M. D. Richardson · D. L. Lavoie · K. B. BriggsMarine Geosciences Division, Naval Research Laboratory,Stennis Space Center, MS 39529-5004, USA

Geo-Marine Letters (1997) 17 : 316—324 ( Springer-Verlag 1997

M. D. Richardson · D. L. Lavoie · K. B. Briggs

Geoacoustic and physical properties of carbonate sedimentsof the Lower Florida Keys

Abstract Near-surface sediment geoacoustic and physicalproperties were measured from a variety of uncon-solidated carbonate sediments in the Lower Florida Keys.Surficial values of compressional and shear speed corre-late with sediment physical properties and near-surfaceacoustic reflectivity. Highest speeds (shear 125—150 m s~1;compressional 1670—1725 m s~1) are from sandy sedi-ments near Rebecca Shoal and lowest speeds (shear40—65 m s~1; compressional 1520—1570 ms~1) are foundin soft, silty sediments which collect in sediment ponds inthe Southeast Channel of the Dry Tortugas. High com-pressional wave attenuation is attributed to scattering ofacoustic waves from heterogeneity caused by accumula-tion of abundant shell material and other impedancediscontinuities rather than high intrinsic attenuation.Compared to siliciclastic sediments, carbonate sedimentshear wave speed is high for comparable values of sedi-ment physical properties. Sediment fabric, rather thanchanges due to the effects of biogeochemical processes, isresponsible for these differences.

Introduction

Sediment structure is a direct result of the biological,geological, biogeochemical, and hydrodynamic processesthat operate at the benthic boundary layer (Richardson1994). This structure, in turn, determines sediment bulkproperties (e.g., porosity, grain size distribution, density,permeability, etc.), sediment behavior (e.g., geoacousticand rheologic) under various stress—strain conditions, andsediment acoustic properties such as propagation and

scattering of high-frequency acoustic sound within or atthe sea floor. Interrelationships among environmentalprocesses, sediment structure, and sediment propertiesand behavior are being studied and modeled for a varietyof shallow-water environments as part of the Office ofNaval Research’s Coastal Benthic Boundary Layer(CBBL) program (Richardson and Bryant 1996). Thiscontribution is part of a special volume dedicated topreliminary results of CBBL experiments in shallow-water, carbonate sedimentary environments of the LowerFlorida Keys.

For carbonate sediments, generalizations about rela-tionships between sediment physical and geoacousticproperties and environmental processes have been de-veloped mostly from deep-water studies of carbonateoozes (Morton 1975; Johnson et al. 1977; Mayer 1979;Demars 1982; Hamilton et al. 1982; Briggs et al. 1985).Geoacoustic and physical properties are profoundly affec-ted by predepositional environmental (climatic) con-ditions that determine surface productivity (speciesabundance and composition), dissolution (depth of thelysocline and the calcite composition depth), and dilutionof carbonate particles by terrigenous particles, pelagicclay, volcanic detritus, or other biogenic matter; by post-depositional biological and biogeochemical processes thatcontrol fragmentation, dissolution, precipitation, andcementation; and by near-bottom hydrodynamic pro-cesses that promote mass wasting, erosion, scour, or win-nowing. Given the complexity of deep-water carbonatesystems, prediction of sediment geoacoustic propertiesrequires knowledge of biological, geological, and bi-ogeochemical processes as well as sediment physicalproperties.

Predictive empirical relationships between sedimentphysical and acoustic properties have not been developedfor shallow-water carbonates, although well-establishedrelationships are available for shallow-water siliciclasticsediments (Hamilton and Bachman 1982; Bachman 1985,1989; Richardson and Briggs 1993). Shallow-water car-bonates normally undergo extensive postdepositional

Fig. 1 Experimental site locations with values of compressional andshear wave speeds for sites north of the Marquesas Keys and nearRebecca Shoal

biogeochemical diagenesis that includes dissolution, pre-cipitation, and subsequent cementation (Bathurst 1975,1993). Significant changes in surficial carbonate fabricresult from subaerial exposure, exposure to fresh water,micritization, bioturbation, and pore-water chemical cha-nges due to the breakdown of organic matter (Bathhurst1980; Walter et al. 1993; Furakawa et al., 1997). At theonset of this project, it was thought that cementationwould significantly increase sediment rigidity and de-crease sediment compressibility resulting in high shearand compressional speeds relative to unaltered sediments;however, neither appreciable precipitation or cementationis evident in these sediments.

