bluuebunny/crossref_metadata_2025 huggingface dataset:
DISCUSSION OF “AN EVALUATION OF BASEMENT DEPTH DETERMINATION FROM AIRBORNE MAGNETOMETER DATA” BY PETER JACOBSEN, JR.
Authors: Ramaswamy G.
Published: October 2002

Mr. Jacobsen’s article and the accompanying discussions on the scope and outlook for the current interpretational practices in aeromagnetic surveys are very timely and deserve the attention of all geophysicists as well as exploration management. Since World War II the aeromagnetic surveys have replaced the ground magnetic surveys as a reconnaissance exploration tool chiefly because of the former’s rapidity and cheapness in data‐gathering. In this process, however, the aerial technique has lost one advantage going with the ground surveys. In land surveys the practice has been to make simultaneous magnetic and gravity observations and the interpretations of basement features are made from these paired observations. I believe that the absence of concurrent information on gravity has been a real handicap with aeromagnetic interpretation in reliably locating basement features in the early stages of exploration. Perhaps the present aerial gravitymeter instrumentation can be soon improved to desired sensitivity for exploration so that simultaneous gravity‐magnetic observations from the air will be possible. In large unexplored sedimentary areas the gravity data are as valuable, sometimes more, to the interpretation of magnetic data as a knowledge of the magnetic properties of any out‐cropping rocks.
On: MINIMUM PENDULUMS
Authors: Beck A. E. , Valliant H. D.
Published: October 2002

In a discussion of the Gulf pendulums in two recent textbooks (Heiskanen and Vening‐Meinesz, 1958; Dobrin, 1960) it is stated that for a pendulum of the minimum type, the point of suspension is so located with respect to the center of mass that the period is least altered by any changes in dimensions resulting from thermal expansion.
REMANENT MAGNETISM AS A CONTRIBUTOR TO SOME AEROMAGNETIC ANOMALIES
Authors: Books Kenneth G.
Published: October 2002

An investigation of some aeromagnetic anomalies in north‐central Montana shows that remanent magnetization is the dominant factor in anomalies over some rocks and a contributing factor in anomalies over others. In volcanic rocks, remanence is commonly south‐seeking down and has an intensity approximately ten times the induced intensity of magnetization. Remanence is shown to be the dominating factor in anomalies over volcanic rocks by the agreement between the profiles of an observed anomaly over a volcanic rock body and an anomaly calculated from remanence data for the same body. In intrusive rocks, the remanence is north‐seeking down and comparable in intensity to the induced intensity of magnetization. That remanence also contributes to the anomalies over some intrusive rocks is shown by the agreement between the profiles of an observed anomaly over an intrusive body and an anomaly calculated from the resultant of induced and remanent magnetizations. A consistent relationship between the axis of the anomaly, or direction between the anomaly high and low, and the horizontal direction of magnetization suggests that the direction of the anomaly axis can be used to indicate the horizontal direction of magnetization in this area.
On: THE ROLE OF THE SURFACE ELECTRICAL METHODS OF GEOPHYSICAL PROSPECTING IN THE PETROLEUM INDUSTRY
Authors: Yungul S. H.
Published: October 2002

Those who have broad backgrounds in exploration geophysics have been saying that the electrical methods in general could be profitably employed by the petroleum industry, that they promise major break‐throughs in the future, and that it is regrettable that we are not making use of them in the U.S. The surface electrical activity for petroleum in the U.S. is so small that it does not make its way into the statistics. It is appreciable in the eastern hemisphere. Outside the USSR, in the eastern hemisphere, the electrical activity in 1958 and 1959 was of the order of 150 crew months per year (The Oil and Gas Journal, 1959; Patrick, 1960).
On: “GRAVITY AND AEROMAGNETIC EXPLORATION IN THE PARADOX BASIN,” BY NELSON C. STEENLAND (GEOPHYSICS, FEBRUARY, 1961, PP. 73–89)
Authors: Joesting H. R.
Published: October 2002

It is generally conceded that estimates of depths to magnetic sources using the methods of Vacquier et al (1951) may be 10 percent or more in error, even for anomalies well‐suited to depth estimates. Where conditions are less favorable—for example, where the anomaly‐producing masses differ in magnetization or shape from the assumed uniformly magnetized vertical prisms—the errors may be much larger. In my opinion, therefore, Dr. Steenland has attempted to obtain more from the magnetic data than is warranted.
On: “EFFECT OF POROSITY, GRAIN CONTACTS, AND CEMENT ON COMPRESSIONAL WAVE VELOCITY THROUGH SYNTHETIC SANDSTONES” BY ANDRIS VIKSNE, JOSEPH W. BERG, JR., and KENNETH L. COOK (GEOPHYSICS, FEBRUARY, 1961, PP. 77–84).
Authors: Stephenson L. P. , Tooley R. D
Published: October 2002

The experimental data reported by Viksne, Berg, and Cook have been reinterpreted in the light of a more rigorous analysis of the variables. Since the experimentally determined velocities of compressional waves in the synthetic cores can be described analytically in terms of the porosity and cement content alone with an accuracy comparable to that expected of the experimental measurements, it is not possible to establish what influence manufacturing pressure may have on velocity (other than that resulting from its effect on porosity). In particular, it cannot be said that the reported experiments have demonstrated that grain‐to‐grain contacts affect the velocity in any way. These conclusions are substantially different from those drawn from the same data by Viksne, Berg, and Cook.
THE “SHOT POPPING” METHOD AND APPLICATION OF DIRECTIONAL SCANNING TO INTERPRETATION
Authors: England C. M.
Published: October 2002

