Project funded by the Jet Propulsion Laboratory,
California Institute of Technology
Principal UNR Investigator: John N. Louie
Seismological Laboratory, The University of Nevada, Reno
The proposed seismic surveys of Upheaval Dome in Canyonlands National Park has two objectives: (1) establish or reject the concept of a salt diapir below the central depression by timing the arrivals of refracted waves passing between the depression and the regional depth of the top of the Paradox Formation; (2) obtain measurements of the change in thickness of the major sedimentary formations at different distances from the center of Upheaval Dome. To achieve these objectives a seismic reflection and refraction survey will be conducted. That work will entail the following:
(1) Conduct a seismic reflection profile on the road between Willow Flat and the central depression. Collect vertical-incidence reflections as close as 0.8 km from the center of the Dome using remote seismic receivers within and on the opposite side of the central depression, to provide refractions and wide-angle reflection data for major structures directly under the center of Upheaval Dome.
(2) Conduct a seismic refraction profile across the structure. This will be accomplished by the placement of remote receivers having accurate absolute timing capabilities. These receivers (10-12 PASSCAL RefTek recording instruments and high-frequency seismometers to be borrowed from IRIS) are designed to be deployed in remote areas for long periods of time, and derive microsecond-accurate timing from the broadcasts of Global Positioning Satellites. The energy source is to be an enhanced weight-drop repeatable seismic source, which will be driven to the end of the road near the central depression. The weight source is to be provided by Prof. J. Schuster of the University of Utah. Over a four- to eight-hour recording period the PASSCAL instruments will continuously record 100-300 repeated actuations of the weight-drop source. The remote instruments will accumulate their data on hard disk, which will remain intact until the recorders are returned and the data downloaded.
(3) Produce a seismic cross section from the resulting data. The data processing will be performed at UNR. The goal of the processing is to produce a detailed cross section showing the geometry of the major sedimentary units and the members of the Paradox Formation, as a function of the radius from the center. The first step will be to reduce the level of coherent noise in the reflection sections through trace editing, filtering, and semblance techniques as necessary. Except for the innermost kilometer of the line, closest to the central depression, and the receivers on the northwest side of Upheaval Dome, the structures are expected to be simple enough that standard processing techniques can be applied. These will include common-midpoint sorting, static corrections, velocity semblances and analysis, and stacking. Once a common-midpoint stack is obtained, it will be converted from time to depth using an appropriate migration. The migration velocities will be derived from both the velocity spectra and analysis of the refracted phases. These techniques should yield depth accuracy on a scale of 25 meters under most of the seismic line. More unusual techniques must be used to image the structure underneath the center of Upheaval Dome. We will use a simulated-annealing Monte-Carlo optimization method (Pullammanappallil and Louie, 1993) to find a set of velocity models below the central depression that match the picked travel times. The synthetic study will allow this experiment to constrain the maximum velocity above the regional depth of the top of the Paradox Formation. Thus, a lack of early arrivals will discount the presence of any substantial salt diapir. Without salt diapirism, all deformation of the Paradox and overlying formations must be the result of impact. If a salt diapir is present, the image of salt structure we will derive from the optimization will allow reverse-modeling of deformation due to salt buoyancy, with a residual due to impact.
(4) Deliverables will include the raw and processed data in the form of magnetic tapes and a report describing the results of the work.