In this paper, we present data on the spatial variationsof surficial geoacoustic and physical properties fromshallow-water carbonate sediments in the Lower FloridaKeys. Empirical predictive relationships among sedimentgeoacoustic and physical properties are compared to ana-logous relationships derived from siliciclastic sediments.The effects of the biogeochemical processes (dissolution,precipitation, and cementation), sediment fabric (particlecharacteristics, spatial arrangements), dilution, and grainsize distribution on sediment geoacoustic behavior areinvestigated.

Experimental setting

Sediments in this study come from a low-to-moderateenergy depositional basin (Dry Tortugas experiment site),a high energy shallow marine environment (Marquesasexperiment site), and a highly mobile, biogenic sand shoal

(Rebecca Shoal) all within the Lower Florida Keys(Fig. 1).

The Dry Tortugas experimental site, located in theSoutheast Channel of the Dry Tortugas in water depths of18—25 m, is approximately 130 km west of Key West andlies within the Fort Jefferson National Monument pro-tected area. The site is well protected from both theprevailing weather and the local trawlers. Bed stresses,primarily due to tidal currents, only occasionally exceedthe threshold for sediment transport (Wright et al. 1997).Thus, the Dry Tortugas site is a low-energy sediment sinkwith most sediments presumably being derived from theDry Tortugas Bank just to the north (Mallinson et al.1997). The Holocene sediments of the Dry Tortugas arevery poorly sorted sand—silt—clays composed of 85—95%carbonate material. Holocene sediment thickness, 2—5 m,thins southward to yield an outcropping of Key LargoLimestone and patch reefs (Fig. 2 top). Surficial sedimentbecomes coarser because of the contribution of particlesfrom the outcrop and patch reefs. Sediments primarilyconsist of aragonite plates and needles derived from thebreakdown of plates of aragonitic green algae (Halimeda,Penicillus, and ºdeota), molluscan shells, benthic andplanktonic foraminifera, echinoid spines, sponge andcoral fragments, diatoms, and less than 5% particles ofsiliciclastic origin (quartz and clay minerals) (Stephenset al. 1997; Briggs and Richardson 1997).

The Marquesas experiment site is located north of theMarquesas Keys in water depths ranging from 10 to 30 m.This area is exposed to the prevailing weather conditions,such as winter storms and unpredictable hurricanes. Sedi-ments in the Marquesas experiment area are derived fromerosion of the Marquesas Keys, a circular accumulation ofHalimeda sand spits and beaches approximately 30 kmwest of Key West, and become progressively finer ina northerly direction (Fig. 2 middle). Sediments, like thosein the Dry Tortugas site, are very poorly sortedsand—silt—clays, but with a greater percentage gravel-size

317

Fig. 2 Stratigraphic profiles using the Acoustic Seafloor Classifica-tion System (ASCS). These seismic records were collected at 4 kHz;depths are in meters below the transducer; and colors representsignal intensity in steps of 6 dB. Seismic profiles were collected in(top) the Dry Tortugas experimental site, (middle) the Marquesasexperimental site, and (bottom) the sea floor near Rebecca Shoal.Sites occupied by ISSAMS are indicated. Profiles corrected byD. Walter

molluscan shells (Briggs and Richardson 1997). Sedimentparticle types are similar to the Dry Tortugas site.

