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Open File Report:
Seismic Reflection
Surveys |
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Introduction |
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Figure 1: Schematic of typical
Karst terrain from Central Florida (modified from Beck, 1988).
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Florida is a carbonate (limestone) platform with a mature karst that is
well known for abundant sinkholes, springs, and caverns. Karstic erosion of
the land surface is controlled by processes occurring in the upper portion of
the limestone where the most intense dissolution occurs (Beck, 1988). In
Florida, features characteristic of karst include dolines (sinkholes), solution
pipes, broad flat bottomed prairies and closed circular depressions that either
drain underground or fill with water to form lakes
(Fig. 1).
The term "sinkhole", or doline, implies a form, a function, and a basic mechanism of
origin. The form is a closed basin having no surface drainage outlet. The
function is to transmit surface water underground into the karstic aquifer.
The origin is basically by solution of the underlying karst host rock.
Sinkholes form primarily on terrain of limestone or dolomite (which in this
context is merely a variety of limestone), or where either of these rocks occur
near the surface. They can, however, form over any rock that is soluble
or cavernous and appear as a variety of structures including solution
sinkholes, subsidence sinkholes, or subsidence over buried sinkholes
(Fig. 2).
Individual sinkholes may be less than one meter, or more than 100 meters, both
in depth and diameter, may be circular or elongate, and can have profiles that
are conical, cylindrical, saucer-shaped or irregular.
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Figure 2: A variety of common sinkhole
structures and terminology (modified from Waltham, 1989).
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The investigation of subsurface and subaqueous karst has historically
proven to be a difficult task. Due to their random, unpredictable distribution,
natural cavities or buried sinkholes are notoriously difficult and expensive to
assess and locate in site investigation. There are several direct and indirect
methods of mapping and identifying features associated with karst, all which
have varied limitations. Waltham (1989) provides an excellent review of the
methods used for the detection of cavities, including geophysical techniques.
Extensive literature about geophysical techniques applied to cave and sinkhole detection
has been reviewed by Bates (1973), McCann and others (1982), Owen (1983), and
McCann and others (1987). The overall conclusion of these authors, however, is
that previously applied geophysical techniques had little reliability for
widespread use, but the potential cost savings compared to other methods
warrant consideration. High-resolution marine seismic reflection profiling
(HRSP) has been successfully used in various lakes and rivers in Florida to
detect subsurface features related to karst (Missimer, 1976; Snyder, 1989; Sacks
and others, 1991; Locher and others, 1988; Subsurface Detection Inc., 1992) and
also offshore in the Atlantic Ocean (Popenoe, 1984; Meisburger, 1976; Snyder,
1989). Technological advances in data acquisition and improvements in field
techniques are rapidly expanding the applications of HRSP to hydrologic
investigations in marine, riverine, and lacustrine settings.
This study was a cooperative investigation conducted by the St. Johns
River Water Management District (SJRWMD) and U.S. Geological Survey Center for
Coastal Geology (USGS). Since 1989 there have been technical and hardware
advances in the digital acquisition of high-resolution seismic data. The
primary objective of this cooperative was to test newly developed digital
high-resolution single-channel marine seismic continuous-profiling-equipment
(HRSP) and apply this technology to identify subbottom features that may enhance
leakage from selected lakes and the St. Johns River. The target features
include: (1) identifying evidence of breaches or discontinuities in the
confining units between the water bodies and the Floridan aquifer, and; (2)
identifying areas where the confining unit is thin or absent.
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Figure 3: Location
of selected north central Florida lakes surveyed.
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The profiles for Orange Lake in Alachua county are emphasized in this
report. Alternatives for managing lake stage are currently being considered
(Robison, 1994). There is at least one doline(s) in the southwest section at
the Heagy-Burry Park that is known to be draining the lake (Roland, 1957;
Spechler, 1992; Pirkle, 1959). Some management alternatives include plugging
or isolating the sinkhole area to prevent the loss of water. A detailed
knowledge of the extent of the features at the park and identification of other
areas that could create a similar high leakage situation is needed to
assess the various management alternatives.
Lake Lowry, Lake Magnolia and Kingsley Lake in Clay county were profiled
to supplement a seismic data base of lakes within the St. Johns River Water
Management District. More information is needed to understand the wide ranges
of lake stage fluctuations from different lakes within a close geographic area
(Figure 3, above).
Those lakes are unique in that their stages remained relatively
constant (less than 1 m) during the period of recorded data (Motz, 1991). Many
of the surrounding lakes have experienced dramatic fluctuations over the same period.
An extreme case is Pebble Lake which has fluctuated over 32 feet
or 9.8 meters (Motz, 1991). As more seismic data is collected for the lakes, a
correlation between lake stages and subbottom conditions may be evident. Sample
sections from these lakes are provided to show the general subbottom character.
Data from a reconnaissance survey around Drayton Island in the St. Johns
River at the north end of Lake George is also presented. This is an area where
faults have been mapped (Bermes and others, 1963) and there is evidence of
abrupt changes in water quality that may be related to the subsurface
structure. A detailed analysis that integrates ground water quality and
subsurface structure is planned but beyond the scope of this report.
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