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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.
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 16 (www.seabird.com/)
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.
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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