In areas where terrain is rugged, or in water‐covered terrain as in Alaska and Canadian Muskeg, the heavy weight‐dropping equipment cannot be used. Under these conditions the weight‐dropper or “Stomper” can be replaced by the firing of small explosive charges in shallow holes while the regular weight‐dropping recording equipment is retained. This method has been called “Shot Popping.” This paper discusses equipment, arrays of shots and detectors, compositing methods, and coverage per day as compared to weight‐dropping and conventional seismic reflection methods. In areas where reflectors are steeply dipping, the compositing over long surface spreads necessary to attenuate noise can also result in distortion and cancellation of the desired reflection signals. Described is a method of directional scanning to help enhance reflection signals under steep‐dip conditions. Examples of records obtained by “Shot Popping” and directional scanning are presented.
A STATISTICAL COMPARISON OF FIVE GEODETIC GRAVIMETERS
Authors: Behrendt John C.
Published: October 2002

A statistical comparison of five LaCoste and Romberg geodetic gravimeters over the 5,129‐mgal North American calibration range shows an estimated standard deviation of 0.17 mgal for the set with a 90‐percent confidence limit of 0.28 mgal. Values for each meter, means and estimated standard deviations are presented for each of the 24 stations included in the set.
On: “THE EFFECT OF SHOTPOINT MEDIUM ON SEISMIC COUPLING,” BY W. M. ADAMS AND L. M. SWIFT (GEOPHYSICS, DECEMBER, 1961, PP. 765–771).
Authors: Kisslinger Carl
Published: October 2002

In their work, Adams and Swift tested the effect of the medium on the resulting seismic signal by observing the signals produced by 100–1,000 lb of granular TNT fired in volcanic tuff and salt. The charges were tightly tamped and placed so that the ambient stress levels were roughly the same in the two media. Their conclusion was that in the frequency range of 30–60 cps, which included the dominant energy, the signal strength in the tuff was 1.6±0.4 times that in the salt.
PROJECT GNOME VOLUNTEER SEISMOLOGICAL TEAMS
Authors: Westhusing Keith
Published: October 2002

Forty‐three volunteer seismological teams participated in monitoring the three‐kiloton nuclear detonation of December 10, 1961. Nearly all the teams were within a radius of 600 miles of GNOME ground zero, located approximately 25 miles southeast of Carlsbad, New Mexico, and most were in or near the Delaware Basin. Standardized methods of calibrating the seismic equipment and synchronizing time references were required to obtain accurate data on the amplitudes and travel times of seismic waves. Calibration of seismograph systems was done by a weight‐drop method. A voice count before and after the shot, as well as audible pulses each second, were transmitted over a special broadcast network to synchronize the timing. Gain settings were based on data from a previous nuclear detonation of comparable size. Seismic wave travel times for the Central United States were used to estimate times of first arrival at each team. As evidenced by the quality of records and considering the limited data available upon which to base operational procedures, the volunteer‐team monitoring of the GNOME event was a successful effort.
GEOPHYSICAL EXPLORATION—AUSTRALIA
Authors: Thyer R. F.
Published: October 2002

The history of geophysical exploration as applied to mineral deposits in Australia is described briefly. The individual methods which have been used are discussed, along with special cases in which they have given useful results. Gravity methods have been used extensively in investigating coalfields, and this work is described. A remarkable increase in the use of geophysical methods in the search for oil over the past two years has been due largely to the encouragement given by means of government subsidy. Under the Petroleum Search Subsidy Act, the Australian government pays part of the cost of approved geophysical surveys and the results are published. Some of the more important results obtained in the southern part of the Perth Basin, Western Australia, and in the Surat Basin, Queensland, where oil has been discovered recently are described.
AN INVESTIGATION OF THE AEROMAGNETIC MAPS OF THE MIDDLEBACK RANGE AREA OF SOUTH AUSTRALIA
Authors: Mumme W. G.
Published: October 2002

Measurement of the magnetic properties of many of the rock types found in the Middleback Range area of South Australia were carried out for the reason that such measurements might aid in the interpretation placed on the airborne and other data obtained by the magnetic method of prospecting in the area.
REPLY OF MESSRS I. D’ERCEVILLE AND G. KUNETZ TO THE DISCUSSION BY J. T. WEAVER
Authors: d’Erceville I. , Kunetz G.
Published: October 2002

The authors have read Mr. Weaver’s remarks with great interest. They would like to point out that their aim has not been to clear up the still debated question of how to define the primary field that generates the telluric currents, but to study the effect of a fault on a special type of field, complying with the laws of electromagnetism (neglecting the displacement currents, as is usual at the frequences considered) and differing little from actual telluric currents.
AN EXAMPLE OF THE INFLUENCE OF REMANENT MAGNETIZATION ON MAGNETIC INTENSITY MEASUREMENTS
Authors: Hays Walter W. , Scharon LeRoy
Published: October 2002

