Elastic properties of salt: laboratory measurements, well-log analysis, and a seismic survey over the Hockley Salt Mine, Texas



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Salt plays a significant part in the geology of the Gulf of Mexico area (GoM). Numerous basins in the world have undergone evaporation sequences that have deposited vast quantities of salt. Sometimes these deposits have remained largely undeformed, which can lead to anisotropic crystal growth, while other salt deposits have undergone significant movement and extrusion. In this thesis, I use lab measurements, well-log data, and surface seismic to determine the properties of salt crystal and rocks. In the lab, we have undertaken ultrasonic measurements on salt samples from various locations. The pure halite crystals from the Goderich salt mine, Canada, demonstrate shear-wave splitting and compressional-wave variations which indicate cubic anisotropy. The stiffness values calculated for that are C_11=48.7 GPa, C_44=13.1 GPa , and C_12=11.9 GPa. Our samples from the Hockley and the Bayou salt mine have fractures, and aligned domains, but no obvious anisotropy. The density ranges from 2.16 - 2.22 g/cc. The confining pressure experiments are conducted on the Louisiana salt cores. The velocities under 0 - 4000 psi are 4.4 - 4.8 km/s for P-waves and 2.5 - 2.8 km/s for S-waves. For a 1 km-thick numerical halite model, the travel-time difference caused by cubic anisotropy is up to 20 ms for P-waves based on the calculated stiffness values. We analyzed 142 well-log from boreholes drilled through salt in the GoM. We find an empirical relationship for P-wave velocity of salt V (km/s) versus depth D (km): V=4.41+0.0145D The RMSE of the fit is 0.10 km/s. This variability may be useful for modeling velocities in “dirty” or inhomogeneous salt deposits. For salt density vs. velocity, our log data are similar to Gardner’s, although we find a cluster, not a monotonic relationship. We acquired a 1.2 km seismic line over Hockley Salt Mine. From refraction crossover analysis of one shot gather, we find depth for the top of anhydrite with the stacking velocity of 5.5 km/s occurs around 50 m (164 ft). These studies of elastic properties of salt provide more information for salt velocity model building and a general understanding of salt properties.



Salt, Gulf of Mexico, Anisotropy, Labortorary measurements, Well logs, Seismic surveying