Shallow seismic reflection section—Introduction

This is the publisher's version, also available electronically from http://library.seg.org/doi/abs/10.1190/1.1444421. Reuse of this article is subject to SEG terms of use and conditions.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

Engineering and environmental geophysics at the millennium

This is the publisher's version, also available electronically from "http://library.seg.org".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 review concerning the status of shallow exploration techniques in the mid-1980s (Dobecki and Romig, 1985)

Structure of the Salina-Forest City Interbasin Boundary From Seismic Studies

As petroleum exploration efforts in the Midcontinent become directed toward smaller fields and the search for minerals is extended into new areas, the edges of the Salina and Forest City basins will become of increased interest to industry. The principal boundary feature between the two basins is the Nemaha ridge, a linear feature that extends from near Omaha, Nebraska, to near Oklahoma City, Oklahoma. Recent seismic studies at the Kansas Geological Survey have revealed a complex array of faulted and folded structures in the vicinity of the Humboldt fault zone (the eastern flank of the Nemaha ridge). Faulting of both normal and reverse types is present, including horsts and grabens. Although some Permian age faulting is present, most of the Permian deformation occurred as monoclinal draping at the flanks of the Nemaha ridge. Recent microearthquake activity suggests that some of the faults are slightly active along a zone 400 kilometers long (north-south) and 50 kilometers wide (east-west) coincident with the Nemaha ridge from southeastern Nebraska to north-central Oklahoma. Seismic reflection evidence suggests that either uplift along the Nemaha occurred contemporaneously with Pennsylvanian deposition or uplift and peneplantation occurred during a period of exposure between the deposition of Mississippian sediments and Pennsylvanian sediments. Analyses of boundary structures and intrabasin structures are not complete without knowledge of basement rock history and of basement structure. Microearthquake arrivals and deep reflection data recently obtained from the Consortium for Continental Reflection Profiling (COCORP) in Kansas reveal intrabasement structures in the 10 to 35 kilometer depth range. Data from aeromagnetic studies and basement drilling reveal block faulting patterns and several episodes of Precambrian intrusive and/or extrusive vulcanism. Much of the data presented in this paper is not yet fully analyzed, but preliminary results suggest that the integrated geological and geophysical techniques will be of increased value to petroleum and minerals exploration and will be of assistance in expanding the scientific knowledge of the Earth’s crust and upper mantle in the Midcontinent. Specifically, it is hypothesized that petroleum deposits are related to localized heating in the upper crust and are associated with igneous intrusions and ascension of mantle fluids into the crust probably during Cretaceous time. This hypothesis is consistent with the existence of known deposits of petroleum

Near-surface seismic reflection applications

Nonintrusive methods of gaining knowledge about the Earth’s subsurface comprise several of the procedures used routinely in near-surface seismology, including reflection, refraction, and surface-wave analysis. During the early 1980s the advent of digital engineering seismographs designed for shallow, high-resolution surveys spurred significant improvements in engineering, mining, and environmental reflection seismology. Commonly, the reflection method is used in conjunction with other geophysical and geological tools and a well-planned drilling verification effort. To the extent that near-surface seismic methods can constrain shallow stratigraphy, geologic structure, engineering properties, and relative permeability, they are useful in groundwater, mining, environmental site characterization, and other civil engineering applications. Much of the improvement in shallow seismic surveys is related to advancements in instrumentation. Challenges remain, however, in developing ways to process near-surface seismic data sets that may contain attributes not seen in deeper petroleum surveys

Far-field aftershocks of the 1906 earthquake

This is the publisher's version, also available electronically from "http://bssa.geoscienceworld.org".During the 24 hr following the great San Francisco, California, earthquake of 18 April 1906, separate seismic events were felt at Paisley, Oregon; Phoenix, Arizona; Los Angeles, California; and Brawley, California (MMIX). Using probability theory, we show that the occurrence of felt earthquakes in each of these widespread locations on the same day would constitute a rare event. Rates of felt-earthquake occurrences over a 9-yr period from 1897 to 1906 were determined for the four different regions that experienced earthquakes within 24 hr after the 1906 event. We modeled the likelihood of occurrence of these aftershocks in the spirit of the “ball-in-the-box” probability problem, and the results indicated a very high probability that the aftershock zone of the great earthquake extended at least 500 km beyond the extent of ground breakage, implying a disturbance of the stress field over an area at least two to three times longer than the fault break itself

Practical modifications to improve the sledgehammer source

This is the publisher's version, also available electronically from "http://onlinelibrary.wiley.com".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

