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Seismic Reflection Surveys
Methods

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Open File Report:
Seismic Reflection
Surveys
  Abstract
  Introduction
  Karst Formation
  Methods You are at Seismic Reflection Surveys - Methods
  Regional Geology
  Orange Lake
  Kingsley Lake
  Lowry & Magnolia Lakes
  Drayton Island
  Conclusions
  References
Contact:
Jim Flocks
  In cooperation with SJRWMD the USGS acquired and upgraded a digital seismic acquisition system. The Elics Delph2 High-Resolution Seismic System was acquired with proprietary hardware (32 bit digital signal processor) and proprietary software ( Ver. 1.22) running in real time on an Industrial Computer Corp. 486/33 PC. Hard-copy data was displayed on an OYO gray scale thermal plotter and a SVGA monitor in TIGA graphics mode. Digital data was stored on a rewritable Sony Magneto-Optical compact disk and a 1 GB hard drive. Navigation data was collected using a Trimble GPS, logged into an Industrial Computer Corp. 486/33 PC and ported real time to the Delph2 seismic system. This navigation data was stored on the Delph2 and navigational computer systems and displayed on both systems, independently. GeoLink XDS mapping software (Ver 3.0) was used for this purpose.

Figure 5A
Figure 5A
Figure 5B
Figure 5B
Figure 5C
Figure 5C
Figure 5: Equipment and deployment. Photo A shows the digital acquisition system: Elics Delph2 High Resolution Seismic System with computers and monitors, IT hydrophone (lower right), Huntec Power SOurce and Boomer Sled. Photos B and C are lake operations and equipment aboard the SJRWMD boat used to survey lakes.

A Huntec Model 4425 Seismic Source Module was used to generate an underwater acoustical pulse. This module consisted of an Energy Storage Unit, a Power Control Unit and a catamaran sled with an electromechanical device (acoustical sound source) (Fig. 5, above). This unit was triggered by the Elics Delph2 system. Occasionally, an ORE Geopulse power supply was substituted for the Huntec Model 4425. The ORE Geopulse power supply replaced the Huntec Power Control Unit and Energy Storage Unit. Power was set at 60 joules or 135 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.

  Figures 6-10
Figures 6-10: Click on the lakes in red to view the location of seismic-reflection profiles collected from each site.
Data was collected during 18 days of field operations conducted during August, 1993 and January-February, 1994. A total survey of 218 line-km was completed with >452 MB of digital data collected from the lakes of Orange (
Fig. 6 and Fig. 7), Kingsley (Fig. 8), Lowry and Magnolia (Fig. 9), and the St. Johns River (Fig. 10). Hard copies of the seismic profiles and track maps have been supplied to SJRWMD and the original digital data archived at the USGS Center for Coastal Geology in St. Petersburg, FL.

A velocity of 1500 meters per second (m/s) was used to calculate a depth scale for the seismic profiles. Measured site specific velocity data is not available for these sites however, other investigators have used velocities ranging from 1500 to 3000 m/s. Emory and Zarudski (1967) measured velocity on the continental shelf of North Carolina in Miocene and Pliocene sediments of 1940 m/s and 3000 m/s in Eocene sediments. Missimer (1976) calculated a velocity of 2300 m/s from clays in Lee County, Florida. A velocity of 1675 m/s was used for Miocene sediments in Onslow Bay offshore of North Carolina (Snyder and others, 1980). Tahanski (written communication, 1992) used 1500 m/s for work in Lake Lucerne, Florida and 1800 m/s was used for work in Lake Barco (Sacks and others, 1991). The high velocities that have been reported are from sediments that are more compacted or lithified and therefore may not be representative of these sites. The lower value of 1500 m/s (velocity in freshwater) was used for this report based on correlation to boreholes near the sites and because the sediments are mostly unconsolidated and saturated with water.

The equation used to convert Two Way Travel Time is as follows:

D = (TWT/2) x (V) x (.001)
D = Depth in meters TWT = Two Way Travel Time in milliseconds V = Velocity (1500 meters per second is used here)

Line drawing interpretations of seismic sections are presented for each site. The strata that produced the high amplitude reflections are high lighted to discern changes in original horizontal bedding. The patterns of the highlighted reflectors provide a cross section of the subsurface that can show folds, faults, slumps and other features that have disturbed the original bedding planes.

The expected depths to geologic contacts are estimated from onshore borehole geophysical data and discussed for each site. These contacts are not identified on the seismic profiles since there is no borehole data along the lines to confirm the interpretation. Seismic refraction surveys at nearby quarries and deep sinkholes such as the Devils Millhopper north of Gainesville, would also be useful to confirm the velocity structure.

These surveys were conducted in part to test the effectiveness of shallow-water marine shown as numbered bold lines. Acquisition techniques were similar but modifications were necessary. Data quality varied from good to poor with different areas and varying conditions. As acquisition techniques improved so did data quality in general. In many areas of Orange Lake an acoustic multiple masked much of the shallow geologic data. An attempt at post acquisition processing of the digital data to reveal more of the shallow geology was not successful, but future processing may bring out geologic data in other areas or lakes.

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Updated December 05, 2016 @ 11:25 AM (THF) Figure 6 - Orange Lake Figure 8 - Kingsley Lake Figure 9 - Lowery and Magnolia Lakes Figure 10 - Lake George