The profiling arm was attached to the existing "flowbee" tripod (figs. 3, 6, 7, and 8), originally built to support instruments for measuring waves, currents, and suspended sediments in shallow environments. The flowbee tripod was used previously off Cape Hatteras, North Carolina in 2009 and at the MVCO in summer 2005 and (with a modified immobile arm) in summer 2007. The flowbee tripod is constructed of stainless steel and is about 3.5 meters (m) tall, with feet about 1.9 m apart. The legs are stainless steel pipes with an outside diameter of 7.6 centimeters (cm). The feet are cylindrical lead weights 41 cm in diameter and 16.5 cm thick; they weigh 136 kilograms (kg). Two decks are located on the upper tripod to support instruments and battery cases; the bottom of the lower deck is approximately 2.6 m above the seafloor. The recovery package normally mounted on the top of the tripod was not used during this OASIS experiment because the recovery line was attached by divers.The profiling arm was designed to move an instrument package up and down between the seafloor and an elevation of about 2 meters above the bottom (mab). The design criteria for the arm were based on the scientific objectives of the OASIS program, as follows:

(1) The instrument package should include the LISST-HOLO, the LISST-100X, other smaller optical sensors, the ABSS transducers, an acoustic doppler velocimeter for measuring flow, and an accelerometer for detecting and allowing correction for arm motions. The arm should be able to support this package, which weighed about 50 kg in air (about 20 kg in water).

(2) The instruments should be exposed to undisturbed flow.

(3) The arm should be strong enough to withstand hydrodynamic forces and move the package without excess motion or vibration.

(4) The instruments should be moved vertically fast enough to complete profiles while wave and tide conditions remain relatively unchanged but slowly enough to allow averages over several individual waves to be estimated for small (about 10-cm) elevation intervals.

(5) The power required to move the instrument package must be less than 90 watts (W) peak. A fail-safe mechanism (circuit breaker) must prevent excess power demand to protect other instruments on the MVCO circuit.

(6) The profiling mechanism must be sufficiently immune to corrosion, fouling, and fatigue to last one month in productive and energetic shallow-water marine conditions.

(7) The position of the arm, and especially the elevation of the instrument package above the seafloor, must be monitored and controlled.

Arm Design

The design of the profiling arm was based on a cantilever arm, with a fulcrum (pivot) on a crossbar of the tripod (figs. 2 and 9). The longer end of the arm extended 1.98 m beyond the tripod and supported the instruments, and the shorter lever end extended 0.71 m into the interior of the tripod. A long threaded stainless-steel rod passed through a gimbaled brass nut on the lever end. This screw was turned by an electric motor mounted high on the interior of the tripod; as the lever end was raised, the instrument end was lowered, and vice versa. The final design of the profiling arm incorporated minor improvements, such as universal mounts for the motor and brass nut, and thrust bearings to protect the motor from axial loads transferred by the screw. Details of the arm geometry are described in appendix 1.

The motor was a Technadyne Model 20 underwater brushless rotary actuator (fig. 10) that delivered a maximum torque of 27 newton-meters at 12 revolutions per minute (rpm). Specification of the electric motor was critical; the motor had to be capable of turning the screw fast enough to move the instrumentation package 2 m in about 20 minutes (min) and to have enough torque to overcome the friction of the threads and the mechanical advantage of the cantilever and lift the 50-kg instrumentation package without drawing excessive power (less than 90 W at 24 volts of direct current (VDC)), even during startup transients. The thread pitch of the screw was 12 threads per inch and with a rotation rate of 12 rpm, the instrument package moved up (or down) about 12 cm per minute.

The instrument package was mounted on the end of the arm with a pivot and a tie-rod mechanism that kept the package oriented vertically (fig. 9).

Arm Interface and Control

The interface between the Moxa main control computer and the Technadyne Model 20 arm actuator was a Modtronix Engineering SBC68EC single-board computer and custom daughterboard to limit power draw. Communication between the Moxa, the Modtronix arm controller, other components, and shoreside computers was via Ethernet. A detailed description of arm interface and control hardware and software is included appendix 2. Software used in the project for arm control and data logging is discussed in appendix 3.