Between Rebecca and Half Moon Shoals, an areaof highly mobile Halimeda sand forms east—west ridgesup to 3 m high in response to strong tidal currents(Fig. 2 bottom). Sediments are well-sorted, mediumsand consisting primarily of Halimeda and otheralgae plates that are abundant on the shallow TheQuicksands west of Halfmoon Shoal (Shinn et al. 1990).The strong tidal currents probably winnow out any fine-grained material.

Methods

Values of near-surface sediment geoacoustic and physicalproperties were determined using in situ probes and fromlaboratory analysis of sediments collected with gravity,box, and diver-collected cores (Figs. 1 and 3). Data werecollected mostly during the February 1995 field season(Tooma and Richardson 1996) with stations 76, 87, and121 occupied during presite surveys in February 1994.

In situ sediment geoacoustic properties were measuredremotely using a hydraulically operated platform thatdrives geoacoustic probes into sediments (Griffin et al.1996) or a diver-deployed version of the same system(Barbagelata et al. 1991). Compressional wave speed andattenuation and shear wave speed were measured overpathlengths ranging from of 30 to 100 cm at depths of5—30 cm below the sediment-water interface. For com-pressional wave measurements, transmit pulses weredriven utilizing 38-kHz pulsed sine waves, and time delays

318

Fig. 3 In situ sampling sites nearthe Dry Tortugas (base map fromH. Fiedler, FWG) with values ofshear and compressional speeds

and voltages were used to determine values of speed andattenuation between identical radial-poled ceramic cylin-ders. Actual values of compressional wave speed andattenuation were calculated by comparison of receivedsignals transmitted through the sediment with thosetransmitted through seawater overlying the sediments.Attenuation measurement techniques do not account fordifferences in transducer sensitivity in water and sedimentor for other insertion losses. Comparison of attenuationmeasurements made by transposition, which eliminatesthe effects of differences in sensitivity and insertion loss,with the methods used in this paper suggest both theseeffects are small (0—10%) and within the natural variabil-ity surficial sediments (Richardson 1997). Compressionalwave speed is also reported as compressional wave velo-city ratio (ratio of in situ sediment sound speed to thesound speed of overlying water) and attenuation is ex-pressed as k, in units of decibels per meter per kilohertz.

Shear wave speed was measured as time-of-flight betweenbimorph bender elements mounted in flexible siliconerubber mounts and driven at 0.25 to 2.0 kHz.

After the cores were acoustically logged (Briggs andRichardson 1997; Stephens et al. 1997), sediments weresubsampled at 2-cm intervals for grain size distribution,grain density, porosity, and wet bulk density. Where pos-sible, sediment physical properties (Table 1) were derivedfrom the diver-collected cores as surficial sediments areless disturbed during diver-collection than by gravity co-ring. Porosity was measured at 2-cm intervals on thesediment core samples by weight loss from samples kept ina drying oven at 105°C for 24 h, or by weight—volumetechniques using the Quantachrome helium gas pycno-meters. Grain density was measured using a Quanta-chrome Penta-pycnometer. Sediment bulk density waseither calculated from the measured values of graindensity, porosity or directly determined from the

319

Table 1 Mean (and range) of sediment physical (porosity, density, mean grain size) and geoacoustic properties (compressional and shearwave speeds and compressional wave attenuation) for carbonate sediments in the Lower Florida Keys

Station V1

Attenuation V4

Wet bulk Porosity Mean grain(m s~1) (dBm~1) (m s~1) density (g cm~3) (%) size (phi)

76 1571 (1557—1587) 59.8 (53—70) 1.73 (1.52—1.83) 59.3 (53.7—70.7) 5.3 (4.7—6.7)87 1547 (1537—1561) 70.2 (70—71) 1.73 (1.57—1.86) 59.1 (52.0—67.6) 6.3 (4.6—8.4)