Near Ironton, Missouri, measurements of the relative vertical intensity of the earth’s magnetic field were made along an 1,800‐ft line over an intrusive rhyolitic rock unit. Sharp variations of the order of 2,500 gammas were obtained along the sample line. These variations cannot be explained on the basis of induced magnetization alone. However, paleomagnetic measurements made on samples collected coincident to the line of magnetic measurements reveal great inhomogeneities in the magnetic properties: the magnetic susceptibility varies between [Formula: see text] and [Formula: see text] units; the ratio of the intensity of remanent magnetization to the intensity of induced magnetization varies from 0.21 to 26.91; the direction of remanent magnetization varies from sample to sample and is significantly different from the direction of the present geomagnetic field; and the polarity of the samples is both normal and reversed. A theoretical magnetic profile, calculated on the basis of the direction of total magnetization in the individual samples along the sample line, has the same gross features as the measured profile. The improved correlation which results upon consideration of the total magnetization properties of the rock unit emphasizes the necessity for considering the effects of remanent magnetization when interpreting magnetic intensity measurements.
THE ERROR IN LEAST‐SQUARES INVERSE FILTERING
Authors: Claerbout J. F. , Robinson E. A.
Published: October 2002

Least‐squares inverse filtering always involves consideration of the error. Under certain conditions the error will go to zero as the length of the filter tends to infinity. It is shown that the error will go to zero if either: (1) the waveform being inverted is minimum‐phase, or (2) if the output is chosen to come after a sufficiently long time delay. If the waveform being inverted is not minimum‐phase and if in addition the output is not chosen to be delayed, then the error will be finite and may be large.
STRESS BEHAVIOR IN THE NEIGHBORHOOD OF SHARP CORNERS
Authors: Karal Frank C. , Karp Samuel N.
Published: October 2002

Knowledge of the behavior of the stress field in the neighborhood of cracks and sharp corners is important for many theoretical and practical reasons. In an earlier paper the authors examined the behavior of the stress field near the edge of an elastic wedge of arbitrary angle with stress‐free boundary conditions prescribed on both surfaces (Karp and Karal, 1962). Other types of boundary conditions, such as those representing rigid and lubricated boundaries, were not considered. In the present paper these boundary conditions are now studied, and the behavior of the stress field in the neighborhood of the tip is determined. In addition, the stress behavior is examined when two different elastic media meet at a sharp edge.
CONDUCTIVITY SPECTRA OF ROCKS FROM THE CRAIGMONT ORE ENVIRONMENT
Authors: Fraser D. C. , Keevil N. B. , Ward S. H.
Published: October 2002

The conductivities of rocks of a suite from the Craigmont ore environment were studied as functions of frequency over the four decades 1,000 to 0.1 cps. The following were observed on a plot of linear conductivity vs log frequency. (1) Rocks mineralized with disseminated sulfides and magnetite yield low‐to‐moderate induced polarization (I.P.) and have conductivity‐frequency spectra which are concave‐up. (2) As veining increases, the spectra tend to straighten, and become sensibly straight when much of the metallics are interconnected; such rocks are characterized by low to large I.P. (3) Massive specular hematite cores yield a large I.P. and have spectra which are markedly concave‐up. For comparison, (4) clay‐containing sandstones yield low to moderate I.P. and have spectra which are slightly convex‐up. Consequently, the combination of I.P. magnitude and spectral analysis permits distinction of the various mineralization types.
TECHNIQUES OF SEISMIC RECORDING
Authors: Limón Antonio C.
Published: October 2002

In order to obtain the best possible seismic field record, it is indispensible on the one hand to know the response of the earth to the explosion, and on the other hand to operate the seismograph equipment with suitable settings.
GEOPHYSICAL ACTIVITY IN 1963
Authors: Smith Neal J.
Published: October 2002

Petroleum geophysical exploration in the Free World, consisting of seismic, gravity, ground magnetic, and other nonairborne geophysical methods, rose 1.6 percent in 1963 over 1962. This is in contrast to the 10 percent drop between 1961 and 1962. It is the first upswing since 1956 and, however modest, is welcome. It is 30 percent down from the 1956 level. Airborne magnetometer activity rose nominally from 433,473 line‐miles in 1962 to 434,943 in 1963.
ON THE STRATEGY AND TACTICS OF EXPLORATION FOR PETROLEUM. II
Authors: Rosaire E. E.
Published: October 2002

The need for statistics relative to the cost of finding oil in the ground has been recently stressed by DeGolyer. In the absence of such statistics, it is suggested that the study of outstandingly successful exploration campaigns will permit the deductions of principles which can be used for general guidance. By showing the parallelism between recent successful exploration campaigns and pertinent illustrations from military history, three of the fundamental Principles of War, relating to the Offensive, to Surprise, and to Security, are found to be applicable to exploration strategy. In conclusion, the present status of exploration in the Gulf Coast is discussed by analogy to a similar period in 1929 and 1930.
MEASUREMENT OF RELATIVE GROUND MOTION IN REFLECTION RECORDING
Authors: Gardner D. H.
Published: October 2002

