Subsurface Void Detection in Oklahoma Evaporite Deposits Using Geophysical Methods
Phil Sirles* and Justin Rittgers*, Zonge Geosciences, Tucson, Arizona;
Jeff Dean, Oklahoma Department of Transportation
Paper – [pdf] INFR_Sirles_2009_Subsurface Void Detection in Oklahoma Evaporite
Surface sinks, distressed highway sections, voids and evaporite bedrock with variable weathering have complicated highway redesign for the Oklahoma Department of Transportation in western Oklahoma. More than 46,000 linear feet of direct-current electrical resistivity data were collected, using the dipole-dipole technique, along Highways US-64 and US-412 in Major County near Woodward, Oklahoma. The main purpose of the survey was to identify and discriminate between sections of highway underlain by solid gypsum or gypsum containing voids (resistivity >1000 ohm-meters) and sections containing combinations of claystone and weathered gypsum (resistivity <100 ohm-meters). The Zonge ZETA system
was used to map subsurface geology and presence of voids.
Preliminary geophysical results were furnished to Terracon, Inc. and, in consultation with the Oklahoma Department of Transportation (ODOT), the locations for eighteen confirming borings were selected. Borehole data correlated very well with the resistivity models and allowed assignment of resistivity ranges to specific lithologies, this correlation became the basis of all data interpretation for the geophysical survey. The results show that the DC resistivity method, alternately called “ERT” (electrical resistivity tomography”, offers an accurate and cost-
effective approach to mapping lateral and vertical variations in material properties that can be directly associated with lithology. This can help alleviate the common issues confronted when making geologic interpretations based on limited data from widely spaced borings. Two useful generalizations can be drawn about this specific project area: 1) The highest values of resistivity more often correlate with gypsum hosting numerous smaller (0.5-1.5 feet diameter) voids than with large voids, and 2) Large sections of the surveyed area (several 1000’s of feet along US-412 and US-64 are underlain by clay, weathered gypsum and gypsum-clay as confirmed by the borings, and will not likely pose as many issues with regards to required mitigation efforts.
In summary, the ERT geophysical method, as confirmed by borings, successfully separated the surveyed sections of highway into distinct areas underlain by claystone and weathered gypsum and into sections with gypsum dissolution features requiring different mitigation tactics. The success of the geophysical project is also related to having a well-integrated geologic, geotechnical and engineering process in which ODOT, Terracon and Zonge personnel worked closely together to assess all of the subsurface data.