Estimates of blast injury and acoustic traumazones for marine mammals from underwater explosions

Darlene R. Ketten

Department of Otology and Laryngology, Harvard Medical School, Boston, Massachusetts, 02114,U.S.A. Email: eplunix!drk@.mit.edu

Summary

The physical parameters of two underwater detonations of Class A explosives (TNT deriv­atives with fast rise-time waveforms) are analyzed for their potential to induce blast injuryand acoustic trauma in marine mammals. Anatomical differences between marine and landmammal ears that may affect the incidence and severity of acoustic trauma and the basicconcepts of temporary versus permanent threshold shifts are summarized. Overpressurecalculations are combined with experimental data from the literature to calculate theoreticalzones for acute trauma, permanent hearing loss, and temporary threshold shifts for mid­water explosions at two charge weights.

Key words: blast injury, threshold shift, ship shock, Cetacea, acoustic trauma

Introduction

This paper is a theoretical overview. In it, thephysical parameters of two hypotheticalhazards, large and small underwater explo­sions, are outlined and analyzed for theirpotential interactions with marine mammalears. For the last decade, there has beengrowing concern about the effects of man­made sounds in the oceans. These concernsare timely, and more stringent reviews areunderway of many activities that couldacoustically impact marine life. However, atthe moment, we have no direct experimentalevidence or controlled measures of energylevels that induce blast or acoustic trauma inmarine mammals.

How labile is the ear of any marine mammalspecies? Given that marine mammals arehighly aural animals, we expect serious im-

pact from even minor acoustic trauma. Thealternative view argues that highly prizedresources, including' sensory systems, aregenerally well protected. To date, there is nodefinitive study of temporary or permanentthreshold shifts in any marine mammal. Pin­niped and cetacean ears provide an interest­ing functional paradox: they are essentiallyland mammal ears immersed in a biological­ly rich but acoustically harsh environment.As marine mammals evolved from landmammals, their ears coupled a terrestrialblueprint to extensive adaptations for rapidpressure changes and large concussiveforces. On one hand, they have basicallyacoustically fragile land mammal ears. Onthe other hand, they evolved in a high noiseenvironment, and some adaptations; e.g.,those that prevent inner ear barotrauma,may deter noise and shock trauma. Little isknown on the effects of intense sounds and

Sensory Systems of Aquatic Mammals (1995)R.A. Kastelein, J.A. Thomas and P.E. Nachtigall (editors)De Spil Publishers, Woerden, The Netherlands ISBN 90-72743-05-9

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concussive forces on hearing in water forvirtually any species. Evidence from humandivers is largely anecdotal and often contra­dictory. There are few audiograms for ma­rine mammals and no published data ontemporary or permanent threshold shifts.Thus, at this stage, there are no direct mea­sures of underwater acoustic trauma for anymarine mammal and detailed descriptionsof the ear are available for fewer than 20% ofcetacean and pinniped species.

The "experiment" presented here is not, inany sense, a definitive analysis of marinemammal acoustic hazards, nor is it an ex­pert opinion on trauma. Rather, it providesan overview of the physiologic aspects ofacoustic trauma and a discussion of the rel­evance and limitations of currently avail­able data (primarily from land mammalears) for predicting the effects of man-madesounds on marine mammals. The scope islimited to two tangible impacts: shockwave and acoustic trauma from blast zones.The values calculated are impact estimatesonly and are largely speculative. Conserva­tive estimates of the zones of potential le­thal, sublethal, transitional, and low to zeroimpacts are extrapolated from arbitrarycharge weights, published anatomical andphysiological data on blast injury, and re­lated anatomical data on marine mammalears. It does not attempt to assess any of thepossible, equally grave, but subtle, effectssound can have; e.g., disruption of commu­nication, breeding behaviour, or naviga­tion. Regrettably, there is little direct fac­tual evidence that can be brought to bear onany of these issues. In essence, this experi­ment is directed not at addressing a single,well defined question but at construing ourcurrent, relatively small volume of knowl­edge into a more coherent form for objec­tive discussion of potential marine acousticimpacts.

392

Materials and methods

The following model assesses the theoreticalhazards of impulse pressures and peak over­pressures that marine mammals may en­counter within a 15 km radius of multi- ton­nage mid-water explosions. The specific det­onations addressed are 1200 lbs (544 kg) and10000 lbs (4535 kg) charges of Class A explo­sives commonly used in inshore and off­shore industrial and military activities(DOC, 1993; Ketten et al., 1993; Czaban et al.,1994; Reeves and Brown, 1994). Class A ex­plosives are TNT-derivative water-gel com­pounds that generate complex, fast-rise timeimpulse waveforms. Detonation velocitiestypically range 4,000 - 10,000 m/sec. The twocharge weights chosen for this model spanthe conventional range used in demolition,construction, and live-fire military testing(DOC, 1993, Ketten et al., 1993). Several sim­plifying assumptions are made in the model:1. detonation occurs at ~ 100m depths.2. bottom depth is ~ 20 times the detonation

depth.3. the bottom is thick sediment with no dis­

cernible terrain.

