Bedrock Velocity Research Articles

High‐resolution seismic traveltime tomography incorporating static corrections applied to a till‐covered bedrock environment

A major obstacle in tomographic inversion is near‐surface velocity variations. Such shallow velocity variations need to be known and correctly accounted for to obtain images of deeper structures with high resolution and quality. Bedrock cover in many areas consists of unconsolidated sediments and glacial till. To handle the problems associated with this cover, we present a tomographic method that solves for the 3D velocity structure and receiver static corrections simultaneously. We test the method on first‐arrival picks from deep seismic reflection data acquired in the mid‐ late to 1980s in the Siljan Ring area, central Sweden. To use this data set successfully, one needs to handle a number of problems, including time‐varying, near‐surface velocities from data recorded in winter and summer, several sources and receivers within each inversion cell, varying thickness of the cover layer in each inversion cell, and complex 3D geology. Simultaneous inversion for static corrections and velocity produces a much better image than standard tomography without statics. The velocity model from the simultaneous inversion is superior to the velocity model produced using refraction statics obtained from standard reflection seismic processing prior to inversion. Best results using the simultaneous inversion are obtained when the initial top velocity layer is set to the near‐surface bedrock velocity rather than the velocity of the cover. The resulting static calculations may, in the future, be compared to refraction static corrections in standard reflection seismic processing. The preferred final model shows a good correlation with the mapped geology and the airborne magneticmap.

Tau-p velocity imaging of regolith structure

The tau-p velocity imaging method, first developed for obtaining the velocity field from marine multichannel seismic data, has been applied to refracted waves from the regolith in regional seismic reflection surveys on land. The technique converts travel time picks from the refracted wavefield into two-dimensional velocity models, by transforming from time-offset into the tau-p domain. Each arrival is mapped individually, and the ?true? velocity and position of the ray turning point is obtained by considering reversed raypaths. Thus the data are transformed directly into a depth or two-way time image of the subsurface displayed in seismic velocity. The method is extremely fast and involves no interpretive steps or iteration. Ideal datasets contain the refracted wavefield sampled densely and equally in the shot and receiver domains. It was therefore decided to test the application of the method for mapping the velocity structure of a portion of regolith and to compare the results with those obtained using more conventional methods. The area chosen for study was part of a regional seismic reflection line across the Lachlan River palaeo-valley in central NSW. The data set consisted of the first break picks for 240 channels with receivers spaced every 40 m and vibration points every 40 m. The velocity images were produced as both time and depth sections and compared with one and two layer refractor models from a refraction tomographic approach. A low velocity region on the image corresponds to the deepest part of the refractor model, interpreted as the thickest part of the palaeo-valley. Bedrock velocity variations are also mapped and agree with the changes along the (lowest) refractor velocity profile. While further tuning may be required for land work, the technique has the advantage that velocities can be directly imaged and potentially related to regolith physical properties.

Site Response in Providence, Rhode Island: Constraints from Ambient Noise Measurements

Abstract We conducted an ambient noise survey to assess the potential for seismic site response in Providence, Rhode Island. Providence is built on bedrock valleys that are partially filled with unconsolidated Holocene sediments. Because similar valley structures exist in other heavily populated coastal and river cities of the northeastern United States, the results of this study are relevant to seismic hazards throughout the region. At sites located in the river valleys beneath downtown Providence, we found peaks in ambient noise frequency spectra that suggest amplification of energy in the range of 2–3 Hz. This spectral peak occurs in the spectra from sediment sites and in spectral ratios that reference sediment to hardrock stations. We used geotechnical borehole records to constrain valley structure and seismic refraction data to determine bedrock velocity, and we calculated the theoretical response to ground motion for one-dimensional models that approximate basin structure. The modeling shows that the 2–3 Hz ambient noise peak can be interpreted as the fundamental resonance frequency of low-impedance surface sediment layers.

A shallow seismic refraction study of the Versailles cryptoexplosion structure, central Kentucky

— The Versailles cryptoexplosion structure, located in central Kentucky, contains a partially brecciated central uplift and a boundary marked by arcuate faulting. Seismic refraction data were used to divide the structure into distinct units on the basis of velocity. A zone of brecciated material shows a decrease in velocity compared with surrounding rocks. Locally thickened strata are present on the flanks of the central uplift. An increase of the bedrock velocity in the area surrounding the structure suggests localized dolomization of fractured bedrock, and a plug of fractured strata near the base of the brecciation appears to be uplifted. Based on the geometry of deformation seen in known impact structures, and the integration of this study with previous geological and geophysical work in the area, it is suggested that our results support the interpretation of the Versailles structure as an eroded astrobleme (Black, 1964b; Seeger, 1972).

