Geologic Framework and Processes of the Lake Pontchartrain Basin

Seismic equipment

The USGS has acquired and upgraded a digital seismic acquisition system. The Elics Delph2 High-Resolution Seismic System (HRSS) was acquired with proprietary hardware and software running in real time on a Kontron Electronics IP Lite running at 132 MHz. Hard-copy data was displayed on a gray scale thermal plotter with digital data stored to disk. Navigation data was collected using a PLGR (Rockwell) GPS and Fugawi mapping software to display navigation.

The acoustic source was the Huntec Model 4425 Seismic Source Module and a catamaran sled with an electromechanical device. During small boat operations an ORE Geopulse power supply was substituted for the Huntec Model 4425. Power was set at 60 -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.

Schematic of towing arrangement

The Elics Delph2 system measures and displays the two-way travel time (TWTT) of the return acoustical pulse in milliseconds (ms). This is related to changes in velocity of the signal as it impeded by variations in lithology in the underlying strata. Laterally consistent changes (lithologic contacts) are displayed as continuos horizons on the seismic profiles. Depth to horizon is determined from the TWTT, depending on the velocity of the signal through the strata.

Coring

Vibracoring

R/V G.K. Gilbert

Vibracores were acquired from Lakes Pontchartrain, Borgne and the surrounding watershed from the R/V G. K. Gilbert 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.

A rapid sediment sampling system has been developed by the Center for Applied Isotope Studies at the University of Georgia. The system is composed of a towed pumping device which samples a slurry of surficial sediments (above, left) as it moves across the sea floor. The sample is transported via a one inch to a shipboard processor (above, center). The processor selectively samples the fine fraction of the slurry and places an aliquot of sediment into a filter media. The sample can then be analyzed for elemental composition by x-ray fluorescence (XRF).