bluuebunny/crossref_metadata_2025 huggingface dataset:
AVA analysis after velocity-independent DMO and imaging
Authors: Gardner Gerald H. F. , Canning Anat
Published: October 2002

A common midpoint (CMP) gather usually provides amplitude variation with offset (AVO) information by displaying the reflectivity as the peak amplitude of symmetrical deconvolved wavelets. This puts a reflection coefficient R at every offset h, giving a function R(h). But how do we link h with the angle of incidence, θ, to get the reflectivity function, R(θ)? This is necessary for amplitude versus angle-of-incidence (AVA) analysis. One purpose of this paper is to derive formulas for this linkage after velocity-independent dip-moveout (DMO), done by migrating radial sections, and prestack zero-offset migration. Related studies of amplitude-preserving DMO in the past have dealt with constant-offset DMO but have not given the connection between offset and angle of incidence after processing. The results in the present paper show that the same reflectivity function can be extracted from the imaged volume whether it is produced using radial-trace DMO plus zero-offset migration, constant-offset DMO plus zero-offset migration, or directly by prestack, common-offset migration. The data acquisition geometry for this study consists of parallel, regularly spaced, multifold lines, and the velocity of propagation is constant. Events in the data are caused by an arbitrarily oriented 3-D plane reflector with any reflectivity function. The DMO operation transforms each line of data (m, h, t), i.e., midpoint, half-offset, and time, into an (m1, k, t1) space by Stolt-migrating each radial-plane section of the data, 2h = Ut, with constant velocity U/2. Merging the (m1, k, t1) spaces for all the lines forms an (x, y, k, t1) space, where the first two coordinates are the midpoint location, the third is the new half-offset, and the fourth is the time. Normal moveout (NMO) plus 3-D zero-offset migration of the subspace (x, y, t1) for each k creates a true-amplitude imaged volume (X, Y, k, T). Each peak amplitude in the volume is a reflection coefficient linked to an angle of incidence.
3-D seismic imaging of complex structures in glacial deposits
Authors: Siahkoohi Hamid R. , West Gordon F.
Published: October 2002

A small‐scale, shallow‐penetration 3-D reflection seismic survey has been carried out over stratified glacial formations to test the method’s potential for delineating fine structural details with a reasonable field effort and cost. Recording was carried out with a 96-channel engineering seismograph. Blank shotgun shells were used as the energy source. Field records were obtained with usable reflection energy up to 500 Hz, exploring approximately a 220-m cubical volume. Although special processing was required to attenuate strongly dispersed and scattered surface waves, a stacked data volume of good quality has been assembled. More than 15 reflecting horizons with vertical resolution of about 1.5 m were clearly delineated within the Wisconsin sediments. Results are in good agreement with local outcrops and core information. The identified reflectors (interbeds) within the Late Wisconsin tills are extremely important. These reflectors may provide potential “hydraulic channels” for the movement of surface contaminants through the till into underlying aquifers.
The interaction of tube waves with borehole fractures, Part I: Numerical models
Authors: Kostek Sergio , Johnson David Linton , Randall Curtis J.
Published: October 2002

We solve, numerically, the equations of elastodynamics that govern the propagation of waves in a fluid‐filled borehole intersected by one or more fluid‐filled fractures, thus extending earlier work on formations presumed to be rigid. The model is axis symmetric, and it allows for arbitrary radial and vertical variations in the elastic properties of the formation. We have developed a novel gridding scheme that takes advantage of the assumed thinness of the fracture compared to any relevant wavelength; this technique obviates the necessity for a mesh within the fracture itself and thereby saves computational time and memory. We illustrate our technique by means of examples with single and double fractures with or without washouts, as well as examples with variable width fractures. We compute the frequency‐dependent reflection and transmission coefficients of tube waves by fractures from the time waveforms generated by the present method. Major conclusions of this work are (1) reflections of tube waves by fractures can exceed the rigid frame prediction by as much as 50% even for “hard” formations, (2) a washout creates its own distinct signature on tube‐wave reflectivity, and (3) reflectivity spectra alone cannot distinguish closely spaced multiple fractures, unless the separation is comparable to the wavelength of the tube wave.
Marine magnetotellurics for petroleum exploration Part I: A sea‐floor equipment system
Authors: Constable Steven C. , Orange Arnold S. , Hoversten G. Michael , Morrison H. Frank
Published: October 2002