A charge depth < 5% total water depth islargely a practical constraint. Given theseassumptions, the pressure spread is spher­ical for 100 m, becoming planar as surfaceinfluences emerge. This model more closelyapproximates military ship shock trials thannear shore industrial activity. In- and near­shore explosions imply multiple irregularinfluences; e.g., bottom topography, artifi­cial structures, etc., that rapidly distort thewave front and produce complex cancella­tion-enhancement patterns.

Ship shock trials are widely accepted proce­dures for live-fire testing of new or existingship designs. Because they are highly regu­lated test procedures, they provide con-

Estimates of blast injury and acoustic trauma zones

trolled measures of a ship's ability to with­stand a near-miss underwater explosion,and, as a consequence, also provide empir­ical data (Czaban et al., 1994) which, in thispaper, act as controls for theoretical esti­mates from ideal models.

sound into neural impulses. Although ma­rine mammal ears clearly follow the landmammal blueprint, they have both grossand microscopic, aquatic-related adapta­tions at all auditory system levels (Ketten,1992, 1993).

Results

Charge parameters in this model are basedon published manufacturer's standards forHBX-l, a common ship shock test explosivein North American trials (DOC 1993; Reevesand Brown, 1994):

Marine mammal earsThere are three essential parts to the mam­malian auditory periphery: 1) an outer earwhich captures sound, 2) a middle ear whichfilters and amplifies, and 3) the inner ear(cochlea) which is a band-pass filter andelectro-mechano-chemical transducer of

Model analyses include calculated pressuresand overpressures at eight arbitrary dis­tances, ranging 1 to 10,000 m from thesource, and estimated distances from thesource for five zones that cover major stagesof blast injury and trauma. These stages aredesignated lethat sublethal with permanentthreshold shifts, temporary to permanentthreshold shift transitions, and minimal tozero injury transitional zones. Related datafrom previous studies are presented in anoverview of marine mammal hearing and ina brief summary of known factors for acous­tic trauma and blast injury in mammals.

External (outer ear) canal adaptations rangefrom extreme (possibly dysfunctional) inCetacea to broad, essentially terrestriat air­adapted canals in the true-eared seals andmustelids. Four outer ear adaptations arecommon to all cetaceans: there are no pin­nae, no air-filled external canals, no encap­sulated pneumatized areas, and exception­ally dense temporal bones. These appear tobe correlated largely with locomotion anddiving; Le., pinnae would provide hydrody­namic drag, and thin-walled, air-filled, bonychambers would be a liability in rapid, deepdives. However, these adaptations are mostderived in echolocating cetaceans, wherethey subserve an acoustic function (Ketten,1992). In odontocetes, the external ear canalsare completely occluded by wax and debris,ending in a blind pouch that does not contactthe middle ear. The temporal bones are sus­pended by ligaments in a peribullar sinusfilled with a spongy mucosa. This ligament­mucosa complex isolates the ear from bonysound conduction and aligns the middle earcavity with two orthogonat specialized fattychannels. Anatomical and behavioral stud­ies suggest these discrete fatty tubes are lowimpedance sound channels (Brill et al., 1988;Ketten, 1992 for review). Mysticetes simi­larly have occluded external canals, but ex­plicit tissue channels to the ear have not beenidentified. Pinniped and mustelid ears areless derived than cetacean. The external pin­nae are reduced or absent. Ear canal dia­meter and closure mechanisms vary widelyin pinnipeds, and the exact role of the canalin submerged hearing has not clearly beendetermined.

HBX-l1,200/10,000 lbs(544/4,535 kg)(non-sequential)1,344/11,200 lbs(609/5,080 kg)8,800 m/sec

ExplosiveCharge Weights

TNT equivalents(HBX factor = 1.12)Detonating velocity

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D.R. Ketten

As in the external ear, most middle ear adap­tations in marine mammals appear to berelated to diving. The dense-walled middleear cavity in whales is lined with a thick,distensible corpus cavernosum which mayregulate middle ear cavity volume or pres­sure. The Eustachian tube in both seals anddolphins is broad, fibrous, and resists col­lapse, thereby assuring rapid middle earequalization and decreasing the probabilityof implosive injury or middle ear barotrau­ma. Ossicular configurations and stiffnesscharacteristics are highly species-specificand therefore may reflect frequency special­izations.