Arm Movement and Sampling Schedule

Four profiles (down, up, down, up) were made every 2 hours (hr) beginning on even hours (fig. 11), except when power to the MVCO node was turned off for maintenance. Each profile took about 16 min, with the end of the arm moving at a somewhat variable rate of approximately 2.2 millimeters per second. Normal profiles started with the arm up at an angle of 37 degrees (°; relative to horizontal) with the motor stopped. At the top of the even hour, a sequence of commands to supply power to the motor and begin arm movement was issued. Starting about 5 seconds (s) after the hour, motor speed was increased gradually over a period of about 20 s until the arm was rotating at a constant rate, causing the arm to move downward for about 16 min. Computers monitored the arm angle, and when it reached a predetermined value (initially, -27°; later reduced to -25.5°, then -24°), the motor was stopped, leaving the arm in the down position, where it remained until 20 min after the hour. At 20 min after the hour, the motor ramped up to a constant rotation rate in the opposite direction, moving the arm upward for about 16 min until it reached the up-position angle (37°). At 40 min past the hour, the sequence was repeated and the arm completed another pair of profiles.

Even though the motor rotation rate was virtually constant, the geometry of the arm caused the vertical profiling speed to vary slightly over the profile range. Arm motion was monitored by two orientation sensors (IG-20 two-axis inclinometer / three-axis accelerometers), one mounted on the arm near the pivot, and one mounted on the instrument package at the end of the arm (figs. 7, 8, and 12).

All the instruments on the arm actively sampled while the arm was moving. Data from the Modtronix arm-controller and from some of the arm instruments (described below) were recorded temporarily by the Moxa and copied back to the shoreside computer. Other instruments had independent internal recorders that logged their measurements; these data were downloaded after the instruments were recovered at the end of the experiment. The logger and the controllers and data from the arm instruments are discussed in the following sections.

Arm Instruments

We describe in this section the instruments that produced data from the profiling arm. For each instrument, we introduce an abbreviated name in parentheses (for example, ADV) for later reference. Numbers in parentheses in the section titles (for example, 9104) are the USGS mooring identification numbers, which link the instrument to data files in USGS archives. The mooring identification numbers are associated with data loggers, so all sensors with output recorded by a single logger (for example, a SonTek Hydra system) are associated with the same mooring identification number. Some instruments are described also by the colors used to mark their cables and sensors (for example, blue ADV), which helps match instruments and data with photographs documenting their location and condition.

Logger and Controllers

The primary logger and controller for the profiling arm was the Moxa Linux computer. This computer issued instructions to the Modtronix arm controller and logged data sent by the Modtronix and several arm instruments.

Modtronix

The Modtronix SBC68EC (http://www.modtronix.com/product_info.php?products_id=196) controlled the arm motor, monitored motor status and arm angle, and reported these data over the internal network in a user datagram protocol (UDP) datagram about 4 times per second. The contents of the datagrams were unpacked by the Moxa as they were received and written to an American Standard Code for Information Interchange (ASCII) data file on the Moxa. This file was retrieved by the shoreside computer after each set of profiles. The datagrams included a time stamp with the system time, data (time since reset, roll and pitch angles, three axes of acceleration, and two onboard temperatures) sent to the Modtronix from the tilt meter on the arm, and data (status of motor power, motor current draw, motor rotation rate, control voltage sent to the motor, and status of the software circuit breaker) acquired by the Modtronix from the motor-controller circuit board.

Moxa

The Moxa UC8418 computer (http://www.moxa.com/product/UC-8418.htm) logged data from several instruments on the arm. These included all data from the SBG Systems IG-20 two-axis inclinometer and three-axis accelerometer at the end of the arm, all data from the YSI 6600 multiparameter sonde on the end of the arm, all data from the SonTek pulse-coherent acoustic doppler profiler (PCADP) mounted on the crossbar of the tripod, and header information from the SonTek acoustic doppler velocimeter (ADV) on the end of the arm. Following each 2-hour profiling period, the data files were written to a directory on the solid-state disk mounted on the Moxa as /var/sda/data and also copied back to the shoreside computer.

