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Open File Report:
Seismic Reflection
Surveys |
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Methods |
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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
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Figure 5B
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Figure 5C
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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.
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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.
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Figures 6-10: Click on the
lakes in red to view
the location of seismic-reflection profiles collected from each site.
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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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