bluuebunny/crossref_metadata_2025 huggingface dataset:
Heat flow in the presence of topography: Numerical analysis of data ensembles
Authors: Henry Steven G. , Pollack Henry N.
Published: October 2002

We present a method of estimating subsurface temperatures and true regional heat flow in the presence of perturbing topography, variable surface temperature, and subsurface thermal conductivity contrasts. The method involves solution of the steady‐state three‐dimensional heat conduction equation by finite‐difference numerical techniques. The topography is represented by an irregular upper boundary and the variable surface temperature as a boundary condition along the irregular upper surface. Internal structural configurations and conductivity contrasts are easily accommodated. The principal variable input into the system is the deep basal (unperturbed) heat flow. The best value of heat flow is obtained by minimizing, in a least‐squares sense, the differences between observed and calculated temperatures. Temperature observations commonly are distributed irregularly in the near‐surface (perturbed) environment, in multiple vertical or inclined boreholes, tunnels, and/or mine galleries. The method is particularly suited to simultaneous analysis of an ensemble of distributed observations, in contrast to methods that focus on the perturbation to the temperature gradient in the vicinity of a single borehole. We used the method to reduce data obtained at fifteen newly established heat flow sites in the Bolivian and Peruvian Andes. We illustrate with three examples—a two‐dimensional model from the Bolivar Mine, Bolivia; (2) a three‐dimensional model using variable conductivity from the Cerro Verde Mine, Peru; and (3) a three‐dimensional model at the Colquiri Mine, Bolivia where temperature measurements were few and the distance between the individual boreholes was fairly large.
Shear waves produced by explosive sources
Authors: Lash C. C.
Published: October 2002

Shear waves (S-waves) created by an explosion at or near an interface have been observed in experimental field studies and in mathematical model studies, but the observations have been separated so widely in time, place, and publication that their relation has not been apparent. Recent papers on the S*-wave are referenced here and study of their implications for field work is recommended. From earlier papers illustrations of S-waves, designated “source shear” waves, at three Gulf Coast sites were selected for examples. The source shear wave was found to be strong, directional, and the likely cause of downgoing compressional waves by wave conversion. The mathematical S*-wave and the source shear waves are SV-waves at observation points along any line through the source position if the line has an inclination of 45 degrees. Along a vertical line through the source, the wave is not detected. Because of difficulties in assembling observations at points both deep and of large offset, it is highly desirable to tie together field studies and mathematical synthetic model studies.
Resolution in seismic trace inversion by parameter estimation
Authors: van Riel P. , Berkhout A. J.
Published: October 2002

An alternative to the conventional time series approach to single‐trace modeling and inversion by convolution and inverse filtering is a parametric approach. To obtain insight into the potential of the parametric approach, the solution of the single‐trace forward problem is formulated in matrix terms. For the nonlinear reflector lag time parameters this is achieved by linearization, which is shown to be a valid approximation over a sufficiently large region. The matrix forward operators are analyzed by means of the singular value decomposition (SVD). The SVD can be considered a generalization of the Fourier transform of convolution operators. On the basis of the SVD analysis, inverse operators are designed which combine stability with high resolving power. A method to determine the resolving power of the parametric inverse operators is presented. Several examples show how wavelet bandwidth, data noise level, and model complexity influence the resolving power of the data for the reflection coefficient and the lag‐time parameters. The most important result is that the resolution obtained in parametric inversion is, in most cases, superior to and, at worst, equal to the resolution obtained with wavelet inverse filtering. The explanation is that in parametric inversion a different representation of the reflectivity function is used which, in practical situations, involves fewer unknowns. In wavelet inverse filtering the reflectivity function is represented as a regularly sampled function where every sample point represents an unknown. In practical applications of parametric inversion the reflectivity function is represented as a model with a limited number of reflectors as unknowns. To formulate parametric models, a priori information is required. The effort of collecting sufficient a priori information is the cost of increasing resolution beyond the resolution offered by wavelet inverse filtering.
Transient analytic point‐source response of a layered acoustic medium: Part II
Authors: Tygel Martin , Hubral Peter
Published: October 2002

In part I (this issue) analytic methods are described for propagating directly in the time domain the full wave field of transient plane waves at arbitrary incidence angles through parallel acoustic homogeneous layers. The theory of part I is used here to compute transient point‐source responses by way of integrating the transient plane‐wave responses over various (real and nonreal) incidence angles. At first the complete transient point‐source reflection (transmission) responses are formulated with the help of the transient Sommerfeld‐Weyl integral that was developed previously by us. This leads to a transient solution in form of an infinite integral over incidence angles involving an (angle‐dependent) integrand that is a time‐convolution between a transient reflectivity (transmissivity) function and an inverse transient analytic square root operator. The transient Sommerfeld‐Weyl integral solution is then proven to be causal. This causality provides the starting point for formulating exact transient point‐source responses in terms of integrals over a finite range of incidence angles only.
The Nth‐root stack: Theory, applications, and examples
Authors: McFadden P. L. , Drummond B. J. , Kravis S.
Published: October 2002

