Terence R. Hammar

Protocols for calibrating multibeam sonar

Development of protocols for calibrating multibeam sonar by means of the standard-target method is documented. Particular systems used in the development work included three that provide the water-column signals, namely the SIMRAD SM2000/90- and 200-kHz sonars and RESON SeaBat 8101 sonar, with operating frequency of 240 kHz. Two facilities were instrumented specifically for the work: a sea well at the Woods Hole Oceanographic Institution and a large, indoor freshwater tank at the University of New Hampshire. Methods for measuring the transfer characteristics of each sonar, with transducers attached, are described and illustrated with measurement results. The principal results, however, are the protocols themselves. These are elaborated for positioning the target, choosing the receiver gain function, quantifying the system stability, mapping the directionality in the plane of the receiving array and in the plane normal to the central axis, measuring the directionality of individual beams, and measuring the nearfield response. General preparations for calibrating multibeam sonars and a method for measuring the receiver response electronically are outlined. Advantages of multibeam sonar calibration and outstanding problems, such as that of validation of the performance of multibeam sonars as configured for use, are mentioned.

A new gas-tight isobaric sampler for hydrothermal fluids

A new gas-tight isobaric sampler for the collection of hydrothermal fluids venting at the seafloor has been designed, constructed, and tested at a ridge-crest vent site. The new device is constructed of chemically inert titanium, is gas-tight to 450 bar and can be used to sample fluids with temperatures up to 400°C. Compressed gas is used to maintain the sample at seafloor pressure before and during sample withdrawal onboard ship, allowing subsampling without degassing the fluid remaining in the sampler. This feature eliminates the need to collect separate gas-tight and major element samples since a single fluid sample can be analyzed quantitatively for major, trace, semi-volatile, and volatile components. The sampler fill rate is regulated to minimize entrainment of ambient seawater during collection of fluids from environments characterized by low fluid flow such as diffuse hydrothermal vents. In addition to deployment at the ridge-crest, the samplers can be used to collect gas-tight samples from other subseasurface environments such as hydrocarbon seeps, areas of methane-gas hydrate formation, cold seeps associated with serpentinites, regions of groundwater egress to the oceans, and the water column.

Calibrating a 90-kHz multibeam sonar

Quantitative use of multibeam sonar requires calibration. Protocols for calibrating multibeam sonar in an absolute sense by means of a standard target are being developed. These protocols are illustrated for the Simrad SM2000 Multibeam Echo Sounder, with 90-kHz operating frequency and 300 beams over a 90-deg sector (/spl plusmn/45 deg), each of nominal beamwidth 1.5/spl times/1.5 deg. In a sea well on Iselin Dock at the Woods Hole Oceanographic Institution, directional characteristics of the sonar were measured at the farfield range of 23 m in a sea well. At the large freshwater indoor tank at the University of New Hampshire, similar characteristics of directivity were measured at the nearfield range of 11.7 m. Beamforming was accomplished by digital signal processing. Beam patterns are presented and compared at both ranges. In addition, measurements of the on-axis response were made for each available time-varied-gain function, enabling linearity and dynamic range to be assessed.

Transducer mounting and rotating system for calibrating sonars in a sea well

A transducer mounting and rotating system for use in calibrating sonars in a sea well beneath a pier at the Woods Hole Oceanographic Institution is described. The transducer is attached to a plate fitted on the end of a shaft that is supported by a sleeve bearing at the lower end of a 6-m-long trussed steel tower. A rotary table for all-weather use with ordinary television antennas has been adapted for precision use, and attached to the surface end of the shaft. Rotation is controlled by software, with user-defined parameters, from a personal computer (PC) platform. The accuracy of the rotation is /spl plusmn/0.1 deg. Initial calibration trials of the transducer mounting and rotating system itself are described.

Measurements of the material properties of live marine organisms and their influence on acoustic scattering

Accurate acoustic interpretation of zooplankton scattering data requires a reliable knowledge of the material properties of the zooplankton. The lack of ability in predicting or measuring in situ material properties of these marine animals greatly affects and limits the capability to estimate the zooplankton biomass acoustically. To provide in situ measurements of the material properties, a new apparatus, the Acoustic Properties Of zooPlankton, (APOP) was developed. The system combines acoustic and resistivity measurements and can be used for in situ applications. This system can be operated under various environmental conditions (temperature and pressure). In addition, a new method (dual-density) of measuring density of live zooplankton is presented. A series of laboratory and in situ measurements were made on various marine organisms using the APOP system. The results from these measurements suggest that the material properties obtained from laboratory measurements, even if they are accurate and reliable under certain environmental conditions, may vary significantly in the ocean under different environmental conditions. Therefore, to correctly model and interpret the zooplankton acoustic survey data, one needs to take into account the variations in zooplankton material properties as a function of the environmental conditions, or oceanographic parameters.

