imagery simultaneously with sub-bot-tom profiler data provides an optimalmix of coverage, cost and accuracy.

There are numerous sub-bottomprofiler configurations with variations

diverse, inhomogeneous and continu-ally in flux, sub-bottom profiling sup-plemented with strategic coring pro-vides a more complete understandingof the seafloor. Acquiring side scan

Parametric Sub-Bottom ProfilerFor AUVsDeep-Penetration, High-Spatial-Resolution Sub-Bottom ImageryIn a Compact, Low-Power Form Factor

By Sean R. GriffinVice PresidentStephen C. KuhnDesign EngineerOmni Technologies Inc.Stennis Space Center, MississippiandKim BenjaminResearch Engineer/ScientistNaval Undersea Warfare Center Newport, Rhode Island

Autonomous underwater vehicles(AUV) are fast becoming viable

survey platforms. With this growth,new survey tools have emerged. TheNaval Oceanographic Office’s (NAV-OCEANO) profiler for the SubsurfaceAutonomous Mapping System(SAMS) AUV (PFRS) was developedby Omni Technologies Inc. (OTI) toprovide high-resolution, sub-bottomimagery (data).

AUVs have stringent requirementsin electrical power budget, size andweight requiring survey sensors thatare smaller, lighter, autonomous andmore energy efficient than typical sur-vey tools. However, AUVs are excel-lent survey platforms due to their sta-bility and ability to operate close to thetarget.

Sub-bottom profilers are used bysurveyors to obtain information onsurficial marine sediments in cableroute surveys, for pipeline route sur-veys, piling installations, pipeline bur-ial and scour surveys, salvaging oper-ations or any operation where knowl-edge of bottom and sub-bottom sedi-ments is critical.

Coring is one way of obtaining sim-ilar information, but is slow, expensiveand provides only minimal coverage.Since marine sediments are incredibly

SAMS deploymentwith PFRS installed.

Reprinted from Sea Technology magazine.For more information about the magazine, visit www.sea-technology.com

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communications. Mis-sion survey enduranceis on the order of 12hours depending on themission configuration.

1

Parametric SonarNAVOCEANO required a

small, deep-ocean-capable sub-bottom profiler to complement thesensor suite on their SAMS AUV. OTIdelivered a high-frequency, parametricsub-bottom profiler that optimizedresolution and accuracy, as well asprovided good penetration. Parametricsonars use the non-linear property ofacoustic wave travel in water (soundspeed varies as a function of pressure)to mix higher frequency primary acou-stic signals (generally two frequen-cies) to achieve sum and differencefrequency components. The differencefrequency component in most cases isthe frequency of interest. The advan-tage of this phenomenon is that thelow-frequency acoustic energy is gen-erated in the narrow beam of the high-frequency primaries. The beam widthof a piston transducer is dictated bythe wavelength of the source signaland the diameter of the transducer.Given a fixed diameter transducer, asthe frequency increases the beamwidth decreases. Since a parametrictransducer transmits at high primaryfrequencies where the beam width isnarrow, the low difference frequencycomponent that evolves during acou-stic propagation in the water has abeam pattern similar to the beam pat-tern of the primary frequencies. Thus,a small transducer is capable of gener-ating narrow beam, low-frequencysignals providing reasonable bottompenetration with excellent spatial reso-lution. The small transducer is idealfor AUVs due to the limited space andweight capacities of the vehicle.

In addition, parametric sonars offerother advantages. Bandwidth is direct-ly proportional to range resolution. Ingeneral, acoustic sources have opera-tional bandwidths of approximately

in operational frequency, bandwidth,beam width and processing methods.In general, lower frequency profilersoperate in the 500 hertz to five kilo-hertz range and high-frequency sys-tems in the five kilohertz to as high as100 kilohertz range. Low-frequencysystems provide better sediment pene-tration depths but sacrifice resolutiondue to the lower bandwidths involved.Higher frequency systems provide

one to two octaves. If a low-frequencysource is used, bandwidth is limitedsince one octave (doubling in frequen-cy) at low frequency has much lowerbandwidth than an octave at higherfrequencies. For example, a typicallow-frequency linear transducer mightoperate from 20 kilohertz to 30 kilo-hertz (one octave) providing a band-width of 10 kilohertz. A parametricsource operating at 200 kilohertz,however, might provide bandwidthson the order of 100 kilohertz. Thereare some limitations in generating dif-ference frequencies parametrically,and one can expect to obtain differ-ence frequency bandwidths on the or-der of 50 to 60 kilohertz. These band-widths are relatively phase and ampli-tude-flat, providing advantages in sig-nal processing.