Multichannel geophysical data are usually stacked by calculating the average of the observations on all channels. In the Nth‐root stack, the average of the Nth root of each observation is raised to the Nth power, with the signs of the observations and average maintained. When N = 1, the process is identical to conventional linear stacking or averaging. Nth‐root stacking has been applied in the processing of seismic refraction and teleseismic array data. In some experiments and certain applications it is inferior to linear stacking, but in others it is superior. Although the variance for an Nth‐root stack is typically less than for a linear stack, the mean square error is larger, because of signal attenuation. The fractional amount by which the signal is attenuated depends in a complicated way on the number of data channels, the order (N) of the stack, the signal‐to‐noise ratio, and the noise distribution. Because the signal‐to‐noise ratio varies across a wavelet, peaking where the signal is greatest and approaching zero at the zero‐crossing points, the attenuation of the signal varies across a wavelet, thereby producing signal distortion. The main visual effect of the distortion is a sharpening of the legs of the wavelet. However, the attenuation of the signal is accompanied by a much greater attenuation of the background noise, leading to a significant contrast enhancement. It is this sharpening of the signal, accompanied by the contrast enhancement, that makes the technique powerful in beam‐steering applications of array data. For large values of N, the attenuation of the signal with low signal‐to‐noise ratios ultimately leads to its destruction. Nth‐root stacking is therefore particularly powerful in applications where signal sharpening and contrast enhancement are important but signal distortion is not.
Traveltime inversion for the flat‐layer model
Authors: Justice James H.
Published: October 2002

Consider a flat‐layer model of the earth with constant velocity in each layer. If primary reflections are recorded at two offset positions on the surface for each reflecting horizon, then the observed traveltime data are sufficient to reconstruct the thickness and velocity of each layer. An exact mathematical formulation for this procedure could be implemented for processing seismic data. The algorithm is sensitive to errors in picked traveltimes.
Pore fluids and frequency‐dependent wave propagation in rocks
Authors: Jones Terry D.
Published: October 2002

Attenuation is the anelastic process which dissipates seismic energy by conversion to heat, thus decreasing the amplitude and modifying the frequency and phase content of a propagating wavelet. Laboratory measurements show seismic phase velocity and attenuation are dependent upon the fluid saturation and the product of frequency and pore‐fluid viscosity, with a peak in attenuation between the seismic and sonic bands. The dominant mechanism by which seismic energy is dissipated in the upper crust is local viscous fluid flow in pores of small aspect ratio. Phenomenologically, this behavior is modeled as a series of linear viscoelastic elements with a narrow distribution of relaxation times, where velocity and attenuation are related through the Hilbert transform. This model may be generalized to include constant-Q behavior, as observed in dry rocks. Solutions to the wave equation may be generated for an arbitrary frequency dependence of phase velocity and Q. When Q is nearly independent of frequency, the impulse response is asymmetric and the power spectrum is a straight line with slope proportional to Q. When Q is roughly proportional or inversely proportional to frequency, the impulse response is nearly symmetrical and the power spectrum is nonlinear. These results indicate that the frequency content, phase spectrum, and velocity may be strong indicators of the type of pore fluid in a formation. Alternatively, if the type of pore fluid is known, these attributes could be used to monitor temperature changes of a formation.
Self‐potential variations with time and their relation to hydrogeologic and meteorological parameters
Authors: Ernstson Kord , Scherer H. Ulrich
Published: October 2002

A systematic investigation of self potentials (SP) shows that they vary considerably with time. To study the influencing parameters, repeated measurements were conducted every 14 days over a period of a year and a half along fixed profiles which extended over different lithological units. An analysis of the recorded SP suggests decomposition into three components of different wavelengths. Variations of the SP with wavelengths in the 0.1–1 m range and amplitudes of up to 150 mV are thought to be induced by vegetation. The mean amplitudes change with a one‐year period, and there is a rough correlation with the soil temperature. A long‐wavelength component (wavelengths, meters and some tens of meters; amplitudes, some 10 mV) is attributed to changes of the underground lithology. The long‐wavelength SP and its pronounced variation with time are assumed to be caused by water movements in layers of different permeability. A morphology‐dependent trend of the SP values which has been much disputed in the literature shows systematic variations with time, too, with gradients changing on the order of .5 mV/m difference in elevation. There is a correlation with precipitation with a one‐month time lag. This effect is assumed to result from depletion and recharge of perched water aquifers. Measurements at the surface and at shallow depth establish the existence of a vertical gradient of the SP which shows variations with time of up to 50 mV/m. With a three‐month time lag, the amplitude of the vertical gradient relates closely to the variation of the potential evapotranspiration. Groundwater recharge from gravitational water is considered the main cause of the vertical gradient. The results suggest that self potentials at the nearsurface must be regarded primarily as being of a streaming‐potential character. Their variation with time should not be neglected when the SP method is applied to problems in engineering geophysics. On the other hand, the results confirm that because of the close relation between self potentials and hydrogeologic parameters, SP measurements may become a valuable and standard technique in hydrogeology.
Exact inversion of plane‐layered isotropic and anisotropic elastic media by the state‐space approach
Authors: Meadows Mark , Coen Shimon
Published: October 2002