Induction in electrically conductive seawater attenuates the magnetotelluric (MT) fields and, coupled with a minimum around 1 Hz in the natural magnetic field spectrum, leads to a dramatic loss of electric and magnetic field power on the sea floor at periods shorter than 1000 s. For this reason the marine MT method traditionally has been used only at periods of 103 to 105 s to probe deep mantle structure; rarely does a sea‐floor MT response extend to a 100-s period. To be useful for mapping continental shelf structure at depths relevant to petroleum exploration, however, MT measurements need to be made at periods between 1 and 1000 s. This can be accomplished using ac-coupled sensors, induction coils for the magnetic field, and an electric field amplifier developed for marine controlled‐source applications. The electrically quiet sea floor allows the attenuated electric field to be amplified greatly before recording; in deep (1-km) water, motional noise in magnetic field sensors appears not to be a problem. In shallower water, motional noise does degrade the magnetic measurement, but sea‐floor magnetic records can be replaced by land recordings, producing an effective sea‐surface MT response. Field trials of such equipment in 1-km-deep water produced good‐quality MT responses at periods of 3 to 1000 s; in shallower water, responses to a few hertz can be obtained. Using an autonomous sea‐floor data logger developed at Scripps Institution of Oceanography, marine surveys of 50 to 100 sites are feasible.
Estimation of fracture height using microseismicity associated with hydraulic fracturing
Authors: Wu Yafei , McMechan George A.
Published: October 2002

The ratio of horizontal‐to‐vertical (H/V) particle velocity in background microseismic radiation associated with hydraulic fracturing is substantially higher in the dilatant, low‐velocity fractured zone than it is outside. This provides a useful diagnostic for determining the height of the fractured zone. Numerical synthesis of guided wave phenomena within the low‐velocity fractured zone accounts for much of the observed behavior, but measured H/V patterns are not totally consistent with either pure tensile or pure shear sources. A composite model containing both tensile‐compressional sources and asperity shear failures appears to satisfy the main observations better than either source type does alone. This composite is consistent with current models of earthquake aftershock sequences, which also have different mechanisms at the edges and in the interior of a fracture zone (tensile and shear, respectively). The H/V phenomenon is consistent with a predominance of energy with shear‐wave polarization traveling at postcritical angles, produced either directly by the source or by P-to-S conversion at the edges of the fracture zone. The H/V ratios are enhanced by increasing dilatancy, which decreases the velocity within the fracture zone.
Comparison of Kirchhoff and reverse‐time migration methods with applications to prestack depth imaging of complex structures
Authors: Zhu Jinming , Lines Larry R.
Published: October 2002

The performance of two popular migration methods—the Kirchhoff integral and reverse‐time migrations—is evaluated through applications to imaging complex structures using prestack shot records. The migration results from the Marmousi model data demonstrate that reverse‐time migration is more accurate than Kirchhoff migration in imaging the steeply dipping faults. However, the improved accuracy of reverse‐time migration requires higher computational costs. In the application to the Alberta Foothills data where a good estimate of the velocity model is available, however, both the Kirchhoff and the reverse‐time migration methods produce almost identical results. This implies that in the real world of exploration seismology, it will be relatively difficult to identify which method performs better because we never know the exact answer of the subsurface.
On resolution and uniqueness in anisotropic crosshole traveltime tomography
Authors: Williamson Paul R.
Published: October 2002

The inclusion of anisotropy in P-wave traveltime tomography has been undertaken by several authors and in all cases some loss of resolution and uniqueness compared to the isotropic problem was observed. The origin of this problem is analysed for straight‐ray tomography using the Radon transform and the projection slice theorem. This analysis shows that the separation of the anisotropy from the isotropic velocity field can only be guaranteed for the dc component. Resolution at higher spatial frequencies depends upon the spatial support of the object, that is the separation of the holes in crosshole work, and the range of ray angles available. Specific calculations in the crosshole case suggest that even in media known to be horizontally stratified, it will probably be difficult to estimate spatially varying elliptical anisotropy at wavelengths less than a few times smaller than the hole separation, and Thomsen’s parameter, delta at wavelengths less than the hole separation itself. These results broadly agree with the empirical observations in the literature.
Shallow seismic reflection section—Introduction
Authors: Steeples Don W.
Published: October 2002

For those interested in shallow seismic reflection (SSR) techniques, this special issue of Geophysics is likely to serve as a useful reference for years to come. The idea for this issue grew out of discussions that took place at the Shallow Seismic Reflection Workshop at the Lawrence Berkeley Laboratory, California, in September 1996. The content of those discussions is the subject of a published report elsewhere (Steeples et al., 1997). Several workshop participants and their colleagues contributed to the papers in this issue as authors and as reviewers. The articles include case histories, novel uses of the SSR technique, state‐of‐the‐art planning considerations for 3-D SSR surveys, and some examples of problems unique to SSR surveying.
High‐resolution shallow‐seismic experiments in sand, Part I: Water table, fluid flow, and saturation
Authors: Bachrach Ran , Nur Amos
Published: October 2002