The beam width of a parametricsonar remains nearly constant over theoperational bandwidth. Thus, targetillumination is constant across band-width. The beam width of a transducervaries as a function of the ratio of thetransducer aperture to the acousticwavelength within the operating band-width. For a fixed transducer apertureand bandwidth, the beam width varia-tion is larger in a low-frequency bandthan in a high-frequency band. As anexample, the beam width of a 12.7-centimeter-diameter transducer oper-ating over a 10-kilohertz bandwidthfrom a 20-kilohertz to 30-kilohertzrange has a beam width variation from32° to 24° (using a three-decibel beamwidth criteria). This is an 8° beamwidth change. The same transducer

(Right) Parametric beam pattern.

(Left) PFRS at the calibra-tion tank.

better resolution at the expense of pen-etration depths. Wider beam widthsystems provide larger coverage areasat the expense of resolution and accu-racy. All systems have unique process-ing methods to improve range, in-crease resolution, penetrate further,better detect layers and improve sedi-ment classification. The PFRS sub-bottom profiler falls into the high-fre-quency, wide bandwidth and narrowbeam width categories, providing ex-ceptional resolution with reasonablepenetration.

SAMS AUVSAMS is the U.S. Navy’s full-

ocean-depth capable oceanographicsurvey AUV designed for two surveymissions: independent physical ocean-ographic data surveys and side scan/sub-bottom mapping surveys. Thevehicle weighs approximately 862kilograms in air, is 4.1 meters long and0.61 meters in diameter at its widestdiameter.

Power is supplied from an eight-kilowatt-hour battery at a nominal 25volts. The vehicle is free-swimming,primarily utilizing longbase-line andinertial navigation methods. Surveysare programmed prior to deployment,but the vehicle is capable of beingredirected mid-survey via acoustic

the bandwidth divided by the centerfrequency and is a measure of howresonant a transducer behaves at a par-ticular frequency. A high Q causespulse ringing or pulse elongation, thus,reducing bandwidth. If a low-frequen-cy and high-frequency transducer havethe same bandwidth, the Q of the high-frequency transducer is much lowerthan the low-frequency transducer be-cause the center frequency in thedenominator is larger for the high-fre-quency transducer. This translates tomore bandwidth (better resolution) inmany cases since high-Q transducers“ring.”

Of course, in physics there are nofree lunches and the parametric effecthas the disadvantage of poor efficien-cy and poor short-range performance.The generated source power level atthe difference frequency is on theorder of one percent of that generatedat the primary frequencies. A paramet-

operating over a 10-kilohertz band-width from 200-kilohertz to 210-kilo-hertz has a beam width variation fromapproximately 3.5° to 3.4°. This is amore desirable 0.1° change. When thechange in acoustic footprint on thebottom is calculated, the effect is dra-matic.

At a 20-meter range, the footprintchange in the 20-kilohertz to 30-kilo-hertz case is approximately 3.1 meters,and in the 200-kilohertz to 210-kilo-hertz case is only 0.06 meters. Thus,when a linear, wide-bandwidth sonaris used, the target illumination varieswith frequency. This creates anomaliesin sub-bottom imagery, especially incomplex seafloors. The parametricsystem illuminates the same sedimentvolume over wide bandwidth, elimi-nating this effect.

A third advantage of parametric so-nar is low Q at the difference frequen-cy. The Q of a transducer is the ratio of

ric sonar also requires a reasonabledistance before the difference frequen-cy is fully formed, limiting how closethe sonar can effectively operate.

PFRS DescriptionOTI’s parametric and linear sub-

bottom profiler development effortsbegan with the Naval Research Lab-oratory (NRL) in 1996. The PFRS isthe latest innovation in this effort.PFRS has proven to be an extremelycapable sub-bottom profiler providingexceptional imagery in the smallest,lowest power package known by theauthors of this article.

The PFRS system was designed asan integrated package (combinedtransducer and pressure housing) tominimize failure points, reduce vol-ume and weight, and simplify installa-tion. On the SAMS AUV, the PFRSand camera system are interchange-able units. The units are similar in sizeand weight, eliminating complicatedballasting requirements. Both unitsalso share a common platform electri-cal interface, alleviating cabling is-sues. The camera and PFRS can beinterchanged on SAMS in less than 30minutes.

The PFRS system is 43 centimetersin length and 17.8 centimeters in di-ameter, weighing 20 kilograms in air.The housing is fabricated from gradeA titanium and can withstand pres-sures of 10,000 pounds per squareinch.