A description of the method employed in determining the relative sensitivity of a reflection seismograph to harmonic displacement of [Formula: see text] inches is given. Results of comparing the sensitivity obtained at this level to the sensitivity used in reflection recording show a ground motion of [Formula: see text] inches for the deeper reflections. Comparisons are made between the sensitivities of the reflection seismograph and a mechanical seismograph taken at a level of [Formula: see text] inches.
OBSERVED REFLECTION AND DIFFRACTION WAVELET COMPLEXES IN TWO‐DIMENSIONAL SEISMIC MODEL STUDIES OF SIMPLE FAULTS
Authors: Harper Delbert Ray
Published: October 2002

The ultrasonic reflection‐diffraction complexes from simple rectangular configurations are compared for various two‐dimensional seismic models. These models represent simple faults and faults with sharp folds for two‐ and three‐layer cases. In particular, the model with cavity and models with extrusions (with square corners) represent simple faults in two‐ and three‐layer cases, respectively. Similarly the model with cavity and models with recesses (with curved corners) correspond to a sharp fold in the vicinity of the fault in two‐ and three‐layer cases. Masking of the fault plane by overlapping of diffraction upon reflection events is shown to be much more prominent for the simple fault with sharp fold than for a simple fault in both the two‐ and three‐layer cases. For the three‐layer cases, models employing recesses and extrusions have vertical dimensions less than the wavelength of the transmitted pulse; hence, waveforms for these models are more complicated due to composition of wavelets from the first interface with those from the second interface. Differences between the models were explained by synthetic waveforms derived using superposition of diffraction wavelets for the two‐layer cases (cavity models), and overlapping wavelets for the three‐layer cases (models with recesses and extrusions).
A LEAST‐SQUARES PROCEDURE FOR GRAVITY INTERPRETATION
Authors: Corbató Charles E.
Published: October 2002

A procedure suitable for use on high‐speed digital computers is presented for interpreting two‐dimensional gravity anomalies. In order to determine the shape of a disturbing mass with known density contrast, an initial model is assumed and gravity anomalies are calculated and compared with observed values at n points, where n is greater than the number of unknown variables (e.g. depths) of the model. Adjustments are then made to the model by a least‐squares approximation which uses the partial derivatives of the anomalies so that the residuals are reduced to a minimum. In comparison with other iterative techniques, convergence is very rapid. A convenient method to use for both the calculation of the anomalies and the adjustments is the two‐dimensional method of Talwani, Worzel, and Landisman, (1959) in which the outline of the body is polygonized and the anomalies and the partial derivatives of the anomaly with respect to the depth of a vertex on the body can be expressed as functions of the coordinates of the vertex. Not only depths but under certain circumstances regional gravity values may be evaluated; however, the relationship of the disturbing body to the gravity information may impose certain limitations on the application of the procedure.
INVESTIGATION OF MICROSEISM SOURCES WITH OCEAN‐BOTTOM SEISMOMETERS
Authors: Bradner H. , Doods J. G. , Foulks R. E.
Published: October 2002

Recordings to depths of 5 km have been made on the Pacific Ocean bottom with self‐rising internally recording seismometers. Simultaneous recordings have been made at land stations. The ocean‐bottom noise spectrum is between one and five orders of magnitude higher power than the land spectrum in the region from 0.1 to 9.0 cps. Coherence between two simultaneous instruments separated one‐quarter kilometer is above the 95 percent confidence level from 0.1 cps to 0.6 cps. Attempts to associate narrow‐beam Love and Rayleigh peaks with large storm‐generating areas or with heavy swell striking shore have not produced consistent results. Although some records show the bulk of the microseism peak energy in well‐defined modes, the energy is carried in different modes at different times and locations. Some of our data fit a model of microseism generation in a 100‐mile strip, by a statistical superposition of incident waves and waves reflected from shore; and the subsequent conversion of the energy to Rayleigh and Love modes propagating away from the generation zone. However, the shapes of the mid‐ocean spectra strongly imply additional sources far from shorelines or recognized storms, unless microseisms attenuate far less in the ocean than on land.
DISPERSIVE BODY WAVES—AN EXPERIMENTAL STUDY
Authors: Wuenschel Paul C.
Published: October 2002

Body‐wave dispersion is documented for a three‐dimensional case using the pulse propagation experiment in the Pierre Shale conducted by McDonal et al. (1958) and for a two‐dimensional case using ultrasonic pulse propagation in a Plexiglas plate. The magnitude of the dispersion for a five‐octave frequency band was only about two to three percent but easily observable by using Fourier‐analysis techniques. Body‐wave dispersion is shown to be predictable from the attenuation coefficient by following the causal transform technique of Futterman (1962). The principle of superposition is shown to be applicable for the prediction of the pulse anywhere in a medium if the complex propagation constant is known and if the pulse is known at one location. Pulse‐form prediction based on the attenuation coefficient alone is inadequate, while prediction based on attenuation and dispersion is in good agreement with observed results.
REFLECTION AND TRANSMISSION OF PLANE COMPRESSIONAL WAVES
Authors: Tooley R. D. , Spencer T. W. , Sagoci H. F.
Published: October 2002

