Rayleigh and Love waves recorded on seismic-shot gathers can be used to determine the thickness and shear-wave velocity of shallow subsurface layers. After the data are transformed into the k-f domain, the dispersion curve for each of the phases can be picked from maxima on the contour plot. This dispersion curve is then inverted for the velocities and depths. Different frequencies in the dispersion curve yield information about different depths. The fundamental mode has proven to be of greater use than higher modes. Both Rayleigh and Love waves are easily inverted. However, the Love waves seem to yield information in a lower portion of the spectrum than the Rayleigh modes. Three examples are given from field experiments conducted near Canton, Texas.

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.

Seismic reflection studies were performed across actively developing sinkholes located astride Interstate Highway 70 in Russell County, Kansas. Results indicate that high‐resolution seismic reflection surveys are useful in the subsurface investigation of some sinkholes. In particular, we were able to delineate the subsurface vertical and horizontal extent of the sinkholes because of the excellent acoustical marker‐bed characteristics of the Stone Corral anhydrite. The seismic reflection evidence presented here, combined with borehole information from 1967, suggest that the Stone Corral anhydrite has been down‐dropped within one of the sinkholes as much as 30 m in 13 years. The seismic reflection method is potentially useful in engineering studies of other sinkholes and karst features. The seismic data presented here were obtained in the presence of relatively heavy highway traffic (i.e., up to a few dozen vehicles per minute) using the MiniSOSIE recording technique.

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. Amplifiers should have low system noise, large dynamic range, and precision of 12 or more bits.

The investigation of zero‐offset response to circular reflectors of increasing Fresnel zone size shows that reflection response is a constant and is independent of reflector size, except when the reflector diameter is so small that the diffractions interfere with the primary reflection. The extent of this effect is dependent upon vertical resolution and the time separation of the primary reflector and the diffraction. Interference occurs for reflectors smaller in diameter than the first Fresnel zone. Migration removes this interference. For broadband data the Fresnel zone solution breaks into two parts: the primary reflector and the edge‐effects diffractor. With broadband seismic data, reflections and diffractions separate in time, except at locations near faults or very small bodies. Reflections are the seismic response to interlayer discontinuity and are independent of reflector size. Diffractions are the seismic response to lateral discontinuities and edges and depend on proximity to—and geometry of—the edge. Except in the locale of an edge, broadband reflections and diffractions are separated physically on the section and mentally by the interpreter. Furthermore, standard CMP processing attenuates diffractions, especially when CMP lateral offset is some distance from the diffractor.

Well log and seismic data indicate that the bedded rock salts (salts) of the Devonian Age Prairie Formation were widely distributed and uniformly deposited in the Lloydminster area, Western Canada (T45-65, R20W3M-R5W4M); however, as a result of extensive leaching, the distribution of these salts is not now what it once was. The Lloydminster area is now bisected by the north‐south trending main dissolutional edge of the Prairie salt. Thick salt (up to 150 m) is preserved to the west of this edge; to the east the salt is mostly absent. Analyses of remnant salt and patterns of subsurface structural relief suggests that the dissolution of the Prairie salt in the Lloydminster area was triggered and/or accentuated in part by several different large scale mechanisms including: near‐surface exposure, centripetal flow of unsaturated waters, regional faulting/fracturing, glacial loading and/or unloading, dissolution of the underlying salt, and salt creep. These mechanisms are supported by the incorporated seismic and well log control that indicate a direct relationship between the thicknesses of remnant salt and post‐salt strata. Well log and seismic data also indicate that the bedded salts of the Devonian Age Black Creek Member were uniformly deposited within the Black Creek sub‐basin, Rainbow Lake area, western Canada (T105-112, R5-R10W6M); however, as a result of extensive leaching, distribution of these salts is not now what it once was. The Black Creek salts are now preserved only as discontinuous remnants with maximum gross thicknesses on the order of 80 m. Seismic and well log control suggests that the dissolution of the Black Creek salt in the Rainbow Lake study area was triggered and/or accentuated in part by several different large scale mechanisms including: centrifugal flow of unsaturated waters, regional faulting/fracturing and salt creep. Bedded salt is preserved within five other Devonian Age evaporitic units in Alberta, Canada: the Lotsberg Formation, Cold Lake Formation, Beaverhill Lake Group, Leduc Formation, and Wabamun Group. Each of these salts has also been extensively leached in places. In the literature, dissolution is generally attributed to one or more of the previously noted large scale mechanisms. Herein we present an overview of the envisioned principal mechanisms of salt dissolution. In support of these hypothesized mechanisms, we present seismic and geologic control from both the Lloydminster and Rainbow Lake areas of western Canada, which illustrate that the dissolution of subsurface salts is accompanied by the subsidence of post‐salt strata and that the analyses of this information can be used to elucidate the timing and large scale mechanisms of salt dissolution.

No abstract

The Kansas Geological Survey has operated a microearthquake seismograph network since mid-1977. The network now consists of fifteen stations located in the eastern half of Kansas and Nebraska. Locatable microearthquakes with duration magnitudes less than 3.2 occur at the rate of roughly 20 per year in the two-state area, with most of the events ranging from 1.4 to 2.5 in local magnitude. The microearthquake pattern observed over the past ten years is consistent with the pattern of historical earthquakes reported since 1867. Much of the activity occurs along the Nemaha Ridge, a buried Precambrian uplift that runs from roughly Omaha, Nebraska, southward across Kansas to near Oklahoma City. This geological structure has been the site of several earthquakes of MM Intensity VII over the past 125 years. Some seismicity is observed along the northwest flank of the Midcontinent Geophysical Anomaly in Kansas, but little is observed in the Nebraska or Iowa portions of this Precambrian feature. The Central Kansas Uplift, which is a buried anticline similar in age to the Nemaha Ridge, has been the site of several felt earthquakes since 1982. A trend of earthquakes extending northeastward across central Nebraska is not associated with any prominent known geologic structure. All the seismicity in central and eastern Kansas can be roughly correlated to known geologic structures.