Sonar

SonarChapter 9HistorySound Navigation And Ranging (SONAR) developed during WW IISound pulses emitted reflected off metal objects with characteristic pingLike radar and lidar time of flight is measured to determine distanceEarly sonar gave only distance and direction to targetModern sonar used for mappingHistoryGIS-compatible bathymetric data now available for many regionsUsed for geologic mapping of seafloorSearch and recovery operationsEnvironmental monitoringPrinciples of SONARSound waves are a mechanical vibrationCan only travel through an elastic medium (air, water, earth)Measured by frequency instead of wavelength ()How is frequency related to ?Principlesf = vw / Vw is the speed and f is frequencyUsually give as cycles/second or hertz (Hz)Human hearing in the rage of 20 20,000 HzSound waves are much slower than EM radiationIn water, sound travels about 1,500 m/sSpeed of light is about 3 x 108 m/sPrinciplesUltrasound is used in medicine and sometimes in engineeringUses frequencies of 1-15 MHzPrinciplesIn environmental work there are three kinds of sonarSide-scanUsually in the range of 100 kHz - 400 kHzSingle beamUsually at 12 kHz - 200 kHzMultibeam to 6,000 m deepUsually at 12 kHz - 200 kHzLower frequency = longer range and reduced spatial resolutionSide-Scan SonarHas been used to detect objects and map bottom characteristicsShows size, shape and texture of featuresCan determine length, width and heightThe sonar is usually mounted on a sled or towfish and dragged close to the bottomAbout 10% of the maximum acoustic rangeLow angle to emphasize surface relief

Image swaths about 100-500 m wideUsually do 200% coverage to eliminate gapsEspecially the track underneath the vessel since that is not imagedFar field image is degraded

Side ScanStrong returns are brighterInfluenced by slope and surface roughnessSmooth surface is darkerObjects sticking up will create shadows much like radar

Container ship

Acoustic Lens SonarProvides high resolution at close range in turbid waterDeveloped in the 1990sFrequencies between 1-2 MHzRange of 1-30 mUses plastic lenses instead of electronics to focus the beams onto a linear array of acoustic transducers (like imaging scanners)Produces the imageAcoustic beamforming modifies the propagation of sound by introducing spatially dependent delays into a wavefront. This focuses incoming sound from a single source or direction into a small volume of space so that it can be detected by a single transducer. Acoustic beamforming can be implemented using electronic circuitry or an acoustic lens

Acoustic Lens SonarUsed to produce video at a rate of 6-20 frames per secondUsed primarily by military to see in dark

Single Beam SonarUsed primarily for mapping channels and for engineering applicationsUses only a single sourceDoes not produce much data, because only a single point for each pulse under the tramsducer

Acoustic data points collected along intertwining transects in Rehoboth Bay, Delaware. The points are classified as algae (green) or no algae (light blue). The points were processed and used to derive a coarse map of the algae distribution (dark green squares). Multibeam SonarSensor uses an array of sources and receiversEnergy focused on a narrow strip beneath the shipMultiple echoes give a profile of depths along a strip perpendicular to ship trackSwath width depends on depth Usually 2-4x water depth

Multibeam SonarCrater Lake flyby