As in other mammals, whale inner ears arefluid-filled membranous labyrinths thathouse two sensory organs: a three-ringedvestibular system (for balance) and a spiralcochlea (for hearing). The vestibular systemis exceptionally small in whales and dol­phins, and vestibular innervation is reducedproportionately; i.e., < 10% of cetacea VIII­th nerve fibers are vestibular, compared to40% in most mammals (Gao and Zhou, thisvolume). Vestibular down-sizing may be acorollary of fused cervical vertebrae inwhales, or alternatively, a valuable adapta­tion for rapid, continuous rotations in water.Maculae are present, implying that geotacticand linear acceleration cues at least are per­ceived.

Dolphin, baleen whale, and seal inner earshave the same general format as other mam­malian ears (Fig. 1a); i.e., a fluid-filled, tri­partate spiral with a resonating membranesupporting mechano-sensory receptors(Ketten, 1992; Ketten, 1993). Mammalianbasilar membranes are essentially tonotopicbivariate resonators built of repeated, rela­tively uniform modules. The range of thestiffness and mass of these modules in eachear determines the range of cochlear reso-

394

nances and therefore the hearing limits ofthat ear. Because all mammalian basilarmembranes have a similar cellular structure,most interspecific differences in stiffnessand mass are determined largely by thick­ness and width distributions along themembrane's length. Highest frequencies areencoded in the base of the spiral where themembrane is narrow and stiff. Progressivelylower frequencies are encoded as the mem­brane becomes broader and more pliant to­wards the spiral apex. The primary soundtransducer is the Organ of Corti (Fig. 1b), anarray of hair cells with stereocilia that aredeflected when the basilar membrane is de­formed by a pressure wave.

Membrane distortions depend on the inter­action of intensity-frequency distributionsof incoming sounds with the resonancecharacteristics of the basilar membrane. In aresponsive membrane region, as the ciliabend, graded chemical changes in the haircells produce a depolarization in the audi­tory nerve, and impulses coding spectraland temporal patterns of the sound are sentto the brain. The absolute range of frequen­cies encoded by the ear varies naturally foreach species according to its average distri­bution pattern of inner ear stiffness andmass characteristics. Some mammals hearwell into the ultrasonic range (> 20 kHz);others, into the infrasonic «50 Hz) (Fay,1988). In addition to differences in absoluterange, species vary in their sensitivity ateach frequency band. Species ranges arefixed by basic structure; however, acuity va­ries widely by individual according to thehealth or integrity of the ear.

Specializations in marine mammal ears re­late not only to extended frequency-intensi­ty ranges, but also to the differences inacoustic power and transmission character­istics for waterborne versus airborne sound.

Estimates of blast injury and acoustic trauma zones

A. X~SECTION OF COCHLEA

SpiralGanglion

B, COCHLEAR DUCTspiral

ligament

Figure 1. Schematized marine mammal inner ear (a) and details of the cochlear duct (b) containing the Organ ofCorti (OC) which is the cochlear structure most directly and frequently impacted by sound. (Sc =scala; M =media;V = vestibuli; T = tympani; sp = spiral prominence)

The odontocete inner ear, like that of bats, isprimarily adapted for echolocation; the dol­phin is capable of producing, perceiving,and analyzing ultrasonics. These ears haveexceptional frequency discrimination abil­ities but are comparatively poor at perceiv­ing lower frequency sounds (Au, 1993). Ba­laenid ears are adapted to the other end ofthe spectrum; i.e., they hear poorly over 20kHz but are probably exceptionally acute atfrequencies in the 10-100 Hz range (Ketten,1992; 1993). Some cetaceans produce sourcelevels as high as 180 to 220 dB re 1 f.lPa(Richardson et al., 1991; Wursig and Clark,1993; Au, 1993). Pinnipeds are equally var­iable. Some, like the high-frequency harbour

seal (Phoca vitulina), are effectively - acousti­cally - sea-going cats; others, like the aptlynamed elephant seal (Mirounga angoustiros­tris), appear to have low to infrasonic adapt­ed ears (Ketten, pers obs.).

In summary, marine mammals are acousti­cally diverse with wide variations not onlyin ear anatomy but also in hearing range andsensitivity. Both mysticetes and odontocetesappear to use soft tissue channels for soundconduction to the ear, which may decreasereceived acoustic power. Whale middle andinner ears are most heavily modified struc­turally from those of terrestrial mammals inways that accommodate rapid pressure

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changes. The end-product is an acousticallysensitive ear that is simultaneously adaptedto sustain moderately rapid and extremepressure changes, and appears capable ofaccommodating acoustic power relation­ships several magnitudes greater than in air.It is possible that these special adaptationscoincidentally may provide protectivemechanisms that lessen the risk of injuryfrom high intensity noise, but no behaviou­ral or psychometric studies are availablewhich directly address this issue.

