High resolution, single channel seismic profiles and side-scan
sonar were acquired using the Elics Delph 2 system, 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.
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. A compressional velocity of 1500 meters per second was used. Contour structure maps were constructed for horizons interpreted from seismic profiles. The digitized surfaces were gridded using CPS3 (commercial contouring package).
Vibracores were obtained using a Bradford pneumatic vibrator
powered by two air compressors delivering 35 scfm at 100 psi
each. The vibracore rig is capable of handling aluminum barrels
three inches in diameter and up to 20 ft in length. Brass
core-catchers were riveted within each barrel to ensure complete
recovery of the sediments. An electric wire line was attached to
the top of the rig and connected to a voltmeter on board to
measure penetration of the barrel into the lake bottom. A Hiab
hydraulic crane onboard the Gilbert was used to position and
recover the rig. Upon recovery, the barrel was removed from the
rig and cut to the core length. The ends of the barrel were
capped and the length measured. The measured length was compared
to the wire line reading to estimate compaction.