Time‐domain state‐space inversion methods for non‐dispersive layered media have shown that acoustic impedance as a function of traveltime can be determined from a normally incident, plane, pressure‐wave source. The state‐space method is used here to determine the density and acoustic velocity separately as functions of depth by simultaneously inverting surface data measured at two precritical angles of incidence. The acoustic state‐space method is also applied to isotropic elastic media with normally incident P-waves and obliquely incident SH-wave sources, whereby the density, shear velocity, and compressional velocity are determined as functions of depth. The same parameters are recovered by modifying Shiva and Mendel's (1983) method for a single, obliquely incident P-wave source to accommodate a single SV-wave source, two P-wave sources, or data obtained from a previous (scalar) SH-wave inversion. Using data from multiple experiments eliminates nonuniqueness in the inversion procedure. We have extended the state‐space approach to generally anisotropic media to solve for the layer thicknesses, densities, elastic stiffness parameters, and symmetry axis rotation angles in each layer. Three‐component recording is required to identify the wave types at each interface and to downward continue the wave fields properly at depth. For transverse isotropy and all higher anisotropies, an [Formula: see text]-parameter search is required at each interface, where 1 ⩽ n ⩽ 25. This adds an inherent nonuniqueness to the inversion. However, for transverse isotropy, nonuniqueness does not appear to be a serious problem since the layer parameters are determined by a single‐parameter search of a monotonic function at each interface. It appears that slant stacking of multicomponent surface seismograms is applicable to generally anisotropic media and that methods that deal with noise and band limitations in acoustic media can be useful to anisotropic inversion as well.
Induced‐polarization and electromagnetic modeling of a three‐dimensional body buried in a two‐layer anisotropic earth
Authors: Xiong Zonghou , Luo Yanzhong , Wang Shoutan , Wu Guangyao
Published: October 2002

The integral equation method is used for induced‐polarization (IP) and electromagnetic (EM) modeling of a finite inhomogeneity in a two‐layer anisotropic earth. An integral equation relates the exciting electric field and the scattering currents in the homogeneity through the electric tensor Green’s function deduced from the vector potentials in the lower layer of the earth. Digital linear filtering and three‐point parabolic Lagrangian interpolation with two variables speed up the numerical evaluation of the Hankel transforms in the tensor Green’s function. The results of this integral equation method for isotropic media are checked by direct comparisons with results by other workers. The results for anisotropic media are indirectly verified, mainly by checking the tensor Green’s function. The calculated results show that the effects of anisotropy on apparent resistivity and percent frequency effect are to reduce the size of the anomalies, shift the anomaly region downward toward the lower centers of the pseudosections, and enhance the effect of overburden; in other words, to shade the target from detection. This is due to the increase of currents flowing horizontally through the earth over the target. The effects of anisotropy on horizontal‐loop EM responses are to reduce the amplitude and lower the critical frequency of the maximum of the quadrature component.
Automatic estimation of large residual statics corrections
Authors: Rothman Daniel H.
Published: October 2002

Conventional approaches to residual statics estimation obtain solutions by performing linear inversion of observed traveltime deviations. A crucial component of these procedures is picking time delays; gross errors in these picks are known as “cycle skips” or “leg jumps” and are the bane of linear traveltime inversion schemes. This paper augments Rothman (1985), which demonstrated that the estimation of large statics in noise‐contaminated data is posed better as a nonlinear, rather than as a linear, inverse problem. Cycle skips then appear as local (secondary) minima of the resulting nonlinear optimization problem. In the earlier paper, a Monte Carlo technique from statistical mechanics was adapted to perform global optimization, and the technique was applied to synthetic data. Here I present an application of a similar Monte Carlo method to field data from the Wyoming Overthrust belt. Key changes, however, have led to a more efficient and practical algorithm. The new technique performs explicit crosscorrelation of traces. Instead of picking the peaks of these crosscorrelation functions, the method transforms the crosscorrelation functions to probability distributions and then draws random numbers from the distributions. Estimates of statics are now iteratively updated by this procedure until convergence to the optimal stack is achieved. Here I also derive several theoretical properties of the algorithm. The method is expressed as a Markov chain, in which the equilibrium (steady‐state) distribution is the Gibbs distribution of statistical mechanics.
A new formulation of dip and depth to a dipping reflector beneath V=V0(1+KZ)
Authors: Kayiran T.
Published: October 2002

The case where the velocity increases linearly with depth over a dipping reflector is considered. The underlying medium is assumed to have a constant velocity. On the time‐distance graph related to this model there is an end point which corresponds both to the direct seismic ray tangent to the reflector and to the maximum reflection distance in the first medium. Starting from this point, a method of finding the dip and depth is developed and a synthetic example is given.
Reduction to the pole as an inverse problem and its application to low‐latitude anomalies
Authors: Silva João B. C.
Published: October 2002