Low velocity zone under Long Valley as determined from teleseismic events

This is the published version. Copyright 1976 American Geophysical Union. All Rights Reserved.A temporary seismograph station network was used to estimate teleseismic P wave residuals in the vicinity of Long Valley geothermal area, California. Relative P wave delays of 0.3 s persist at stations in the west central part of the Long Valley caldera after regional and near-surface effects have been removed. Ray tracing indicates that low-velocity material exists beneath the caldera at depths greater than 7 km and less than 40 km, probably less than 25 km. The velocity contrast with normal crust must be at least 5% to satisfy the data and is probably in the range 10–15%. We believe that the low velocity indicates anomalously hot rock at depth and that relative teleseismic P residuals may be useful for investigation of sources of geothermal energy

Kansas refraction profiles

Historically, refraction surveys have been conducted in hopes of mapping distinct layers within the earth. Refraction is a useful tool provided its limitations and the assumption that layers increase in seismic velocity with increasing depth are kept in mind. A traditional reversed-refraction profile was conducted along a 500-km (300- mi)-long east-west line extending from Concordia, Kansas, to Agate, Colorado. Analysis of the data showed an average crustal velocity of 6.1 kmlsec (3.7 milsec) and an average upper-mantle P phase velocity of 8.29 kmlsec (4.97 milsec) with a Moho depth calculated to be 36 km (23 mi) on the eastern end and 46 km (29 mi) on the western end. Some evidence suggests velocities as high as 7.2 kmlsec (4.3 milsec) in the crust at various locations along the survey line. The strong east-west regional gravity gradient of -0.275 mgalb supports the seismically drawn conclusion of a thinning of crust in north-central Kansas. In order to supplement the data from this refraction survey, we took advantage of the Kansas earthquake seismograph network. A crustal study using earthquakes as energy sources and a regional earthquake network as seismometer locations resulted in a crustal-velocity model that will improve determination of local earthquake locations. A large anomalous body in the upper mantle/lower crust, assumed to be related to the Precambrian-aged Midcontinent Geophysical Anomaly (MGA), resulted in early Pwave arrivals from refracted energy from the Moho recorded at Concordia, Salina, Tuttle Creek, and Milford. An omnidirectional positive P residual zone near El Dorado may be related to the Wichita geomagnetic low. Some evidence suggests the presence of a lower velocity material on the western and eastern flanks of the MGA, possibly representing the Rice Formation. A P velocity of 8.25 krn/sec±0.1k m/sec (4.95 mi/sec+0.09m i/sec) with the crust thinning from west to east and an apparent thinning from the north and from the south was determined from the 16 regional earthquakes studied. Crustal thickness from central Kansas through western Missouri seems to be relatively consistent

Avoiding pitfalls in shallow seismic reflection surveys

This is the publisher's version, also available electronically from "http://library.seg.org".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 exploration. Consequently, when using shallow seismic reflection, an interpreter must be exceptionally careful not to misinterpret as reflections those many coherent waves that may appear to be reflections but are not. Evaluating the validity of a processed, shallow seismic reflection section therefore requires that the interpreter have access to at least one field record and, ideally, to copies of one or more of the intermediate processing steps to corroborate the interpretation and to monitor for artifacts introduced by digital processing

Shallow structure from a seismic reflection profile across the Borah Peak, Idaho, fault scarp

This is the publisher's version, also available electronically from “http://onlinelibrary.wiley.com”.A short 12-fold CDP seismic-reflection survey was performed along the road to Doublespring Pass across the fault scarp formed by the October 28, 1983, magnitude-7.3, Idaho earthquake. This high-resolution reflection survey was conducted to determine the feasibility of using reflection seismology to delineate shallow structures in a fault zone. Field-recording parameters were designed to optimize seismic reflections in the 30-150 msec range corresponding to 10-100 m in depth. A modified 30-06 hunting rifle was used as the energy source. Single 100-Hz geophones at 1.5-m group intervals in conjunction with 220-Hz low-cut recording filters (24 dB/octave) provided dominant frequencies above 150 Hz on field records. As would be expected from geologic considerations, the processed data suggest the existence of faulting in the subsurface. Strong events between 30 and 80 msec on the upthrown side of the scarp are of distinctly different character and frequency than those on the downdropped side at similar times. This indicates different geologic units are present at approximately the same reflection time on opposite sides of the fault zone. The northeastern edge of the scarp may not represent the true subsurface boundary of the upthrown block. Projection to the surface of the northeasternmost edge of the seismically determined subsurface graben is 10-15 m farther northeast than expected from surface faulting. High-frequency energy present within the subsurface expression of the graben is primarily noise and is related to the deformed and incoherent nature of materials within the graben