigraphy of the Quaternary infill of this structure was mapped. The spatial extent of several prominent gravel‐sandy aquifers, which represent the hydrogeologic framework of the region, was outlined by the survey. The seismic signatures also established the hydraulic independence of three major aquifers along the survey line.

The complex heterogeneous lithology of the surface cover limited effective elastic‐wave generation to surface sources. This geologic framework also caused propagation of strong diverse coherent‐noise patterns which severely degraded reflected signal. The suppression of those overhelming interfering events required the design of noise‐specific filters and their sequential multistep implementations. Results of forward modeling of background geologic information were crucial factors in the design of the data acquisition program and preliminary choices of the processing parameters, and (along with borehole data) were the primary guidance in the geologic interpretation of the final seismic section.

Fundamental procedures were developed for mapping of glacial tills in the Western Canadian Basin, techniques that can be applied in other regions with similar near‐surface glacial stratigraphy. The experiment revealed that even closely spaced borehole information could never duplicate the detail of the subsurface images of the seismic data.
Seismic inversion for the parameters of two orthogonal fracture sets in a VTI background medium
Authors: Bakulin Andrey , Grechka Vladimir , Tsvankin Ilya
Published: October 2002

Characterization of naturally fractured reservoirs often requires estimating parameters of multiple fracture sets that develop in an anisotropic background. Here, we discuss modeling and inversion of the effective parameters of orthorhombic models formed by two orthogonal vertical fracture sets embedded in a VTI (transversely isotropic with a vertical symmetry axis) background matrix.

Although the number of the microstructural (physical) medium parameters is equal to the number of effective stiffness elements (nine), we show that for this model there is an additional relation (constraint) between the stiffnesses or Tsvankin's anisotropic coefficients. As a result, the same effective orthorhombic medium can be produced by a wide range of equivalent models with vastly different fracture weaknesses and background VTI parameters, and the inversion of seismic data for the microstructural parameters is nonunique without additional information. Reflection moveout of PP‐ and PS‐waves can still be used to find the fracture orientation and estimate (in combination with the vertical velocities) the differences between the normal and shear weaknesses of the fracture sets, as well as the background anellipticity parameter ηb. Since for penny‐shaped cracks the shear weakness is close to twice the crack density, seismic data can help to identify the dominant fracture set, although the crack densities cannot be resolved individually.

If the VTI symmetry of the background is caused by intrinsic anisotropy (as is usually the case for shales), it may be possible to determine at least one background anisotropic coefficient from borehole or core measurements. Then seismic data can be inverted for the fracture weaknesses and the rest of the background parameters. Therefore, seismic characterization of reservoirs with multiple fracture sets and anisotropic background is expected to give ambiguous results, unless the input data include measurements made on different scales (surface seismic, borehole, cores).
Three‐dimensional induction logging problems, Part 2: A finite‐difference solution
Authors: Newman Gregory A. , Alumbaugh David L.
Published: October 2002

A 3‐D finite‐difference solution is implemented for simulating induction log responses in the quasi‐static limit that include the wellbore and bedding that exhibits transverse anisotropy. The finite‐difference code uses a staggered grid to approximate a vector equation for the electric field. The resulting linear system of equations is solved to a predetermined error level using iterative Krylov subspace methods. To accelerate the solution at low induction numbers (LINs), a new preconditioner is developed. This new preconditioner splits the electric field into curl‐free and divergence‐free projections, which allows for the construction of an approximate inverse operator. Test examples show up to an order of magnitude increase in speed compared to a simple Jacobi preconditioner. Comparisons with analytical and mode matching solutions demonstrate the accuracy of the algorithm.
On: “THE EFFECT OF MOISTURE ON THE DIRECT CURRENT RESISTIVITIES OF OIL SANDS AND ROCKS”
Authors: Wyckoff R. D.
Published: October 2002

The authors do not explain certain details in the technique of measuring with d.c. which would be of interest in view of the well‐known difficulties usually encountered. Moreover, there are no data from duplicate observations given whereby the reader can judge the probable accuracy obtained. However, in view of the senior authors’ wide experience in resistivity measurements this reviewer assumes that the question of technique is not involved in the results.
On: “Shallow Stratigraphic Variations over Gulf Coast Structures”
Authors: Weatherby B. B.
Published: October 2002

One of the purposes of a scientific paper is to provoke thought and discussion and Dr. Rosaire’s paper has admirably performed these purposes. Moreover, he offers a plausible explanation for some effects which have been found with the electrical transient method—an explanation which does not do violence to the knowledge of and experience with previously existing electrical current methods. How it was possible to pick up the effects of deeply buried structure and faults with such current and potential spreads as are used in transient work was a most difficult question to answer and the feat, if possible, amounted almost to legerdemain. By bringing the effect of deeply buried structure much nearer the surface with his zone of mineralization Dr. Rosaire has clarified the problem greatly. Even so there may be some skeptics who will wonder whether it is possible, after taking into account the effects of surface and near surface conductivity changes which most certainly will profoundly alter the form of the electrical transient to interpret in terms of geological structure any residual effect which may be due to somewhat deeper physical changes.
TO THE EDITOR
Authors: Rosaire E. E.
Published: October 2002