Traditionally, reduction to the pole has been accomplished either by space‐ or wavenumber‐domain filtering. In the two‐dimensional case, this procedure is stable regardless of the latitude, as long as the source strike is not parallel to the horizontal projection of the geomagnetic field. In the three‐dimensional case, however, reduction‐to‐the‐pole filtering is stable only at high magnetic latitudes. At latitudes lower than 15 degrees, it is of no practical use due to a sharply increasing instability toward the magnetic equator. The three‐dimensional instability of this filtering technique is demonstrated, and the reduction‐to‐the‐pole problem is formulated in the context of a general linear inverse problem. As a result, stable solutions are found by using well‐known stabilizing procedures developed for the inverse linear problem. The distribution of magnetization of an equivalent layer of doublets that reproduces the observed data is computed. The magnetic doublets are parallel to the magnetization direction which is assumed constant throughout the sources. The magnetic field reduced to the pole is then obtained by changing the inclinations of the geomagnetic field and the doublets to 90 degrees and recalculating the total field. The usefulness and limitations of the method at low magnetic latitudes are assessed using theoretical data. The effects of noise and anomaly truncation are also investigated for both high and low latitudes. In all cases, application of the proposed method produced meaningful results regardless of the latitude. The method is applied to field data from two different low‐latitude anomalies. The first anomaly is due to a seamount in the Gulf of Guinea with reversed magnetization. The geomagnetic field at this location is about −23 degrees. The second anomaly is an intrabasement anomaly from Parnaiba Basin, Brazil, where the magnetization is assumed to be induced by a geomagnetic field with −1.4 degree inclination. The results obtained confirm that the proposed method produces stable, meaningful, reduced‐to‐the‐pole maps.
Inversion of magnetotelluric data using a practical inverse scattering formulation
Authors: Whittall Kenneth P. , Oldenburg D. W.
Published: October 2002

We present a flexible, one‐dimensional magnetotelluric (MT) inversion algorithm based on inverse scattering theory. The algorithm easily generates different classes of conductivity‐depth profiles so the interpreter may choose models that satisfy any external geologic or geophysical constraints. The two‐stage process is based on the work of Weidelt. The first stage uses the MT frequency‐domain data to construct an impulse response analogous to a deconvolved seismogram with or without a free‐surface assumption. Since this is a linear problem (a Laplace transform), numerous impulse responses may be generated by linear inverse techniques which handle data errors robustly. We minimize four norms of the impulse response in order to construct varied classes of limited‐structure earth models. We choose such models to prevent overinterpreting the limited number of inaccurate MT observations. The second stage of the algorithm maps the impulse response to the conductivity model using any of four Fredholm integral equations of the second kind. We evaluate the performance of each of the four mappings and recommend the Burridge and Gopinath‐Sondhi formulations. We also evaluate three approximations to the second‐stage equations. These approximations are fast and easy to implement on small computers. We find the one which includes first‐order multiple reflections to be the most accurate.
Archaeological applications of resistivity and magnetic methods at Fort Wilkins State Park, Michigan
Authors: Young Charles T. , Droege David R.
Published: October 2002

Geophysical measurements were conducted to locate building sites at a nineteenth‐century fort in Michigan. The survey area, Fort Wilkins, was built during the copper boom of the 1840s and is located at the tip of the Keweenaw Peninsula in the Upper Peninsula of Michigan. Resistivity and magnetic surveys were conducted over the locations of former privies, a guard house, a carpenter house, and a blacksmith shop. The resistivity surveys were made using a half‐Schlumberger array with electrode spacings of 0.3, 0.6, 1.0, 1.5, and 2.0 m. A masonry foundation and the privies are indicated by high‐low resistivity pairs. Analog modeling was used to create anomalies similar to those seen over the privies. Measurements of the vertical magnetic gradient yielded anomalies which are associated primarily with modern objects.
Evaluation of archaeological site potential on the Gulf of Mexico continental shelf using high‐resolution seismic data
Authors: Stright Melanie J.
Published: October 2002

Prehistoric archaeological sites dating between 12 000 B.P. and 6 000 B.P. are expected on that portion of the Gulf of Mexico continental shelf which was subaerially exposed at 12 000 B.P. Although the archaeological sites are too small and their contrast to the surrounding sediments is insufficient to be resolved by high‐resolution seismic profilers, geomorphic features, such as fluvial channels, bays, lakes, etc., with which these sites are frequently associated are easily detected with seismic methods. High‐resolution seismic reflection data from the Gulf of Mexico have produced evidence of relict fluvial systems with well‐preserved terraces, point bars, and levee ridges, occurring within 9.0 m or less of the present seafloor. Regional studies which detail the late Pleistocene and Holocene geology of the Gulf of Mexico, and published sea level curves, indicate that these fluvial features date from the late Wisconsin and early Holocene periods when prehistoric man’s presence in the Gulf Coast Region is well documented. Verifying the presence of archaeological sites in association with these fluvial features will require physical and chemical analysis of sediment cores from each location. The Minerals Management Service is presently conducting an archaeological study designed to investigate relict fluvial features similar to those discussed in this paper. The physical and chemical analyses of the sedimentary cores collected during this study are presently incomplete; however, initial inspection of the cores indicates the presence of well‐preserved lithosomes. The preservation potential of any archaeological sites occurring within these lithosomes also would be excellent.
Anisotropy effects in P-wave and SH-wave stacking velocities contain information on lithology
Authors: Winterstein D. F.
Published: October 2002