A high‐resolution, very shallow seismic reflection and refraction experiment was conducted to investigate the seismic response of groundwater level changes in beach sand in situ. A fixed 10-m-long receiver array was used for repeated seismic profiling. Direct measurements of water level in a monitoring well and moisture content in the sand were taken as well. The water table in the well changed by about 1 m in slightly delayed response to the nearby ocean tides. In contrast, inversion of the seismic data yielded a totally different picture. The reflection from the water table at high tide appeared at a later time than the reflection at low tide. This unexpected discrepancy can be reconciled using Gassmann’s equation: a low‐velocity layer must exist between the near‐surface dry sand and the deeper and much faster fully saturated sand. This low‐velocity layer coincides with the newly saturated zone and is caused by a combination of the sand’s high density (close to that of fully saturated sand), and its high compressibility (close to that of dry sand). This low‐velocity zone causes a velocity pulldown for the high‐frequency reflections, and causes a high‐tide reflection to appear later in time than low‐tide reflection. The calculated velocities in the dry layer show changes with time that correlate with sand dryness, as predicted by the temporal changes of the sand’s density due to changing water/air ratio. The results show that near‐surface velocities in sand are sensitive to partial saturation in the transition zone between dry and saturated sand. We were able to extract the saturation of the first layer and the depth to the water table from the seismic velocities. The high‐resolution reflections monitored the flow process that occurred in the sand during the tides, and provided a real‐time image of the hydrological process.
Seismic reflection imaging of a geothermal aquifer in an urban setting
Authors: Liberty Lee
Published: October 2002

A seismic reflection survey that was conducted in downtown Boise, Idaho, to help city planners site a new well for injection of spent geothermal water illustrates some methods to safely and successfully employ a seismic reflection survey in an urban setting. The objective of the seismic survey was to estimate the depth and continuity of a basalt and rhyolite volcanic sequence. Well siting was based on geothermal aquifer depth, location of interpreted faults, projected thermal impact of injection on existing wells, surface pipe extension costs, and public land availability. Seismic acquisition tests and careful processing were used to ensure high‐quality data while minimizing the potential for damage along city streets. A video camera placed in a sewer and a blast vibration monitor were used to confirm that energy from the seismic source (a 75-in3land air gun) did not damage nearby buildings, street surfaces, or buried utilities along the survey lines. Walkaway seismic tests were also used to compare signal quality of the air‐gun source to an explosive source for imaging targets up to 800 m depth. These tests show less signal bandwidth from the air‐gun source compared to the buried explosive source, but the air‐gun signal quality was adequate to meet imaging objectives. Seismic reflection results show that the top of this rhyolite/basalt sequence dips (∼8–11°) southwest away from the Boise foothills at depths of 200 to 800 m. Seismic methods enabled interpretation of aquifer depths along the profiles and located fault zones where injected water may encounter fracture permeability and optimally benefit the existing producing system. The acquisition and processing techniques used to locate the Boise injection well may succeed for other hydrogeologic and environmental studies in urban settings.
Large near‐surface velocity gradients on shallow seismic reflection data
Authors: Miller Richard D. , Xia Jianghai
Published: October 2002

Extreme velocity gradients occasionally present within near‐surface materials can inhibit optimal common midpoint (CMP) stacking of near‐surface reflection arrivals. For example, abrupt increases in velocity are observed routinely at the bedrock surface and at the boundary between the vadose and the saturated zone. When a rapid increase in near‐surface velocity is found, NMO correction artifacts manifested on CMP gathers as sample reversion, sample compression, or duplication of reflection wavelets can reduce S/N ratio on stacked data or can stack coherently. Elimination of these nonstretch‐related artifacts using conventional NMO-stretch muting requires near‐vertically incident reflection arrivals and allowable stretch ratios as small as 5% in some shallow environments. Radical allowable stretch mutes are not a feasible means to subdue these artifacts if high‐amplitude coherent noise on near‐offset traces inhibits identification and digital enhancement of shallow reflections. On most shallow seismic reflection data, long‐offset reflection arrivals (but less than wide angle) are critical to the generation of an interpretable stacked section. The difference in offset between the optimum window for shallow reflections within unsaturated sediments and reflections from the underlying saturated or consolidated‐material portion of the section inherently limits the effectiveness of conventional NMO corrections. Near‐surface average velocity increases of 200% in less than two wavelengths and at two‐way traveltimes less than 60 ms are not uncommon on shallow reflection data. Near‐surface reflections separated by large velocity gradients can rarely be accurately or optimally CMP processed using conventional approaches to NMO corrections. Large velocity‐gradient shallow reflection data require segregation of shallow lower velocity reflections from higher velocity reflections during processing to maximize the accuracy and resolution potential of the stacked section, as shown by examples herein.
Effects of soil‐moisture content on shallow‐seismic data
Authors: Jefferson Robert D. , Steeples Don W. , Black Ross A. , Carr Tim
Published: October 2002