In his article on “Concentrations of Hydrocarbons in the Earth, Geophysics, July 1939, Mr. E. McDermott says “it is known that ethylene in the presence of chlorine, and especially aluminum chloride, will polymerize even at a faster rate.” On page 198, the statements are made “based on the assumption that the wax is formed from ethylene in the presence of chloride and sunlight, this is not surprising” and, also, “That ethylene in the presence of chlorine will polymerize was demonstrated in the laboratory by bubbling the gas through a concentrated salt solution for two days.”
On: Association of Magnetic and Density Contrasts with Igneous Rock Classifications by L. L. Nettleton and T. A. Elkins
Authors: Reno Duane H.
Published: October 2002

This excellent article is an ingenious approach to a very pertinent problem. However, there are many pitfalls in the computations of “norms” which should be pointed out. In this computation, it is largely a matter of chance that there is any resemblance between the calculated minerals—the “norms”—and of the actual minerals—the “mode.” It should be noted that the physical properties of a rock are determined by the mode rather than the norm. In Clark’s Data of Geochemistry (Frank Wigglesworth Clark, The Data of Geochemistry, U.S.G.S. Bul. 770 (1924), pp. 432) the mineralogic irregularities introduced by the calculations of norms are illustrated by Table I.
Author’s Reply
Authors: Ion D. C.
Published: October 2002

I was and am grateful for the support given to my thoughts by Dr. Woods and, in his enlarged written comments, by Dr. Richards.
Solicited Reply to Professor Amalendu Roy
Authors: Nettleton L. L.
Published: October 2002

Professor Roy’s letter points out the mathematical similarity of expressions for “residual” values and those for derivatives and that what appear to be essentially the same quantities have been expressed, in the geophysical literature, in different units.
AFMAG—Airborne and Ground: Author’s Reply
Authors: Ward S. H.
Published: October 2002

Dr. Holzer’s comments are very appropriate and much appreciated.
Discussions on Induced Polarization: Authors’ First Reply
Authors: Frische Richard H. , Von Buttlar Haro
Published: October 2002

Dr. James R. Wait invited us to comment on his discussion of our paper in the July, 1957, issue of Geophysics. We are grateful for this opportunity, since a clarification of some basic concepts appears necessary.
Reply by author to discussion by Michael Schoenberger.
Authors: Burg John P.
Published: October 2002

The principal assertion of Schoenberger’s discussion appears to be that (3), which is correctly derived from equations in Section IV and corresponds to a space shift, should instead be written as (4), corresponding to a space sample. However, the space‐convolution operator corresponding to a seismometer is indeed meant to be a space‐shift operator. An array of seismometers is used as a weighted sum of space‐shift operators, just as a time‐domain, sample‐point operator is made up of a weighted sum of time‐shift operators. Equation (5.2), which Schoenberger indicates as being related to his (4), actually comes from (3) with x and y set to zero.
Reply by author to Discussion by Cheh Pan
Authors: Robinson J. C.
Published: October 2002

This discussion seems nonobjective from start to finish from a technical standpoint, and it is especially unclear in the discussion of so called “reverse representation” and in the general discussion toward the end. It appears that the primary objection in the discussion to the subject paper is the representation of a seismic data array, as viewed through a finite two‐dimensional data window, as a periodic, discrete function. The data were so represented in order that use could be made of the “fast Fourier transform” algorithm (see Cochran et al, 1967). It can be shown that the differences in the continuous, 4 periodic Fourier transform of a continuous, finite function (with zero value outside of the data window) and the discrete, periodic Fourier transform of the discrete, periodic representation of that function are insignificant at the discrete points in the transform domain if aliasing is not a problem. Due to the length of the subject paper, the details of this aspect were not included, but a thorough discussion of it can be found in Appendix A of Robinson “1968”. I also strongly recommend that a study of the entire issue of the IEEE Transaction on Audio and Electroacoustics, v. AU‐15, no. 2, June, 1967, will illuminate the subject. Now, let me be more specific in my reply to the discussion, in much the same as presented in the discussion.
Dimensions and Units of Electromagnetic Quantities: Author’s Reply
Authors: Baker Glenn J.
Published: October 2002

In the course of theoretical work and experimentation in the field it was my experience that the use of the m.k.s. system of units resulted in a saving of labor and also helped to clarify some concepts. The purpose of preparing my paper was, as stated, to present a conversion table and to bring to the attention of others the advantages of the m.k.s. system. These considerations, and an enterprising program committee, constituted my only “irresistible urge to publish something on the subject.”
Reply to Comments of Mr. P. L. Lawrence
Authors: Savit Carl H.
Published: October 2002