Depths calculated from S-wave stacking velocities and event times almost always exceed actual depths, sometimes by as much as 25 percent. In contrast, depths from corresponding P-wave information are often within 10 percent of actual depths. Discrepancies in depths calculated from P- and S-wave data are attributed to velocity anisotropy, a property of sedimentary rocks that noticeably affects S-wave moveout curves but leaves the P-wave relatively unaffected. Two careful studies show that discrepancies in depths, and hence in constituent layer thicknesses, correlate with lithology. Discrepancies ranged from an average of 13 percent (Midland basin) to greater than 40 percent (Paloma field) in shales, but were within expected errors in massive sandstones or carbonates. Hence anisotropy effects are indicators of lithology. Analysis of seismic data involved determining interval velocities from stacking velocities, calculating layer thicknesses, and then comparing layer thicknesses from S-wave data with thicknesses from P-wave data. When the S-wave thicknesses were significantly greater than the P-wave (i.e., outside the range of expected errors), I concluded the layer was anisotropic. I illustrate the technique with data from the Paloma field project of the Conoco Shear Wave Group Shoot.
Phase offset indicator (POI): A study of phase shift versus offset and fluid content
Authors: Rafipour B. , Herrin E.
Published: October 2002

There is limited published experimental and theoretical work related to offset‐dependent reflection and phase changes at the interface between porous media. An appropriate relationship between seismic parameters and phase shift has not been well‐established. This investigation measures variations of phase for nonnormal incidence reflection as a function of frequency at the interface of a horizontally layered porous (perforated) medium, using a two‐dimensional Plexiglas model. In both parts of this study, phase‐matched filters (PMF) are used to analyze the data. Phase‐matched filters previously have been applied to digital records of Rayleigh waves and Love waves. The purpose of the first part of the study is to demonstrate application of the PMF process to reflected compressional waves and to use this technique in discriminating multiples from primaries. By using this linear process, the complex spectrum of the primary signal can be recovered from the multiple arrivals. The reduction of reflected compressional pulse wavelets to a zero‐phase wavelet provided a tool to examine the change in phase (and amplitude) as the wave travels through a lossy medium. The second part of the study deals with the relationship of phase shift Δϕ at an interface of a horizontally layered porous medium, as a function of fluid content, angle of incidence (offset), and frequency. The result indicates that, at midband frequency of 60 kHz, an air‐ (gas‐) saturated medium would cause a large increase in Δϕ with offset, whereas a water‐saturated medium would show less change in Δϕ with offset. The phase shift‐versus‐offset response for oil is assumed to be intermediate between the responses for gas and water.
Hole‐to‐hole resistivity signatures of cylindrical cavities: Dipole‐dipole electrode array
Authors: Parra Jorge O. , Owen Thomas E.
Published: October 2002

A numerical procedure for predicting cavity signatures for a dipole‐dipole array configuration in hole‐to‐ hole resistivity measurements has been developed. This electrode geometry is implemented from the general solution for a point source of current near an air‐filled cylindrical cavity or an air‐filled cylinder surrounded by a concentric conductive or resistive halo region embedded in a homogeneous conducting host medium to simulate hole‐to‐hole resistivity measurements. Cavity signatures obtained for several vertical offset distances between the source and detector dipoles as well as multiply spaced dipole‐dipole responses suggest that a processing technique may be devised to identify directly the position of the cavity inhomogeneity with respect to the boreholes. The results also show that the presence of a concentric halo region more conductive than the host medium influences the overall signature by either reducing or enhancing the effect of the air‐filled cavity depending upon the halo size and conductivity contrast. In comparison, a halo region more resistive than the host medium always influences the composite signature by enhancing the effect of the air‐filled cavity.
Theoretical far‐field radiation from a low‐frequency horizontal acoustic point force in a vertical borehole
Authors: Kurkjian Andrew L.
Published: October 2002

