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Oceanographic Observations, Hudson Shelf Valley, U.S. Geological Survey Open-File Report 02-217


A variety of sensors from several manufacturers were used to measure current, temperature, conductivity, light transmission, optical backscatter, acoustic backscatter, suspended sediment concentration, and pressure. These sensors were deployed on a variety of platforms (see descriptions below and Table), and the data were sampled (see summary of sampling schemes) and recorded by various data logging systems.

BASS tripod (figures 5, 7A, 8B)

Tripods equipped with 4-axis Benthic Acoustic Stress Sensors (BASS) at 0.4 and 1.0 meters above the bottom (mab), optical backscatter (OBS) sensors at .4 and 1.0 mab, temperature at 1.9 mab, conductivity at 1.8 mab, and pressure at 1.7 mab were deployed at Stations A, B and C. The tripods were also equipped with an upward looking Acoustic Doppler Current Profiler (ADCP) and a downward looking Acoustic Backscatter Sensor (ABS) (see below). Downward looking 35 mm Benthos cameras to photograph the seafloor every four hours failed in this experiment.

The BASS, temperature, conductivity, OBS, and pressure data were sampled and recorded using a Multiparameter Intelligent Data Acquisition System (MIDAS) that recorded data using a Tattletale (Martini and Strahle, 1992). Every second, the MIDAS data logger recorded pressure and 4 velocity components from the two BASS sensors (Williams and others, 1987). Every 3.75 minutes, MIDAS computed cumulative sums of pressure and current, and recorded them, along with temperature, conductivity, and light transmission measured at the center of the 3.75 minute averaging interval. Average pressure and current were calculated during post processing.

BASS current meters are capable of resolving 0.03 cm/s currents, however this requires a field determination of the zero. Accuracy is affected by the capacitance of the long cables which connect the data logger to the sensor pods. A new calibration must be obtained each time the data logger and sensor wiring is attached to a tripod frame. An accuracy of 0.3 cm/s can be achieved when the offsets generated by these capacitance changes are measured and removed from the data. The BASS current meter voltages were measured when there was no current through the measurement volume, and this 'zero' offset was subtracted from measurements made during the deployment. A set of experiments were performed to determine the most efficient method of calibrating the BASS to 0.3 cm/s accuracy (Morrison and others, 1993). The BASS was calibrated in still water on the pier prior to deployment and after recovery.

MAVS Tripod (figures 6A, 6B)

At Stations C and E near-bottom current was measured at 0.4 mab using a Modular Acoustic Velocity Sensor (MAVS - Thwaites and Williams, 1997). The MAVS was mounted on a small tripod frame, along with upward-looking ADCP, OBS sensors at .5 and 1.2 mab, temperature and conductivity at 0.6 and 0.7 mab, and bottom pressure. The MAVS was recorded with a MIDAS system.

ADCP (figures 7A, 7B)

RD Instruments ADCP's (300 Khz Workhorse) were deployed at Stations A-F to obtain profiles of currents from about 5 mab to near the water surface, with measurements spaced 1 m apart in the vertical. The instruments measure currents from the doppler shift of sound reflected from the water column from two pairs of orthogonal acoustic beams (Gordon, L., 1996). The instruments recorded 5-minute averages of current every 15 minutes. To obtain an accuracy of at least 0.4 cm/s for each 5 minute measurement, 300 pings emitted at a rate of 1 ping per second were averaged together. At Station A, B and D, the ADCP was deployed on a large tripod frame (figure 5, 7A). At Stations C and E, the ADCP was deployed on a small tripod frame (figure 6A), and at Station F on a micropod (figure 7B).

ABS (figures 8A, 8B)

To measure the vertical structure of suspended sediment concentration and cm scale seafloor elevation changes three acoustic backscattering systems (ABS) were deployed on the Hudson Shelf Valley tripod array. Two three frequency (1.0, 2.5 and 5.0 Mhz) AQUATEC ABS systems were deployed, one (serial number 016) at Station A in Christiansen Basin (40 m water depth) and the other (serial number 015) at Station B in the upper valley (55 m water depth). A third (serial number 041) older custom designed two frequency (2.5 and 5.0 Mhz) ABS was deployed on the Long Island shelf at Station D. All ABS transducers were mounted 115 cm above the base of the tripod feet. The ABS samples 128 range bins of acoustic intensity. In order to sample wave frequencies and tidal/mean currents the AQUATEC ABS sampled a 10 minute burst at 1 Hz every half hour. The older ABS sampled a 12 minute burst once per hour at 1 Hz.

