Don W. Steeples

Avoiding pitfalls in shallow seismic reflection surveys

Acquiring shallow reflection data requires the use of high frequencies, preferably accompanied by broad bandwidths. Problems that sometimes arise with this type of seismic information include spatial aliasing of ground roll, erroneous interpretation of processed airwaves and air‐coupled waves as reflected seismic waves, misinterpretation of refractions as reflections on stacked common‐midpoint (CMP) sections, and emergence of processing artifacts. Processing and interpreting near‐surface reflection data correctly often requires more than a simple scaling‐down of the methods used in oil and gas exploration or crustal studies. For example, even under favorable conditions, separating shallow reflections from shallow refractions during processing may prove difficult, if not impossible. Artifacts emanating from inadequate velocity analysis and inaccurate static corrections during processing are at least as troublesome when they emerge on shallow reflection sections as they are on sections typical of petroleum ...

Seismic reflections from depths of less than two meters

Three distinct seismic reflections were obtained from within the upper 2.1 m of flood-plain alluvium in the Ar- kansas River valley near Great Bend, Kansas. Reflections were observed at depths of 0.63, 1.46, and 2.10 m and confirmed by finite-difference wave-equation modeling. The wavefield was densely sampled by placing geophones at 5-cm intervals, and near-source nonelastic deformation was minimized by us- ing a very small seismic impulse source. For the reflections to be visible within this shallow range, low seismic P-wave ve- locities (<300 m/s) and high dominant-frequency content of the data (-450 Hz) were essential. The practical implementa- tion of high-resolution seismic imaging at these depths has the potential to complement ground-penetrating radar (GPR), chiefly in areas where materials exhibiting high electrical con- ductivity, such as clays, prevent the effective use of GPR. Potential applications of these results exist in hydrogeology and environmental, Quaternary, and neotectonic geology.

A workshop examination of shallow seismic reflection surveying

In September 1996 the Office of Basic Energy Sciences of the U.S. Department of Energy (DOE) sponsored a research workshop in Berkeley, California, where approximately 20 participants analyzed the potential and limitations of near‐surface seismic‐reflection methods.

Field comparison of shallow seismic sources near Chino, California

Data from a shallow seismic-source comparison test conducted in an area with a water-table depth in excess of 30 m and near-surface velocities less than 330 m/s were acquired from 13 different sources at a single site near Chino, California. The sources included sledgehammer, explosives, weight drop, projectile impacts, and various buffalo guns. A possible reflecting event can be interpreted at about 70 ms. At this particular test site, the lowly sledgehammer is among the best sources to provide data to see the possible reflection. Our previous work and that of our colleagues suggests that any source could dominate the comparison categories addressed here, given the appropriate set of site characteristics.

Joint analysis of refractions with surface waves: An inverse solution to the refraction-traveltime problem

We describe a possible solution to the inverse refraction-traveltime problem (IRTP) that reduces the range of possible solutions (nonuniqueness). This approach uses a reference model, derived from surface-wave shear-wave velocity estimates, as a constraint. The application of the joint analysis of refractions with surface waves (JARS) method provided a more realistic solution than the conventional refraction/tomography methods, which did not benefit from a reference model derived from real data. This confirmed our conclusion that the proposed method is an advancement in the IRTP analysis. The unique basic principles of the JARS method might be applicable to other inverse geophysical problems.

Swept impact seismic technique (SIST)

A coded seismic technique is developed that can result in a higher signal‐to‐noise ratio than a conventional single‐pulse method does. The technique is cost‐effective and time‐efficient and therefore well suited for shallow‐reflection surveys where high resolution and cost‐effectiveness are critical. A low‐power impact source transmits a few to several hundred high‐frequency broad‐band seismic pulses during several seconds of recording time according to a deterministic coding scheme. The coding scheme consists of a time‐encoded impact sequence in which the rate of impact (cycles/s) changes linearly with time providing a broad range of impact rates. Impact times used during the decoding process are recorded on one channel of the seismograph. The coding concept combines the vibroseis swept‐frequency and the Mini‐Sosie random impact concepts. The swept‐frequency concept greatly improves the suppression of correlation noise with much fewer impacts than normally used in the Mini‐Sosie technique. The impact con...