121 1532 (1506—1543) 56.8 (50—68) 1.73 (1.55—1.86) 56.8 (53.4—69.4) 6.5 (6.2—7.1)149 1541 (1533—1547) 19.1 (18—20) 1.77 (1.68—1.84) 56.6 (52.7—61.3) 5.9 (6.2—5.6)162 1563 (1562—1564) 23.7 (23—24) 70.6 (61—76) 1.79 (1.56—1.92) 55.9 (50.9—64.4) 5.0 (4.5—5.0)173 1536 (1534—1537) 23.1 (19—27) 55.1 (50—64) 1.75 (1.48—1.85) 59.3 (54.3—73.9) 5.6 (5.2—5.9)202 1549 (1546—1550) 19.9 (19—23) 59.1 (50—69) 1.78 (1.55—1.87) 63.1 (57.6—70.1) 6.6 (6.2—6.9)211 1542 (1533—1547) 19.0 (14—29) 51.3 (47—60) 1.79 (1.66—1.86) 57.5 (53.4—64.5) 6.2 (4.9—7.8)236 1537 (1524—1555) 12.9 (10—18) 40.2 (28—56) 1.77 (1.53—1.85) 57.9 (54.4—71.0) 6.8 (6.1—6.8)241 1545 (1526—1563) 12.7 (9—18) 52.5 (38—66) 1.77 (1.53—1.85) 57.9 (54.4—71.0) 6.8 (6.1—6.8)242 1544 (1532—1559) 12.3 (9—17) 52.1 (46—60) 1.78 (1.55—1.87) 63.1 (57.6—70.1) 6.6 (6.2—6.9)251 1541 (1530—1555) 14.7 (6—23) 57.7 (49—63) 6.8253 1558 (1544—1571) 13.9 (7—18) 73.5 (62—88) 5.9255 1581 (1566—1592) 23.2 (19—27) 92.8 (83—110) 5.2257 1551 (1537—1562) 14.3 (9—18) 72.8 (61—97) 1.75 (1.57—1.86) 58.1 (53.7—69.0) 6.1265 1672 (1642—1697) 28.8 (21—43) 98.8 (75—116) 2.00 (1.99—2.01) 45.3 (44.7—46.9) 1.1 (1.4—1.0)266 1576 (1546—1637) 19.9 (8—35) 78.2 (51—118) 1.76 (1.66—1.87) 57.8 (51.4—63.7) 6.5 (5.8—6.8)274 1573 (1552—1602) 15.4 (7—24) 75.1 (52—91) 1.84 (1.56—1.92) 53.2 (50.9—64.4) 6.1280 1536 (1518—1548) 10.8 (3—16) 54.3 (39—67) 1.75 (1.51—1.87) 59.0 (53.0—72.5) 6.9 (6.4—7.5)284 1567 (1551—1589) 14.6 (8—23) 85.1 (74—91) 1.85 (1.77—1.81) 53.8 (52.8—58.1) 5.9286 1600 (1589—1616) 23.2 (19—28) 82.2 (77—91) 3.4288 1697 (1668—1725) 18.1 (12—25) 143.8 (129—154) 2.02 (2.00—2.05) 43.7 (42.1—45.1) 1.3 (1.1—1.5)290 1708 (1684—1728) 25.5 (14—33) 129.6 (123—140) 2.06 (2.00—2.09) 41.5 (39.7—45.1) 1.2 (1.3—1.9)

weight—volume techniques. Sediment grain size distribu-tion was determined by dry sieving for gravel- and sand-sized particles and with a Micromeritics sedigraph forsilt- and clay-sized particles.

Results and discussion

The highest wave speeds (shear 125—150 ms~1; compres-sional 1670—1725 m s~1) were measured in sandy sedi-ments near Rebecca Shoal and the lowest (shear40—65 m s~1; compressional 1520—1570 ms~1) measuredfrom ponded, soft sediments near the Dry Tortugas(Figs. 1 and 3; Table 1). In the Dry Tortugas, where themajority of in situ measurements were made, wave speedsdecrease with depth of the ponded Holocenic sedimentsand distance from underwater reefs and increase withacoustic reflectivity (see Wever et al. 1997; Chotiros et al.1997; Haynes et al. 1997). The distribution of geoacousticproperties correlates with the areal distribution of surficialsediment physical properties, including grain size, poros-ity, and sediment bulk density, which supports the pro-posed links between environmental process, sedimentphysical properties, and sediment geoacoustic behavior.Sediment physical and geoacoustic properties in thesecarbonate sediments appear to be controlled primarily bydepositional processes, including proximity to the sourceof particles and to hydrodynamic processes that distributeparticles. Higher compressional and shear speeds are as-

sociated with higher energy environments and shorterdistances from the particle source.