The reflected and transmitted waves generated by a plane, monochromatic, compressional wave incident at a plane interface between two half‐spaces are analyzed. Energy ratios and the phase angles for the vertical displacement have been tabulated for over two thousand choices of the elastic parameters and densities. Computations for the solid‐fluid and solid‐air cases show that a large fraction of the incident energy goes into the shear wave over a large range of angles of incidence. The conditions for perfect reflection (with no conversion and no transmission) at the critical angles are derived. Within the critical angle, the requirement that the instantaneous energy flux be continuous is given by Knott’s equation. Beyond the critical angle, Knott’s equation gives continuity of the net flux but not continuity of the instantaneous flux. To achieve continuity of the instantaneous flux, Knott’s equation and two additional equations must be satisfied.
THEORETICAL INVESTIGATION OF MODAL SEISMOGRAMS FOR A LAYER OVER A HALF‐SPACE
Authors: Laster Stanley J. , Foreman Joe G. , Linville A. Frank
Published: October 2002

The use of normal modes to represent the propagation of elastic waves at large horizontal offset is well known. By addition of the relevant leaky modes, the mode theory is shown to be useful for representation of the seismograms at shorter range. A theoretical model consisting of a 2‐cm brass layer over a steel half‐space is considered. Dispersion curves and excitation functions are computed for the first four normal modes and the first three PL modes. Attenuation as a function of frequency also is computed for the PL modes. A suite of seismograms is computed for the distance range 50–70 cm, showing each mode individually and their sum (the total seismogram). It is found that, for the distances used, the individual modes do not approximate transients with a definite “arrival” time. Only their sum is required to exhibit this physical behavior. In addition, at short distances, the dispersion of a single mode is not visually obvious although the dispersion curve may be recovered by use of Fourier transform methods. Determination of the dispersion curves from the total seismogram is more difficult and requires some seperation of the modes, as they overlap in frequency and velocity. This work shows the preponderance of the leaky modes in the early part of the seismogram and indicates their importance in the later part of the seismogram for short horizontal offset.
A THEORETICAL PREDICTION OF SALT DOME PATTERNS
Authors: Selig Franz
Published: October 2002

Z. F. Daneš’s mathematical formulation of salt dome dynamics was modified to the extent that explicit solutions can be obtained. For both the two‐dimensional and three‐dimensional model, a formula is derived relating the rate of growth and the wavenumber of a fundamental mode with the physical and geometrical parameters. It is shown that there exists one wavenumber for which the rate of growth reaches a maximum. In time this component will dominate and control the final pattern.
GRADIENT MEASUREMENTS IN AEROMAGNETIC SURVEYING
Authors: Hood Peter
Published: October 2002

The recent development of highly sensitive magnetometers, such as the optical‐pumping varieties, has made feasible the measurement of the first vertical derivative of the total field (∂ΔT/∂h) in aeromagnetic surveys. This is accomplished by using two sensitive magnetometer heads separated by a constant vertical distance, and recording the difference in outputs. The effect of diurnal is thus eliminated in the resultant differential output, and this is an especially desirable feature in northern Canada where the diurnal variation is usually much greater than is found in more southerly magnetic latitudes. Moreover, steeply dipping geological contacts in high‐magnetic latitudes are outlined by the resultant zero‐gradient contour. It is also possible to obtain the depth of burial of the contact from the graph of (∂ΔT/∂h) versus (x∂ΔT/∂x) where x is the horizontal distance measured from the contact. Similar quantitative interpretations may be made for the point pole and dipole. The data reduction necessary to produce a vertical‐gradient map is much simpler than with the total‐field case because no datum levelling is necessary. Since the aircraft track will be available from the main compilation it is only necessary to plot the resultant vertical‐gradient values on the track map and contour. Thus, two maps will be obtained for little more than the price of one but with a greatly increased gain in geophysical information concerning the geometry of the causative bodies. Actually, a first‐derivative map is difficult (and therefore costly) to produce by any other means. The measurement of the vertical gradient would appear to be the main advantage to using hundredth‐gamma magnetometers in aeromagnetic surveys, since those types presently in service are sensitive enough for the effective delineation of total‐field anomalies.
DENSITY DETERMINATIONS BY UNDERGROUND GRAVITY MEASUREMENTS—SEQUEL
Authors: Hammer Sigmund
Published: October 2002

The foregoing paper (McCulloh, 1965) reports an exhaustive study and extension of a previous investigation bearing the captioned title (Hammer, 1950) of the problem of determining rock densities in the crust of the earth. Hammer reported that laboratory measurements on core samples gave densities which were systematically about 5 percent lower than the values determined by underground gravity measurements in 1945 in the vertical shaft of the Pittsburgh Plate Glass Company limestone mine at Barberton, Ohio. After considering the various factors which might affect the gravity determinations, Hammer concluded that the systematic difference was to be ascribed to nonrepresentative cores and core measurements. McCulloh suggests and adduces convincing evidence that the discrepancy was, in fact, due to an unrecognized gross error of about 12 percent in the gravimeter calibration.
DOUBLE‐TRACK PROFILING WITH THE GRAVITY METER
Authors: Thyssen‐Bornemisza Stephen
Published: October 2002

This technique, providing, simultaneously, gravity values and horizontal gravity gradients, is briefly discussed because of its possible advantage in marine exploration using the underwater gravity meter. Operating these remote‐control meters at greater depth is rather cumbersome and costly if compared to surveys at the ground surface (Dobrin, 1960). However, a profile of average horizontal gradients, in addition to a regular gravity profile, would indicate positive or negative anomalies located away from the profile itself, and would help avoid gravity stations with insignificant interpretative value.
INTERPRETATION OF AEROMAGNETIC ANOMALIES BY THE USE OF CHARACTERISTIC CURVES
Authors: Grant F. S. , Martin Luciano
Published: October 2002

