546 IEEE JOURNAL OF OCEANIC ENGINEERING, VOL. 27, NO. 3, JULY 2002 Fine-Scale Volume Heterogeneity Measurements in Sand Dajun Tang, Kevin B. Briggs, Kevin L. Williams, Darrell R. Jackson, Eric I. Thorsos, and Donald B. Percival Abstract—As part of the effort to characterize the acoustic en- vironment during the high frequency sediment acoustics experi- ment (SAX99), fine-scale variability of sediment density was mea- sured by an in situ technique and by core analysis. The in situ mea- surement was accomplished by a newly developed instrument that measures sediment conductivity. The conductivity measurements were conducted on a three-dimensional (3-D) grid, hence providing a set of data suited for assessing sediment spatial variability. A 3-D sediment porosity matrix is obtained from the conductivity data through an empirical relationship (Archie’s Law). From the porosity matrix, sediment bulk density is estimated from known average grain density. A number of cores were taken at the SAX99 site, and density variations were measured using laboratory tech- niques. The power spectra were estimated from both techniques and were found to be appropriately fit by a power-law. The expo- nents of the horizontal one-dimensional (1-D) power-law spectra have a depth-dependence and range from 1.72 to 2.41. The vertical 1-D spectra have the same form, but with an exponent of 2.2. It was found that most of the density variability is within the top 5 mm of the sediment, which suggests that sediment volume variability will not have major impact on acoustic scattering when the sound fre- quency is below 100 kHz. At higher frequencies, however, sediment volume variability is likely to play an important role in sound scat- tering. Index Terms—Acoustic scattering, seafloor, sediment density, sediment porosity, velocity measurement. I. INTRODUCTION O NE of the major goals of the sediment acoustics experi- ment (SAX99) [1], [2] is to investigate the mechanisms responsible for scattering high-frequency sound from sandy seafloors. The hypothesized mechanisms include scattering from seafloor roughness and scattering from sub-bottom volume heterogeneity [1], [2]. In past studies some investi- gators have focused attention on the potential contribution of sediment volume scattering to bottom backscattering in their field measurements, either by appealing to physical models for volume scattering [3], [4], or by attributing poor agreement between acoustic model predictions and measured backscat- tering data at small grazing angles to lack of consideration of sediment volume scattering [5]–[8]. Scattering from the sediment volume is created by spatial variability in sediment density or sound velocity. Characterizing this heterogeneity Manuscript received December 3, 2001; revised February 7, 2002. This re- search was supported in part by the Office of Naval Research and in part by the Naval Research Laboratory Program Element 0601153N. D. Tang, K. L. Williams, D. R. Jackson, E. I. Thorsos, and D. B. Percival are with the Applied Physics Laboratory, University of Washington, Seattle, WA 98105 USA. K. B. Briggs is with the Seafloor Sciences Branch, Naval Research Labora- tory, Stennis Space Center, MS 39529-5004 USA. Publisher Item Identifier S 0364-9059(02)06419-1. within the sediment fabric is therefore essential for accurate model predictions of scattering from the sediment volume [3], [5], [8]. In perturbation theory [3], [4], [8] the natural statistical characterization of heterogeneity is through three-dimensional (3-D) spatial power spectra for sound-speed spatial fluctuations, or equivalently, compressibility fluctuations, and for density fluctuations. In addition, the cross-correlation between density and compressibility is also an input to acoustic scattering models [8]. Issues concerning the cross correlation will be investigated in a future paper. In this paper, we describe measurements and models of surfi- cial sediment heterogeneity. Such variability must be measured at the acoustic wavelength scale in order that backscattering from the heterogeneity can be quantitatively modeled. Two ap- proaches to the measurement of surficial sediment heterogeneity are described. The first approach is a laboratory analysis of sediment sound-speed and density variability in cores collected during SAX99. The second is an in situ measurement of sedi- ment density variability through determination of electrical con- ductivity variability within the sediment using a system called in situ measurement of porosity (IMP). The results from the two approaches complement each other and, along with other envi- ronmental and acoustic backscatter measurements collected in the experiment, constitute a data set capable of validating pre- dictions of acoustic backscatter. Because a major impetus of this study is to provide input to acoustics models, we have conducted extensive analysis of the data to obtain statistical results of the sediment density, in- cluding mean profiles, variances, and power spectra of fluctua- tions in both vertical and horizontal directions. II. CORE DATA ANALYSIS A. Methodology Cores were collected at several locations to assess the variability of surficial geoacoustic properties for the SAX99 site. Cores for geoacoustic and physical property measurements were collected with 6.1-cm diameter, polycarbonate plastic, cylindrical core liners. The inner diameter of the core liner is 5.9 cm. Each core liner was beveled at one end to facilitate the careful manual penetration into the sediment. Cores were capped at both ends immediately upon collection to retain the water overlying the sediment and were kept in an upright position during transport to the laboratory for analysis. Collec- tion, measurement, and handling procedures were designed to minimize sampling disturbance and to maintain an intact sed- iment-water interface within the core samples. Measurement of sediment sound speed was made within 24 h of collection, 0364-9059/02$17.00 © 2002 IEEE