Dean Keiswetter

A field investigation of source parameters for the sledgehammer

We examined amplitude and frequency changes in shallow seismic-reflection data associated with simple source-parameter modifications for the sledgehammer. Seismic data acquired at three sites with different near-surface geology show the potential effects of varying the hammer mass, the hammer velocity, the plate mass, and the plate area. At these study sites, seismic amplitudes depend on plate-surface area and on hammer mass but not heavily on hammer velocity or plate mass. Furthermore, although the total bandwidth of the recorded data was independent of source parameter changes, the peak frequency at one site was increased approximately 40 Hz by increasing the area of the plate. The results indicate that the effects of modifying the source parameters for the sledgehammer are site-dependent. The experiments described are quick, cheap, and simple, and can be duplicated by others at prospective sites to answer site-specific questions.

Seismic reflection analysis of the Manson Impact Structure, Iowa

Our combined interpretation of new, high-resolution seismic reflection data and reprocessed, but previously published, industrial Vibroseis data indicates that the Manson Impact Structure, Iowa, has an apparent crater diameter of 35 kin, an annular trough diameter of around 21 km, a shallow floor (0.6-0.7 kin), and a central uplift that has a minimum diameter of 7.5 kin. The two reflection lines are coincidentally located along an east-west radial transect and are consmined by shallow drill information. Results from the two data sets are correlative; both data sets were instnunental to the final interpretation due to the trade-offbetween resolution and depth of energy penetration. Based on the combined interpretation, structural uplift of the central peak is estimated to be around 2.8 kin. Onlapping seismic sequences are present at the eastern edge of the central uplift. These seismic packages, observed only in the high-resolution line, are interpreted to represent impact breccia or debris material that was shed from the central peak or dynamically transported from outside of the crater.

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.

Practical modifications to improve the sledgehammer seismic source

We have examined frequency and amplitude changes in high-resolution seismic-reflection data associated with practical modifications to the sledgehammer method. Our seismic data, acquired at three sites with different near-surface geology, demonstrate the effects of seating the plate prior to recording, of centered versus noncentered impacts, of subsurface plate emplacement, of various plate-surface covers, and of aluminum versus steel impact plates. Impacts on well-seated plates produced as much as 4 dB higher seismic amplitude than data recorded using unseated plates, and increased the ratio of high-to-low frequencies. Sledgehammer impacts on the edge of the plate decreased seismic amplitude by 6 to 12 dB for frequencies above 100 Hz compared to centered impacts. Placement of the impact plate 1 meter below the ground surface produced a 12 dB amplitude increase for frequencies above 130 Hz at one test site. Plates made of either steel alloy or aluminum produced equivalent seismic signals. The site-dependent nature of some of our results suggests that other investigators may benefit from conducting similar experiments prior to acquiring shallow seismic-reflection data when using the sledgehammer source.

Mapping bedrock beneath glacial till using CDP seismic reflection methods

This paper is a case history demonstrating the applicability of the common depth point (CDP) seismic reflection method to image bedrock beneath glacial till in northwestern Iowa. Reflections from the base of the 40-m thick glacial till are clearly observable on field files at around 45 to 50 ms two-way traveltime and possess a dominant frequency of around 100 Hz. The bedrock reflection is confirmed by drill data. The seismic data are of sufficient quality to detect local bedrock topographic changes and to interpret discontinuities along the till-bedrock interface. Finite-difference synthetic seismograms substantiate the interpreted reflections and the diffraction signatures from faults observed on the field files. At some locations along the seismic line, intra-till reflections are apparent on the field files. These Intra-till features are on the order of tens of meters in length along the line traverse and reflections from them are not enhanced by common depth point processing. Intra-till reflections could be indicative of gravels or other alluvial materials that may serve as local aquifers.

Detecting Burial Trenches Using Magnetic And Electromagnetic Techniques: A Case History

Magnetic and electromagnetic surveys, conducted over a known radioactive burial facility, accurately and efficiently delineated the spatial location of individual burial trenches and provided estimates regarding the volume of buried waste material. The geophysical data were acquired over the seven-acre site in three days, processed in the field, preliminary color-contour maps were produced, and the client was briefed on the results before leaving the site. Although the interpreted trenches correspond, in general, to “as-built” sketches of the site (provided after our final interpretations were made), they do differ markedly for a few of the trenches. For example, one of the trenches was shown to extend beyond the property boundary. Our final interpretations included estimated of the width, length, depth to the center of the trench, and approximate thickness of each trench.

Practical vertical resolution limits of CDP data targeting reflectors less than 125 m deep near Independence, Kansas

Shallow high-resolution reflection surveys can be used to study hydrocarbon reservoir analogs on a scale of 1 m vertically and 5 m horizontally. This scale is two orders of magnitude up from laboratory specimens and two orders of magnitude down from typical production reservoirs. A 12-fold CDP survey designed to determine practical resolution limits imaged geometric changes in an oolitic limestone near Independence, Kansas, at a depth of approximately 75 m. The dominant frequency of the stacked data is in excess of 180 Hz. The predicted resolution limits at this site, according the the l/

Improved Shallow Seismic-reflection Source: Building a Better Buffalo

-wavelength criterion, is on the order of 2 to 3 m. Practical resolution limits at this site could be as small as 1 to 1 l/2 m. Seismic reflection can be utilized at this site to accurately place stratigraphic core drilling holes for determining depositional environments and geologic processes responsible for the present geologic setting.

Significance of sledgehammer source parameters: A high‐resolution seismic reflection study

An improved design of the buffalo gun, incorporating hole drilling and shooting into one operation with almost total blast containment, provided nearly 50 percent more recordable seismic signal than the traditional buffalo gun. The ‘auger gun’ is designed to optimize field efficiency, source couple, and safety. The auger gun consists of three main parts: power source, auger/screw, and modified buffalo gun. Amplitude spectra indicate the auger gun delivers approximately 20 percent more total energy and 50 percent more reflection energy with no noticeable increase in recorded ground roll at a test site in Lawrence, Kansas. Subtle changes in source environment, such as a 15 cm gap between the detonation point and the base of the hole or inclusion of water as opposed to air at the detonation point, have a significant effect on the quality of the recorded data. The auger gun should increase field efficiency by over 50 percent on most shallow surveys, while improving the signal-to-noise ratio and total energy in comparison to the buffalo gun.