Multichannel analysis of surface wave method with the autojuggie

Abstract The shear (S)-wave velocity of near-surface materials and its effect on seismic-wave propagation are of fundamental interest in many engineering, environmental, and groundwater studies. The multichannel analysis of surface wave (MASW) method provides a robust, efficient, and accurate tool to observe near-surface S-wave velocity. A recently developed device used to place large numbers of closely spaced geophones simultaneously and automatically (the ‘autojuggie’) is shown here to be applicable to the collection of MASW data. In order to demonstrate the use of the autojuggie in the MASW method, we compared high-frequency surface-wave data acquired from conventionally planted geophones (control line) to data collected in parallel with the automatically planted geophones attached to steel bars (test line). The results demonstrate that the autojuggie can be applied in the MASW method. Implementation of the autojuggie in very shallow MASW surveys could drastically reduce the time required and costs incurred in such surveys.

High‐resolution common‐depth‐point seismic reflection profiling: Instrumentation

Seismic recording hardware must be a deliberately designed system to extract and record high‐resolution information faithfully. The single most critical element of this system is the detector. The detector chosen must be capable of faithfully generating the passband expected and furthermore, must be carefully coupled to the ground. Another important factor is to shape the energy passband so that it is as flat and broad as possible. This involves low‐cut filtering of the data before A/D conversion so the magnitude of the low‐frequency signal does not swamp the high‐frequency signal. The objective is to permit boosting the magnitude of the high‐frequency signals to fill a significant number of bits of the digital word. Judicious use of a low‐cut filter is the main element of this step, although detector selection is also a factor because detectors have a −6 dB/octave velocity response at frequencies less than the resonant frequency of the detector. Finally, recording instrument quality must be good. Amplifi...

Useful resorting in surface‐wave method with the Autojuggie

The multichannel analysis of surface wave (MASW) method (Park et al., 1999; Xia et al., 1999, 2002a,b) is a relatively new technique. This technique consists (1) acquiring wide-band (∼2 to ∼100 Hz), high-frequency ground roll using a multichannel recording system; (2) creating efficient and accurate algorithms designed to extract and analyze 1D multimodal Rayleigh-wave dispersion curves from ground roll using a basic, robust, and pseudoautomated processing sequence; (3) developing stable and efficient algorithms (Xia et al., 1999) incorporating the minimum number of assumptions necessary to obtain 1D near-surface S-wave velocity profiles using the generalized linear inversion (GLI) method (Xia et al., 1999; Tian and Goulty, 1997); and (4) combining a standard common midpoint (CMP) roll-along acquisition format (Mayne, 1962) with surface-wave inversion of each shot gather to generate a cross-section of S-wave velocity (Xia et al., 1998; Miller et al., 1999). Based on published results (Xia et al., 2002a,b), when calculated with high accuracy, the fundamental mode phase velocities generally can provide reliable S-wave velocities with ±15% relative error.

Engineering and environmental geophysics at the millennium

Near‐surface geophysics is being applied to a broader spectrum of problems than ever before, and new application areas are arising continually. Currently, the tools used to examine the near‐surface environment include a variety of noninvasive methods employing electrical, electromagnetic, or mechanical energy sources, along with passive techniques that measure the physical parameters of the earth. Some of the advances of recent years have emerged from breakthroughs in instrumentation and computer‐processing techniques, and some have been driven by societal needs, such as the increasing demand for the accurate geophysical characterization of polluted sites. Other compelling factors, such as the ever‐expanding need for groundwater, the enactment of laws that have spurred geophysical surveying for archaeological purposes, and the necessity for better soil‐physics information in geotechnical engineering and agriculture, are present worldwide. For historical context, the reader is referred to an excellent revi...