In fitting simple models such as prisms or tabular bodies to magnetic anomaly patterns, certain dimensionless groupings of measurements from the anomaly—termed “characteristic estimators”—appear to be more effective than others in achieving satisfactory solutions. Characteristic curves are a concise, graphical presentation of the behaviour of characteristic estimators with changes in the parameters of the model, and are convenient for making rapid interpretations of anomalies. In this paper, illustrations of characteristic curves are given for the prism and for the tabular body, together with illustrations of their use from field examples.
A COIL OF IMPROVED SENSITIVITY FOR ROCK MAGNETISM
Authors: Grenet G.
Published: October 2002

The sensitivity of a coil‐type magnetometer can be increased by varying the cross‐sectional area of the winding for different parts of the inner coil and by increasing the radius of the outer coil. The axis of the coil should be in the direction of the earth’s magnetic field in order to reduce vibrational noise.
THEORY OF NEUTRON LOGGING II
Authors: Tittle C. W. , Allen L. S.
Published: October 2002

A new version of an analytical theory of epithermal neutron logging is developed by application of steady‐state one‐group diffusion theory to a central point neutron source in concentric cylindrical regions. An axial source expansion is employed in order to determine the coefficients of the series solution for two regions and for three regions. Both are compared with experiments in laboratory models. Favorable results are obtained notwithstanding the limitations of diffusion theory and its inherent approximations. Application of the theory is made to (1) a theoretical justification of the dual‐spaced neutron method of gas location, and (2) a theoretical investigation of the effect of the rock matrix in clean limestone and sandstone. Additional flux groups and source energy groups are briefly discussed.
POSSIBLE APPLICATION OF THE ANOMALOUS FREE‐AIR VERTICAL GRADIENT TO MARINE EXPLORATION
Authors: Thyssen‐Bornemisza Stephen
Published: October 2002

Recently it could be shown (Thyssen‐Bornemisza, 1965) that a vertical lithologic unit cylinder generates a relatively strong anomalous free‐air vertical gravity gradient F′ along the cylinder axis. The following simple example may serve as a demonstration. A small vertical cylinder made of gold or tungsten, where radius r and length L are identical, would generate the anomalous average gradient F′∼3,223 Eötvös units over the interval h=r=L going from the cylinders top surface upward. Suppose r=l=1 cm, then an average gradient exceeding the earth’s normal free‐air vertical gradient F is present over the interval h=1 cm.
SYNTHETIC SEISMOGRAMS OF P WAVES PROPAGATING IN SOLID WEDGES WITH FREE BOUNDARIES
Authors: Fuchs Karl
Published: October 2002

A numerical method for the synthesis of seismograms for body wave propagation in solid wedges is presented. The method is based on the superposition of multiple reflections arising from the entrance of a plane primary wave. Therefore the method is restricted to that part of the time domain where no diffracted waves from the wedge axis occur. In spite of this restriction, dispersion of body waves in wedges can well be studied by this method. Seismograms have been synthesized which show the dispersion of a primary p‐signal propagating in a solid 10‐degree and a 5‐degree wedge with free boundaries. For wedge angles less than 10 degrees the signal front (to be distinguished from the wavefront) suddenly decreases its velocity from that in the infinite medium to about that of the plate wave as the signal approaches the wedge axis. Simultaneously in this transition zone a decrease of the dominant period of the interference signal occurs. These observations are concordant with previous model studies. Particle motion diagrams disclose elliptical polarization of the interference signal in the neighborhood of the wedge axis; the polarization changes its sense from prograde to retrograde on passing through the transition zone.
A GENERALIZED METHOD OF NUMERICAL RESISTIVITY ANALYSIS
Authors: Bukhari S. A. , Lennox D. H.
Published: October 2002

The purpose of this note is to direct attention of geophysicists to the Research Council of Alberta Bulletin 19 which reports on continuing work in the subject field.
TOUR OF PETROLEUM GEOPHYSICS ACTIVITIES IN THE U.S.S.R.
Authors: Keller George V. , Caldwell Richard L. , Dobrin Milton B. , Holekamp Otto G. , White James E. , Zietz Isidore
Published: October 2002

Six American geophysicists spent three weeks in the U.S.S.R. during the early fall of 1965 as members of an exchange delegation in petroleum geophysics set up under the terms of a treaty on technical exchanges between the U. S. and the U.S.S.R. The group visited eleven different activities located in four cities, Moscow, Oktyabr’skiy, Krasnodar, and Leningrad. These activities included a number of geophysical and geological research institutes, the Moscow State University, and the Gubkin Institute. The principal difference between exploration geophysics in the U.S.S.R. and in the Western countries is the great emphasis in Soviet oil exploration on regional studies as a preliminary to the more direct search for petroleum deposits. As a result of such emphasis, their exploration program involves much greater use than ours of long refraction lines, electrical prospecting, and aeromagnetic surveys with close line spacing. Reflection work follows conventional lines without such recent refinements as common depth point shooting, impact sources, or vibrators. A possible reason these newer field techniques are not used is that no processing equipment appears to be generally available for compositing signals from more than one tape on to a single channel. Seismic shear waves generated at the surface have given reflections from depths as great as 5,000 ft but they have thus far been used only on an experimental basis. A technique of recording “exchange” (PS) waves from earthquakes has been employed for mapping basement depths in regional investigations.
ARCTIC OCEAN GEOPHYSICAL STUDIES: THE SOUTHERN HALF OF THE SIBERIA BASIN
Authors: Kutschale Henry
Published: October 2002