The electrical interface to the PFRSsystem consists of 100BaseT Ethernet,a 24-volt DC nominal (18-volt DC to36-volt DC) supply voltage, a two-wire flood sensor interface and a trig-ger input. The entire system operateson an average power of just under 40watts.

PFRS is configured with a Win-dows-based program and can also becontrolled/monitored via the SAMScontrol/status program. The embeddedhard disk is shared providing easy dataaccess with standard file managementmethods. Once off-loaded to a massstorage device, data is reviewed usingthe PFRS playback software.

Parametric TransducerThe PFRS transducer is a piston

transducer 17.8 centimeters in diame-ter and 8.9 centimeters thick. Thetransducer is rated to an operationalpressure of 10,000 pounds per squareinch and offers two operationalmodes: a 200-kilohertz parametricmode and a 30-kilohertz linear mode.

Sub-bottom and side scan imagery of a fault line.

McGovern and Martin Ammond atNAVOCEANO for their support.Doug Lambert, Dale Bibee and DonWalters of the NRL have been innova-tors pressing forward with research tofurther the science.

The inclusion of names of any spe-cific commercial product, commodityor service in this article is for informa-tion purposes only and does not implyendorsement by the U.S. Navy orNAVOCEANO.

ReferencesFor a complete list of references,

contact author Sean Griffin at srg@otiengineering.com. /st/

For more information, visit ourwebsite at www.sea-technology.com.

The parametric mode is the normal op-erating mode providing bandwidthfrom 10 kilohertz to 60 kilohertz witha constant beam width of approxi-mately 3°. The 30-kilohertz linearmode provides an 18° beam width pro-viding a larger bottom footprint thataides sediment classification process-ing.

The active material in the PFRStransducer is injection molded 1-3piezoceramic polymer compositematerial from Materials Systems Inc.The 1-3 composite material offers theadvantage of low Q, wide bandwidthand minimal lateral mode resonancesthat can produce non-uniform respon-se characteristics and side lobes.Using this material in a parametrictransducer provides textbook beampattern curves with extremely low sidelobes.

ResultsDuring a recent SAMS survey, sub-

bottom data was collected in conjunc-tion with side scan data. A fault linewas discovered during this survey thatdemonstrates the PFRS system’s capa-bilities.

PFRS was operating at a 10-meterrange with a 35-kilohertz differencefrequency and obtained bottom pene-trations between five and 10 meters insilty clay sediments. Acoustic interfer-ence from an acoustic modem isobserved in the imagery and remainsan item of concern.

ConclusionsThe PFRS sub-bottom profiler is an

exceptionally-high-resolution and ac-curate sub-bottom profiler requiringminimal space and power. Parametricacoustics is the enabling technology.By careful design, the efficiency limi-tations of parametric acoustics can beovercome providing an ideal match forAUVs. Future plans are to transitionthis technology to man portable AUVsfor use in shallow-water surveys.

AcknowledgementsThe NAVOCEANO, Stennis Space

Center Code N9, provided funding forthe development of the PFRS. Specialthanks goes to Andrew MacNaughtonand Michael Bendzlowicz at NAVO-CEANO for their contributions to thisarticle and their support. Funding forearly development efforts was provid-ed from the NRL and Office of NavalResearch. The authors would like tothank Kenneth Sharp, Rick Swanson,Randy White, Joseph Sparcella, Dan

Sean Griffin is afounding partner ofOmni TechnologiesInc. and has devel-oped oceanographicinstrumentation overthe last 15 years.Griffin has devel-oped a wide array of acoustic, geo-acou-stic, geo-technical, and specialized sen-sors and systems for oceanographic re-search. He received a B.S. degree in elec-trical engineering from Louisiana StateUniversity in 1987 and performed gradu-ate studies at the University of NewOrleans.

Stephen Kuhn hasbeen working as adesign engineer atOmni TechnologiesInc. since 1998 and isinvolved in hardwareand software devel-opment of oceano-graphic instrumentation. Kuhn received aB.S. degree in electrical engineering aswell as a B.S. degree in computer engi-neering from Louisiana State University in1997.

Kim C. Benjamin isemployed as a re-search engineer/sci-entist with the Acou-stic MeasurementTransducers Branchof the Naval Under-sea Warfare Center’sSubmarine Sonar Department. His recentwork has focused on injection molded 1-3piezocomposite. He has designed, fabri-cated and calibrated planar as well ascurved single elements and multi-elementarrays. Benjamin received an M.S. inocean engineering in 1980 and a B.S. inphysics in 1977 from the University ofRhode Island.