Noise traumaFor this discussion, sound-related trauma isdivided into two simplistic divisions: lethaland sublethal impacts. Lethal impacts arethose that result in the immediate death orserious debilitation of the majority of ani­mals in or near an intense source; i.e., pro­found injuries from shock wave or blast ef­fects. Lethal impacts are not, technically,pure acoustic trauma; they are discussed atthe end of this section. Sublethal impacts inwhich a hearing loss is caused by exposuresto perceptible sounds are correctly termedacoustic or noise derived trauma. In thesecases, some sound parameters exceed theear's tolerance; i.e., auditory damage occursfrom metabolic exhaustion or over-exten­sion of one or more ear components. In somecases, sublethal impacts may ultimately beas devastating as lethal impacts, causingdeath through impaired foraging or preda­tor detection, but they do not carry the im­mediate, broad devastation of a blast injury.

To determine whether an animal is subject tohearing loss from a particular sound re­quires understanding how its hearing sensi­tivity interacts with that sound. Basically, ifyou can hear a noise, at some level it candamage your hearing by causing decreasedsensitivity. The minimal level at which asound is perceived is the threshold.

396

If an individual requires a significantly grea­ter intensity (than the species norm) to per­ceive a particular frequency, there is a hear­ing deficit marked by a threshold shift.Noise induced threshold shifts are a moder­ately well investigated phenomenon for air­adapted ears (Lehnhardt, 1986; Lipscomb,1978; Richardson et al., 1991 for detailed re­views). Any noise at sufficient levels willsignificantly shift hearing thresholds, but allnoises at the same levels do not cause equiv­alent shifts. The important issue is whetherthe impact produces a recoverable (TTS ­Temporary Threshold Shift) or permanent(PTS - Permanent Threshold Shift) loss.

Major research efforts have been directed atunderstanding the relationships of frequen­cies, intensities, and duration of exposuresin producing damage; that is, what sounds,at what levels, for how long, or how oftenwill produce temporary versus permanenthearing loss. A comprehensive discussion ofthe complexities involved in TTS versus PTSis not possible in the scope of this paper.Excellent reviews of noise trauma mecha­nisms are available in Lehnhardt (986);Miller et aI. (987); and Liberman (990). On­ly the briefest, and therefore necessarily sim­plistic, introduction is given here. Two fun­damental effects are generally accepted:1. for pure tones, the loss centers around the

incident frequency.2. for all tones, at some balance of noise

level and time, the loss is irreversible.

TTS may be broad or punctate, according tosource characteristics, subject hearing sensi­tivity, and relative health of the ear impact­ed. The majority of studies have been con­ducted with cats and rodents, using rela­tively long duration stimuli (> 1 hr) and midto low frequencies 0-4 kHz) (Lehnhardt,1986). The extent and duration of suchthreshold shifts generally are proportional

Estimates of blast injury and acoustic trauma zones

to the extent of inner ear damage. Virtuallyall studies show that losses from short tomoderate term, narrowband stimuli « 1 hr,CF ± 2 kHz) are centered around the peakspectra of the stimulus and are largely re­coverable although recoverability is strong­ly influenced by individual sensitivity of thesubject (Henderson et a!., 1983; Miller et a!.,1987). Histologic data from impacted earsshowed very discrete, localized cell damageat short-duration exposures progressing towidespread lesions with increasing intensi­ty and bandwidth (Liberman, 1987). Recov­erable (TTS) to non- recoverable (PTS) losseswere characterized by a progression fromsimple stereocilia fatigue to loss of hair cellbodies to broadening patches of strial, liga­ment, and neuronal degeneration.

In the typical air-adapted ear, a pure tonewith an intensity 80 dB (re 20 IlPa) higherthan the normal minimum response thresh­old for that frequency is generally sufficientfor stereocilia and hair cell damage. Mostdamaged cells will recover from this impact,but repeated exposures compound the in­sult. Recovery periods can vary from hoursto weeks among individuals. This findingled to the current allowable limit of 90 dB re20 IlPa for human workplace exposures(Lehnhardt, 1986); i.e., a 90 dB limit for allsignals reduces the probability of encounter­ing a broadband signal with components 2':80 dB over threshold at the most sensitivefrequency range (500-5000 Hz) for humans.

Repeated exposures to TTS level stimuliwithout adequate recovery periods can in­duce permanent, acute threshold shifts(PTS). Liberman (1987) showed that losseswere directly correlated with levels of dam­age to hair cell stereocilia, the hair cell bod­ies, stria vascularis, the spiral ligament, andnerve fibers. The duration of a thresholdshift, is correlated with both the length of

time and the intensity of exposure. This doesnot, however, mean that TTS and PTS aresimply gradations of the same damagemechanisms. A major complication in anyTTS versus PTS comparison is that signalrise- time and duration of peak pressure aresignificant factors in PTS. If the exposure isshort, hearing is recoverable; if long, or has asudden, intense onset and is broadband,hearing, particularly in the higher frequen­cies, can be permanently lost. Experimental­ly, PTS is induced in air with multi-hourexposures to narrowband noise. In humans,PTS typically results accidentally from pro­tracted, repeated intense exposures (contin­uous occupational background noises) orsudden onset of intense sounds (repeatedgun fire). Sharp rise-time signals have beenshown also to produce broad spectrum PTSat lower intensities than slow onset signalsboth in air and in water (Lipscomb, 1978;Lehnhardt, 1986; Liberman, 1987).

