GEOLOGIC INVESTIGATION OF SINKHOLE LAKES NEAR ORLANDO, FL USING SEISMIC REFLECTION AND DIRECT-CURRENT RESISTIVITY

This study was initiated in cooperation with the St. Johns River Water Management District (SJRWMD) to investigate ground/surface water interaction in designated sentinel lakes near Orlando, FL. These sentinel lakes have become important water bodies in the Minimum Flows and Levels program and will be used to define long-term hydrologic and ecologic performance measures. The U.S. Geological Survey investigated 16 of the 21 water bodies on the 2008/2009 SJRWMD priority list.

The primary objective of this study was to provide geologic information through the tandem use of high-resolution seismic reflection profiling (HSRP) and direct-current resistivity to isolate both geologic framework (structure) and composition (lithology). Understanding the structure and lithology beneath these lakes will lead to a better understanding of why water levels fluctuate in some lakes more than others.

Geologic control on lake hydrology remains poorly understood. Past HRSP surveys from various lakes in the study area have been successful in identifying karst features, such as subsidence sinkholes. However, it is difficult to image highly irregular or chaotic surfaces, such as collapse sinkholes. Resistivity profiling complements HRSP by detecting porosity change within fractured or collapsed structures.

Saunders Lake (Lake County) is composed of a series of north-south trending sinkholes that have joined to form one lake body. HRSP shows surface depressions and deformation in the substrate. Resistivity data likewise shows areas in the southern area of the Lake where resistivity shifts abruptly from approximately 400 ohm-m along the edges to approximately 12 ohm-m in the center, These well-defined areas may indicate a 'ravel' zone of increased porosity or clay content. Within Helen Lake (Volusia County), a parallel set of seismic reflectors within a host of chaotic reflectors may represent fill within a large sinkhole. The feature extends to over 50 m deep and contains very steep pinnacles within the center. This is supported by high resistivity values from adjacent continuous resistivity profiles that show possible center collapse within the lake and infilling of sandy material. When used together, HRSP and resistivity techniques provide a composite image of structure and lithology to detect potential conduits for fluid flow.