A subsurface point body force can be realized using a body force on the axis of a small‐radius borehole drilled perpendicular to the force. This observation, originally made by Kitsunezaki (1980), is having a significant impact in shear‐wave logging, and may potentially affect downhole seismic sources. Applying the principle of reciprocity to this finding, a horizontal vibration sensor on the axis of a vertical hole will be unaffected by the existence of the hole at low frequencies. Numerical methods determine both the frequency below which, and the offset beyond which, borehole‐related effects are negligible. If the shear wavelength is greater than ten times the diameter of the hole and if the measurement is made at least one shear wavelength from the point force, then the borehole effects will be minimal.
A transient electric dipole‐dipole method for mapping the conductivity of the sea floor
Authors: Edwards R. N. , Chave A. D.
Published: October 2002

The electrical conductivity of the sea floor is usually much less than that of sea water, and not all electrical measurements made on the sea floor are particularly sensitive to the electrical conductivity value. The analytic impulse and step‐on transient responses of two conductive, adjoining half‐spaces (with a large conductivity contrast) to an in‐line electric dipole‐dipole electromagnetic system located on the interface are derived. The shape of the transient at relatively early time is seen to be independent of the conductivity of the more conductive half‐space and is indicative of the conductivity of the less conductive haft‐space. Based on this observation, a simple, practical system can be designed to measure sea floor conductivity.
Geophysical case history, Prudhoe Bay field
Authors: Specht R. N. , Brown A. E. , Selman C. H. , Carlisle J. H.
Published: October 2002

In June 1968 ARCO‐Exxon completed the Prudhoe Bay State No. 1 well, discovering the largest oil accumulation in the United States. This discovery was the result of a [proposed] three‐year seismic program begun in 1963. The three predecessor companies of Atlantic Richfield were involved in separate geophysical programs and by 1964 each program had delineated two major structures on the North Slope coastal plain: Colville and Prudhoe. During the first state sale on the North Slope (late 1964), Sinclair, in partnership with British Petroleum, leased the entire Colville structure. The critical state lease sale covering Prudhoe Bay was held in July, 1965. This sale determined the eventual ownership of the Prudhoe Bay field. ARCO‐Exxon acquired the top tracts, with British Petroleum acquiring flank acreage. In January, 1967, ARCO‐Exxon acquired additional offshore tracts and began drilling the Prudhoe Bay State No. 1 in April.
A gravity model of the basement structure in the Santa Maria Basin, California
Authors: Kieniewicz Paul M. , Luyendyk Bruce P.
Published: October 2002

The Santa Maria Basin in southern California is a lowland bounded on the south by the Santa Ynez River fault and on the northeast by the Little Pine‐Foxen Canyon‐Santa Maria River faults. It contains Neogene sedimentary rocks which rest unconformably on a basement of Cretaceous and older clastic rocks. Analysis of over 4 000 gravity stations obtained from the Defense Mapping Agency suggests that the Bouguer anomaly contains a short‐wavelength component arising from a variable‐density contrast between the basin’s Neogene units and the Cretaceous basement. A three‐dimensional inversion of the short‐wavelength component (constrained by wells drilled to basement) yields a structure model of the basement and the average density of the overlying sediments, assuming that the basement does not contain large‐scale density variations. The density anomalies modeled in the Neogene sediments, showing higher densities in the basin troughs, can be related to diagenetic changes in the silica facies of the Monterey and Sisquoc formations. The basement structure model shows the basin as composed of parallel ridges and troughs, trending west‐northwest and bounded by steep slopes interpreted as fault scarps. The basin is bounded on the west by a north‐south trending slope which may also represent a fault scarp.
Uniform norm minimization in three dimensions
Authors: Huestis Stephen P.
Published: October 2002

Linear programming is the standard tool for constructing solutions to linear inverse problems which have the smallest possible maximum absolute value, or uniform norm. For problems in three dimensions, however, the tableaus are usually so large as to be computationally prohibitive. An iterative scheme can replace a single, very large linear programming tableau with a sequence of much smaller tableaus, making such problems tractable.
On: “Vertical seismic profiling: Separation of upgoing and downgoing acoustic waves in a stratified medium” by B. Seeman and L. Horowicz (GEOPHYSICS, 48, 555–568, May 1983)
Authors: Stainsby S. D. , Worthington M. H.
Published: October 2002

Seeman and Horowicz devised an elegant procedure for the separation of upgoing and downgoing waves in VSP data. Their method is based upon a least‐squares solution of the frequency‐domain equations which relate the upgoing and downgoing signals at a reference level to the observed signals at other levels in the Earth. The coefficients of these equations are time‐shift operations. Unfortunately, for frequencies [Formula: see text] where δt is the vertical time sampling interval, the denominator of the solution equations is zero. For this reason the authors only applied the method over a passband: [Formula: see text] where the cutoff frequencies [Formula: see text] and [Formula: see text] are chosen to reflect the useful frequency band of the signal.
The effect of the extent of freezing on seismic velocities in unconsolidated permafrost
Authors: Zimmerman Robert W. , King Michael S.
Published: October 2002