Since at least one erroneous conclusion has apparently been drawn from our paper, “The Moveout Filter,” and this by an obviously non‐casual reader, we must apologize for having failed to set forth clearly the viewpoint and fundamenal hypothesis of this paper.
Reply to author to Discussion by Ronald Green
Authors: Dampney C. N. G.
Published: October 2002

The improvements found by Green to my paper are noted with considerable interest. An important advantage of Green’s technique is that by specifying the smallest anomaly that can be represented, the “ill‐posed” nature (e.g., Meshcheryakov, 1969) of determining a source for a given potential field is overcome.
Reply by author to the discussion by S.‐E. Hjelt
Authors: Åm Knut
Published: October 2002

We fully agree with James Clerk Maxwell and Sven‐Erik Hjelt that long rods should be used if one wishes to measure accurately susceptibilities, because this will eliminate the effect of demagnetization. However, this holds only if the material under measurement is homogeneous with one and the same susceptibility throughout. If the sample is not homogeneous, the magnetization will be inhomogeneous, and we are back to where we started.
Reply by S. A. Arcone to D. V. Thiel
Authors: Arcone Steven A.
Published: October 2002

I apologize for the confusion rendered by discussion of Stratton’s (1941) integral equation [equation (17)]. However, I believe the mathematical handling of the discontinuity is correct as discussed on p. 942 in conjunction with equations (20) and (21).
Reply by authors to H. Rüter and R. Schepers
Authors: Schoenberger M. , Levin F. K.
Published: October 2002

We agree with Dr. Rüter and Dr. Schepers—there are methods of generating synthetic seismograms that are less sensitive to the choice of sample rate than that obtained by summing transit times. We tried to make that clear in our paper, but apparently failed.
Reply to H‐M Maurer by the authors
Authors: Bartel L. C. , Jacobson R. D.
Published: October 2002

In our reply to the discussion by Szarka (this issue), we examined the near‐field correction scheme for two‐layer cases with a basal layer more conducting and less conducting than the surface layer where we only considered the broadside configuration. Maurer points out that for the collinear configuration the correction scheme does not produce an accurate correction particularly for the less conducting basal layer case. We wish to comment on the correction scheme.
Reply by the authors to R. P. Singh
Authors: Shen Liang C.
Published: October 2002

We disagree with Singh on all three points that he is attempting to clarify. First, we stand by our original statement about the low‐frequency tool. Singh’s statement seems to suggest that it is impossible to measure the dielectric constant below 10 MHz. We stated that at the conventional induction tool or resistivity tool frequencies (20 kHz or below) it is rather difficult to measure accurately the dielectric constant of the formation. We prefer not to say “impossible” because there has been progress made by researchers at Exxon and Shell to measure dielectric properties of rocks in the [Formula: see text] range.
Reply by the author to C. F. O’Brien
Authors: Tucker Paul M.
Published: October 2002

I am particularly pleased with O’Brien’s constructive comments on “Techniques of Hand Contouring”, which appear in his discussion of my article on contouring. His is an excellent contribution to contouring technology. O’Brien provides a systematic approach to contouring which should be useful to less experienced interpreters. It is a well‐defined step‐by‐step approach and a most useful addition to the art of contouring. May I suggest that O’Brien expand his subject of hand contouring and submit it to Geophysics for publication. His ideas would be a welcome addition to the subject.
Reply to N. C. Steenland by the authors
Authors: Pilkington M. , Crossley D. J.
Published: October 2002

In response to Steenland’s points we offer some comments. The spectral division results of Figure 6 reproduce the original model to a greater degree of accuracy due to the longer (implicit) filter length used (256 points) compared to 37 points for the Kalman filter run of Figure 4b. Obviously, the spectral division method advocated by previous workers (e.g., Grant, 1973) could have been used to calculate an apparent susceptibility map similar to Figure 9. However, it was our intent to provide an alternative approach that would not suffer from the limitations inherent to processing in the frequency domain.
Reply by the authors to E. Szaraniec
Authors: Niwas Sri , Israil M.
Published: October 2002

We thank Dr. Szaraniec for taking a keen interest in our paper, but we are aware of the work of Santini and Zambrano (1981), Kumar and Chowdary (1982), and subsequent comments (Santini and Zambrano, 1982; Straub, 1984; Szaraniec, 1982, 1984.) The papers by Santini and Zambrano (1981) and Kumar and Chowdary (1982) are in fact referenced in our earlier paper (Niwas and Israil, 1987) dealing with resistivity‐data interpretation in the kernel domain.
Reply by the author to Doo Jung Jin
Authors: Ziolkowski A. M.
Published: October 2002

