Chris M. Schmeissner

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.

Toward the Autojuggie: Planting 72 geophones in 2 sec

This is the publishers version, also available electronically from “http://onlinelibrary.wiley.com”.

In‐situ, high‐frequency P-wave velocity measurements within 1 m of the earth’s surface

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‐...

Near‐surface imaging using coincident seismic and GPR data

In many near-surface applications, detailed subsurface characterization is important. Characterization often is obtained using ground-penetrating radar (GPR) or shallow seismic-reflection (SSR) imaging methods, depending upon depth of interest and surficial geology. Each method responds to different physical properties; thus, each may produce different images of the same near-surface volume. By incorporating the two methods, we generated a cross-section of the subsurface at an alluvial test site and identified the depths of three interfaces accurately to ±5 cm. We present here experimental results and examples of SSR and GPR images obtained along the same traverse, showing coincident and noncoincident reflections from multiple interfaces within 3 m of the surface.

Geophones on a board

We examined the feasibility of using seismic reflections to image the upper 10 m of the earth’s surface quickly and effectively by rigidly attaching geophones to a wooden board at 5-cm intervals. The shallow seismic reflection information obtained was equivalent to control‐test data gathered using classic, single‐geophone plants with identical 5-cm intervals. Tests were conducted using both a .22-caliber rifle source and a 30.06-rifle source. In both cases, the results were unexpected: in response to our use of small, high‐resolution seismic sources at offsets of a few meters, we found little intergeophone interference that could be attributed to the presence of the board. Furthermore, we noted very little difference in a 60-ms intra‐alluvial reflection obtained using standard geophone plants versus that obtained using board‐mounted geophones. For both sources, amplitude spectra were nearly identical for data gathered with and without the board. With the 30.06 source, filtering at high‐frequency passbands...

The evolution of shallow seismic exploration methods

Near‐surface seismic methods have developed considerably and have been applied much more widely since the 1970s. Improvements in instrumentation, along with cheaper computer power, have greatly affected the capabilities of these methods in recent years. Based on these new capabilities we offer suggestions for future research in and applications for shallow‐seismic exploration methods. We present our recommendations in the context of significant developments in shallow‐seismic techniques from the 1920s to the mid‐1990s, concentrating on seismic reflection and, to a lesser degree, refraction and surface‐wave studies. The recent advent of hardware capable of collecting as well as processing high‐resolution, near‐surface seismic data opens up new opportunities for three‐component recording and multimode analysis.

Contrasting near-surface and classical seismology

A statement in the article “The velocity domain” by Dave Marsden (TLE, July 1993) motivated this paper. On page 747 he states, “From seismic wave tests, we see that air waves are the slowest of all, the direct wave and ground roll often have comparable speeds, and reflected P-waves are the fastest.” Those who have spent their careers in oil‐exploration seismology probably would not question this assertion and would not necessarily be concerned by it. However, those experienced in near‐surface seismology are aware of cases in which Marsden’s statement does not hold true. Thus, we present data from two seismograms to illustrate that near‐surface seismology sometimes differs from that used to explore deeper targets.

A program for seismic wavefield modeling using finite-difference techniques

Abstract We discuss and present a general purpose computer program that uses finite-difference techniques to approximate the solution of the two-dimensional heterogeneous acoustic wave equation. The program (FDMODEL) uses explicit approximations of second-order accuracy for spatial and temporal sampling intervals and the energy-absorbing boundary conditions. In addition to creating snapshots and common-shot seismograms in standard data formats, the program can generate multiple synthetic seismograms in a common-depth-point data-acquisition mode. Synthetic seismograms produced by FDMODEL are examined analytically herein and verify the accuracy of the program. A comparison between high-resolution, seismic-reflection field data and synthetic seismic data demonstrates the programs usefulness. The program is suitable for demonstrating the concepts of acoustic wave-field spreading and providing synthetic seismograms for use as an interpretive tool. The code was written in FORTRAN 77 for a PC-compatible computer but can be modified easily for use on other computing environments.

Coincident GPR And Ultra-shallow Seismic Imaging In the Arkansas River Valley, Great Bend, Kansas

Summary The primary objective of the work-in-progress reported here is to compare near-surface imaging results using ultra-shallow (upper 3 m) seismic-reflection techniques and ground-penetrating radar (GPR) at an alluvial site near Great Bend, Kansas. Although various seismic and GPR surveys have been used for near-surface imaging, little i s known about how the two techniques might work i n concert for very-high-resolution surveys at depths where both techniques work well. We show examples from the test site demonstrating that imaging is possible in the 1- to 6-m range using both seismic and GPR techniques and then discuss the implications of each.

On coincident seismic and radar imaging

Recent advances in understanding near-surface seismicwave phenomena and data-acquisition techniques have allowed seismic P-wave reflections to be collected from depths as shallow as 60 cm. Therefore, shallow seismicreflection (SSR) and ground-penetrating radar (GPR) techniques should no longer be considered mutually exclusive, a bias in which SSR imaging is considered a “deep” tool and GPR a “shallow” one. Because of improvements in ultrashallow (< 3 m) seismic-reflection data collection methods, it has become possible to gather seismic data more routinely from 0.5 to 10 m, which is also the typical working range for GPR. The purpose of this paper is to show empirical relationships between seismic P-wave and EM-wave propagation velocities for various media and suggest how improvements in site analysis may be possible when both SSR and GPR data can be obtained.