Elastic Properties of Rock Salt: Lab Measurements, Numerical and Physical Modeling, and Gulf of Mexico Well Logs and VSP
To achieve high-resolution subsurface imaging and lithology description, the elastic properties of rock salt are investigated using ultrasonic lab measurements, well logs, and a salt proximity VSP survey. In the lab, I analyzed the effects of composition, crystalline structure, pressure, and temperature on the elastic behavior of a variety of rock salt samples, includ- ing pure halite (>95 wt.%) from the Gulf Coast, argillaceous rock salt from the Zipaquir ́a Mine, Colombia, and crystalline salt from the Goderich Mine, Canada. At room-dry conditions, with 0.5-1 MHz transducers, the Gulf Coast cores behave isotropically in general: Vp and Vs range from 4325-4747 m/s and 2655-2730 m/s, respectively. The attenuation quality factor Q for a selection of Gulf Coast salt samples were measured at 34-57 and 34-41 for Qp and Qs at room conditions using a spectral ratio method. The Zipaquira salt samples show velocity and density variations due to their heterogeneous composition. The Goderich crystals display distinct cubic anisotropy: up to 7% and 18% differences in Vp and Vs, respectively. Increasing pressure elevates both V and Q while increasing temperature decreases them. An empirical relationship between the intrinsic Qp and Vp under varying pressure and temperature was established. Further velocity and density values were studied from 147 log suites from boreholes drilled through rock salt in the northern Gulf of Mexico. Within the salt formations, P-wave velocities increase slightly with depth (around 5 m/s per km). In addition, a set of numerical and physical models were studied for the salt flank delineation using VSP acquisition geometries. A single-shot salt proximity VSP survey in Markham salt dome, Texas, was processed to extract salt properties and image the structures inside salt as well as the salt flank. Wave conversion between P- and S-waves in the presence of ultra-high impedance layer (e.g., anhydrite) around salt structures was analyzed and can be significant. These studies further inform us about the physics of rock salt as well as assist with salt structure model building and imaging.
Rock salt, Gulf of Mexico, Rock physics, Borehole geophysics, Vertical seismic profile (VSP), Seismic, Well logs, Ultrasonic transmission, Physical modeling
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