Historically, high-resolution single-channel seismic profiling (HRSP) has been used to determine the regional distribution of stratigraphic units having distinct acoustical characteristics. In this study, the lakes are well distributed and have a relatively small diameter, making stratigraphic correlation difficult. HRSP data has been used primarily here to map the shallow subsurface features found beneath selected lakes of northeastern Florida. Subsurface diagnostic features are used to define the structural history and to locate possible breaches in the confining layer that maintains the perched lakes above the Floridan aquifer. In many cases the acoustical records show fine details of karst (>10 m) and karren (<10 m) features (Ford and Williams, 1992). Compilation of these features from seismic profiles acquired from the lake surveys have shown that certain acoustic patterns reoccur from lake to lake. Figure 14 shows similar acoustic patterns from three lakes located in separate geomorphologic regions. In general, low angle, parallel reflections are down warped to form a depression. These reflections are accompanied by discontinuous or segmented reflections that suggest structural displacement and subsurface subsidence. Horizontal reflections overlying the subsidence indicate subsequent fill.

The reoccurrence of these features in seismic sections from the more than 39 sites profiled (Fig. 1, Introduction) led to the identification of six acoustical signatures of commonly found karst or geologic features. These features are characterized in Figure 16. Included in the summary are patterns indicative of no acoustic return (Fig. 16 type 1). Negligible or noisy acoustic return unfortunately is common in the lake surveys and are typically the result of various environmental and geomorphologic factors. Such factors include organic material collecting in depressions that disperse the acoustic signal, or a lithologically “hard” lake bottom of packed homogeneous sands. A karst surface near the lake bottom may also disperse the signal or cause ringing (multiples) throughout the record. Side-wall reflections from the shoreline or slope of a depression may further obscure return from subsurface features. Acquisitional deficiencies such as electrical noise or faulty grounding may affect entire surveys, as do lake surface wind, chop or waves.

When the record is not obscured, a number of patterns have been identified that relate to karst features. Types 2 and 3 (Fig. 16) represent depressions that have been subsequently filled to the present lake bottom. The fill is represented by horizontal reflections that may onlap the depression or completely cover the subsided area. Evidence of stress fractures, slumping, faulting, or dissolution fractures around the depression (type 3) differentiate the two dolines and may indicate more rapid or continuous subsidence, or a more competent overburden. These breaches within the depression may provide a significant hydraulic connection between surface waters and the underlying aquifer. Most of the sinkholes detected using HRSP are of the buried base-level type (Fig. 13, Sinkhole Evolution) and should be a common occurrence beneath dry land as well. Only when these features develop a transitional phase (Fig. 13), reactivate and cause a surface subsidence or collapse, do they become evident at the surface.