SCM 330 Ocean Discovery through Technology Area F GE

Sensors - Physical Physical Sensors: Temperature Salinity Pressure Acoustics Passive Active Radar Theory Application Sensor

Acoustic Theory  Sound is produced by a vibrating source in an elastic medium  Causes compressions and rarefactions in the medium  Causes detectable pressure changes  Speed of propagation depends on the medium  Sound travels at 1500m/s in water, approx 390m/s in air

Transmission Ping is generated by a piezo-electric ceramic projector Responds to an applied voltage with oscillation Frequency of oscillation depends on input and ceramic characteristics Oscillation causes a series of compressions and rarefactions in the surrounding water; a sound pressure wave.

The speed of a wave is the rate at which vibrations propagate through the medium. Wavelength and frequency are related by: = c/f where lambda = wavelength, c = speed of sound in the medium, and f = frequency. The speed of sound in water is approximately 1500 m/s while the speed of sound in air is approximately 340 m/s. Therefore, a 20 Hz sound in the water is 75 m long whereas a 20 Hz sound in air is 17 m long.

Sound in Water In 1822 Daniel Colloden used an underwater bell to calculate the speed of sound under water in Lake Geneva, Switzerland.

Sound velocity in water  Can vary considerably from point to point in the ocean  Dependent on 3 main factors: Salinity (Conductivity) Temperature Pressure (Depth)

Salinity Average salinity ppt (parts per thousand) Change in salinity causes a density variation which affects velocity Varies geographically (Baltic 7ppt, Dead Sea 300 ppt) Oceanic fronts River mouths, estuaries Monsoon areas Ice Change of 1ppt = approx 1.3m/s velocity change

Temperature Predominant factor affecting velocity Usually decreases with depth Approx 3m/s decrease per 1ºC Below 1000m pressure becomes predominant Temperature change has a large effect: 165m depth change has the same affect as a 1ºC temp change

Typical SV at surface Off the coast of Calif

Transmission losses  Spreading loss  Attenuation AbsorptionAbsorption ScatteringScattering

Spreading loss Note that there is no true loss of energy. The energy is simply spread over a progressively larger surface area, thus reducing its density. Spherical spreading Cylindrical spreading

Spreading loss It should be noted that the loss of intensity of a sound wave due to spreading is a geometrical phenomenon and is independent of frequency. As range increases, the percentage of intensity lost for a given distance traveled becomes increasingly less.

Absorption  Transmission loss, expressed in dB/km, due to conversion of acoustic energy into heat.  Dependent upon amount of MgSO4 and MgCO3 present.  Frequency dependent  Temperature dependent

Frequency dependence  Low frequency has greater range due to less attenuation.  High frequency suffers from more attenuation which reduces range.  Greater definition (resolution) with higher frequencies but with corresponding loss of range.

Frequency dependence of absorption coefficient Frequency Fresh water Absorption coefficient Salt water Absorption coefficient 12kHz 100kHz5dB/km30dB/km 240kHz20dB/km70dB/km 455kHz70dB/km110dB/km

Scattering Sound striking bodies in the water and being reflected from: o Surface, bottom and land o Organic particles o Marine life o Bubbles o Temperature variations The amount of energy scattered is a function of the size, density, and concentration of foreign bodies present in the sound path, as well as the frequency of the sound wave

Scattering Part of the reflected sound is returned to the source as an echo, i.e, is backscattered, and the remainder is reflected off in another direction and is lost energy. Backscattered energy is known as reverberation  Surface reverberation  Volume reverberation  Bottom reverberation

Reverberation Volume Fish / marine organisms Suspended solids, bubbles, temperature variations Surface Waves / bubbles. Related to wind speed Bottom Bottom roughness Frequency

Bottom Absorption

Bottom absorption  Varies from 2dB – 30dB per bounce  Varies with frequency, bottom type and grazing angle  Losses increase with frequency and grazing angle For 24kHz, 15º grazing angle Rock approx 20dB/m2 Sand approx 30dB/m2 Mud approx 40dB/m2

Sensors - Physical Physical Sensors: Temperature Salinity Pressure Acoustics Passive Active Radar Theory Application Sensor