The Moxa also ran programs to control the arm and to update a hypertext markup language (HTML) page that displayed the status of the arm. These programs are discussed in detail in appendix 3.

Arm accelerometer (IG-20)

Two IG-20 tilt meter and accelerometer units (http://www.sbg-systems.com/products/ig-20) were mounted on the arm, one to monitor the angle of the arm and the other to monitor the motion of the instrument package at the end of the arm (figs 7 and 8). The IG-20s were compact (36 × 49 × 22 millimeters (mm)) and lightweight (38 grams (g)) and required small amounts of power (less than 150 milliwatts (mW)) (fig. 12). According to the manufacturer, they could measure pitch and roll over a range of ±80° and ±180° (respectively) with a static accuracy of ±0.2° and a resolution of less than 0.05°. The accelerometers measured over a range of ±2 g, with nonlinearity of less than 0.2 percent of the full scale, bias stability of ±0.002 g, and alignment error of less than 0.1 over a bandwidth of 0.1 to 100 hertz (Hz).

The IG-20, mounted on the leveled instrument package, was oriented so that positive pitch and roll axes corresponded to those of the nearby arm ADV. Power was provided to the IG-20 by the logger and controller unit, and output was logged to ASCII files by the Moxa using the log_ig20.c program. The data logging program was initiated every 2 hr by the cron scheduler on the Moxa. One file was written every 2 hr; a detailed description of the contents and format of the data files is documented in the source code (appendix 3).

Sequoia Scientific LISST-100X (LISST-100X) - 9109

The Sequoia Scientific Laser In Situ Scattering and Transmissometry 100X (LISST-100X; http://www.sequoiasci.com/products/susp_LISST_100.aspx) laser particle sizer (fig. 13) uses a collimated laser and an annular ring detector to measure forward scattering by particles along the optical path. The forward scattering data are mathematically inverted to provide an estimate of the volume concentration (in microliters per liter, µL/L) of particles in 32 log-spaced-size classes. This system can resolve particles ranging from 2.5 to 500 microns (micrometers, µm) in diameter. In addition to particle size distribution, the LISST-100X measures the percentage of transmission along the optical path, temperature, and pressure.

The LISST-100X was mounted horizontally next to the Sequoia Scientific digital holographic particle imaging system (LISST-HOLO) on the bottom side of the profiling arm with its sample volume about 25 cm from the end of the arm (figs. 7 and 8). It collected bursts of 960 samples at 0.2 Hz every 7,200 s (80 min every 120 min). The LISST-100X operated autonomously, using power from internal batteries and logging to internal flash memory, and had a serial connection to the Moxa computer so its operation could be verified from the shore-based computer. The LISST-100X data were inverted using the Sequoia Scientific LISST-SOP (version 4.65) software. Data from the out-of-water periods were trimmed from the record.

Temperature data from the LISST-100X were inaccurate when the instrument was powered by the MVCO node. All LISST temperature data were replaced with fill (placeholder) values of 1e35.

Sequoia Scientific LISST-HOLO (LISST-HOLO) - 91010

The LISST-HOLO (http://www.sequoiasci.com/products/fam_LISST_HOLO.cmsx) in situ in-line holographic camera (fig. 14) illuminates a 5-cm optical path with a red (658-nanometer (nm)-wavelength) collimated laser. Particles in the water scatter the laser light along the optical path. A charge-coupled device (CCD) opposite the laser captures particle silhouettes and the interference pattern created by the interaction of scattered and unscattered light. Subsequent processing uses the interference patterns to identify and digitally focus on particles in narrow slices along the length of the optical path. Image analysis algorithms can be used to characterize the size and shape of the imaged particles.

