ACQUISITION OF SUBSURFACE STRATIGRAPHY AND SHEAR WAVE VELOCITY DATA FOR CRITICAL INFRASTRUCTURE FROM EXISTING WELL LOG DATABASES: REDUCING THE NEED FOR COSTLY DEEP DRILLING PROGRAMS

A generalized stratigraphic sequence from ground surface to Precambrian basement, and associated S-wave velocity (Vs) structure was estimated for three nuclear power plant sites in the eastern United States. Generalized stratigraphic sections were developed by correlating data from nearby, deep well lithologic and geophysical logs obtained from well-log databases from state geological surveys. Stratigraphic unit names on the logs required correlation and standardization due to the use of antiquated, generic, or colloquial naming conventions used by disparate logging entities. This was addressed by researching regional geological formations, sequence stratigraphy, formation thickness variations, and structural geology, ultimately resulting in a generalized regional stratigraphic column for each site. Seismic velocities of the subsurface materials were obtained from downhole sonic log interval transit times (microseconds ft^-1; i.e., the inverse P-wave velocity [Vp] data). Analysis involved assignment of an average, representative transit time value to consecutive depth intervals on hard-copy sonic logs. Depth intervals were of variable length, with boundaries based on marked visual changes in interval transit time, typically indicative of significant lithologic or porosity changes. The Vp values were then converted to Vs equivalents using lithology-based ratios derived from published experimental data. Upper and lower bounds as well as average Vs profiles were calculated and fit to stratigraphic sequences at each well location. The combination of the stratigraphic column and the velocity profiles were then extrapolated to create an estimated, generalized Vs structure for the subsurface at each site. The data were used to determine the depths at which the subsurface materials exceed a threshold minimum Vs for seismic stability (2.8 km sec^-1) and for defining associated epistemic uncertainty. Results from this study, which relied on previously collected and publicly available data, exemplifies relatively inexpensive research and analysis that can be used to support seismic stability analyses for stakeholders of critical infrastructure, in lieu of costly and time-consuming deep borehole drilling and downhole seismic velocity data collection.