TTS has been produced in humans with un­derwater sound sources at levels of 150 - 180dB re 1 IlPa for frequencies between 700 ­5600 Hz (Smith and Wojtowicz, 1985; Smithet al., 1988). These intensities are similar tothose that induce TTS in air in humans be­cause: a) intensity is a power ratio that de­pends on sound speed and density of themedium and b) the absolute value dependson the reference pressure used. The follow­ing calculations illustrate this difference:

1= pv = p2/epIail- = p2j(340 mlsee)(O.0013 glee)Iwata = p2j(1530 mlsee)(1.03 glee)

where I = intensity, v = vibration velocity, p= sound pressure, c = the speed of sound inthat medium, p = the density of the medium.Appropriate comparisons can be made interms of acoustic power (W1m2); however,in many texts the reference pressures will be

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D.R. Ketten

dB re 20 /-lPa for airborne sound versus dB rel/-lPa for in-water thresholds. Ears are essen­tially sound energy difference detectors. Ifan auditory percept depends on the samelevel of intensity in air and in water, theunderwater sound level that will produce anequivalent sensation must be 61.5 dB greaterthan its airborne counterpart; i.e., 150 dB re 1/-lPa "" 90 dB re 20 /-lPa.

Blast injurySimple, intensity related loss is not synony­mous with blast injury. Blast injuries can bedivided into three groups based on the se­verity of the symptoms:

1. MILD - RecoveryPain, vertigo, tinnitus, hearing loss, tym­panic tears

2. MODERATE - Partial lossOtitis media, tympanic membrane rup­ture, tympanic membrane hematoma, se­rum or blood in middle ear, dissection ofmucosa

3. SEVERE - Permanent loss or deathOssicular fracture or dislocation, round/oval window rupture, CSF leakage intomiddle ear, cochlear and saccular dam­age.

Moderate to severe losses result most oftenfrom blasts, extreme intensity shifts, andtrauma in which explosions or blunt cranialimpacts cause sudden, massive systemicpressure increases and surges of circulatoryor spinal fluid pressures (Schuknecht, 1993).Hearing loss in these cases results from aneruptive injury to the inner ear; i.e., with therarefactive wave of a nearby explosion, cere­brospinal fluid pressures increase and theinner ear window membranes blowout dueto massive surges in the inner ear fluids.

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Inner ear explosive damage frequently coin­cides with fractures in the bony capsule ofthe ear or middle ear bones and with ruptureof the eardrum. Although technically a pres­sure induced injury, hearing loss and theaccompanying gross structural damage tothe ear are more clearly thought of as theresult of the inability of the ear to accommo­date the sudden, extreme pressure differ­entials and over-pressures from the shockwave.

At increasing distance from the blast, theeffects of the shock wave lessen. Eventhough there is no overt tissue damage, milddamage with some permanent hearing lossoccurs (Burdick, 1981, in Lehnhardt, 1986).This type of loss generally is called anasymptotic threshold shift (ATS) because itderives from a saturation effect. Like TTS,the hair cells are damaged, but as in any PTS,no recovery takes place. Because ATS de­pends on complex interactions of rise- timeand waveform, not simply intensity at peakfrequency, asymptotic hearing losses typ­ically are broader and more profound thansimple PTS losses.

There is no well-defined single criterion forsublethal ATS (Roberto et al., 1989), but ear­drum rupture, which is common to all stagesof blast injury, has been moderately wellinvestigated. Rupture per se is not synony­mous with permanent loss; eardrum rup­tures have occurred at as little as 2.5 kPaoverpressure. However, the incidence oftympanic membrane rupture is strongly cor­related with distance from the blast (Kerrand Byrne, 1975). As frequency of ruptureincreases, so does the incidence of perma­nent hearing loss. In zones where > 50%tympanic membrane rupture occurred, 30%of the victims had long-term or permanentloss.