Repeated shallow‐seismic experiments were conducted at a site on days with different near‐surface moisture conditions in unconsolidated material. Experimental field parameters remained constant to ensure comparability of results. Variations in the seismic data are attributed to the changes in soil‐moisture content of the unconsolidated material. Higher amplitudes of reflections and refractions were obtained under wetter near‐surface conditions. An increase in amplitude of 21 dB in the 100–300 Hz frequency range was observed when the moisture content increased from 18% to 36% in the upper 0.15 m (0.5 ft) of the subsurface. In the time‐domain records, highly saturated soil conditions caused large‐amplitude ringy wavelets that interfered with and degraded the appearance of some of the reflection information in the raw field data. This may indicate that an intermediate near‐surface moisture content is most conducive to the recording of high‐quality shallow‐seismic reflection data at this site. This study illustrates the drastic changes that can occur in shallow‐seismic data due to variations in near‐surface moisture conditions. These conditions may need to be considered to optimize the acquisition timing and parameters prior to collection of data.
Downhole seismic logging for high‐resolution reflection surveying in unconsolidated overburden
Authors: Hunter J. A. , Pullan S. E. , Burns R. A. , Good R. L. , Harris J. B. , Pugin A. , Skvortsov A. , Goriainov N. N.
Published: October 2002

Downhole seismic velocity logging techniques have been developed and applied in support of high‐resolution reflection seismic surveys. For shallow high‐resolution reflection surveying within unconsolidated overburden, velocity‐depth control can sometimes be difficult to achieve; as well, unambiguous correlation of reflections with overburden stratigraphy is often problematic. Data obtained from downhole seismic logging can provide accurate velocity‐depth functions and directly correlate seismic reflections to depth. The methodologies described in this paper are designed for slimhole applications in plastic‐cased boreholes (minimum ID of 50 mm) and with source and detector arrays that yield similar frequency ranges and vertical depth resolutions as the surface reflection surveys. Compressional- (P-) wave logging uses a multichannel hydrophone array with 0.5-m detector spacings in a fluid‐filled borehole and a high‐frequency, in‐hole shotgun source at the surface. Overlapping array positions downhole results in redundant first‐arrival data (picked using interactive computer techniques), which can be processed to provide accurate interval velocities. The data also can be displayed as a record suite, showing reflections and directly correlating reflection events with depths. Example applications include identification of gas zones, lithological boundaries within unconsolidated sediments, and the overburden‐bedrock interface. Shear- (S-) wave logging uses a slimhole, well‐locked, three‐component (3-C) geophone pod and a horizontally polarized, hammer‐and‐loaded‐plate source at ground surface. The pod is moved in successive 0.5- or 1-m intervals downhole with no redundancy of overlapping data as in the P-wave method. First‐arrival data can be obtained by picking the crossover onset of polarized energy or by closely examining particle‐motion plots using all three components of motion. In unconsolidated sediments, shear‐wave velocity contrasts can be associated with changes in material density or dynamic shear modulus, which in turn can be related to consolidation. Example applications include identification of a lithological boundary for earthquake hazard applications and mapping massive ice within permafrost materials.
Refraction tomography over a buried waste disposal site
Authors: Lanz Eva , Maurer Hansruedi , Green Alan G.
Published: October 2002

Determining the depth and geometry of a landfill’s lower boundary is a difficult task. Potential field methods generally lack the necessary depth resolution, and seismic reflection data are usually contaminated by source‐generated noise in the time range of interest (<50 ms). To address this problem, we have developed a surface 2-D tomographic refraction scheme that is based on a fast finite‐difference eikonal solver and an inversion method that incorporates appropriate damping and smoothing constraints. This new scheme has been applied to a first‐arrival traveltime data set collected across adjacent landfills in northern Switzerland. High‐quality seismic data were collected along five profiles that crossed the landfills and two that sampled undisturbed natural sediments. Seismic waves generated from multiple shots were recorded on large numbers of closely spaced receivers during quiet evening periods. Reliability of the resultant velocity tomograms was estimated on the basis of (1) ray diagrams, (2) plots of synthetic and observed traveltimes, (3) traveltime residual analyses, (4) comparisons of coincident velocity‐depth profiles computed from intersecting profiles, (5) inversions with diverse input models, and (6) quantitative error analyses using a bootstrap technique. At our study site, the base of the near‐surface natural layer and the lower boundaries of the landfills were defined by rapid increases in velocity from <1000 m/s to >1500 m/s, with velocities in the upper parts of the models determined to within about ±100 m/s. The thickness of the near‐surface natural layer varied between 2 and 6 m, with occasional thickening to ∼7 m. In contrast, low velocities associated with the landfills could be traced to 9 to 11 m depth. Although our results have demonstrated that the tomographic refraction scheme may be an efficient and cost‐effective means of studying the very shallow subsurface (<20 m depth), complementary geological and other geophysical data were required to discriminate between velocity anomalies attributed to the landfills and those attributed to natural variations in the near‐surface geology.
3-D instantaneous frequency used as a coherency/continuity parameter to interpret reservoir compartment boundaries across an area of complex turbidite deposition
Authors: Hardage B. A. , Pendleton V. M. , Simmons J. L. , Stubbs B. A. , Uszynski B. J.
Published: October 2002