We develop a model to relate velocities of seismic waves in unconsolidated permafrost is idealized as an assemblage of spherical quartz grains imbedded in a matrix composed of spherical water inclusions in ice. The theory of Kuster and Toksöz, based on wave‐scattering considerations, is used to determine the effective elastic moduli, and hence the wave speeds. The Hasin-Shtrikman theoretical bounds on the elastic moduli of heterogeneous materials and considerations establish the plausibility of the model. The model predicts [Formula: see text] and [Formula: see text] to be decreasing functions of both the porosity and the water‐to‐ice ratio, and the ratio [Formula: see text] to be an increasing function of these two parameters. The theory is then applied to laboratory measurements of shear‐ and compressional‐wave velocities in 23 permafrost samples from different sites in the Beaufort Sea, Mackenzie River Valley, and Canadian Arctic islands. Although no direct measurements were made of the extents of freezing in these samples, the data are consistent with the predictions of the model. We show the theory can be used to predict the extent of freezing of the water in the pore spaces, based on knowledge of the porosity and either of the two wave speeds.
Electromagnetic surveying in current Middle Eastern archaeology: Application and evaluation
Authors: Frohlich Bruno , Lancaster Warwick J.
Published: October 2002

Since 1977, the Smithsonian Institution has had a major research program on the human biological history of the Near East. As part of this program, electromagnetic (EM) surveying methods have been extensively used to identify anomalies of potential archaeological significance below the surface. An EM-31 noncontacting terrain conductivity meter was used in Jordan, Bahrain, Kuwait, and Egypt with excellent results. In addition to the successful application of EM equipment to archaeological problems, we investigated the equipment’s response to changing environmental and climatic conditions. We have developed efficient methods for recording EM data. The following results were obtained. Recording accuracy is better than previously reported (0.97 percent repeatability error between two observers); the degree of accuracy is primarily a function of operator error and less a function of equipment reliability; and the EM-31 produces helpful, highly reproducible results in the arid and semiarid environment of the Middle East.
MAGSAT scalar anomaly map of South America
Authors: Ridgway J. R. , Hinze W. J.
Published: October 2002

A scalar magnetic anomaly map for South America and adjacent marine areas was prepared from MAGSAT data. Preparation of the map posed problems, notably in separating external field and crustal anomalies, and in reducing data to a common altitude. External fields are manifested in a long‐wavelength ring current effect, a medium‐wavelength equatorial electrojet, and short‐wavelength noise. The ring current is corrected through use of a standard ring current equation, augmented by further wavelength filtering. Because the electrojet is confined primarily to dusk profiles, its effect is minimized by drawing the data set from dawn profiles only. The noise is reduced by selecting profiles from “quiet” periods [Formula: see text]. The optimum filter cutoff level is determined by a statistical regression technique to be approximately 50 degrees wavelength. When wavelengths longer than this are rejected, resultant profiles from redundant satellite orbits correlate well, with differences due primarily to satellite altitude. These differences are eliminated by normalizing the data to a constant altitude by the equivalent point‐source inversion technique. The resultant map differs significantly from the original 2 degree averaged version, most notably in low geomagnetic latitudes. A version of this map reduced to the pole shows correlations with several regional South American and Caribbean tectonic features.
Wave‐field extrapolation by the linearly transformed wave equation
Authors: Li Zhiming
Published: October 2002

Many approximations to different order of the one‐way scalar wave equation have been suggested in seismic imaging or modeling. Of these approximations, the first‐order approximation, usually called the 15‐degree equation, is most commonly used in industry because of its high efficiency. However, one common constraint of all these approximations is that they cannot handle large‐angle events exactly. Through a linear transformation of the wave equation, the LInearly Transformed Wave EQuation (LITWEQ) is obtainable, without approximation. The LITWEQ has the form of the 15‐degree equation. The solution to the LITWEQ is still a two‐way wave solution. By imposing the boundary condition for upcoming (or downgoing) waves, the LITWEQ can be applied to seismic imaging (or modeling). Implementing the LITWEQ with a finite‐differencing algorithm gives a 180‐degree, or all‐dip, finite‐difference wave‐extrapolation operator, which solves the angle limitation problem of conventional finite‐difference methods.
A simple method for the absolute measurement of thermal conductivity of drill cuttings
Authors: Lee Tien‐Chang , Henyey Thomas L. , Damiata Brian N.
Published: October 2002

We present a method for absolute measurement of thermal conductivity of drill cuttings. The simplicity of the apparatus makes it suitable for nondestructive use of cuttings and for sample sizes too small to be measured with a needle probe. Because the measurement is absolute, no calibration standards are required. Samples are placed in a Plexiglas cup with a lid containing an electric heat source. The base of the cup is placed in good thermal contact with an aluminum‐block heat sink. Upward and radial heat losses are minimized with styrofoam insulation surrounding the cup. The accuracy of the method was estimated by cross‐measurement of selected samples with a well‐calibrated needle probe. Results indicate that errors in measurement are less than 5 percent for sample conductivities greater than 0.8 W/m ⋅ K if the heat loss through the styrofoam insulation is accounted for. Reproducibility is typically within 3 percent. An axisymmetric finite‐element model which simulates the temperature distribution of the measurement apparatus further demonstrates its viability.
An investigation of the potential for geothermal energy recovery in the Calgary area in southern Alberta, Canada, using petroleum exploration data
Authors: Lam H. L. , Jones F. W.
Published: October 2002