Estimates of blast injury and acoustic trauma zones

Recent experimental work showed thatweighted sound exposure level is a morerobust predictor of permanent loss thanpeak pressure (Patterson, 1991). Data withweighted levels are rare; overpressure dataare more common and are highly correlatedwith received levels (Roberto et al., 1989).Table 1 summarizes the experimental resultson overpressures associated with> 50% rup­ture. The data indicate complex and fast rise­time sounds cause ruptures at lower over­pressures than slow rise-time waveforms,and smaller mammals are injured by smalleroverpressures than larger animals. Of theanimals listed, sheep and pigs have ears ana­tomically closest to those of whales andseals. The air-based data for pigs and sheepimply that overpressures> 70 kPa are need­ed to induce 100% tympanic membrane rup­ture. However, cross-study comparisonsand extrapolations are risky because of rad­ically different experimental conditions aswell as differences in power transmission inair and water.

Data available for submerged terrestrial andaquatic vertebrates imply that lower pres­sures in water than in air induce serioustrauma (Myrick et al., 1989; Richardson et al.,1991). For submerged terrestrial mammals,lethal injuries occurred at overpressures>55 kPa (Yelverton, 1973, in Myrick et al.,1989; Richmond et aI., 1989). In a blast studyin Lake Erie using Hydromex, a TNT-deriv­ative explosive, the overpressure limit for100% mortality for fish was 30 kPa (McAnuffand Booren, 1976), suggesting overpres­sures between 30 and 50 kPa are sufficientfor a high incidence of severe blast injury.Minimal injury limits in both land and fishstudies coincided with overpressures of 0.5to 1 kPa.

Marine mammal blast injuriesVery few reports of blast induced trauma inmarine mammals detail injuries to the headregion. Bohne et al. (1985) reported on innerear damage in Weddell seals (Leptonychotesweddelli) that survived blasts, but they didnot have access to exposure levels or numberof exposures per animal. Scattered reports ofopportunistic examinations of animals ex­posed to large blasts include one on sea ot­ters (Enhydra lutris) with extensive traumafrom nuclear explosions (cited in Richard­son et al., 1991). The study concluded thatpeak pressures of 100-300 psi were invar­iably lethal.

Recently, several humpback whales (Mega­ptera novaeangliae) exposed to TOVEX blastswere shown to have severe blast injuries(Ketten et al., 1993; Lien et al., 1993). TOVEX,like Hydromex, is a TNT clone explosivesimilar to HBX-1 with an average detonationvelocity of 4,500 m/sec. Received levels inthe whales could not be calculated with con­fidence; however, the charge weights associ­ated with the injuries ranged between 1700to 5000 kg. The animals died within threedays of the blasts, and the extent of the in­juries implied they were close to the blastsite. Mechanical trauma in these ears includ­ed round window rupture, ossicular chaindisruption, bloody effusion of the peribullarspaces, dissection of the middle ear mucosawith pooled sera, and bilateral periotic frac­tures (Fig. 2). These observations are consis­tent with classic blast injuries reported inhumans, particularly with victims near thesource who had massive, precipitous in­creases in cerebrospinal fluid pressure andbrain trauma. There was no evidence of shipcollision or prior concussive injury in thesewhales, and no similar abnormalities werefound in ears from humpback whales notexposed to blasts.

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D.R. Ketten

Figure 2. A three-dimensional reconstruction from CT scans of an ear from a humpback whale (Megaptera71oveae71gliae) with blast injuries shows multiple fractures throughout the periotic, which are consistent withintracochlear blood. Blood, serum, and cellular debris of both intra- and extra-cochlear origin filled the middle earand surrounding peribullar region (reproduced with permission, Ketten et aI., 1993).

Table 1. Peak overpressures producing 50% tympanic membrane rupture (Phillips et aI., 1989; Richmond et aI.,1989; Bruins and Carwood, 1991)

Species

Sheep (Ovis spp.)

Pig (Sus spp.)Dog (Canis familiaris)

Monkey (Macaca mulatta)

Human (Homo sapiens)Rabbit (Sylvilagus spp.)

Guinea Pig (Cavia porcella)

400

Peakpressure

(kPa)

16575-12915278205296103172138-17257-34564155115

Waveform

Fast-rising, reflected shockComplexComplexFast-rising, reflected shockComplexSmooth-risingStaticFast-rising, reflected shockFast-rising, incident shockFast-rising, incedent shockComplexFast-rising, incident shockComplexFast-rising, incident shock

Estimates of blast injury and acoustic trauma zones

Discussion and conclusions

The few data points available for blast trau­ma in marine mammals indicate that ex­ternal and middle ear structural adapta­tions in cetaceans and pinnipeds that mayminimize barotrauma do not provide im­munity to blast trauma. Considering tli.esimilarities of seal and whale ears to landmammal ears, it is not surprising that ex­plosions, and the resulting intense transientsound field and shock wave, produce bothblast injury and asymptotic acoustic trau­ma in marine animals. More important,even though the whale ear is a fluid-to­fluid coupler, all marine mammals retainedan air-filled middle ear, which makes themsubject to all ranges of compressive-rare­factive blast injury.