In 1962, ice island Arlis II drifted over a portion of the southern half of the Siberia Basin. Depth recordings made between 81°N, 170°E and 82°30′N, 160°E show that the ocean floor in this area is an abyssal plain at about 2,825 m depth dissected by several interplain channels. This abyssal plain, here called the Wrangel Abyssal Plain, is bounded on the north by Arlis Gap, which joins it with the Siberia Abyssal Plain at about 3,946 m depth. The Siberia Abyssal Plain occupies the northern half of the Siberia Basin. Seismic reflection profiles show that a prominent subbottom basement ridge exists in the vicinity of Arlis Gap. This ridge strikes in an approximately northwest direction and appears to connect with Alpha Ridge, which bounds the Siberia Basin on the east and north, and with Lomonosov Ridge, which bounds the Siberia Basin on the west. The seismic reflection profiles also show that at least 3.5 km of subhorizontal, stratified sediments underlie Wrangel Abyssal Plain south of the ridge. Each layer within these sediments appears to correspond to a fossil surface of Wrangel Abyssal Plain. This thick sequence of stratified sediments shows the influence of the Asian continent, which bounds the Siberia Basin on the south, on sedimentation within the Siberia Basin. Presumably the buried basement ridge forms a dam which has permitted the accumulation of a thick sequence of sediments under the higher‐level Wrangel Abyssal Plain. Turbidity currents moving through Arlis Gap presumably carried the overflow of sediments from Wrangel Abyssal Plain into the lower‐level Siberia Abyssal Plain. The structure of the sediments suggests that the Siberia Basin has been free from deformation during the deposition of the sediments, except for possible broad crustal down warping. A crustal model based on the water depth measurements, seismic reflection profiles, gravity measurements, and magnetic measurements yields a crustal thickness of 15 km south of the buried ridge and 22 km under the ridge measured from sea level.
DIPOLE METHOD FOR DEEP RESISTIVITY STUDlES
Authors: Keller G. V.
Published: October 2002

The dipole method of measuring resistivity is practical for use in studying the electrical properties of the earth’s crust. In such studies, electrical current is driven into the ground with a pair of electrodes spaced a few kilometers apart, and the electric field caused by this current is mapped to distances as great as 100 km. The principal advantage of the dipole method is the relatively small amount of cable which has to be laid out in comparison with that required by other electrode arrays. The results obtained from dipole surveys are very similar to the results obtained from other arrays, provided that the spacing factor is defined in an appropriate way, and that the earth is laterally homogeneous. If the electrical properties of the earth vary laterally, the dipole method provides results which differ widely from the data obtained with other arrays. This sensitivity to lateral effects is probably the most serious drawback to the use of dipole methods in crustal resistivity surveys.
DETAIL GRAVITY PROFILE ACROSS SAN ANDREAS FAULT ZONE
Authors: Domenico S. N.
Published: October 2002

A gravity profile was obtained from closely spaced readings along a traverse approximately nine miles in length across the San Andreas fault zone immediately south of Palmdale, California in the western Mojave Desert. Corrected gravity values show a slight but distinctive minimum associated with the fault zone which may be attributed to the reduced density of the shattered rock masses in the fault zone. The existence of this minimum suggests that major fault zones may be traced across terrain, on which surface expression of the fault does not exist, by successive profiles across the suspected position of the fault zone.
ACCURACY IN MODEL SEISMOLOGY
Authors: Schwab Fred
Published: October 2002

The most common technique currently used in model seismology makes use of a fixed source of elastic waves and a single receiver. This receiver is moved from position to position on a model in order to determine the relative spatial response to the source excitation. Inaccuracies occur due to the difficulty in obtaining the same receiver‐model coupling at each position. This coupling problem can be solved through the use of two source positions, the simultaneous use of two receivers, and the application of a general symmetry condition to the source‐receiver, and model geometries. Basic relations are established for this method. Independently of whether displacement, velocity, or acceleration detectors are used as receivers, these relations yield relative amplitude and phase spectra of the displacement at two different points on the model. These basic relations, and thus the validity of the method, are established by experiment. When a receiver is coupled to a model, the incident pulse is scattered as it passes the receiver position. This effect is negligible when bimorph receivers are used with two‐dimensional, steel‐sheet models. Edge receivers, used in conjunction with the same type models, produce significant, frequency‐dependent scatter. Quantitative results for this latter type of receiver‐model combination are obtained by experiment. These results show the amplitude‐spectrum of the displacement pulse to be attenuated by as much as 41 percent. Over the entire frequency range treated, the attenuation is never less than 4 percent. If it is necessary to use scattering receiver‐model combinations, the technique described for obtaining improved accuracy can be modified to correct for this effect.
A MULTILAYER SEISMIC REFRACTION NOMOGRAM
Authors: Meidav Tsvi
Published: October 2002