In previous unpublished work, we found that anomalous values of instantaneous frequency (that is, frequency values that are negative or that have positive magnitudes greater than Nyquist limit) are valuable indicators of alterations in reflection waveshape that occur commonly at stratigraphic terminations. Inspection of 3-D seismic data across Nash Draw Field on the northwest slope of the Delaware Basin showed that appreciable wavelet alterations occurred at the boundaries of distinct seismic facies within the targeted Brushy Canyon reservoirs that are being produced in this field. Based on this observation, we used instantaneous frequency as the fundamental database to define the edge positions and areal shapes of individual reservoir facies within this complex, slope‐basin distribution of siltstones and sandstones, commonly thought to be a succession of turbidite depositions. We compared the compartmentalization detail derived from this frequency‐based approach with compartmentalization models provided by an amplitude‐based interpretation and by coherency/continuity cube technology. These comparisons led us to conclude that a properly executed 3-D interpretation of instantaneous frequency behavior can provide a good first guess of the internal compartmented structure of many reservoirs. We offer our work here for peer evaluation by people who support reservoir characterization studies similar to what we describe. We have used instantaneous frequency behavior to successfully detect lateral disruptions in stratigraphic continuity for several years—long before the current concept of a coherency (or continuity) cube was publicized for this same purpose. This study shows that interpreters who do not have ready access to coherency‐cube technology, but who do have Hilbert transform algorithms available to them, can create 3-D volumes of instantaneous frequency that provide valuable indications of reflection discontinuity, much in the same way that continuity/coherency cube technology does.
An analytical method to interpret self‐potential anomalies caused by 2-D inclined sheets
Authors: Sundararajan N. , Rao P. Srinivasa , Sunitha V.
Published: October 2002

The first‐order horizontal and vertical derivatives of the self‐potential (SP) anomalies caused by a 2-D inclined sheet of infinite horizontal extent are analysed to obtain the depth h, the half width a, the inclination α and the constant term containing the resistivity ρ and the current density I of the surrounding medium. The vertical derivative of the SP anomaly is obtained from the horizontal derivative via the Hilbert transform, which is also redefined to yield a modified version, a 270° phase shift of the original function. The point of intersection of these two Hilbert transforms corresponds to the origin. The amplitudes constitute a similar case. The practicability of the method is tested on a theoretical example as well as on field data from the Surda area of Rakha mines, Singhbhum belt, Bihar, India. The results agree well with those of other methods in use. Since the procedure is based on a simple mathematical expression involving the real roots of the derivatives, it can easily be automated.
Sediments with gas hydrates: Internal structure from seismic AVO
Authors: Ecker Christine , Dvorkin Jack , Nur Amos
Published: October 2002

We interpret amplitude variation with offset (AVO) data from a bottom simulating reflector (BSR) offshore Florida by using rock‐physics‐based synthetic seismic models. A previously conducted velocity and AVO analysis of the in‐situ seismic data showed that the BSR separates hydrate‐bearing sediments from sediments containing free methane. The amplitude at the BSR are increasingly negative with increasing offset. This behavior was explained by P-wave velocity above the BSR being larger than that below the BSR, and S-wave velocity above the BSR being smaller than that below the BSR. We use these AVO and velocity results to infer the internal structure of the hydrated sediment. To do so, we examine two micromechanical models that correspond to the two extreme cases of hydrate deposition in the pore space: (1) the hydrate cements grain contacts and strongly reinforces the sediment, and (2) the hydrate is located away from grain contacts and does not affect the stiffness of the sediment frame. Only the second model can qualitatively reproduce the observed AVO response. Thus inferred internal structure of the hydrate‐bearing sediment means that (1) the sediment above the BSR is uncemented and, thereby, mechanically weak, and (2) its permeability is very low because the hydrate clogs large pore‐space conduits. The latter explains why free gas is trapped underneath the BSR. The seismic data also indicate the absence of strong reflections at the top of the hydrate layer. This fact suggests that the high concentration of hydrates in the sediment just above the BSR gradually decreases with decreasing depth. This effect is consistent with the fact that the low‐permeability hydrated sediments above the BSR prevent free methane from migrating upwards.
A bootstrap procedure for high‐resolution velocity analysis
Authors: Sacchi Mauricio D.
Published: October 2002

A method is given for further improving velocity estimates derived from high‐resolution velocity analysis. In conventional velocity analysis, a set of tentative velocities is used to apply a normal moveout (NMO) correction to a set of spatio‐temporal windows, the coherence measure is evaluated for each velocity and finally, the velocity estimate is retrieved from the peak of the coherence measure. Because analytical expressions for velocity uncertainties are difficult to derive, I propose an intensive statistical procedure, the bootstrap method, to assess the accuracy of the velocity estimate. In the bootstrap method, I create a data sample by randomly drawing seismic traces with replacement from a window of the common midpoint gather (CMP). Next, I calculate the velocity that maximizes the coherence measure for each bootstrap realization. The variation of this velocity provides a means to compute standard errors. I also use the bootstrap method to construct an average coherence measure and a kernel density estimator of the velocity that maximizes the coherence. The average coherence exhibits an important attenuation of spurious events while retaining enough resolution to model reflections properly with similar moveout curves. The latter is illustrated with synthetic and field data examples.
Finite‐impulse‐response reduction‐to‐the‐pole filter
Authors: Lu Richard S.
Published: October 2002