Acoustic wave speeds increase with decreasing porosity,increasing bulk density, and increasing mean grain dia-meter following the general trends already reported byHamilton and Bachman (1982), Bachman (1985, 1989),and Richardson and Briggs (1993) for surficial, shallow-water sediments. However, detailed comparison of rela-tionships between values of physical and geoacousticproperties for the carbonate sediments with the well-es-tablished empirical relationships developed for siliciclasticsediments suggest fundamental differences in structure ofthese two types of sediments (Fig. 4; Table 2). These struc-tural differences should be traced to differences in effectsof biogeochemial, biological, hydrodynamic, and geologi-cal processes on sediment origin, deposition, and sub-sequent diagenetic changes in carbonate and siliciclasticsediments.

Shear wave speeds, a measure of the shear modulus orrigidity of sediments, are consistently higher in carbonatesediments than siliciclastic sediments for a given porosityor bulk density (Fig. 4a, b). Given the propensity forcementation to increase sediment rigidity in carbonatesediments (Bathhurst 1975), sediment microfabric wasexamined for evidence of cementation. TEM micrographsof sediment microfabric from these carbonate sedimentsshowed no evidence of inter- or intraparticle cementationup to depths of 1 m below the sea floor. Most sedimentsin the study site consist of a large percentage of frag-mented Halimeda plates in various stages of mechanical

320

Fig. 4 Relationships between sediment physical properties (poros-ity, bulk density, and mean grain size) and sediment geoacousticproperties (compressional wave velocity ratio, shear wave speedand compressional wave attenuation) for carbonate sediments in theLower Florida Keys. For comparison, regressions based on in situdata from siliciclastic sediments are included (Richardson andBriggs 1996; Richardson et al. 1991)

degradation. These plates are made up of aragoniteneedles (Fig. 5), which contain approximately 30% in-traparticle porosity. This intraparticle porosity probablyhas little effect on sediment rigidity if grain-to-grain con-tact primarily controls sediment rigidity. Based on TEMexamination of surficial sediments from ponded regions of

the Dry Tortugas study site, 10—15% of the total porosityis tied up within Halimeda plates (Lavoie 1997). Thisreduction of average porosity (10—15%) is sufficient toeliminate the differences between siliciclastic and carbon-ate sediment without affecting the shear wave speed versusmean grain size relationships (Fig. 4c). The reduction ineffective porosity should also decrease expected shearwave velocity for a given sediment bulk density. The highstandard deviation in empirical prediction of shear wavevelocity from porosity and density may be a function ofthe variation in percentages of skeletal material and thusintra- and interparticle porosity.

Compressional wave speeds (V1-ratio), primarily

a measure of bulk modulus or incompressibility, are only

321

Table 2 Predictive relationships between laboratory measured sediment physical and in situ geoacoustic properties for carbonate sedimentsof the Lower Florida Keys and for siliciclastic sediments

