High-Frequency Sound Interaction in Ocean Sediments
2 July 1999

Sediment Volume Inhomogeneities:
Patterns, Mechanisms and Rates of Change

Rob Wheatcroft
Oregon State University
(541) 737-3891, -2064 (fax), raw@oce.orst.edu

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The primary objective of this proposal is to quantitatively document the patterns of, and processes leading to, sediment volume inhomogeneities at the Panama City study site. A secondary objective is to quantify temporal rates of change of the volume inhomogeneity field. A two-pronged approach will be taken to accomplish these objectives.

First, an instrumented bottom tripod will be deployed at the study site during the October 1999 experiment. Time-lapse instrumentation on the tripod will include a stereocamera and various physical sensors. In addition, deliberate-tracer bioturbation experiments will be conducted prior to, and during, the main experiment. The purpose of these measurements is to document physical and biological processes leading to the measured sediment volume inhomogeneities.

The second approach will be to quantitatively measure the sediment volume inhomogeneity field using digital radiography. Precisely located and oriented cores will be collected by divers at multiple separation lengths, transported to the ship, and immediately x-rayed. Digital brightness data will be transformed to bulk density, based on empirical laboratory studies, supported by field measurements made using an in-situ resistivity profiler (IRP). The sediment volume inhomogeneity field will be described and analyzed using a variety of spatial statistical measures (e.g., Tang and Wheatcroft, in prep.), as well as classical sedimentological nomenclature.

The following describes the operational details of these various measuring systems/approaches.

Time-Lapse Tripod

A fully autonomous bottom tripod will be deployed at the study site during the October 1999 experiment. [Previous plans called for deployment of the tripod for several months (i.e., July–October) prior to the main experiment. However, warnings from CSS personnel regarding boat traffic and fouling has led me to reconsider this plan. I still believe that it would be beneficial to have prior information from the site, and am exploring alternative (safer) deployment strategies. Suggestions are welcome.] Instrumentation on the tripod will include a PhotoSea 2000 stereocamera that will image a roughly 1 by 2 m patch of seafloor every 6 hours. In addition, a Sontek Hydra system comprising a 5 MHz Acoustic Doppler Velocimeter (u, v, and w velocity), a Druck pressure sensor, Seabird conductivity and temperature sensors, and an optical backscatter sensor (turbidity) will be deployed. These various sensors will provide information on geophysical sediment transport prior to (hopefully), and during the experiment, as well as a glimpse at the types, abundances, and behavior of epibenthic megafauna (i.e., those animals most responsible for creating cm-scale surface and subsurface biogenic structures).

Bioturbation Experiments

Running in parallel with the tripod measurements will be a series of bioturbation (= sediment displacement by organisms) experiments. Because there is unlikely to be sufficient radionuclide (Th-234, Pb-210) activities in the sandy sediments off Panama City, I will use previously developed deliberate tracers. Briefly, natural sediment grains from the study site will be labeled with noble metals (e.g., Au and Ag) using a thermal diffusion technique. The labeled sediment grains will then be spread on the seafloor by divers and cored at various time intervals. Enumeration of tracer concentrations (at ppb concentrations) as a function of depth in the sediment will be via instrumental neutron activation. These experiments will provide information on sediment mixing intensity, which will in turn provide estimates on the rates of change of the volume inhomogeneity field.

Digital Radiography

Small-diameter (<15 cm) cores will be collected by divers at a variety of spacings (centimeters to hundreds of meters), as well as on features of interest (e.g., mounds, ripple crests and troughs) in both the insonified (e.g., adjacent to the APL/UW BAMS and XBAMS tripods) and "background" areas. [I propose to use an acoustic triangulation system to locate cores within the study area. Briefly, this system comprises three transponders (moored roughly 500 m apart in a triangular configuration) and a diver-held interrogator. The interrogator pings at 26 kHz and the various transponders transmit at 25–32 kHz. Will this system interfere with your measurements?] The cores will be carefully transported to the ship and immediately x-rayed using a digital x-radiography system. Shipboard x-radiography is important in that it minimizes biological and geotechnical (e.g., compaction) artifacts that occur during transport. A digital x-radiography capability will allow a near real time view of the volume inhomogeneities within the study site, which will allow adaptive acoustical and environmental sampling. Once radiographed, the cores will be provided to colleagues (e.g., R. Bennett, K. Briggs, and P. Jumars) for additional characterization.

Calibration of the digital radiographs will be based on empirical laboratory studies, supported by field measurements made using an in-situ resistivity profiler (IRP). Similar in design to the IMP described by DJ Tang, but consisting of only one probe, the IRP can be precisely positioned by divers over features of interest (e.g., mounds, ripple crests, and troughs). A complete profile is collected in < 4 minutes, thereby permitting acquisition of several profiles during a dive.

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