The LISST-HOLO was mounted horizontally next to the LISST-100X on the bottom side of the profiling arm with its sample volume about 25 cm from the end of the arm (figs. 7 and 8). It collected 1 image every 30 s for 80 min of every 120 min. The LISST-HOLO had neither the memory nor the battery capacity to sample autonomously at this rate for an entire month so power was supplied from the MVCO node, and images were downloaded from the instrument after every 80-min set of samples. The instrument was controlled and downloaded via a shell script called by a cron job on a shore-based Linux computer.

The output of the LISST-HOLO was in grayscale images in portable graymap (pgm) format, which is a lossless compression format. Each pgm file has some basic environmental data and image metadata appended to the end of the file including date, time, water depth, temperature, input voltage, image size, exposure duration, laser power, laser photodiode, camera brightness, camera shutter, camera gain, and imager code version.

YSI 6600 Multiparameter Sonde (YSI) - 91011

The YSI model 6600 multiparameter sonde (http://www.ysi.com/productsdetail.php?6600V2-1) is a water-quality sensor that uses interchangeable probes to measure a variety of parameters (fig. 15). For this study, the sonde was equipped with conductivity, temperature, pressure, dissolved-oxygen, and turbidity probes.

The YSI was mounted on the end of the profiling arm in a vertical orientation, with the sensors pointed down (figs. 7 and 8) . The sensors were guarded by a 6-mm mesh (about 1/4-inch-diameter) plastic screen, and most surfaces were wrapped in copper tape to minimize fouling.

The YSI was deployed in discrete sample mode and sampled every 5 s for 90 min out of every 120 min from September 17 to October 18, 2011. The instrument was destroyed when the tripod capsized on October 18. The YSI did not have sufficient battery power or memory to maintain this sampling scheme for the duration of this study. Instead, power was supplied to the YSI from the MVCO node, and data were streamed from the instrument by a serial communication port and recorded on the Moxa. A time stamp from the Moxa, designated as "Moxa time" and indicated as "yyyymmdd," was appended to the sonde data stream to aid in synchronizing YSI data with the other arm instruments.

The turbidity sensor on the YSI was wiped automatically every 2 hr. The conductivity, dissolved-oxygen, and turbidity sensors on the YSI 6600 were calibrated in the lab before deployment using the manufacturer's recommended calibration procedures. Postrecovery calibrations could not be performed because the instrument was damaged when the tripod capsized.

Salinity data from the YSI are reported in parts per thousand (ppt). The data were despiked by replacing values less than 31.4 ppt with fill values (1e35). Data from the out-of-water periods were trimmed from the record.

Aquatec Acoustic Backscatter System (ABSS) - 91012

The Aquatec acoustic backscatter system (ABSS; http://www.aquatecgroup.com/index.php/products/aquascat) emits pulses of high-frequency sound from up to four transducers at a variety of frequencies (fig. 16). The pulses are scattered by suspended material in the water column. The ABSS measures the scattered sound returned from particles in the water column and produces vertical profiles of acoustic backscatter intensity (a proxy for suspended sediment concentration). This system uses three transducers of differing frequency (1 megahertz (MHz), 2.5 MHz, and 4 MHz) and measures profiles of backscatter intensity in very small bins (mm to cm). In general, lower frequency sound is more responsive to larger particles so comparing the response of multiple frequencies can provide information on the size of particles in suspension.

The ABSS was mounted vertically on the end of the profiling arm with its transducers pointing downward (figs. 7 and 8). It sampled in vertical profiles of 0.01-m bins at a rate of 32 Hz. Each saved profile was an average of 32 profiles (that is, one profile per second) and each burst lasted 5,040 s (84 min). The ABSS was synchronized with the SonTek PCADP so each burst was triggered when the PCADP began sampling. Data from the out-of-water periods were trimmed from the ABSS data.

SonTek Acoustic Doppler Velocimeter-Ocean and Hydra logger (Blue ADV) - 91013

The SonTek Acoustic Doppler Velocimeter-Ocean (blue ADV) was connected to a Hydra system, an electronics package in an underwater housing that controls instruments, provides power, and records data. The Hydra system accommodates the 5-MHz ADVOcean probe (fig. 17) as well as numerous user-configurable external sensors (for example, pressure sensors or optical turbidity sensors). For this study, several instruments were associated with a color, blue, for example, used on cases and cables to distinguish all sensors logged by a particular Hydra logger. The Hydra logger and associated sensors used on the arm array were color-coded blue.