A quantitative approach to bedrock velocity resolution and precision: The Lithoprobe Vancouver Island Experiment

Bedrock velocities obtained from the inversion of first‐break picks of multichannel seismic reflection data have been used to aid the interpretation of shallow reflectors. The trade‐off between resolution and stability of inversion results has been examined. Models have been optimized by minimizing the variability of static corrections, and a Monte Carlo method has been used to estimate their precision. A velocity of 4.6 ± 0.4 km/s for the Leech River Complex (LRC) compares with velocities of 5.9 ± 0.6 and 5.8 ± 0.3 km/s for the neighboring geological units. Velocity contrasts associated with the change in lithologies may therefore be a contributing factor for generating reflections from the faults that bound the LRC. Velocities of 5.5 ± 0.4 and 4.6 ± 0.6 km/s on either side of the San Juan fault suggest that an event identified on several shot gathers may be a reflection from a steeply dipping fault.

Seismic refraction at Horse Mesa Dam: An application of the generalized reciprocal method

Horse Mesa Dam is located in the Mazatzal Mountains about 60 miles east of Phoenix, Arizona, and is a part of the Salt River Project. Early in 1981, the management of the Salt River Project decided to solve problems with the existing roadway to the dam by building an improved roadway and retaining wall. Geotechnical engineers on the project originally planned to obtain depth‐to‐bedrock information for the design of these structures by drilling a large number of boreholes to bedrock. However, we decided that more complete information could be obtained less expensively by using fewer drill holes tied together with seismic refraction lines. The complicated physical setting of the site made planning for the survey very important. The bedrock surface at the site is quite irregular, dipping as steeply as 30 degrees across the axis of the roadway, and the space between the stream bank on one side of the roadway and the canyon wall on the other side is very narrow. Because of these constraints, we calculated that head waves refracted along the bedrock surface would never be observed on refraction lines shot across the roadway. Therefore, we decided to shoot two long refraction lines along the axis of the existing roadway—one each on the inside and outside shoulders of the road bed—to provide two points on the bedrock surface at any particular cross‐section of the road. Field work at the site was done between June 10 and June 15, 1981. We obtained reversed coverage on 914 m (3 000 ft) of 70 m (230 ft) spreads along the road. Because of the attenuation of the coarse overburden material, 0.45 kg charges of Kinestik were necessary to deliver seismic energy across these spreads, and 0.15 kg charges were necessary at the center shots. To avoid overwhelming background noise at the site, we performed much of the survey in the evening or early morning when the turbines at the dam were idle. Survey results showed not only a highly irregular bedrock surface, but also indicated substantial velocity variations within both the overburden and bedrock. Overburden velocities varied from 366 m/s (1 200 ft/s) to 549 m/s (1 800 ft/s), while bedrock velocities varied from 1 829 m/s (6 000 ft/s) to nearly 7 620 m/s (25 000 ft/s). We interpreted the results with the generalized reciprocal method (GRM), which is widely used in Australia but is not so well‐known in the United States. This method proved to be particularly well‐suited for situations involving large lateral velocity variations and highly irregular refractor surfaces. When compared with the seven borings that were eventually done on the roadway, the refraction interpretation was accurate to within a few feet; or, in terms of the nominal depth to bedrock, the results were accurate to within about 10 percent. This is remarkable agreement considering the irregularity of the bedrock surface. The Horse Mesa Dam study shows the importance of planning a survey with the physical characteristics of the site and the goals of the survey in mind. It also shows the value of a powerful, flexible interpretation method—in this case, the GRM. Most important, it demonstrates that engineering geophysics is capable of providing useful, accurate information at a lower cost than drilling alone.

Seismic stratigraphy and bedrock forms in perialpine lakes

Reconnaissance air-gun seismic reflection profiles and sonobuoy refraction measurements were recorded in 17 perialpine lakes. Quaternary fill velocities derived from sonobuoy refraction records suggest a stratigraphic sequence consisting of two general elements. The upper interval with velocities between 1.5 and 1.9 km/sec may represent normal sedimentation of glaciolacustrine sediments, whereas higher velocities of 1.9 to 2.7 km/sec in a lower element probably result from glacier overriding and compaction. The refraction data also show bedrock velocities from 3.6 to 6.7 km/sec in concert with underlying geological formations. Reflection sections from the deep basins show modest overdeepening of the lakes north of the Alps and deep incision of the southern perialpine lakes. The data are consistent with an interpretation of glacial overdeepening of fluvially cut bedrock valleys, overriding of older glaciolacustrine deposits (Riss?), and limited erosion by the Wurm glacier streams that seem to scoop but not downcut the basins. We found no evidence for late Quaternary faults.

A Total-time Approach for Estimating Depth to Bedrock in Glaciated Mountain Valleys

A seismic method is described by which depth to bedrock in valleys can be estimated without the need to traverse the valley surface with conventional refraction or reflection techniques. A short detector array on rock outcrop on one side of the valley is used to measure compressional waves from a shot on outcrop on the opposite side, and to measure the seismic velocity of the rock. The shortest travel-time path across the valley follows the bedrock overburden interface, and when the bedrock velocity is known, the length of the path can be computed. The ratio of the length of the bedrock path to the direct distance between source and receiver is a function of the depth to bedrock at the valley center. The success of the method requires that certain valley-shape configurations be reasonably predictable and that the bedrock velocity be uniform throughout. Depth calculations for several field examples have been made using graphical techniques.