Acoustics Measurement Facility

The NEAR-Lab measurement facility is located in suite 25-00 at 1900 SW 4th Ave, Portland OR 97207. The facility contains equipment for:

• Scaled underwater acoustics experiments

• Passive sensors

• Off-shore seaglider

Scaled Underwater Acoustics

Underwater acoustics is the study of the propagation of sound in the water and its interaction with bottom sediments. Unlike electromagnetic waves, acoustic waves can propagate in water for long ranges on the order of kilometers, and this allows for a large variety of applications such as remote sensing of the seabed, detection of ships or targets of interest in the water column and study of the behavior of marine animals, to mention a few.

At the NEAR-Lab measurement facility, experiments are conducted for studying waveguide propagation and scattering from rough surfaces and random media. The experiments are performed at ultrasound frequencies, which allow scaling down the physical dimensions of field experiments which are typically performed at much lower frequencies.

Our facility is equipped with:

• A DAQ board capable of recording four simultaneous channels at 5 MS/s.
• Broadband acoustic sources with frequencies of 250 kHz and 500 kHz.
• Broadband hydrophones (receivers).
• A 10x10x1 feet tank is utilized to re-create shallow water environments, in which cylindrical waves can be propagated.
• A 5x7x3 feet tank, used to transmit spherical waves and to study the scattering and interaction of sound with rough surfaces, sediments with random scatterers or complex scatterers in the water column such as fish.
• An adjustable aluminum frame for sensor positioning that can be configured for monostatic and bistatic measurements.

Validation experiments of algorithms for target tracking and demonstrations for academic purposes can be performed in our facility using scaled down scatterers such as steel ball bearings, brass cylinders and aluminum plates. Similarly, wave propagation in random media can be studied using slabs of elastic background material with well characterized embedded scatterers, such as glass or metal beads.















Experiments in shallow water (depth 130 m) performed at frequencies around 10 kHz (wavelength~0.15 m) can be scaled down and reproduced in our 10x10x1 feet water tank, using higher frequencies around 250 kHz and higher (wavelength~6 mm). In both cases, the dimension of most of the relevant features (targets, water depth, etc) relative to the wavelength is preserved.















Propagation of spherical waves and scattering from rough surfaces and random media are performed in our 5x7x3 feet acrylic tank. The experimental set-up can also be used for instrument calibration, study of scattering from simple geometries like spheres, cylinders or more complex scatterers such as fish or rocks.

Passive Acoustic Sensors

At the NEAR-Lab measurement facility, sensors are being developed to listen and record underwater sound. These sensors could be used in a number of different applications including noise monitoring, boat detection and fish detection.

(Above) Laptop based hydrophone system block diagram and picture.

The first system is a laptop based system. This system is easily portable and is used frequently to listen to boats on the Willamette River. The system includes:

• Hydrophone and preamp with 1Hz-10kHz bandwidth.
• Signal conditioning board powered by a 9V battery.
• Laptop with 16-bit line-in input.



















The second system is a completely autonomous data collection system that can be deployed for up to a week at a time.

NEAR-Lab Seaglider

The NEAR lab took delivery of its first glider in late April in anticipation of greatly expanding the lab’s ability to gather data in real world environments. The Slocum Coastal model sea glider is capable of carrying various payloads for up to 30 days at depths of 200 meters with a 1000 km range. The coastal model is better suited to operating in shallow, near shore environments than other models as well as still being capable as an off shore platform, providing a large amount of flexibility.

A sea glider is an autonomous Unmanned Underwater Vehicle (UUV) that does not require any form of motorized propulsion. Instead of the conventional method of using a propeller that is attached to a motor for forward thrust, the sea glider can change its buoyancy between being slightly positive to slightly negative in order to glide through the water. Using the same principles that allow an airplane glider to fly - weight, lift and thrust - in both vertical directions, the sea glider has small wings that provide lift in the opposite direction of its buoyancy (weight in water) in order to propel itself forward. This means that the sea glider must move through the water column in a series of dives and accents, or yos. Viewed from the side this gives the sea glider’s path a saw-toothed profile.

Except for a small servo used to actuate the rudder and an electric motor that changes the buoyancy of the UUV, there are no moving parts in the Slocum Sea Glider. The buoyancy motor only needs to be actuated at the top or bottom of each yo. Therefore power consumption is very minimal and the Solcum Sea Glider has very long range and endurance in comparison to UUVs that use motorized propulsion.

NEAR-Lab Tests Seaglider in Northern California

March, 2009

In early March the NEAR lab, in association with various members of the Space and Naval Warfare (SPAWAR) Systems Center, participated in sea trials of the Slocum Sea Glider off the coast of Northern California. Departing Eureka harbor aboard the Research Vessel Coral Sea - acoustic data was collected over a five-day period in the vicinity of the Eel River outflow area. At times the crew endured wind speeds approaching 40 knots with 15ft seas.

A typical day would start with meeting at the boat around 6 am, getting underway shortly thereafter and running out to the operation (op) area. While en-route, profiles would be programmed into the gliders while they were still on board for the desired depths and distances. The first step upon reaching the op area was to collect Conductivity, Temperature, and Depth (CTD) data.

Next, the final preparations were made to the gliders to get them ready for launching. Launching was easily accomplished with good communication between the launch crew of 3 and the ship’s captain. In the case of the Coral Sea, the articulating A-frame and a hydraulic winch along steady manipulation by the launch/recovery crew got the task done without drama or anyone getting wet.

During the time between launching and recovering CTD data was collected at hourly intervals and data from the previous days collection would be processed – sea state permitting. There were a couple of days when the crew spent most of the time just trying not to get seasick. The Coral Sea would typically take a broad circular path towards the intended recovery site so as not to interfere with the sensitive acoustic receivers onboard the gliders.

Once the gliders reached the end of their profile, they would come to the surface and “call” home with their onboard freewave line-of-sight transmitters. The freewave would also send a GPS position incase the actual location differed from the intended recovery point. This was the point at which the launch/recovery crew and the captain earned their pay. Now the captain would skillfully back the Coral Sea up to the glider and the crew would snag the recovery bail on top of the glider with a long boat hook and attach the line to the winch. This was fairly straightforward in normal seas, but it became much more interesting, and wet, in heavier seas.

Despite the heavy seas and an intrusive crab pot line that temporarily entangled one of the gliders, the trip was an enormous success. The SPAWAR team was able to gather some very useful data and the NEAR lab members gained a lot of experience and insight into sea glider operations. The crew of the R/V Coral Sea was very professional and proficient. The NEAR lab intends to return to Eureka in the summer with its own glider and continue the success that was enjoyed on this trip.