Sensors attached to the blue Hydra are discussed in the following sections. All are associated with the same USGS mooring identification number, 91013. Only burst data from the blue ADV and associated sensors are included in this report; we did not calculate burst statistics because the sensors were profiling during the bursts. Data from the out-of-water periods were trimmed from the record.

SonTek ADVOcean Acoustic Doppler Velocimeter (ADV)

The 5-MHz SonTek ADVOcean (ADV; http://www.sontek.com/advocean-hydra.php) makes three-component vector measurements of water flow using doppler principles and records them with synchronous measurements from an array of complementary external sensors.

The ADV uses pulse-to-pulse coherent processing of pings from a single acoustic source and three receivers to obtain precise, three-dimensional (3D) velocity measurements within a small sample volume (about 2 cubic centimeters). When pointed downward, this unit acoustically measures the distance from the sensor to the seabed. The sensor orientation is measured with a compass and a two-axis tilt sensor providing heading, pitch, and roll information. Temperature is also recorded. Velocity, temperature, pressure, and optical turbidity measurements were recorded at 8 Hz. Range and orientation were measured and logged once per burst at the beginning of the burst.

The blue ADV (91013) was mounted vertically, pointing down, at the end of the profiling arm (figs. 7 and 8). It was programmed to sample in a multiburst sampling scheme to maximize the number of range measurements taken when the arm was in the down position. The multiburst sampling scheme consisted of a 20-min burst (9,600 samples at 8 Hz), followed by a 40-min burst (19,200 samples at 8 Hz), and ended with another 20-min burst

Paroscientific, Digiquartz Pressure Sensors (Paros)

The Paroscientific, (Paros) Digiquartz pressure sensor (http://www.paroscientific.com/Depthsensors.htm) measures pressure using a quartz crystal resonator. The frequency of oscillation of this resonator varies with pressure induced stress. The output pressure is compensated for temperature by using the signal from temperature-sensitive crystals within the instrument.

The Paros pressure sensor (fig. 18) was powered and logged by the blue SonTek Hydra and was mounted near the end of the profiling arm and oriented vertically (figs. 7 and 8).

Major pressure spikes from the end of the Paros pressure record associated with the blue Hydra were removed by replacing values less than 1600 or greater than 2600 with fill values (1e35).

D&A Instrument Company Optical Backscatter Sensor (OBS)

The D&A Instrument Company OBS-3 sensor (fig. 19) measures optical backscatter, a proxy for suspended solids concentration (SSC), in the water column. The sensor emits a beam of infrared light that is scattered by suspended solids. Optical backscatter sensors can be calibrated in the laboratory to provide data in units of turbidity or by using suspended solid samples collected in situ to provide information about suspended solid concentrations. Prior to deployment, all OBS sensors were intercalibrated in the laboratory using a Formazin calibration standard and a multistep (0 nephelometric turbidity units (NTU), 5 NTU, 15 NTU, 30 NTU, 50 NTU, 100 NTU, 300 NTU, and 500 NTU) calibration procedure.

The OBS was powered and logged by the blue SonTek Hydra and was mounted vertically near the end of the profiling arm with the optics facing horizontally (figs. 7 and 8). Optical faces on the OBS sensors were painted with clear antifouling paint to minimize sensor drift due to fouling. The sensors were not equipped with automatic wipers, but were cleaned manually by divers on four occasions during the deployment.

Sea Tech Transmissometer

The Sea Tech transmissometer (fig. 20) measures the transmission of red collimated light (660-nm wavelength) through the water. The percent transmission of light and the length of the optical path (5 cm or 25 cm) can be used to calculate an attenuation coefficient. Transmission and attenuation are often used as proxies for SSC in the water column.

The 5-cm transmissometer was powered and logged by the blue SonTek Hydra and was mounted horizontally on the top of the profiling arm with its sample volume about 20-cm from the end of the arm (fig. 7). The transmissometer was not equipped with an automatic wiper but was cleaned manually by divers on four occasions during the deployment.

