The Use of Downhole Geophysical Methods to Detect Zones of Poor-Quality Rock or Voids

摘要

The primary objective of the study was to evaluate existing downhole geophysical capabilities to detect poor-quality rock or voids at depths up to hundreds of metres and at distances on the order of tens of metres from the borehole. After an extensive literature review and contacts with the geophysical community, it was determined that no single borehole device offered the desired capability. However, crosshole testing did provide some of the directionality and detection capabilities sought and this approach was considered to be the only viable and cost-effective alternative remaining. Accordingly, a series of crosshole method field evaluations were carried out at two WES research sites in Florida. Both sites are in extensively solutioned limestone, with the Medford Cave site typifying dry (air-filled) cavities and the Manatee Springs site typifying wet (water-filled) cavities. In the course of these studies conventinal single borehole logging methods were used to typify the site materials, and crosshole acoustic, electromagnetic, and resistivity methods were evaluated on known cavity features. Results of the investigation indicate that the crosshole methods can detect known cavity features with borehole spacing on the order of 20 to 50 ft (6.1 to 15.2 m), depending on site conditions.