Structure

A search of Railroad Commission and Water Commission files shows that eighteen wells are available for determining the structure of the Austin Chalk within 10 miles of the DTS property. Elevations on the base of the Austin Chalk vary from 1,081 ft below mean sea level (MSL) at a distance about 5 mi north of the DTS to 3,690 below MSL about 10 mi south of the DTS. Well separations in the map grid vary from 2 to 10 mi. Dip gradients are homoclinal, being about 160 ft/mi in the north and increasing uniformly to about 250 ft/mi in the south. No abrupt elevation changes are discernible with the sparse well coverage. Only one fault has been mapped in the immediate DTS area by Bureau researchers, and that fault is about 1.5 mi northwest of the site (Figure 18). This fault evidence will be helpful in studies of S-wave splitting across the DTS property.

Figure 18. Regional structural map of the base of the Austin Chalk as defined by well control. DTS is the Devine Test Site. These data imply that there are no significant faults near the test site. The one fault that is shown has been extracted from surface geology work done in the area by Bureau scientists.

We know from Bureau work at the abandoned Superconducting Collider site near Waxahachie, Texas (Ellis County) that there is a definitive relationship between the thickness of the Austin Chalk and the spatial frequency of fracture swarms. Any fractures that exist are likely to be at relatively high angles (>45°) to horizontal and will probably not be intercepted by DTS boreholes. It has been recognized that spacing of fracture swarms is systematically related to the thickness of the fractured interval (Nance and others, 1994). Applying the thickness-fracture spacing relationship shown in Figure 19 to the 320-ft thickness of Austin Chalk at the DTS indicates that fracture swarms, if present, would be expected to be spaced at intervals of about 750 ft between the most densely fractured portions of such swarms. Given the 1000-ft distance between the No. 4 Wilson and No. 9 Wilson wells, the interwell space at the DTS could accommodate parts of up to three fracture swarms.

Figure 19. Fracture-swarm spacing versus brittle-bed thickness. The brittle-bed thickness for the Austin Chalk is the formation thickness. Pictured Cliffs and Frontier Formation sandstone data are from Laubach (1992), and the Chapin Wash Formation sandstone data are from Laubach et al. (1991).