Lake Pontchartrain Basin:  Bottom Sediments and Related Environmental Resources

Geophysical Surveys

High-Resolution Seismic Profiling

Objectives and Accomplishments

High-resolution seismic profiling (HRSP) provides a means to remotely image subsurface features continuously and relatively easily. Resolution is about 1 m and extends to about 1 km in depth. Unlike land based seismic-reflection techniques, marine HRSP is acquired continuously, greatly reducing post-processing. HRSP provides continuous two- dimensional profiles, whereas invasive techniques such as borehole drilling can only be inferred through correlation between boreholes. HRSP is also more cost effective at depth than drilling. Remote sensing is subjective, however, and without close correlation to direct sediment sampling, characteristics such as lithology and texture can only be assumed.

Since 1994, over 650 line-kilometers of HRSP have been obtained, tied to Differential Global Positioning System (DGPS) navigation (fig. 2). The surveys were conducted aboard the USGS research vessel G. K. Gilbert (fig. 3). In 1994, the effort was to establish a reconnaissance gridwork of Lake Pontchartrain. In addition, one line was run through Pass Manchac into Lake Maurepas. The following year, reconnaissance lines were run off of the Pearl River and Rigolets area and one was run along the southeastern shore of Lake Pontchartrain. In 1996, transects were run to fill in gaps left by previous surveys and to provide insight into issues brought up by interpretations of the earlier surveys and sediment core samples. Further reconnaissance along the Rigolets area of Lake Borgne was included. The effort in 1997 was to complete coverage in easternmost Lake Pontchartrain and Lake Borgne and to tie together core transects obtained the previous year. In 1998, efforts were focused on a series of dredge pits located on the south shore of Lake Pontchartrain in conjunction with side- scan sonar surveys (described next). Additional surveys were conducted in the Rigolets and Chef Menteur passes.

Figure 2

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 Figure 3

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 Figure 4

Figure 4 (click for larger view)

The task objective for the HRSP surveys was to establish a recent (Quaternary) geologic framework for the basin through interpretation of the seismic profiles and their correlation with invasive field investigations. Current accomplishments using the HRSP include (1) the identification and location of subsurface features such as faults, barrier trends, paleo-deltas, and paleo-incised valleys throughout the basin and (2) resolving the spatial distribution of various regional geomorphologic features such as the unconformable contact between Pleistocene and Holocene sediments (fig. 4) and the transitional relationship between open and closed-basin sediments during the Late Wisconsin sea-level rise. Contacts are verified through correlation with direct sampling techniques, such as vibracoring.


High- resolution, single channel seismic profiles were acquired using the Elics Delph 2 system, running in real time on a Kontron Electronics IP Lite laptop computer. Hard-copy data were displayed using a gray- scale thermal plotter with digital data backed up on removable 1-gigabyte hard disks. Navigation data were collected using a Rockwell International Precision Lightweight GPS receiver (PLGR) with Geolink mapping software (fig. 5). Figure 5
Figure 5 (click for larger view)

The acoustic source was a Huntec Model 4425 Seismic Source Module, which triggered an electromechanical device deployed on a catamaran sled. 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 Delph 2 system for storage and processing.

Figure 6
Figure 6 (click for larger view)
The Elics Delph 2 system measures and displays two-way travel time (TWTT) of the acoustical pulse in milliseconds. 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 (fig. 6). Depth to horizon is determined from the TWTT, adjusted to the subsurface velocity of the signal. An averaged compressional velocity of 1,500 m/s was used.

Reflective horizons on the seismic profiles were correlated to stratigraphic horizons based on corroboration with invasive techniques, previous studies, and the development of the geologic framework of the basin. The horizons were mapped out on paper copies and digitized using conventional table-top digitizing techniques. Navigation was tied to each digitized point using a FORTRAN program that interpolated geographic coordinates between navigational fixes. The resulting latitude, longitude, and depth to horizon coordinates for an interpreted horizon, for all the profiles digitized, were combined and gridded using the commercial software package CPS-3. From the gridded data set, contour maps of the interpreted horizon were developed using the same software.


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New Field Sampling Techniques