Transmissometer (figure 9)

Sea Tech transmissometers measure the transmission of red light (at a wavelength 650 nm) from a Light Emitting Diode (LED) along a 25 cm water path. The voltage output by a photovoltaic detector was recorded by a SEACAT. These instruments were deployed beneath the surface moorings at 30 and 45 mbs at Station B (25 and 10 mab) and at 30 and 60 mbs (40 and 10 mab) at Station C. The power supply for the transmissometer at 60 m at Station C failed in this experiment.

OBS (figure 10)

Suspended sediment concentrations were measured at Stations A-E using optical backscatter sensors (OBS) mounted at heights of 0.5 and 1.2 mab. Fouling of some OBS sensors gradually increased backscatter intensity during the deployment. The OBS readings were converted to suspended sediment concentration based on calibration curves obtained in the laboratory generated using sediment collected in tripod-mounted sediment traps at Station A. The trap material was used because it is more representative of suspended material than sea-bed samples.

SEACAT (figure 9)

SEACAT 16 ( measure conductivity and temperature, and record the voltage signals produced by a transmissometer. SEACAT's were operated with a sampling interval of 300 seconds (3.75 minutes). SEACATs were deployed on the mooring beneath the surface buoy at Stations B and C at 10 and 30 mab.

MicroCAT (figure 11)

The MicroCAT is a simpler version of the SEACAT 16. The MicroCAT uses the same sensor technology as the SEACAT to collect salinity and temperature data. MicroCATS were used to obtain temperature and salinity measurements at 1 mbs buoys at Stations A-E. The recording interval was every 3.75 minutes.

Time-series sediment sampler (figure 12)

A McLane Labs Water Transfer System (WTS 6-24-47FH) was deployed on the tripod at Station A to collect in situ suspended particulate matter. A dual multi-port valve sequentially directs water through 24, 47-millimeter filters to obtain a time series of suspended sediments. The positive displacement pump is placed downstream from the filters to prevent sample contamination. Both the multi-port valve and the positive displacement pump are controlled by an internal computer.

The menu-driven software allows the user to enter the initial flow rate, total volume, maximum pumping time, and minimum flow rate. In order to prevent the sample from being crushed onto the filter and to conserve the battery life, the software is designed to decease the flow rate when back pressure on a filter is increasing. The internal computer records the instantaneous flow rate and total volume at a constant interval of time for each filter.

In this experiment, the WTS was set to sample in a fixed interval mode and in a conditional mode based on the standard deviation of the bottom pressure (PSDEV), a proxy for surface wave amplitude. Three samples were allocated to fixed interval samples, and 20 samples to conditional sampling (4 events with 5 samples each). Samples were obtained in the fixed interval mode at one day following deployment, and at 44 and 88 days into the deployment. The objective for the conditional samples was to obtain a series of suspended sediment samples during large storms. As a proxy for the amplitude of surface waves, the WTS computes the standard deviation of bottom pressure (PSDEV), sampled at one hz, over a 50 second interval every 10 minutes. In the conditional mode, the first sample in a storm event is taken when the average PSDEV, computed over a running 4-hour average, exceeds 25 cm. After a wave event sampling is begun, successive samples are initiated at 6 hour intervals. During this timed interval, the system searches for a maximum in PSDEV by comparing the average PSDEV calculated over successive 5 hour intervals. If the PSDEV level continues to increase, the timed sample is ignored until the maximum is reached and sampled. After a maximum is found the timed interval is resumed and the search for a successively higher maximum continues. This cycle continues until all samples for a given storm are collected. At 50%, 75% and 80% of the deployment period, the WTS checks the expected number of storms, allocating more samples to fixed interval samples if the expected number of storms have not occurred, and reduces the PSDEV threshold by 20 and 30% respectively. After 80% of the deployment period, the remaining samples are obtained evenly in time over the remaining period.

Tube sediment traps (figures 6A, 7A, 8B, 9)

Traps made from standard core tubing were deployed on the bottom tripods to obtain samples of sediment in the near-bottom water. These traps are 60 cm long polybuterate tube with a 6.6 cm internal diameter and a wall thickness of 3.2 mm. The bottom of the tube was sealed with a securely taped plastic cap. Baffles consisted of an aramid fiber/phenolic resin honey comb (trade name: HEXCELL) with a cell diameter of 1 cm and a length of 7.5 cm. The material showed no apparent deterioration during exposure to seawater, although it is subject to biofouling. Traps were filled to within 7.5 cm of the top with a filtered solution of 5% sodium azide in seawater.

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Title Page / Contents / Tables / Figures / Abbreviations / Introduction / Field Program / Observations / Instrumentation / Data Processing / Fouling / Mooring/Data File ID / Results / Digital Data / Acknowledgements / References / Matlab / Supplementary / Metadata
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