R. William Keach

Shallow subsurface structure of the Wasatch fault, Provo segment, Utah, from integrated compressional and shear-wave seismic reflection profiles with implications for fault structure and development

Integrated vibroseis compressional and experimental hammer-source, shear-wave, seismic reflection profiles across the Provo segment of the Wasatch fault zone in Utah reveal near-surface and shallow bedrock structures caused by geologically recent deformation. Combining information from the seismic surveys, geologic mapping, terrain analysis, and previous seismic first-arrival modeling provides a well-constrained cross section of the upper ∼500 m of the subsurface. Faults are mapped from the surface, through shallow, poorly consolidated deltaic sediments, and cutting through a rigid bedrock surface. The new seismic data are used to test hypotheses on changing fault orientation with depth, the number of subsidiary faults within the fault zone and the width of the fault zone, and the utility of integrating separate elastic methods to provide information on a complex structural zone. Although previous surface mapping has indicated only a few faults, the seismic section shows a wider and more complex deformation zone with both synthetic and antithetic normal faults. Our study demonstrates the usefulness of a combined shallow and deeper penetrating geophysical survey, integrated with detailed geologic mapping to constrain subsurface fault structure. Due to the complexity of the fault zone, accurate seismic velocity information is essential and was obtained from a first-break tomography model. The new constraints on fault geometry can be used to refine estimates of vertical versus lateral tectonic movements and to improve seismic hazard assessment along the Wasatch fault through an urban area. We suggest that earthquake-hazard assessments made without seismic reflection imaging may be biased by the previous mapping of too few faults.

Investigating fault continuity associated with geologic carbon storage planning in the Illinois Basin

AbstractBecause the confinement of CO2 in a storage reservoir depends on a stratigraphically continuous set of seals to isolate the fluid in the reservoir, the detection of structural anomalies is critical for guiding any assessment of a potential subsurface carbon storage site. Employing a suite of 3D seismic attribute analyses (as opposed to relying upon a single attribute) maximizes the chances of identifying geologic anomalies or discontinuities (e.g., faults) that may affect the integrity of a seal that will confine the stored CO2 in the reservoir. The Illinois Basin, a major area for potential carbon storage, presents challenges for target assessment because geologic anomalies can be ambiguous and easily misinterpreted when using 2D seismic reflection data, or even 3D data, if only conventional display techniques are used. We procured a small 3D seismic reflection data set in the central part of the basin (Stewardson oil field) to experiment with different strategies for enhancing the appearance of ...

Fine-scale structure of the Precambrian beneath the Illinois Basin

Increasing our understanding of the heterogeneity of Precambrian crust continues to be a focus for deep seismic reflection studies. High-resolution two-dimensional (2D) seismic profiles and a high-resolution 3D seismic volume, all centered on Decatur (Illinois, USA), provide new insights on the structure and composition of Precambrian basement beneath the Illinois Basin of the central USA midcontinent. The new data reveal a pattern of strong and coherent reflections and associated diffractions deeply buried within the eastern Granite-Rhyolite Province. This pattern is dominated by a thick seismic stratigraphic sequence, which is wedge or bowl shaped in cross section and has an angular unconformity with the overlying Paleozoic section. Deeper intrabasement bowl-shaped sequences or series are also observed in the same area. We interpret these features to be a northward continuation of analogous basement sequences located 75 km to the south below the southern part of the Illinois Basin. This correlation indicates a vast Precambrian province with a north-south dimension of >200 km. Although multiple explanations are admissible for the Precambrian reflectivity, the most likely for our study area is igneous intrusion of broad mafic igneous (diabase?) sills possibly underlain by small plutons. The concentration of such mafic (or bimodal) igneous activity within or coeval with the eastern Granite-Rhyolite Province suggests an episode of Proterozoic crustal extension and rifting.

Petroleum industry techniques yield new insights into 3D GPR data

We have adapted petroleum industry technologies to 3D GPR datasets from a variety of geological and archaeological settings. Seismic attribute analysis designed for 3D seismic volumes of oil fields has been used to map a 19th century cemetery and to prepare a Roman archaeological site for detailed study by mapping the prior disturbances of the site. Specific seismic attributes that we have successfully applied include connectedness (“geoanomalies”), semblance (or dissimilarity), volume rendering (transparency visualization), and waveform classification. An especially powerful strategy is to use two or more attributes simultaneously via volume rendering that allows multiple visualizations.

Integrating seismic exploration methods into a geological sciences curriculum at Brigham Young Unversity

The continuing expansion in petroleum and other resource exploration and in geological hazard assessment for infrastructure development have combined to dramatically increase the need for training of university students in seismic imaging methods. This need is being met at Brigham Young University (BYU) by forming alliances or collaborations with private industry and government in order to provide financial support for research using seismic techniques, to obtain access to proprietary datasets, and to place students in the workplace as part of their university experience. Infrastructure support has been provided by BYU in the form of acquisition of seismic recording equipment, procuring of state‐of‐the‐art software for data processing and geologic mapping, and building of a dedicated 3D visualization lab. This infrastructure creates an environment that mimics research and exploration programs in private industry. Seismic geophysical research foci at BYU include (1) seismic characterization of deep reservo...