In solving seismic refraction problems, the solution of layer thickness becomes increasingly cumbersome as the number of layers increases. We have derived a simplified multilayer seismic refraction nomogram, which is based on some approximations, and leads to a much simpler thickness and depth computation for any number of layers. This nomogram should prove more useful in the multilayer case than a previous one (Meidav, 1960). The principles of nomogram construction have been briefly described before (op. cit.), and will not be restated here.
DISCOVERY CASE HISTORY OF THE PYRAMID ORE BODIES PINE POINT, NORTHWEST TERRITORIES, CANADA
Authors: Seigel H. O. , Hill H. L. , Baird J. G.
Published: October 2002

In the fall of 1965 a geophysical exploration program, in the Pine Point area, Northwest Territories, Canada, revealed indications which led to the discovery of 11.2 million tons of ore grading 11.7 percent combined lead and zinc in two separate ore bodies. The ore bodies are relatively flat‐lying although irregular in plan and section. They lie in limestones and dolomites, some reefoidal, of Middle Devonian age and are generally conformable to these sea sediments. The basic discoveries were effected by a time‐domain, induced‐polarization survey. A subsequent gravity survey proved to be of considerable value in guiding the extensive drilling program which followed and in predicting the amount of ore to be expected in each body. Despite a high marcasite content, the ore bodies did not respond appreciably to electromagnetic induction methods. This is attributed to the mode of distribution of the various sulfide minerals in the deposits.
On: “An inverse slope method of determining absolute resistivity” by P. V. Sanker Narayan and K. R. Ramanujachary (GEOPHYSICS, vol. 32, no. 6, p. 1036–1040, December 1967)
Authors: Keller George V.
Published: October 2002

It is important to point out that the method of cumulative conductances described by the authors of this note does not provide a means for determining the “absolute resistivity” of a sequence of layers. The basic technical fallacy in this note is the use of Hummel’s “principle,” stated in equation (1) without consideration of the limits for its applicability. The development of Hummel’s principle is based on a very specific model, that of a sequence of conducting layers resting on an insulating substratum. In such a case, at very large distances from a current source, current must flow entirely horizontally because none can leave either the upper or lower boundaries of the conducting layers. In order that the current can flow in horizontal planes, the rate of potential drop along any horizontal plane must be the same. This condition can be satisfied only if the current density is inversely proportional to the resistivity in each of the conducting layers. Then, the layers may be treated as resistances in parallel, and Hummel’s principle applies.
NOTE ON A USEFUL APPROXIMATION FOR REFRACTION SHOOTING
Authors: Snell Francis A.
Published: October 2002

The following paper represents a summary distilled from the paper written by the author in 1928 and retained in unpublished form to the present time. It is of some interest to note that the useful approximation derived forty years ago apparently has not been published elsewhere. In pre‐reflection days, forty years ago, the problem of subsurface profiling by refraction shooting in foreign wildcat areas was an interesting one. In such areas there might be, at best, a few rather questionable drillers’ logs to give some advance idea of the formations to be encountered. There was no communication between the shot‐point and the recorder, frequently a mile or more distant, to give the instant of firing. Shots were fired and recorded with the aid of synchronized watches and had to be captured on a fifteen‐inch length of oscillograph film, for continuous rolls of paper were not yet available. On the phenomenal side, the prevalence of a velocity increase with depth, which caused a large deviation from elementary straight‐ray theory, was a factor just beginning to be recognized.
On: “The Equivalent Source Technique” by C. N. G. Dampney (GEOPHYSICS, February 1969, p. 39–53)
Authors: Roy Amalendu
Published: October 2002

The following comments should not be taken as indicative of my overall appreciation of the paper, which I have read with immense interest: (1) The title of a paper should be a little more expressive, where possible. In this case, I would have suggested “Equivalent Source Technique in Gravity Interpretation.” This can prevent some readers from making a mistake in choosing titles of interest from the cover. (2) The equivalent source technique, in a more elementary form, was used for upward continuation of gravity data in a paper by Roy & Burman (1960).
EXPLOSIVE SEISMIC SOURCES FOR THE MOON
Authors: Kovach Robert L. , Ahrens Thomas J.
Published: October 2002

The coupling of seismic energy under vacuum conditions, such as the moon, using an untamped surface charge is different from coupling in air. In vacuum, the explosive gas blast and the detonation products continuously expand outward and interact with the solid surface. A series of model experiments was performed to investigate the effect of vacuum on coupling seismic energy. HNS charges of 0.2 gm each were detonated in contact with a plate and block of acrylic plastic in vacuum and in air. The amplitudes of the first and second arrivals (longitudinal and shear plate wave) are about 50 percent greater in vacuum than in air because the plate velocity (∼2.4 km/sec) more closely matches the gas‐blast velocity (∼3.5 to 7.5 km/sec) than the sound‐wave velocity (∼0.35 km/sec). When the charges are detonated in contact with the block to generate direct body waves, little difference is noted in the first arrival amplitudes in air and vacuum; suspending the charge one charge‐diameter above the surface produces about 25 percent lower first amplitudes in a vacuum. Large scale experiments were also performed in air to examine the effect of the detonation configuration on seismic coupling.