Corroborating Collapse Structures from Land/Marine Surveys in Carbonate Systems Using Borehole Geophysical Logs

Subsurface carbonate systems are inherently complex as a result of dissolution of sedimentary rocks and collapse within the groundwater-flow system. These collapse structures may penetrate upper confining units that may have previously protected the aquifer. Increased resolution of subsurface collapse structures at depth from modern land and marine seismic surveys has greatly enhanced the ability to both detect and delineate these features that potentially control the upward migration of groundwater. The use of borehole geophysical logs to corroborate the surface geophysical survey interpretations is essential to understanding the physical collapse systems, including secondary fractures and other features.

In Miami-Dade County, Florida, USA, wastewater injection operations at the South District plant have been ongoing since the late 1970s. The target injection horizon has been the Boulder Zone, a permeable dolomitic unit present at depth from about 2,700-3,000 feet. A review of more than 500 borehole geophysical logs collected from the original pilot holes and injection wells in comparison to recent seismic surveys in the area have revealed the potential delineation of collapse structures. Such collapse structures may affect upward migration of injected wastewater from the Boulder Zone into shallower carbonate zones of varying permeability. Delineation of fractures, both within and potentially outward away from the wells, which are potentially associated with collapse structures tapped by the 17 injection wells, is part of a recent effort being conducted by the U. S. Geological Survey in cooperation with Miami-Dade County. Because groundwater modeling tools have evolved since publication of the original flow and transport model for the South District, the use of this more detailed physical framework information toward future modeling efforts should greatly improve the accuracy of those simulations.