University of Maine ECO BB2F Spectral Backscattering Meter and CDOM Fluorometer (BB2F)

The WETLabs ECO BB2F (http://www.wetlabs.com/products/ebb/bbvsfindex.htm) provided by the University of Maine is a combination scattering meter and fluorometer that measures backscattered light (at an angle of 117°) at two wavelengths (532 nm and 650 nm) and colored dissolved organic matter (CDOM) fluorescence (excited at 380 nm and emitted at 460 nm). The BB2F was mounted near the end of the profiling arm. Data were recorded at 2 Hz. Computation of the particulate backscattering coefficient from single measurements in the back direction was done following Boss and others (2004). The CDOM fluorometer channel did not work during the deployment (no change in very noisy data) and those data are not included in this report. The BB2F was not equipped with automatic wipers and was too delicate to be cleaned by divers.

University of Maine AC-9 Spectral Absorption and Beam Attenuation Meter (AC-9)

The WetLabs AC-9 spectral absorption and beam attenuation meter (http://www.wetlabs.com/products/ac/acall.htm) provided by the University of Maine measures attenuation and absorption of light at nine wavelengths (412 nm, 440 nm, 490 nm, 510 nm, 532 nm, 555 nm, 650 nm, 676 nm, and 715 nm). Water sampled by the AC-9 was pumped from an intake mounted near the end of the arm. A valve allowed the flow to be routed directly to the AC-9 or through a filter and then to the AC-9. The filter removed particles larger than 0.2 µm from the sample, which allowed for calculations of particulate absorption and attenuation that were calibration independent (see Boss and others, 2007; Slade and others, 2011). Data from the sensor was recorded at 6 Hz. Chlorophyll-a concentration (Chl) was computed from the absorption line height at 676 nm (Boss and others, 2007). Mean size tendency or power-law slope (γ) of the power-spectral density was computed from attenuation spectra (Boss and others, 2001).

The filter was changed by divers on three occasions during the experiment. The optical surfaces of the AC-9 were not cleaned during the experiment, but the comparison of filtered and unfiltered water compensated for fouling.

Tripod Instruments

We describe in this section the instruments that were mounted in fixed positions on the main frame of the tripod. For each instrument, we introduce an abbreviated name in parentheses, (for example, ADCP) for later reference. The numbers in parentheses in the section titles (for example, 9101) are the USGS mooring identification numbers, which link the instrument to data files in USGS archives. In addition, some instruments are described by the colors used to mark their cables and sensors (for example, green ADV), which helps match instruments and data with photographs documenting their location and condition.

Teledyne RD Instruments Acoustic Doppler Current Profiler (ADCP) - 9101

The 1,200-kilohertz (kHz) Teledyne RD Intruments (RDI) ADCP (abbreviated to ADCP here; http://www.rdinstruments.com/sen.aspx) measures the speed and direction of water flow using doppler principles. Acoustic pulses are transmitted by the ADCP transducer assembly along two pairs of orthogonal beams (four beams total). Scatterers in the water column, such as small sediment particles and plankton traveling with the water flow, reflect the acoustic pulses. The ADCP transducer assembly receives the reflected pulses and, using the doppler effect and basic trigonometry, converts the pulses into eastward, northward, and vertical components of water flow. When installed with waves acquisition firmware, RDI ADCPs record three different types of time series from which wave properties may be computed: pressure, range to surface along each orthogonal beam (that is, water level), and orbital velocities of the surface waves taken from three bins nearest the surface in each of the four beams. It is possible to estimate nondirectional wave energy spectra, and thus wave height and period, from any of the three time series, but the orbital velocity time series are required for definition of the directional distribution of the wave energy.

The ADCP was mounted 3.27 mab near the top of the tripod facing up. The instrument was programmed to collect velocity and acoustic backscatter profiles in 0.5-m bins as well as temperature, pressure, and orientation (heading, pitch, and roll) measurements every 6 min and was programmed to collect wave data once per hour.