Convolving a finite‐impulse‐response (FIR) filter with a magnetic anomaly map produces a reduction‐to‐the‐pole (RTP) that is superior to that of the conventional Fourier‐transform approach. The conventional approach, in which the map’s Fourier transform is multiplied by the frequency response of the RTP filter, is flawed by not accounting properly for the dimensions of the respective Fourier transforms. The resultant wraparound effect of circular convolution degrades the RTP map. The FIR filter, combined with linear convolution and appropriate choices for dimensions of data and filter, eliminates the wraparound effect, minimizes contamination of the result by noise, and improves stability. These properties are illustrated by a synthetic example and by application to an actual data set.
Multichannel processing for airborne gamma‐ray spectrometry
Authors: Minty Brian R. S. , McFadden Phil , Kennett Brian L. N.
Published: October 2002

The conventional approach to the processing of airborne gamma‐ray spectrometric data is to first sum the observed spectra over three relatively broad energy windows. These three window count rates are then processed to obtain estimates of the potassium (K), uranium (U), and thorium (Th) elemental abundances. However, multichannel spectra contain additional information on the concentrations of K, U, and Th in the source, on the distance between the source and the detector, and on the relative contribution of atmospheric radon to the observed spectrum. This information can be extracted using multichannel processing procedures. The observed spectrum is considered as the sum of three terrestrial and three background component spectra, which are determined through suitable airborne and ground calibrations. The background components can be calculated independently and removed from the observed spectra. A parametric model based on a principal component analysis of the terrestrial components as functions of simulated detector height is then used to find the effective heights at which the K, U, and Th terrestrial components best fit the background‐corrected airborne data. The component spectra for these heights are then fitted to the background‐corrected observed spectra to obtain elemental count rates. The multichannel processing results in significant reductions in the fractional errors associated with the estimated elemental count rates. For three surveys processed using the new methodology, the average deviations of the K, U, and Th elemental count rates from the estimated mean elemental count rates at each observation point are reduced by 12.4%, 26.5 %, and 20.3 %, respectively, when compared with the conventional three‐channel method. This results in a better structural resolution of small anomalies in enhanced images of the processed data.
Multichannel models for the estimation of radon background in airborne gamma‐ray spectrometry
Authors: Minty Brian R. S.
Published: October 2002

Adequate background correction is a crucial step in processing airborne gamma‐ray spectrometric data because any errors are amplified during subsequent processing procedures. Two multichannel models for the estimation of atmospheric radon background are proposed. The spectral‐ratio method uses the relative heights of uranium (U) series photopeaks to estimate the contribution of atmospheric radon to observed spectra. The full‐spectrum method estimates the atmospheric radon contribution through the weighted least‐squares fitting of potassium (K), U, thorium (Th), and radon component spectra to the observed spectra. Both the spectral‐ratio and full‐spectrum methods are adequately calibrated through the estimation of component spectra from calibration experiments on the ground using radioactive calibration sources and wood to simulate the attenuation of gamma rays by air. The simulated heights used in these calibrations must be mapped onto real heights through calibration flights over an airborne calibration range. The spectral‐ratio method is also adequately calibrated using a heuristic calibration procedure. An iterative minimization method is used to find the optimum values of the calibration constants such that the radon background over suitable calibration lines is best removed.
Measurement of air‐gun bubble oscillations
Authors: Ziolkowski Anton
Published: October 2002

In this paper, I provide a theoretical basis for a practical approach to measuring the pressure field of an air gun array and present an algorithm for computing its wavefield from pressure measurements made at known positions in the vicinity of the gun ports. The theory for the oscillations of a single bubble is essentially a straight‐forward extension of Lamb’s original paper and provides a continuous, smooth transition from the oscillating wall of the bubble to the far‐field, preserving both the fluid flow and the acoustic radiation, all to the same accuracy and valid for bubbles with initial pressures up to about 200 atm (3000 psi or 20 MPa). The simplifying assumption, based on an argument of Lamb, is that the particle velocity potential obeys the linear acoustic wave equation. This is used then in the basic dynamic and kinematic equations to lead, without further approximations, to the nonlinear equation of motion of the bubble wall and the wavefield in the water. Given the initial bubble radius, the initial bubble wall velocity, and the pressure variation at any point inside or outside the bubble, the algorithm can be used to calculate the bubble motion and the acoustic wavefield. The interaction among air‐gun bubbles and the resultant total wavefield is formulated using the notional source concept, in which each bubble is replaced by an equivalent notional bubble obeying the same equation of motion but oscillating in water of hydrostatic pressure, thus allowing the wavefields of the notional bubbles to be superposed. A separate calibration experiment using the same pressure transducers and firing the guns individually allows the initial values of the bubble radius and bubble wall velocity to be determined for each gun. An appendix to the paper provides a test of the algorithm on real data from a single gun.
Coherency calculations in the presence of structural dip
Authors: Marfurt Kurt J. , Sudhaker V. , Gersztenkorn Adam , Crawford Kelly D. , Nissen Susan E.
Published: October 2002