The geothermal gradient in the Calgary area of southern Alberta is about 24 °C/km. Although this is only an average geothermal gradient, the city lies on the flank of a deep portion of the western Canadian basin with sediment thickness of over 4 km so that a wide temperature range exists in the sediments. This factor, and the substantial population of the city and surrounding area, satisfy two prerequisites for successful recovery and use of low‐grade geothermal energy. The Calgary area is, therefore, a suitable candidate for an investigation of its geothermal energy potential. The results of a study of petroleum exploration data show that good aquifers exist in the carbonate rocks of the Elkton formation of the Mississippian and the Wabamun formation of the Upper Devonian. The water temperatures range from 60 °C to 90 °C with salinities of 70 000 to 100 000 mg/l, which is two to three times that of average sea water. High water flow rates up to [Formula: see text] from the Elkton formation at moderate depths may be obtained in areas northwest and south of the city. Although the flow rates for the Wabamun formation are lower, the Wabamun and the deeper Leduc formations could supplement the Elkton as the main target for geothermal purposes. The availability of hot water of reasonable quality and at reasonable depth, together with the large population, means that Calgary presents an attractive location for geothermal energy recovery and use.
On: “Application of Gauss’s method to magnetic anomalies of dipping dikes” by I. J. Won (GEOPHYSICS, 46, 211–215, February, 1981).
Authors: Rao C. V.
Published: October 2002

Dr. Won chose to comment on some of the earlier computerized interpretation schemes on the “inversion of a magnetic profile across a dipping dike” where he remarked that the schemes are often susceptible to (1) difficulty in absolute convergence, and (2) dependence of the final solution on the initially assumed model. Dr. Won described an iterative scheme which combines the differential correction and least‐squares methods, and he claimed rapid convergence to minimum rms error. The scheme he proposed seems to be substantially the same as the one described by Rao et al. (1973), who also employed differential correction and least‐squares criteria for the convergence of the solution. The flow chart given by Rao et al. (p. 714) clearly shows this. The only difference I could observe in the subject paper from that of Rao et al. is the imposition of certain allowable limits on each parameter of the dike to be determined so that the solution does not diverge. This is outside the principle of the method. Furthermore, no new scheme has been developed to eliminate the dependence of the final solution of the dike on the initial model; without such a scheme the conclusion arrived at in the paper, “that the final solution is independent of the initial choice of the model,” is not realistic.
Polymetallic sulfide exploration on the deep sea floor: The feasibility of the MINI‐MOSES experiment
Authors: Wolfgram P. A. , Edwards R. N. , Law L. K. , Bone M. N.
Published: October 2002

The recent discoveries of polymetallic sulfide deposits on the deep sea floor have created an interest in geophysical techniques for mapping them. A magnetometric resistivity method (MINI‐MOSES) has been developed for detecting the distribution of sub‐sea‐floor electrical resistivity. A vertical, long‐wire bipole feeds electric current into the ocean. Some of the current enters the sea floor, and its magnetic field is measured at various distances by a low‐frequency induction coil magnetometer. The method is sensitive to conductive and resistive zones at depth, and information about three‐dimensional resistivity structures can be obtained. The small load resistance of an electric transmitter grounded in the ocean allows high current to be driven by a battery‐powered instrument. The induction coil receiver is more sensitive and less noisy than conventional flux‐gate magnetometers over the selected frequency band. This receiver is therefore well‐adapted to the almost noise‐free deep sea‐floor environment. Both the transmitter and the receiver are completely self‐contained, recording averaged transmitted and received signals in random access memory. The technique was tested near a sulfide deposit at 47° 57′N, 129°06′W on the axis of the Juan de Fuca Ridge. The deep submersible ALVIN positioned and moved the instruments about on the sea floor. Just enough data were obtained to demonstrate the viability of the technique for this kind of geologic mapping and mineral exploration. The electrical resistivity of 14 Ω ⋅ m obtained for a uniform half‐space model is consistent with Deep Sea Drilling Project (DSDP) results for the seawater‐saturated basalt.
Time‐domain solution for horizontal coaxial dipoles on a half‐space
Authors: Bartel David C.
Published: October 2002

The practice of transforming frequency‐domain results into the time domain is fairly common in electromagnetics. For certain classes of problems, it is possible to obtain a direct solution in the time domain. A summary of these solutions is given in Hohmann and Ward (1986). Presented here is another problem which can be solved directly in the time domain—the magnetic field of horizontal coaxial dipoles on the surface of a homogeneous half‐space. Solutions are presented for both an impulse transmitter current and a step turnon in the transmitter current. The solution in the time domain is obtained by taking the inverse Laplace transform of the product of the frequency‐domain solution and the Laplace‐domain representation of the current waveform.