Passive Acoustics Bioacoustics Seismicity Environmental Noise

Methods SOSUS

Autonomous Hydrophones

Bioacoustics Whales/Shrimp

Blue Whale - Atlantic Fin Whale - Atlantic Humpback Whale - Pacific Minke Whale - Pacific Bowhead Whale Spotted Dolphin

Snapping Shrimp

Seismicity TremorEarthquake

Environmental Noise ShipAirgun

Sensors - Physical Physical Sensors: Temperature Salinity Pressure Acoustics Passive Active Radar Theory Application Sensor

Common Applications Passive Acoustics –Just Listening Seal, Whales & Dolphins Military Applications Active Acoustics –Seafloor Mapping –Seismic –Waves & Currents –Diver Detection –Fish Finding

Single-Beam Echosounders Single Beam Echo Sounder  Developed in the 1920s  Using an acoustic transmitter and receiver to measure echo receive time

Sidescan Sonar Side Scan Imagery  Developed in the 1920s  Using an acoustic transmitter and receiver to measure echo receive time  Side scan developed in the 1960s to assist in bottom type determination

Generally a series of survey lines that are run parallel to each other Dense data along track but NO data is available in between each survey line = coverage Single Beam Single Beam Survey

Multi-Transducer Echo Sounder Multichannel  Developed in the mid-1960s  Using booms to mount multiple transducers  Limited to small ranges of depth  Hard to navigate  Difficult to compensate for vessel motion

If designed well and spaced correctly & survey lines designed to overlap, a 100% coverage can be obtained Most effective in very shallow water (<10m)  Disadvantages:  very slow survey speed  limited weather window  reduced coverage versus depth range  tight line spacing Multichannel Multi-Transducer Survey

Swath Bathymetry Multibeam Echo Sounder  Developed in the mid-1970s  Larger range of operating depth  Easier to navigate  Wider coverage in deeper water  Backscatter Imagery for bottom analysis

Shallow gas deposits in the surficial sediments of Strangford Lough, N. Ireland. Seismic

System Design Objectives Resolution horizontal ~0.5 m horizontal ~0.5 m vertical ~0.3 m vertical ~0.3 m acoustic impedance ~1% acoustic impedance ~1% Penetration max. 40 m Surface towed from small vessels Produce real 3D seismic data volume 3D Chirp sub-bottom profiling system

You hear the Doppler effect whenever a train passes by - the change in pitch you hear tells you how fast the train is moving. The profiler uses the Doppler effect to measure current velocity by transmitting a short pulse of sound, listening to its echo and measuring the change in pitch or frequency of the echo. Acoustic Doppler Current Profiler Principles An acoustic echo reflected from moving particles is shifted in frequency (Doppler shifted) in proportion to the particle velocityAn acoustic echo reflected from moving particles is shifted in frequency (Doppler shifted) in proportion to the particle velocity

Typical Ocean Scatterers The acoustic waves are reflected by scatterers in the water. These are typically small phytoplankton and zooplankton that are assumed to be moving at the same speed as the currents. Pteropod Copepod Euphasiid 1 cm 1 mm

High resolution velocimeters Self-contained ocean models –Focused acoustic beams for true 3D measurements –Signal scattering from small particles –High frequency (>25 Hz) and small sampling volume ( 25 Hz) and small sampling volume (<1 cm 3 )

Velocimeter applications: –Process studies –Orbital wave motion studies –Surf-zone dynamics –Boundary layer studies –Natural low flow studies in lakes and marshes –Turbulence measurements

Acoustic Doppler Profilers Transmits acoustic pulseTransmits acoustic pulse Listens for return echoListens for return echo Measures velocity along beamMeasures velocity along beam Converts to coordinate systemConverts to coordinate system Velocity measured at multiple levels (cells) from bottom to surfaceVelocity measured at multiple levels (cells) from bottom to surface Profiling range depends on frequency and the amount of scattering materialProfiling range depends on frequency and the amount of scattering material

The Doppler technology is superior to conventional technologies because: a) One instrument measures the whole water column a) One instrument measures the whole water column b) No physical obstructions in measuring volume b) No physical obstructions in measuring volume c) No moving parts c) No moving parts d) No calibration d) No calibration e) No drift due to fouling e) No drift due to fouling