We have used crosscorrelation, semblance, and eigenstructure algorithms to estimate coherency. The first two algorithms calculate coherency over a multiplicity of trial time lags or dips, with the dip having the highest coherency corresponding to the local dip of the reflector. Partially because of its greater computational cost, our original eigenstructure algorithm calculated coherency along an implicitly flat horizon. Although generalizing the eigenstructure algorithm to search over a range of test dips allowed us to image coherency in the presence of steeply dipping structures, we were somewhat surprised that this generalization concomitantly degenerated the quality of the fault images in flatter dip areas. Because it is a local estimation of reflector dip (including as few as five traces), the multidip coherency estimate provides an algorithmically correct, but interpretationally undesirable, estimate of the best apparent dip that explained the offset reflectors across a fault. We ameliorate this problem using two methods, both of which require the smoothing of a locally inaccurate estimate of regional dip. We then calculate our eigenstructure estimate of coherency only along the dip of the reflector, thereby providing maximum lateral resolution of reflector discontinuities. We are thus both better able to explain the superior results obtained by our earliest eigenstructure analysis along interpreted horizon slices, yet able to extend this resolution to steeply dipping reflectors on uninterpreted cubes of seismic data.
Simultaneous inversion for model geometry and elastic parameters
Authors: Wang Yanghua
Published: October 2002

Both traveltimes and amplitudes in reflection seismology are used jointly in an inversion to simultaneously invert for the interface geometry and the elastic parameters at the reflectors. The inverse problem has different physical dimensions in both data and model spaces. Practical approaches are proposed to tackle the dimensional difficulties. In using the joint inversion, which may properly take care of the structural effect, one potentially improves the estimates of the subsurface elastic parameters in the traditional analysis of amplitude variation with offset (AVO). Analysis of the elastic parameters estimated, using the ratio of s-wave to P-wave velocity contrasts and the deviation of this parameter from a normal background trend, promises to have application in AVO analysis. The inversion method is demonstrated by application to real data from the North Sea.
Dip moveout in the Radon domain
Authors: Wang Chengshu
Published: October 2002

I consider a new dip‐moveout (DMO) processing technique in the Radon domain called Radon DMO. The Radon DMO operator directly maps data from the NMO-corrected time domain to the DMO wavefield in the Radon domain. The method is built upon a process that transforms a single NMO-corrected trace into multiple traces spread along hyperbolas in the Radon domain. These hyperbolas are a linear Radon map of the DMO ellipses in the time domain. In this paper, I introduce the amplitude‐preserving Radon DMO and compare some examples of Radon DMO and Fourier DMO for both synthetic and real data. I also show the better frequency preservation properties of the Radon DMO method. Three‐dimensional data are often irregularly sampled with respect to fold, azimuth, and offset. Population deficiencies are exaggerated in the common‐offset domain. Radon DMO does not require that input traces belong to one common‐offset bin as does the Fourier method. Input traces can be organized from multiple offset bins grouping to perform Radon DMO, which is well used in 3-D surveys. Some synthetic and real data examples show these properties.
New scheme for computing the magnetic field resulting from a uniformly magnetized arbitrary polyhedron
Authors: Guptasarma D. , Singh B.
Published: October 2002

The magnetic field at any point outside a uniformly magnetized polyhedron of arbitrary shape is obtained by adding the fields resulting from the effective free magnetic poles on each of the polygonal surfaces of the polyhedron. For each polygonal surface, the components of the field at the point of observation are expressed in terms of new line integrals around the edges of the polygon and the solid angle subtended by the polygon at the point of observation. The line integrals are standard elementary forms. This new approach makes the numerical evaluation of the magnetic fields for such models much simpler and faster than previously published methods.
In‐situ, high‐frequency P-wave velocity measurements within 1 m of the earth’s surface
Authors: Baker Gregory S. , Steeples Don W. , Schmeissner Chris
Published: October 2002