The instrument failed at 03:18:20 UTC (Coordinated Universal Time) on September 18, 2011, less than a full day into the deployment.

Sea-Bird Electronics 16plus V2 SEACAT (SEACAT) - 9102 and 9107

The Sea-Bird Electronics 16plus V2 SEACAT recorder (SEACAT; http://www.seabird.com/products/spec_sheets/16plusdata.htm) is a conductivity and temperature (CT) sensor. Salinity can be determined from the conductivity and temperature record. The SEACATs are equipped with pumps to flush the sensor ducts and reduce salinity spiking and powered by internal batteries; data are recorded internally on flash random-access memory (RAM), allowing the instrument to operate autonomously.

Two SEACAT CT sensors were deployed on the tripod at elevations of 0.40 mab (9107; fig. 21) and 3.16 mab (9102; fig. 6). Both sensors were programmed to sample every 5 min. The sensor mounted 0.40 mab (9107) sampled for the entire deployment. The sensor mounted 3.16 mab (9102) stopped sampling at 01:01:17 UTC on October 18, 2011, most likely due to insufficient battery power.

Salinity values were generated by postprocessing raw data with the Sea-Bird software. Conductivity and salinity records from the SEACAT sensor mounted 3.16 mab (9102) were despiked by replacing values where salinity exceeded 31.4 practical salinity units (psu) with fill values (1e35). Data from the out-of-water periods were trimmed from the record.

Nortek Aquadopp High-Resolution Acoustic Profiler (Aquadopp HR) - 9103

The Nortek AS Aquadopp 1-MHz high-resolution acoustic profiler (Aquadopp HR; http://nortekusa.com/usa/products/current-profilers/aquadopp-hr-profiler) uses acoustic doppler technology to measure 3D water flow velocity profiles. The Aquadopp HR is an upgraded version of the standard Nortek Aquadopp profiler. The Aquadopp HR uses a pulse-coherent processing technique to measure three components of velocity in small depth bins (2-30 cm) over a limited part (less than 6 m) of the water column.

The Aquadopp HR was mounted facing down on the cross beam that supported the profiling arm (figs. 3 and 22). Its transducers were 1.08 mab. It measured velocity profiles in 0.065-m bins at 2 Hz for 15 min every 2 hr. It also measured temperature, pressure, and orientation (heading, pitch, and roll) and collected profiles of acoustic backscatter intensity (a proxy for SSC) at 2 Hz. The instrument was deployed autonomously with power provided by internal batteries and data logged to internal memory.

Inspection of the data indicates that the heading information from this instrument was not reliable. The compass calibration was performed with the instrument looking up, but the instrument was deployed facing down. Flow data from the Aquadopp HR (9103) were compared with those of green ADV (9105) to determine the relative orientation of the two instruments (ADV direction - Aquadopp HR direction = -54.5°). The heading of the ADV+54.5° was interpolated to the Aquadopp HR profiler time base and used for the Aquadopp HR orientation. Data from the out-of-water periods were trimmed from the record.

SonTek Pulse Coherent Acoustic Doppler Profiler (Red PCADP) - 9104

The 1.5-MHz SonTek PCADP has three acoustic beams and uses acoustic doppler technology to measure profiles of all three components of water velocity. Pulse-to-pulse processing allows this unit to measure velocity in small depth (about 6 cm) bins over a limited part (less than 5 m) of the water column.

The SonTek PCADP (color-coded in red) is connected to a SonTek Hydra system, which is an electronics package in an underwater housing that controls instruments, provides power, and records data. The Hydra system accommodates the 1.5-MHz PCADP sensor as well as numerous user configurable external sensors (for example, pressure sensors and optical turbidity sensors). The PCADP and all other sensors logged by this Hydra are associated with mooring 9104.