Relationship Siliciclastic sediments R2 Carbonate sediments R2

Vs vs. porosity y"0.0635x2!9.940x#399.006 0.918 y"0.149x2!19.509x#690.415 0.829Vs vs. wet bulk density y"225.910x2!612.945x#424.151 0.933 y"586.474x2!1981.68x#1731.49 0.805Vs vs. mean grain size y"0.0148x2!9.965x#118.546 0.91 y"0.281x2!13.847x#138.22 0.725Vp-ratio vs. porosity y"0.000138x2!0.020x#1.703 0.947 y"0.000304x2!0.0372x#2.150 0.948Vp-ratio vs. wet bulk density y"0.463x2!1.327x#1.917 0.933 y"1.0791x2!3.741x#4.257 0.948Vp-ratio vs. mean grain size y"0.000233x2!0.0188x#1.144 0.975 y"0.00212x2!0.034x#1.146 0.931Attenuation vs. porosity y"0.0244x2!3.699x#142.035 0.766 y"0.000442x2!0.0573x#2.275 0.289Attenuation vs. wet bulk density y"79.360x2!218.433x#151.68 0.735 y"2.282x2!7.954x#7.370 0.291Attenuation vs. mean grain size y"!0.381x2#1.019x#29.071 0.867 y"!0.0162x2#0.0798x#0.565 0.553

!Based on the data from Richardson et al. (1991) and Richardson and Briggs (1996)

Fig. 5 TEM micrographs offragments of Halimeda plates.Left: entire fragment showingboth inter- and intraparticleporosity, right: highmagnification reveals theporous nature of aragoniteneedles that make up thefragments of Halimeda plates

slightly higher in carbonate sediments than siliciclasticsediments for a given porosity and nearly the same fora given bulk density (Fig. 4 d,e). A reduction of 10—15% inporosity yields values of compressional wave speed lowerthan predicted for siliciclastic sediments. The higher com-pressional wave speeds probably result from the contribu-tion of increased shear modulus (shear wave speeds) fora given porosity and the much higher bulk modulus ofcarbonate grains (7.5]107 kPa) compared to siliciclasticsediment grains (3.6]107 kPa) (Sumino and Anderson1984).

Compressional wave attenuation in carbonate sedi-ments is higher than empirical predictions based on silicic-lastic data for given porosity or bulk density (Fig. 4g,h).Attenuation, as measured by pulse techniques, is a sum ofthe intrinsic attenuation (due to internal pore fluid viscos-ity and internal friction) and losses due to scattering. Theabundant clusters of coarse sand- and gravel-size shellsand other larger-sized heterogeneities found in most ofthese carbonate sediments (see Briggs and Richardson1997) scatter these short wavelength (&4 cm) compres-sional waves, which contributes to high measured-attenu-ation values.

Most sediments in the study areas are very poorlysorted, sand—silt—clays (standard deviation"3.5—4.5)consisting of a mixture of sand- and gravel-sized skeletalremains and silt- and clay-sized aragonite fragments andneedles derived by chemical and mechanical breakdownof larger coralline algae (Furakawa et al 1997). If thesediments are matrix supported (larger particles not incontact), sediment geoacoustic properties such as rigidityand compressibility should be controlled by properties ofthe matrix, i.e., the larger carbonate particles contribute tomean grain size values but not compressional and shearwave speeds. It follows that mean grain size predictiveregressions based on siliciclastic sediments would over-estimate wave speeds in carbonate sediments. In fact, thisis the case for compressional wave speed but not shearwave speed.

Conclusions

The distribution of geoacoustic properties correlateswith the areal distribution of surficial sediment physical

322

properties including grain size, porosity, and sedimentbulk density. Higher wave speeds are found in more ener-getic environments where coarser sized particles dominateand lower values of sediment porosity and density arefound. The distribution of sediment physical andgeoacoustic properties is primarily controlled by the bio-logical processes that generate carbonate skeletal materialand by hydrodynamic processes that erode, fragment, andredistribute those particles. Postdepositional bi-ogeochemical processes appear to have little effect onsurficial sediment physical and geoacoustic properties inthe Lower Florida Keys.

Empirical relationships between sediment physical andgeoacoustic properties for these carbonate sediments aresignificantly different than for siliciclastic sediments.Higher shear speeds relate to the high percentage of inter-particle porosity, the higher grain bulk modulus, and thevery poorly sorted nature of these carbonate sedimentscompared to siliciclastic sediments. High attenuation incarbonate sediments is probably the result of increasedscattering from heterogeneity caused by the accumulationof gravel- and sand-size shells and other impedance het-erogeneities caused by bioturbation rather than from dif-ferences in intrinsic attenuation.

