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Coastal & Marine Geology Program > Center for Coastal & Regional Marine Studies > Geologic Characterization of Lakes and Rivers of Northeast Florida > OFR 00-180

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

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Methods

Seismic Profiling | Geophysical Well Logs | Map Generation

Equipment used to acquire high-resolution single-channel subbottom seismic reflection profiles.
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. Click on the image for a larger version.
The Elics Delph2* High-Resolution Seismic Profile System (HRSP) was acquired with proprietary hardware and software running in real time on an Industrial Computer Corporation 486/33 PC (Fig. 2). A gray scale thermal plotter was used to display hard-copy data. Digital data were stored on a rewritable magneto-optical compact disk. Navigation data were collected using a Trimble Global Positioning System (GPS) or Rockwell Precision Lightweight GPS Receiver (PLGR) these systems provide navigational accuracies of ±10 m. GeoLink XDS mapping software was used to display navigation. The acoustic source was an electromechanical device, the Huntec Model 4425 Seismic Source Module mounted on a catamaran sled (Fig. 2). Occasionally, an ORE Geopulse power supply was substituted for the Huntec Model 4425 due to operational limitations. Power settings were 60 joules or 135 joules depending upon data quality during acquisition. 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.

Comparison of depth-to-horizon in milliseconds on seismic profiles to depth-to-peak in meters of a correlative horizon on natural gamma logs.
Figure 3: Comparison of depth-to-horizon in milliseconds on seismic profiles to depth-to-peak in meters of a correlative horizon on natural gamma logs. The resulting equations describing 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. Click on the image for a larger version.

The Elics Delph2 Geophysical 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 1500 to 1800 meters per second (m/s) (Tihansky, pers. comm.; Sacks and others, 1991). Refraction studies conducted in areas within Alachua County, Florida (Wiener, 1982) yielded velocities of 1707 to 4939 m/s for the Hawthorn Group sediments. Wiener, (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).

More than 750 line-km of data were collected from >40 lakes, rivers and offshore sites, only 34 are presented in this study (Table 1). Best-fit-curves (Figure 3) were used to compare well-log depths and seismic depths but an approximate velocity of 1500 m/s was used as a general calculation for depth scales on the HRSP data. Data quality varied from good to poor with different areas and varying conditions. As acquisition techniques improved, data quality in general also improved. The interbedded nature of the lake bottom sediments provides good reflecting surfaces for acoustic signals. These layers appear on the seismic records as convergent, divergent, or parallel bands. Folds, faults and facies changes can be recognized as bands, lateral and vertical discontinuities, and truncations of the bands by other reflections. In some areas, acoustic multiple-reflections masked much of the shallow geologic data. Multiple reflections, an artifact of the acquisition system, are caused by a number of possible factors that reflect the acoustic signal to the water surface and back down more than once.

Coastal & Marine Geology Program > Center for Coastal & Regional Marine Studies > Geologic Characterization of Lakes and Rivers of Northeast Florida > OFR 00-180

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Updated December 02, 2016 @ 11:25 AM (JSS)