The 1.5-MHz PCADP probe was mounted facing down on the cross beam that supported the profiling arm (figs. 3 and 23). The transducers were mounted 1.03 mab. The PCADP was programmed to sample velocity profiles with 0.063-m bins at a rate of 1 Hz. The instrument sampled in 4,800-s bursts every 7,200 s. The PCADP also measured profiles of acoustic backscatter intensity (a proxy for SSC) and temperature with every velocity sample and collected orientation information (heading, pitch, and roll) once per burst. In addition, this SonTek PCADP and Hydra system powered and logged data from a 25-cm Seatech transmissometer (fig. 24) mounted 2.75 mab and an OBS mounted 1.18 mab on a tripod leg.

Major spikes in OBS and transmissometer data were removed by replacing values greater than 5 standard deviations from the burst mean with fill values (1e35). Data from the out-of-water periods were trimmed from the record.

SonTek Hydra ADVs (Green ADV 9105 and Yellow ADV 9106)

A pair of SonTek ADVOcean acoustic doppler velocimeters (ADV) and Hydra systems, each with an external Paros pressure sensor, a D&A OBS, and a 5-cm Seatech transmissometer, were mounted on the tripod frame (individual sensors are described in previous sections). The green ADV (9105; fig. 25) had an ADVOcean probe mounted 0.42 mab, a Paros pressure sensor mounted 1.53 mab, a 5-cm Sea Tech transmissometer mounted 1.20 mab, and an OBS mounted on a tripod leg 0.23 mab. The yellow ADV (9106; fig. 26) had an ADVOcean probe mounted 0.42 mab, a Paros pressure sensor mounted 1.53 mab, a 5-cm Seatech transmissometer mounted 1.19 mab, and an OBS mounted on a tripod leg 0.61 mab. Both ADVOcean probes were mounted on stiff metal poles extending downward near the center of the tripod to minimize disturbance of the flow being measured by the ADVs (fig. 3).

The two systems were connected and synchronized. The yellow ADV (9106) was the master, and the green ADV (9105) was the slave. Both units sampled at 8 Hz with 3,480-s bursts every 3,600 s (27,840 samples per burst, sampling 58 out of every 60 min).

For the green ADV (9105), heading data and pressure data were despiked by replacing samples greater than 5 standard deviations from the burst with fill values (1e35). Range from transducer to boundary for ADV 9105 was despiked by replacing values less than 1 mm or greater than 420 mm with values from a 15-sample running median. Range from sample volume to boundary for ADV 9105 was despiked by replacing values less than 1 mm or greater than 260 mm with values from a 15-sample running median.

For the yellow ADV (9106), heading data and pressure data were despiked by replacing samples greater than 5 standard deviations from the burst with fill values (1e35). Range from transducer to boundary for ADV 9106 was despiked by replacing values less than 1 mm or greater than 450 mm with values from a 15-sample running median. Range from sample volume to boundary for ADV 9106 was despiked by replacing values less than 1 mm or greater than 300 mm with values from a 15-sample running median.

Nortek Aquadopp Acoustic Profiler (Aquadopp) - 9111

The 1-MHz Nortek Aquadopp acoustic profiler (Aquadopp; http://nortekusa.com/usa/products/current-profilers/aquadopp-profiler-1) measures three-component water-velocity profiles using acoustic doppler principles. The instrument measures the doppler shift that occurs when three acoustic beams are reflected by scatterers suspended in the water column. Because doppler shift is proportional to the component of water flow along the beam, trigonometry can be used to convert the returned signal into eastward, northward, and vertical components of water flow.

The Aquadopp was mounted 0.16 mab facing up on a small monopod (fig. 27) deployed about 5 m southeast of the tripod. It collected velocity and acoustic backscatter profiles in 0.5-m bins as well as temperature, pressure, and orientation (heading, pitch, and roll) data. A measurement was recorded every 600 s, and each measurement was collected by averaging over a period of 540 s.

As a result of high wave energy, a sandy seabed, and the small footprint of the monopod, scour occurred around the base of the monopod, resulting in significant (more than 20°) tilt during some periods of the deployment. According to the manufacturer, data from the Aquadopp may be unreliable when tilt exceeds 20° to 30°. Waves and currents also caused the monopod to rotate. Data from the out-of-water periods were trimmed from the record.