Advances in developing a high-frequency sonar calibration facility

Advances in developing an acoustic calibration facility at the Woods Hole Oceanographic Institution are documented in the course of giving a brief overview of the calibration venue: a sea well on Iselin Dock, and previously described systems for transducer-mounting and rotation, and target suspension. New instrumentation consisting of a precision target-positioning system and external triggering system to synchronize transducer rotation and sonar pinging is described.

Design and operation of automated ice-tethered profilers for real-time seawater observations in the polar oceans

Funding was provided by the National Science Foundation under Contract Nos. OCE-0324233 and ARC-0519899.

Development and assessment of a new dermal attachment for short‐term tagging studies of baleen whales

Summary 1. Current studies of fine-scale baleen whale diving and foraging behaviour rely on archival suction cup tags that remain attached over time scales of hours. However, skin irregularities can make suction cup attachment unreliable, and traditional pole deployment of suction cup tags is challenging in moderate sea conditions or when whales are evasive. 2. We developed a new tag attachment to overcome these limitations. The attachment features a short (6� 5– 7� 5 cm) needle that anchors in the whale’s dermis (epidermis and blubber) to which a free-floating tag is attached via a severable tethered link. The needle, tag and a detachable ‘carrier rocket’ with fletching are fitted together to form a projectile that can be deployed at distances of up to 20 m using a compressed-air launcher. A corrosive release mechanism allows the tag to separate from the needle after a specified period of time so that the tag can be recovered. 3. The dermal attachment was evaluated during a study of humpback whales (Megaptera novaeangliae )i n the Gulf of Maine and then subsequently deployed on bowhead whales (Balaena mysticetus) near Barrow, Alaska. Monitoring of tagged humpback whales indicated that the needle was shed several days after deployment, the attachment site healed shortly thereafter, and there were no discernible behavioural or health effects over time scales of days to months after tagging. Bowhead whales showed little immediate reaction to tagging; the most common response was a prolonged dive right after tag deployment. On average, respiration rates of tagged bowhead whales were elevated after tag attachment, but returned to the same rate as undisturbed bowheads within 1–1� 5h . 4. When compared to suction cups, the dermal anchor provided a more reliable attachment and it can be applied from greater distances and in rougher sea conditions; it is therefore a useful alternative in circumstances where suction cup tags cannot be easily deployed.

Calibrating two scientific echo sounders

The Simrad EK500 has been the state-of-the-art scientific echo sounder for surveying marine fish stocks; the EK60 is its successor. Both echo sounders have been calibrated with the same 38-kHz, 12-deg-beamwidth, split-beam transducer by the standard-target method at the acoustic calibration facility on Iselin Dock at the Woods Hole Oceanographic Institution. The principal measurements were on-axis target strengths and the two-way directivity patterns of the main lobe, measured with a 60-mm-diameter copper sphere. For each echo sounder, the respective split-beam-determined and directly measured angles of the standard target are compared. The directivity pattern as approximated by Simrad firmware is fit to the experimental data, and both the split-beam-determined and newly compensated values of target strength are expressed through histograms. Target strength distributions are compared for two spheres: a 60-mm-diameter aluminum and 38.1-mm-diameter tungsten carbide with 6% cobalt binder spheres.

Calibrating a 90-kHz multibearn sonar: illustrating protocols

Protocols developed for calibrating multibeam sonar have been applied to the calibration of the Simrad SM2000/90-kHz multibeam echo sounder at the WHOI acoustic test facility in a sea well on Iselin Dock. An external transmitter was oriented perpendicularly to the equatorial plane of the receiving array, thus forming a Mills cross. A total of 128 beams, each with nominal beamwidth 1.5times1.5 deg, were formed, collectively spanning an equatorial sector of 100 deg. The stability in performance was evaluated through time series of echoes from an on-axis target These and other measurements, excepting mainly those of the time-varied gain function, were made with a transmit pulse duration of 300 mus and receiver gain function of 20 log r, where r denotes the range. Vertical beam patterns were measured to identify the equatorial plane with high precision. The dependence of the received level on the time-varied-gain function was measured for gain coefficients from 0 to 50, demonstrating linearity over the range from 0 to 30 for the particular target strengths, both about -35 dB, and ranges, 11.7 and 23.2 m, enabling extrapolation for other target strengths and ranges. The directionalities of beams in the equatorial plane and in the 2D central region were both measured. The calibration protocols are sufficient for quantitative applications of the sonar, as in fish-surveying