Seismic P-wave velocities in near‐surface materials can be much slower than the speed of sound waves in air (normally 335 m/s or 1100 ft/s). Difficulties often arise when measuring these low‐velocity P-waves because of interference by the air wave and the air‐coupled waves near the seismic source, at least when gathering data with the more commonly used shallow P-wave sources. Additional problems in separating the direct and refracted arrivals within ∼2 m of the source arise from source‐generated nonlinear displacement, even when small energy sources such as sledgehammers, small‐caliber rifles, and seismic blasting caps are used. Using an automotive spark plug as an energy source allowed us to measure seismic P-wave velocities accurately, in situ, from a few decimeters to a few meters from the shotpoint. We were able to observe three distinct P-wave velocities at our test site: ∼130m/s, 180m/s, and 300m/s. Even the third layer, which would normally constitute the first detected layer in a shallow‐seismic‐refraction survey, had a P-wave velocity lower than the speed of sound in air.
Regional aquifer mapping using combined VES-TEM-AMT/EMAP methods in the semiarid eastern margin of Parnaiba Basin, Brazil
Authors: Meju M. A. , Fontes S. L. , Oliveira M. F. B. , Lima J. P. R. , Ulugergerli E. U. , Carrasquilla A. A.
Published: October 2002

As part of a program to evaluate the utility of integrated geoelectrical methods for subsurface structural mapping and groundwater resource investigation in the semiarid eastern margin of the Parnaiba basin in Brazil, several vertical electrical soundings (VES) and transient electromagnetic (TEM) and tensorial audiomagnetotelluric (AMT) measurements were carried out along a 250-km-long east‐west transect (passing through major towns and cities) and a 127-km-long north‐south profile (passing through small farm settlements). The various data sets have been jointly processed using a novel integration scheme and a constrained 1-D inversion technique to yield the resistivity structure underneath each observation station. Regularized 2-D inversion of static‐shift‐corrected, dual‐mode AMT data provided additional deep structural information and, together with the joint 1-D results, enabled an assessment of the distribution of aquifers and major structural controls in the region. The east‐west regional geoelectrical model evokes a picture of a gently dipping succession of conductive and resistive units in good agreement with the alternating shaly and sandy formations evinced from preexisting borehole data and previous geological studies. The geoelectric models also show the presence of the large‐scale Transbrazilian lineament and other graben‐like structures, previously inferred from aeromagnetic data, which may have some control on groundwater distribution. The agreement with geology and aeromagnetic interpretation emphasizes the importance of integrated geoelectrical surveying as a complementary or independent means of obtaining useful stratigraphic and structural information for hydrogeological studies in this region.
Geophysical surveys of burial sites: A case study of the Oaro urupa
Authors: Nobes David C.
Published: October 2002

A Maori family burial site was surveyed using shallow electromagnetic (EM), magnetometer/gradiometer, and ground‐penetrating radar (GPR) techniques. The locations of some older graves are no longer known, partly through the loss of markers and partly through the loss of the oral record. There are at least 150 such burial sites throughout New Zealand, and similar problems occur elsewhere in the world. The knowledge that Maori graves are traditionally set out facing east made survey design easier. Background trends are removed, and possible anomalous features are identified, despite the complicating presence of metal fencing. The EM in‐phase response, the vertical gradient of the total magnetic field, and the GPR response were particularly useful in combination. GPR surveys were carried out using two frequencies: 450 and 200 MHz. The higher frequency radar surveys were limited in extent because these data had a limited depth of penetration and were prone to signal “ringing” in the surficial clay. The lower frequency radar surveys were also troubled by ringing. Nonetheless, anomalous features, especially diffractions, can be recognised. The radar anomalies were enhanced by removal of an average background response. The comparison and correlation of the individual data sets are used to estimate the positions of unmarked graves, using marked graves for calibration. Some interpreted positions of graves are at first surprising, but their locations are consistent with elements of the oral history of the site.
Combined application of dc and TEM to sea‐water intrusion mapping
Authors: Yang Chieh-Hou , Tong Lun-Tao , Huang Ching-Feng
Published: October 2002

A geoelectric survey using a dc resistivity method and a transient electromagnetic induction (TEM) method was carried out from 1986 to 1988. It was used to help map the lateral and vertical distributions of the freshwater/salt‐water interface in the Pei‐kang area on the west coast of Taiwan. The dc and TEM soundings were performed at 79 localities over an area of 240 km2 of Quaternary alluvium. Significant changes in pore‐water conductivity at some places were detected by these two methods. A low resistivity value (<1.5 ohm-m) implies salt‐water contamination of groundwater. The results of spatial distribution of apparent resistivity indicate the salt‐water‐affected aquifers are confined to the southwest part of the study area, and the affected aquifers are confined to the top of two aquifers. A geoelectric model with three to four layers is inferred from the joint inversion of dc and TEM data. Based on a modified Archie’s law suggested by the authors, an empirical relation between pore‐water resistivity of the stratum and formation resistivity can be obtained. It can be used to convert the computed resistivity of each geoelectric layer (aquifer) to the resistivity of the water contained in these layer, thus leading to the assessment of groundwater contamination. Repeated dc resistivity measurements have been carried out at selected locations once every year from 1986 to 1988. These data were used to monitor the temporal variation and a possible spreading of the salt‐water invasion. During 1995, a dc survey was conducted in the same area to compare with the dc sounding results obtained in 1988. The final results provide an evaluation of the efficiency of groundwater management in the study area.