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Subsurface Characterization of Selected Water Bodies in the St Johns River Water Management District, Northeast Florida

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Identification of Karst Features from Seismic Profiles

Figure 15: High-resolution seismic profile examples from three lakes located in separate geomorphologic regions of northeastern Florida. Colors are for interpretive purposes and do not indicate correlation between profiles. Click on the image for a larger version.

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 15 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 ³hard² 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.

Seismic profiles with line drawing interpretations of six types of features described from the lakes of northeastern Florida.
Figure 16: Seismic profiles with line drawing interpretations of six types of features described from the lakes of northeastern Florida. Click on the image for a larger version.
Other common features are high-frequency or chaotic reflections interspersed between horizontal reflections (Type 4, Fig. 16). These reflectors indicate a disturbance within a relatively intact stratigraphic sequence and may represent solution pipes or fractures through the overburden. The features may connect to dissolution systems in the underlying limestone and could represent direct hydraulic connection through the semi-confining layer to the underlying aquifer. The disturbed reflections indicate areas of potential subsidence or collapse. These features have a high potential for reactivation since the plugs that fill solution pipes may dislodge during periods of major rainfall variations. There are many examples of this from Marion County (Cain and Hornstine, 1991). Solution pipes and related features commonly occur in areas where cohesive overburden is moderate to thin. Dissolution is focused and material directly over the cavity is washed into the void during the piping process. Type 4 features are widespread throughout the lakes surveyed, they occur in all phases of karst development and are commonly associated with poljes.

The Type 5 (see Fig. 16) feature represents the classic collapse sinkhole, with steep walls that show evidence of slumping and active development along the periphery of the collapse. Freshwater plumes have been imaged emerging from similar collapse features found in marine environments (e. g. Crescent Beach Spring). In seismic profiles, areas of negligible acoustic return below the collapse have been postulated to represent subterranean cavities. These active phase collapse sinkholes are typically evident at the surface without imaging and occur in areas of minimal overburden. They also indicate areas of internal drainage or discharge depending on the location of the potentiometric surface of the Floridan aquifer.

Finally, the Type 6 (see Fig. 16) feature does not necessarily include a karst-related structure but rather represents intact bedding or undisturbed section. A moderate to thick overburden overlies a deeper limestone surface that may not be within the imageable area of the HRSP, in which case depth to limestone is estimated from other methods, such as gamma profiles of well logs. This type of stratigraphy may occur over the entire survey area if there is a thick overburden, or as fill within karst features. Communication between the surface and groundwaters may be minimal in these areas.

All of the lakes surveyed that have legible seismic profiles show at least one of the features noted in the summary reflections but usually there are multiple features present. Where these features have been identified in the profiles, their corresponding number has been annotated on the index map for each individual lake. The extent of the coverage, along with correlation from other sensing techniques such as gamma logs, and general knowledge of Florida geology, has allowed for some inference as to the type of material associated with the acoustic return. Parallel reflections or "transparent" return infer a stratigraphy of sand and clays. A jagged or noisy return indicates the limestone surface is near the lake bottom. As mentioned earlier, the type of feature present is probably a function of type and amount of overburden, proximity of karst surface to the lake bottom and maturity of karst development. Each of the identified features influence leakage between surface waters and the Floridan aquifer. Studies using seepage meters are being conducted to quantify variations in leakage related to a particular subsurface feature (Hirsch, 1998).

Coastal & Marine Geology Program > Center for Coastal & Regional Marine Studies > Geologic Characterization of Lakes and Rivers of Northeast Florida > OFR 00-180 U. S. Department of the Interior | U.S. Geological Survey
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Updated May 06, 2013 @ 09:24 AM (JSS)