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Seismic Stratigraphy of the Central Indian River Region
Methods

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  Figure 2
Figure 2: Equipment used to acquire high-resolution single-channel subbottom seismic reflection profiles. Figure includes sound source (A), receiver (B), power supply (C), hard copy output (D) and computer (E) to process, display and store digital signal.
This study is part of a series of cooperative investigations conducted from 1993 to 1997 by the SJRWMD and U.S. Geological Survey St. Petersburg Coastal and Marine Science Center (USGS). Areas of study include inland and offshore waters and adjacent terrain throughout much of the SJRWMD. In cooperation with SJRWMD, the USGS has acquired and upgraded a digital seismic acquisition system. The Elics Delph2 High-Resolution Seismic System (HRSS, Fig. 2) was acquired with proprietary hardware and software running in real time on a Kontron Electronics IP Lite laptop computer. Hard-copy data was displayed using a gray scale thermal plotter with digital data backed up on removable 1 Gigabyte hard disks. Navigation data was collected using a PLGR (Rockwell) GPS with Fugawi mapping software.

The acoustic source was a Huntec Model 4425 Seismic Source Module and a catamaran sled equipped with an electromechanical device (Fig. 2). An ORE Geopulse power supply was substituted for the Huntec Model 4425 for small boat operations . Power settings ranged from 60 to 265 joules depending upon conditions. An Innovative Transducers Inc. ST-5 multi-element hydrophone was used to detect the return acoustical pulse. This pulse was fed directly into the Elics Delph2 system for storage and processing.

Figure 3
Figure 3: Plot of depth-to-horizon in milliseconds on seismic profiles, versus depth-to-peak in meters on natural gamma logs. The resulting equations from the best fit curve (blue) or the best fit curve with zero origin (red) can be used to determine sound velocity for a given depth. Averaged velocity for 100 to 200 meters depth is 1,955 meters per second.
 
The Elics Delph2 system measures and displays two-way travel time (TWTT) of the acoustical pulse in milliseconds (ms). Amplitude and velocity of the signal are affected by variations in lithology of the underlying strata. Laterally consistent amplitude changes (lithologic contacts) are displayed as continuous horizons on the seismic profiles. Depth to horizon is determined from the TWTT, adjusted to the subsurface velocity of the signal. Suggested compressional velocities for Hawthorn Group sediments for the Florida Platform range from 1,500 to 1,800 meters per second (m/s) (Tihansky, pers. comm.; Sacks and other, 1991). Refraction studies conducted in areas within Alachua County Florida (Weiner, 1982) yielded velocities of 1,707 to 4,939 m/s for the Hawthorn Group sediments. Weiner, (1982) reported lower velocities for the sand and clay sediments and higher velocities for the carbonate sediments. To correlate horizons from gamma logs to seismic profiles, best-fit-curve plots were used to determine local velocities (Fig. 3). Contour structure maps were constructed for horizons interpreted from seismic profiles (Fig. 5, 6, 7, 8). The digitized surfaces were gridded using CPS3 (commercial contouring package).

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