Sediment Sampling Surveys
Objectives and Accomplishments:
Vibracoring is one of many subsurface sediment acquisition (sediment coring) techniques, each of which fills a role dependent on task objective and environmental parameters. Vibracoring obtains sediment samples by vibrating a core barrel into the sediment (Figure 9). The advantage of vibracoring is that core depths (up to twenty feet with this system) can exceed most small-boat gravity-driven type corers without adding cumbersome weight. Likewise, vibracoring is easier to accomplish and maintain than rotary-drill core methods. The vibracoring process, however, makes penetration success dependent on lithology; pure sands tend to attenuate the vibration of the barrel and slow descent, rock of course will stop descent of the barrel. The vibration will also translate to the core sample itself, and may compact the sediments or disrupt laminations or bedding in the sediment. Other coring techniques should be considered if these factors become an issue.
From 1995 to 1998 over one hundred vibracores have been obtained from the Pontchartrain Basin (Figure 10) using a rig on board the G. K. Gilbert. In 1995 twelve cores were taken from the tidal delta sediment east of Pass Manchac, along with nine cores taken along seismic tracklines acquired the year before near Goose Point and the southern part of Lake Pontchartrain. A series of short barrel cores were taken near the outfall canals and bayous of the southern shoreline for geochemical and isotope work. The following year cores again were taken along the outfall canal. Cores were also taken in the northern part of the lake in attempts to identify point and line features such as relict beach ridges and paleo-deltaic sequences. Several short cores were taken in Bayou Bonfouca for geochemical studies. In addition, exploratory cores were taken in the Middle Grounds area of easternmost Lake Pontchartrain and in Lake Borgne. The vibracore survey in 1997 was designed to hit key features interpreted from seismic profiles and to verify location of the Pleistocene subsurface in Lakes Pontchartrain and Borgne. In addition, a series of six cores were taken along Northshore Beach for sediment budget studies, several in Bayous Bonfouca and Lacombe, and eight in the tributaries of the Pearl River delta. In 1998, cores were acquired from the Lacombe Bayou area, north shore Lake Pontchartrain. The cores were extracted by researchers from the University of New Orleans, using a land-based tripod. Later that same year 17 additional cores were acquired from the G. K. Gilbert throughout the lake in order to resolve further depth to Pleistocene issues and to identify relict barrier trends. Nine cores were obtained from the dredge pits indentified in bathymetric and side scan sonar surveys. The water content of the sediments in these pits was so high that loss of material from the vibracores during recovery could not be avoided. Most of the cores acquired thus far are currently archived at the University of New Orleans Geology department.
The task objective of the vibracore surveys was to develop high resolution sedimentation patterns and stratigraphy of the post Pleistocene sediments in the Basin, and to provide down core samples for geochemical and processes studies. The cores were also intended to corroborate with the HRSP surveys to identify and map key reflective horizons. Accomplishments include the development of a geologic framework related to sea level rise and subsequent geomorphologic development of the Basin. Stratigraphic features within the framework that have been identified by the cores and HRSP include the Pleistocene surface across the basin, as well as, marine to lacustrine transitional sediments associated with the closing of the basin. Additional features include barrier island and deltaic sediments, oyster bioherms and marsh deposits.
Vibracores are 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 are riveted at the base of each barrel to ensure complete recovery of the sediments. An electric wire line is 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 is used to position and recover the rig. Upon recovery, the barrel is removed from the rig and cut to the core length. The ends of the barrel are capped and the length measured. The measured length is compared to the wire line reading to estimate compaction. A complete instructional course on the construction, preparation and use of the vibracore rig used in this survey, as well as barrel preparation and core curation, is available on CD-ROM and can be obtained from the USGS Center for Coastal Geology.
Researchers at the University of New Orleans have collected additional cores using a land-based tripod and portable vibrator. The vibrator, a STOW Model G550H, is powered by a Honda 5.5 horsepower gas motor. This unit spins a flexible cable at high speed that causes the unevenly weighted head to vibrate. The head is mounted by U-bolts to a weighted adapter that clamps to 3 inch aluminum core barrels. The setup is capable of handling barrels of any length since the adapter can easily be repositioned up the shaft as the core is driven in. Thirty foot barrels have been the standard maximum length for this operation.
Once the barrel has been driven in, the excess shaft is cut off with a hacksaw and the compaction is noted by measuring the inside distance from the barrel top to the sediment and the outside distance to the ground. The barrel is then filled to the rim with water and sealed with a 3 inch plumbers plug. This will create a suction when the core is pulled out that will keep the column of sediment in the core tube. This method of sediment retention works as long as the weight of the water column does not exceed the relative support provided by the sediment in the barrel.
The core barrel is extracted using a 1 ton come-along attached to the adapter and to a 12 foot aluminum tripod. This tripod has a 1 foot diameter swiveling feet attached 1 foot above the bottom of each tripod shaft to minimize sinking problems. In extremely marshy conditions, the tripod can be further supported by a 4 foot diameter plywood disk with a 4 inch hole in the center and three holes for the tripod feet.