The level of impact from blasts depends onboth an animal's location and, at outerzones, on its sensitivity to the residual noise.Important factors for trauma from explosivesources are the following:

1. Topography2. Proximity of ear to the source3. Anatomy and health of ear4. Charge weight and type5. Rise time6. Overpressure7. Pressure and duration of positive pres­

sure phase

In the specified model, topographic effectsare minimal. The bottom is effectively non­reflective; therefore, with a 100 m detona­tion, surface reflections are the primary in­terference source.

The health of individual ears that may beimpacted cannot be estimated in advance. Itis reasonable to assume an average distribu­tion.

HBX-1 has a rapid detonation velocity (8800m/sec), a relatively long peak pulse (5 to 10msec), and a complex waveform. It is effec­tively an instantaneous onset, high peakpressure, broad spectrum blast. Therefore,the explosive front and acoustic signature ofthe proposed charges (1200 and 10000 lbs)are likely to cause equivalent damage to allspecies in the target area; i.e., individualhearing ranges and thresholds are largelyirrelevant. High impedances and reflectionsat the air-sea boundary make calculations ofreceived levels for surface animals unrelia­ble, but it is possible that some individualsabove or at the boundary layer will be par­tially protected.

Overpressure is based on a scaled distancenormalized to charge weight in TNT equiv­alents (Bruins and Cawood, 1991) and is cal­culated as:

Z = s/m1/3

PoverlkPIl) = (11.3/z-186/z2+19210/z3)(101.3)

Poverlps;) = PoverlkPIl) /6.9

where Z =scaled distance; s =distance (cm);m = charge mass (g) in TNT equivalents;PoverlkPa) = overpressure (kPa);Poverlps;) = overpressure (psi).Peak pressure (psi) is calculated using astandard algorithm (Czaban et al., 1994):

P = (8.22)(103) [W033/s]1-15

where W = weight (kg) in TNT equivalentsand s = distance from source (m).

Table 2 compares these calculations for theo­retical maximal pressures and overpres­sures with semi-empirical peak pressure da­ta reported by Czaban et al. (1994) forHBX. Under stable field conditions, there is

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D.R. Ketten

Table 2. Pressure level estimates at the charge depth contour (100 m) for 1200 lb and 10000 lb HBX charges withsimultaneous detonations (dB and measured maximum pressure data excerpted from Czaban et al., 1994)

-1200 II:> Charge-

Distance dB Maximum Theoretical Scaled Overpressure Overpressure(m) pressure pressure distance (psi) (kPa)

(psi) (psi)

1 257 95700.0 96121.8 1.18 167753.13 1157496.5710 233 6790.0 6804.9 11.85 164.17 1132.75100 206 481.0 481.8 118.48 1.38 9.52500 188 75.5 75.7 592.42 0.27 1.891000 180 34.0 34.1 1184.83 0.14 0.964500 164 5.99 6.0 5331.75 0.03 0.215000 162 5.4 5.4 5924.17 0.03 0.1910000 155 2.4 2.4 11848.34 0.01 0.10

-10000 II:> Charge-

1 263 216000.0 216577.0 0.58 1426284.83 9841365.3410 237 15300.0 15332.5 5.82 1381.99 9535.76100 211 1080.0 1085.5 58.15 3.49 24.08500 193 170.0 170.5 290.77 0.55 3.811000 185 76.8 76.8 581.53 0.28 1.934500 168 13.6 13.6 2616.89 0.06 0.445000 167 12.1 12.1 2907.65 0.06 0.3910000 159 5.4 5.4 5815.31 0.03 0.20

no significant difference between the empir­ical and theoretical values for peak pressurefor either charge weight.

Although multiple parameters are associ­ated with both lethal and sublethal effects,studies of lethal or compulsory injury zonesfor fast-rise time, complex waveforms agreeon baseline criteria: 30-50 kPa peak over­pressure in water and/or> 180 dB re 20 IlPain air (240 dB re 1 IlPa in water) (McAnuffand Booren, 1976; Yelverton and Richmond,1981; Phillips et al., 1989: Richmond et al.,1989; Myrick et ai., 1989). Given the pressuredistributions calculated for HBX-l detona­tions at 100 m, the 100% lethal impact zonelimits for a 1200 lb source are between 40 m(absolute minimum -land mammal) and 300m (conservative estimate based on otter psidata).

402

For a 10000 lb charge, the equivalent min­ima-maxima limits for the killing ground is70 m to 800 m. Figures 3 and 4 illustrates thevariables that are most relevant to these limitestimates. For either charge, within 100 m ofthe source, death would likely be immediateor would follow in days as a result of concus­sive brain damage, cranial fractures, hemor­rhage, etc. for the majority of animals. Forthe remainder, permanent, profound deaf­ness from massive inner ear trauma wouldseriously impair the animal's ability to sur­vive (Lehnhardt, 1986; Bruins and earwood,1991; Ketten et al., 1993; Schuknecht, 1993).