Acknowledgments Ship support was provided by the ¼FS Planetfrom Forschungsantalt der Bundeswehr fur Wasserschall- und Geo-physik (FWG), the R/» Columbus Iselin from University of Miami,and the R/» Pelican from LUMCON. Special thanks is given toSean Griffin and Kevin Stephens for their support during in situmeasurements. This paper benefited from the careful reviews of N. P.Chotiros and M. J. Buckingham. The research was supported by theCoastal Benthic Boundary Layer program, funded by the Office ofNaval Research (ONR) and ONR program element N0601153N.The NRL contribution number is NRL/JA/7431-97-0002.

References

Barbagelata A, Richardson MD, Miaschi B, Muzi E, Guerrini P,Troiano L, and Akal T (1991) ISSAMS: An in situ sedimentgeoacoustic measurement system. In: Hovem JM, RichardsonMD, Stoll RD (Eds.), Shear Waves in Marine Sediments. Dor-drecht: Kluwer Academic Publishers. pp 305—312

Bachman RT (1985) Acoustic and physical property relationships inmarine sediments. Journal of the Acoustical Society of America78 : 616—621

Bachman RT (1989) Estimating velocity ratio in marine sediment.Journal of the Acoustical Society of America 86 : 2029—2032

Bathurst RGC (1975) Carbonate Sediments and Their Diagenesis.Developments in Sedimentology 12. New York: Elsevier. 657 pp

Bathhurst RGC (1980) Lithification of carbonate sediments. ScienceProgress Oxford 66 : 451—471

Bathhurst RGC (1993) Microfabrics in carbonate diagenesis: a criti-cal look at forty years of research. Deep Water Carbonates froma Microfabric Perspective. In: Rezak R, Lavoie D (Eds.), Carbon-ate Microfabrics. New York: Springer-Verlag. pp 3—14

Briggs KB and Richardson MD (1997) Small-scale fluctuationsin acoustic and physical properties. Geo-Marine Letters 17 :306—315

Briggs KB, Richardson MD, and Young DK (1985) Variability ingeoacoustic and related properties of surface sediments from theVenezuela Basin, Caribbean. Marine Geology 68 : 73—106

Chotiros NP, Altenburg R, and Piper J (1997) Analysis of acousticbackscatter in the vicinity of the Dry Tortugas. Geo-MarineLetters 17 : 325—334

Demars KR (1982) Unique engineering properties and compres-sional behavior of deep-sea calcareous sediments. In: DemarsKR, Chaney RC (Eds.), ASTM STP 777. American Society forTesting and Materials, Philadelphia. pp 97—112

Furakawa Y, Lavoie DL, and Stephens K (1997) Effect of bio-geochemical diagenesis on sediment fabric in shallow marinecarbonate sediments near the Dry Tortugas, Florida. Geo-Mar-ine Letters 17 : 283—290

Griffin SR, Grosz FB, and Richardson MD (1996) ISSAMS: A re-mote in situ sediment acoustic measurement system. Sea Techno-logy 37(4) : 19—22

Hamilton EL and Bachman RT (1982) Sound velocity and relatedproperties of marine sediments. Journal of the Acoustical Societyof America 68 : 1891—1904

Hamilton EL, Bachman RT, Berger WH, Johnson TC, and MayerLA (1982) Acoustic and related properties of calcareous deep-seasediments. Journal of Sedimentary Petrology 52 : 733—753

Haynes R, Huws DG, Davis AM, and Bennell JD (1997) Geophysi-cal sea-floor sensing in a carbonate sediment regime. Geo-Mar-ine Letters 17 : 253—259

Johnson TC, Hamilton EL, and Berger WH (1977) Physical proper-ties of calcarous ooze: control by dissolution at depth. MarineGeology 24 : 259—277

Lavoie DM (1997) Microfabric of Dry Tortugas and MarquesasSediment. In: Lavoie DL and Richardson MD (Eds.), Proceed-ings of the Key West Campaign Workshop, Stennis SpaceCenter, MS, Naval Research Laboratory Report NRL/MR/7431-97-8044. pp 183—192

