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 nearsurface 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.

Seismic reflection surveys were used to follow the drawdown in a shallow aquifer during a pumping test. Using severe analog low‐cut filters and 1/4‐m geophone spacings, 335 Hz reflections were obtained from the top of the saturated zone 2.7 m deep. The reflections moved down in time as the saturated zone dropped in response to pumping. The dominant frequency and bandwidth both dropped during pumping indicating a more diffuse reflecting boundary. Slight pullups of reflectors at specific locations on the CDP sections may indicate a higher elevation of the capillary fringe and therefore finer sediments in those locations. Other potential applications of this technique include mapping cones of depression and detecting and delineating perched‐water tables.

The results of a seismic reflection profiling exercise are strongly dependent upon parameters used in field recording. The choice of parameters is determined by objectives of the survey, available resources, and geologic locality. Some simple modeling and/or a walkaway noise survey are helpful in choice of field parameters. Filtering data before analog‐to‐digital conversion in the field can help overcome limitations in the dynamic range of the seismograph. Source and geophone arrays can be used to a limited extent in high‐resolution surveys to help attenuate ground roll. Proper planting of geophones can be an important factor in obtaining the flattest spectral response. Various seismic energy sources provide the flattest spectral response. Various seismic energy sources provide different spectral character and varying degrees of convenience and cost.

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Shallow seismic‐reflection techniques were used to image the bedrock‐alluvial interface, near a chemical evaporation pond in the Texas Panhandle, allowing optimum placement of water‐quality monitor wells. The seismic data showed bedrock valleys as shallow as 4 m and accurate to within 1 m horizontally and vertically. The normal‐moveout velocity within the near‐surface alluvium varies from 225 m/s to 400 m/s. All monitor‐well borings near the evaporation pond penetrated unsaturated alluvial material. On most of the data, the wavelet reflected from the bedrock‐alluvium interface has a dominant frequency of around 170 Hz. Low‐cut filtering at 24 dB/octave below 220 Hz prior to analog‐to‐digital conversion enhanced the amplitude of the desired bedrock reflection relative to the amplitude of the unwanted ground roll. The final bedrock contour map derived from drilling and seismic‐reflection data possesses improved resolution and shows a bedrock valley not interpretable from drill data alone.

The seismic-reflection method is a powerful geophysical exploration method that has been in widespread use in the petroleum industry for more than 60 years. Since 1980, it has been increasingly used in applications shallower than 30 m, and that is the principal subject of this paper. The seismic-reflection method measures different parameters than other geophysical methods, and it requires careful attention to avoid possible pitfalls in data collection, processing, and interpretation. Part of the key to avoiding the pitfalls is to understand the resolution limits of the technique, and to carefully plan shallow-reflection surveys around the geologic objective and the resolution limits. Careful planning is also necessary to make the method increasingly cost effective relative to test drilling and/or other geophysical methods. The selection of seismic recording equipment, energy source, and dataacquisition parameters are often critical to the success of a shallow-reflection project. It is important to carefully follow known seismic reflections throughout the data-processing phase to avoid misinterpretation of things that look like reflections but are not. The shallowreflection technique has recently been used in mapping bedrock beneath alluvium in the vicinity of hazardous waste sites, detecting abandoned coal mines, following the top of the saturated zone during a pump test in an alluvial aquifer, and in mapping shallow faults. As resolution improves and cost-effectiveness increases, other new applications will be added.Downloaded 08/09/15 to 128.42.202.150. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/

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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 kin, probably less than 25 kin. 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. INTRODUCTIONthe ith station. Expected travel times TEt from each hypocenter to each station are read (by computer in the presThis paper reports on the results of a study using teleseismic ent work) from the Herrin [1968] tables. The absolute P