The criteria for differentiating PTS or ATSzones from TTS are less clear. The transition­al lethal zones in which serious sublethalinjury predominates are estimated as 100­500 m (1200 lb) and 150-900 m (10000 lb).

Estimates of blast injury and acoustic trauma zones

overpressure (kPa) 1200 Ibs

l!!I - overpressure (kPa) 10000 Ibs

and even minimal injury is rare (Yelvertonand Richmond, 1981; Smith et al., 1985, 1988;Myrick et al., 1989; Roberto et al., 1989).Therefore, between 1-10 km of the source thepotential for any acoustic impact drops pre­cipitously (Figs. 3 and 4). Because intensitydecays exponentially, 5 km is a reasonablyestimate of a safe outer limit for the 10000 lbcharge for minimal, recoverable auditorytrauma versus 2 km for the 1200 lb charge.Figure 4 provides a schematic of all of thezones extrapolated from the data outlined.

403

100

Distance (m)1 0

100% lethalo~- for

mammalsfish .__. ... _.__

~~ 1000(/)/l)..a...~ 100o

Figure 3. Overpressures at the 100 m depth contour are plotted for each charge weight versus distance from source.The gray vertical bars show the differences in distance from source for the outer edge of 1200 vs 10000 lb chargezones based on the following categories: 100% lethal overpressure for submerged land mammals (57 kPa) and100% lethal overpressure for fish (30 kPa).

Beyond 500 meters and 900 m, the relativeincidence of PTS to TTS largely depends onindividual susceptibility. That is, the var­iables that will determine TTS vs PTS arehighly species dependent and the animalsare sufficiently mobile that no global, per­manent division of these intermediate zoneswill be correct.

There is general consensus in the literatureon the criteria for assigning an outer limit tothe mild/moderate TTS zone. Generally, 5­15 psi is accepted as the value at which TTS

D.R. Ketten

LI ,,1200 los. psi

ll1l 10000 Ibs. psi

100% lethalrange for otters

300 psi 100 psiQ)...~ 1000IIIl!!0..

100

0:'

1 0 100

Distance (m)1000

50% TMruptlire

Figure 4. Peak pressures at the 100 m depth contour are plotted for each charge weight versus distance from source.The gray vertical bars show the differences in distance from source for the outer edge of 1200 vs 10000 lb chargezones based on reported pressures for 100% mortality in otters (Enhydra lutris) from blast and minimal peakpressure for 50% incidence of eardrum rupture,

It must be recalled here that these conclu­sions are highly speculative. They dependlargely on limited anatomical comparisons.Depending upon its actual, and currentlyunknown, function, the same specializedstructure in a marine ear could aggravate orameliorate the effects of blasts. For example,dolphins have soft tissue sound conductionpathways and fibro-elastic tissue beds thatacoustically isolate the inner ear. Either ad­aptation could significantly attenuate or en­hance conduction of potentially damagingsounds. Without more explicit data, defin­itive guidelines for safe limits on underwa-

404

ter signals are not possible. Substantiallymore research is needed on all aspects ofmarine mammal hearing before wholly re­liable estimates, much less solid answers, areavailable.

Acknowledgements

Moira Brown (East Coast Ecosystems), Ran­dall Reeve, and Jan Czaban (NDHQ, Onta­rio) were instrumental in the creation of thisreport. Their assistance and encouragementare gratefully acknowledged, as is the coop-

Estimates of blast injury and acoustic trauma zones

>50%TTS ~

toNo Injury

"I

1 04

L. 'I

>50% TTSto ...~

No InjuryTransition

1200 Ib charge zones

10000 ib charge zones

Distance from source 1 000(m)10010

Figure 5. Impact zone summary for 1200 and 10000 lb charges. The 5 divisions shown represent distances from thesource in which a) 100% mortality is expected (lethal), b) permanent hearing loss is common and some mortality isexpected (sublethal), c) > 50% animals have some permanent hearing loss (all ranges of acoustic trauma), d) themajority of animals have temporary hearing loss but some permanent auditory damage will be found (transitionaltrauma), and e) little or no clinically detectable auditory damage (low impact). The zones are synthesized based onconservative extrapolations of data from fish, submerged terr~strial mammals, and humans.

eration of the Canadian National DefenseHeadquarters and Nodeco, Newfoundland.An early version of this paper was preparedat the request and with support from EastCoast Ecosystems, Quebec, and was submit­ted previously to Bio Acoustics. Original re­search on mysticete and odontocete ears andparticipation in this symposium was sup­ported by Office of Naval Research contractsN00014-92-J-4000 and N00014-93-1-0940.

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