Mallinson D, Locker S, Hafen M, Naar D, Hine A, Lavoie D, andSchock S (1997) A high resolution geological and geophysicalinvestigation of the Dry Tortugas carbonate depositional envi-ronments. GeoMarine Letters 17 : 237—245

Mayer LA (1979) Deep-sea carbonate: acoustic, physical and strati-graphic properties. Journal of Sedimentary Petrology 49 :819—836

Morton RW (1975) Sound velocity in carbonate sediments from theWhiting Basin, Puerto Rico. Marine Geology 19 : 1—17

Richardson MD (1994) Investigating the coastal benthic boundarylayer. EoS 75 : 201—206

Richardson MD (1997) Attenuation of shear waves in near-surfacesediments. In: Pace NG, Pouliquen E, Berquem O, and Lyows,AP (Eds.), High Frequency Acoustics in Shallow Water.Saclamanton Conference Proceedings Letters CP-45. Nato Sac-lamant Underwater Research Centre, La Speria, Italy.pp 451—457

Richardson MD and Briggs KB (1993) On the use of acousticimpedance values to determine sediment properties. In: PaceNG, Langhorne DN (Eds.), Acoustic Classsification and Map-ping the Seabed. Institute of Acoustics, University of Bath, Bath,UK. pp 15—25

Richardson MD and Briggs KB (1996) In-situ and laboratorygeoacoustic measurements in soft mud and hard-packed sandsediments: implications for high-frequency acoustic propagationand scattering. Geo-Marine Letters 16 : 196—203

Richardson MD and Bryant WR (1996) Benthic boundary layerprocesses in coastal environments: an introduction. Geo-MarineLetters 16 : 133—139

Richardson, MD, Muzi E, Troiano L, and Miaschi B (1990) Sedi-ment shear waves: a comparison of in situ and laboratorymeasurements. In: Bennett RH, Bryant WR, and Hurlbert MH(eds), Microstructure of Fine Grained Sediments. New York:Springer-Verlag. pp 403—415

Richardson MD, Muzi E, Miaschi B, and Tugutcan F (1991)Shear wave gradients in near-surface marine sediments. In:Hovem, JM, Richardson MD, Stoll RD (Eds.), Shear Waves inMarine Sediments. Drodrecht: Kluwer Academic Publishers.295—304

323

Shinn EA, Lidz BH, and Holmes CW (1990) High-energy carbonate-sand accumulation, the Quicksands, Southwest Florida Keys.Journal of Sedimentary Petrology 60 : 952—967

Stephens KP, Fleischer P, Lavoie D, and Brunner C (1997) Scale-dependent physical and geoacoustic property variability of re-cent, shallow-water carbonate sediments from the Dry TortugasKeys, Florida. Geo-Marine Letters 17 : 299—305

Sumino Y and Anderson AL (1984) Elastic constants of minerals.In: Carmichael RS (Ed.) Handbook of Physical Constants ofRocks, Volume III. Boca Raton, Florida: CRC Press. pp 39—138

Tooma SJ and Richardson MD (1996) The Key West campaign. SeaTechnology 36 : 17—25

Walter LM, Bischof SA, Patterson WP, and Lyons TW (1993)Dissolution and recrystallization in modern shelf carbo-nates: evidence from pore water and solid phase chemistry.Philosophical Transactions of the Royal Society of London344 : 27—36

Wever TK, Fiedler HM, Fechner G, Abegg F, and Stender IH (1997)Side-scan and acoustic subbottom characterization of the sea-floor near the Dry Tortugas, Florida. Geo-Marine Letters17 : 246—252

Wright LD, Friedrichs CT, and Hepworth DA (1997) Effects ofbenthic biology on bottom boundary layer processes, Dry Tor-tugas Bank, Florida Keys. Geo-Marine Letters 17 : 291—298

.

324