I thank Doo Jung Jin for pointing out that my remarks on homomorphic deconvolution were badly explained and I am very glad to have this opportunity to clarify them.
Reply by the author to J. H. McBride
Authors: Louie John N.
Published: October 2002

The discovery of structural features within the deep crust across the United States by the COCORP consortium through the use of conventional stacking is a highly significant achievement. The data sets they recorded are a valuable national resource that can be analyzed in many ways to address diverse geologic problems. Conventional stacking of the data provided initial glimpses of the nature of the crust. Other types of analysis may, however, be required to explain particular aspects of the data and to resolve more difficult structural problems.
On: “The shape of a marine streamer in a cross current” by P. M. Krail and H. Brysk (GEOPHYSICS, 54, 302–308, March 1989).
Authors: Johnson Daniel
Published: October 2002

The work of these authors represents a substantial advance in the technology of marine‐streamer position measurement, by providing a physically realistic fitting and interpolation model. Their work, however, confuses the issue of the equilibrium streamer shape, since they claim that “…currents caused the streamer to drift out of the line of travel of the vessel in a curved shape” (p. 302).
Reply by the authors to Douglas W. McCowan
Authors: Mithal Rakesh , Vera Emilio E.
Published: October 2002

In his discussion, McGowan directs his attention exclusively to which method should be used to produce a plane-wave decomposition of point-source seismic data. Although the choice of method is an important point, it was not the main emphasis of our paper which, as its title indicates, was the comparison between plane-wave decomposition (cylindrical slant stacking) and simple slant stacking. We demonstrated the differences between these two processes and clearly indicated the necessity of using cylindrical slant stacking in order to get the correct plane-wave reflection response of point-source data. McGowan criticizes our method because it makes use of the standard asymptotic approximation of the Bessel function [Formula: see text] and considers only outward traveling waves. In our paper we acknowledged that these simplifications do not produce accurate results for ray parameters near zero and explicitly mentioned the method of Brysk and McGowan (1986) as a suitable alternative to deal with this problem.
Reply by the authors to C. I. Blyth
Authors: Lishman F. W. , Christos M. N.
Published: October 2002

We agree with the main point of Cameron Blyth’s discussion. A data processing approach is a more rigorous method for solving many of the problems associated with subsea erosion than a nonprocessing approach and, therefore, should be included when identifying drilling sites in areas where sea‐floor erosion is evident. However, because data processing is expensive, we want to stress the importance of our nonprocessing solution, both as a reconnaissance tool for minimizing the amount of data processing needed, as well as a building block for any subsequent data processing that is done.
Reply by the author to Ru‐Shan Wu
Authors: Lerche I.
Published: October 2002

Wu provides above a discussion of my paper (Lerche, 1986). Several points are relevant.
Reply to discussion by G. P. Hersir, J. O. Nielsen, and L. B. Pedersen by authors
Authors: Bhimasankaram V. L. S. , Nagendra R. , Rao S. V. Seshagiri
Published: October 2002

We are thankful to Hersir et al for bringing to our notice the work of Sharma et al (1970) along similar lines. We have considered our results and conclusions in view of the interesting discussions presented by Hersir et al, and in comparison with the method of interpretation suggested by Sharma et al and their results. We give below our replies to the comments.
Reply to K. Kunaratnam by the author
Authors: Won I. J.
Published: October 2002

It appears to me that the comment raises two issues: (1) The inversion method which I devised will not work if the dike has permanent magnetization in an unknown direction; and (2) there is no need of treating two linear parameters, namely the susceptibility and the datum, on the same footing as the four remaining nonlinear parameters.
Reply by J. A. Pitkin and J. S. Duval to W. C. Kellogg
Authors: Pitkin J. A. , Duval J. S.
Published: October 2002

We thank Mr. Kellogg for his generally complimentary view of our paper, but we would like to discuss the subject of analog versus digital recording of aerial spectrometric data.
Reply by the author to N. C. Steenland
Authors: Hammer Sigmund
Published: October 2002

Dr. Steenland’s principal criticism arises from an unfortunate overstatement, in my paper, of the precision and anomaly resolving power of the Carson Airborne Gravity method. This criticism is well deserved. My calculation of the probable error of an airborne gravity measurement was based on many thousands of Δg gravity differences at grid‐line intersections, but it made the implicit assumption that the two reported gravity values at each grid intersection were independent. This is incorrect because the grid system of intersection differences is used for controls in the data processing. A realistic value for the probable error of an airborne gravity measurement is of the order of 1 mgal (standard deviation of 1.5 mgal). The associated resolving power for gravity anomalies, above this magnitude, is of the order of 2 to 3 miles (3 to 5 km) at flight speed of 50 knots. Smaller anomalies may be resolved at lower speeds.