Marinna A. Martini

Standard-target calibration of an acoustic backscatter system

The standard-target method used to calibrate scientific echo sounders and other scientific sonars by a single, solid elastic sphere is being adapted to acoustic backscatter (ABS) systems. Its first application, to the AQUAscat 1000, is described. The on-axis sensitivity and directional properties of transducer beams at three operating frequencies, nominally 1, 2.5, and 4 MHz, have been determined using a 10-mm-diameter sphere of tungsten carbide with 6% cobalt binder. Preliminary results are reported for the 1-MHz transducer. Their application to measurements of suspended sediment made in situ with the same device is described. This will enable the data to be expressed directly in physical units of volume backscattering.

Measurements of echo stability of an acoustic backscatter system

Acoustic backscatter from acoustic profiling instrumentation is commonly used as a proxy for suspended sediment concentrations. Past calibration procedures have used suspended beads or natural sediment to relate system output to sediment concentration. A new approach seeks to separate the instrumentation system constant from parameters that are dependent on the particles in the water column. Protocols are being developed for the standard-target calibration of an acoustic backscatter system (ABSS). These were initially applied to the Aquatec Acoustic Backscatter System, model AQUAscat 1000, with three transducers at the nominal operating frequencies of 1, 2.5, and 4 MHz. The standard target was a 10-mm-diameter sphere of tungsten carbide with 6% cobalt binder. In addition to measurements of sensitivity and angular response, the operating stability of the system was gauged over four successive nights, with total data collection periods between 16 and 20 hours. Pinging was performed at a constant 64-Hz rate for either 1200 or 1800 s during each hour. The nominal pulse duration was 6.7 μs at each frequency. Each series of 64 echo signals was averaged to form an average echo profile. A total of 1200 or 1800 echo profiles were stored during each hour. The resulting echo data have been processed by integrating the squared echo amplitudes in each profile over range intervals containing the target and over adjacent range intervals without the target, allowing an analysis of both signal and noise. Resulting energy-based measures of signal and noise are presented as time series together with their basic statistics. Enough data were collected to demonstrate that signal to noise ratios are high enough and the standard deviation of measurements recorded over several hours stable enough to allow a rigorous standard target calibration of this instrument in a test tank 3×4×4 m in dimension.

A Multi-sensor Oceanographic Measurement System For Coastal Environments

An instrument system has been developed for long-term sediment transport studies that uses a modular design to combine off the shelf components into a complete and flexible package. A common data storage format is used in each instrument system so that the same hardware can be assembled in different ways to address specific scientific studies with minimal engineering support and modification. Three systems have been constructed and successfully deployed to date in two different coastal environments.

Water samples in support of oceanographic and water-quality measurements in Chincoteague Bay, Maryland and Virginia, 2014-15, U.S. Geological Survey Field Activity 2014-048-FA

U.S. Geological Survey scientist and technical support staff measured oceanographic, water quality, seabed elevation change, and meteorological parameters in Chincoteague Bay, Maryland and Virginia, during the period of August 13, 2014 to July 14, 2015 as part of the Estuarine Physical Response (EPR) to Storms project (GS2-2D) supported by the Department of the Interior Hurricane Sandy Recovery program. These measurements provide time-series and supplemental data to quantify the response and understand the resilience of this back-barrier estuarine system to storm events. The Assateague Island National Seashore (U.S. National Park Service) and the Chincoteague National Wildlife Refuge (U.S. Fish and Wildlife Service) are both situated on the east side of Chincoteague Bay, and serve as the barrier between Chincoteague Bay and the Atlantic Ocean as well as providing globally important migratory bird habitat and a popular recreation destination.

Observations of ocean circulation and sediment transport experiment offshore of Fire Island, NY

Researchers from the U.S. Geological Survey (USGS) Woods Hole Coastal and Marine Science Center (WHCMSC), in collaboration with Coastal Carolina University (CCU) and University of South Carolina (USC), conducted a scientific field study to investigate the ocean circulation and sediment transport processes offshore of Fire Island, NY. Although the physical processes along the entire linear extent of Fire Island (~50 km) are of interest to the project, one particular region of focus is at the western end of the island where offshore sand ridges out to depths of 20 m extend across the inner shelf and connect to the near-shore bar system. The primary objective was to measure the physical processes around the sand ridges, including circulation patterns, wave parameters, bottom stress, and suspended sediment. Transects of instrumentation were positioned along and across the crests and troughs of the ridge field. A site at the top of a ridge and a site at the bottom of an adjacent trough were each populated with two tripods designed to provide high-resolution measurements near the sea-bed to record sediment re-suspension events. Measurements at these two sites include near bottom velocity profiles, acoustic Doppler velocimeters, pressure, optical transmission and backscatter at high sampling rates. Other measurements include upward looking velocity profiles, temperature, salinity, sonar images and profiles, and sediment size classes. Five smaller tripods were deployed to complete lines alongshore and across shore over a 5 km area to provide a regional picture. These tripods recorded upward looking velocity profiles and near bottom temperature, pressure and salinity. Surface buoys marked the position of the tripods and collected surface measurements at six of the sites. One buoy gathered meteorological measurements. The sites were occupied from January to April, 2012. This deployment was similar to previous efforts off Cape Hatteras, NC, in 2009, and is part of an ongoing effort to understand regional patterns in circulation and sediment transport and the interaction of inner shelf and near shore processes. New instrumentation for the USGS was introduced, including a variety of current and wave measurement equipment, acquisition and telemetry in near-realtime of the weather data, time series sonar imaging equipment, and anti-fouling wipers. Preliminary results suggest a complex and subtle relationship between wind and across shore current velocity in this region, and a more straightforward relationship between winds and alongshore currents. This paper also includes a preliminary report on the effectiveness of new measurement techniques used during this experiment.