Measurement of Attenuation and Speed of Sound in Soils

required. These basic properties of the soil can then be evaluated to assess the acoustic imaging tradeoffs for The potential application of this work is the detection and imaging detecting and characterizing buried artifacts. The acousof buried objects using acoustic methodology. To image buried artifacts, it is vital to know speed and attenuation of sound in the particular tic attenuation coefficient is used to assess the tradeoff soil being examined because they vary in different soil types and at between imaging depth and resolution. The acoustic different moisture contents. To that end, our research involved six propagation speed is used to assess resolution and to soils representing a range of properties expected to influence acoustic evaluate the acoustic impedance for transducer design response. Clay ranged from 2 to 38%, silt from 1 to 82%, sand from considerations. 2 to 97%, and organic matter from 0.1 to 11.7%. Signals from an Acoustic wave propagation has been utilized to image acoustic source were passed through soil samples and detected by an and characterize properties of different porous materiacoustically coupled hydrophone. From a total of 231 evaluations, we als. Ultrasonic waves have been used to image and chardetermined the acoustic attenuation coefficient and the propagation acterize living tissues. Seismic waves of frequency below speed of sound in the soil samples as a function of four levels of 100 Hz have been used to explore deep into the earth. soil moisture and two levels of compaction. Attenuation coefficients determined over frequencies of 2 to 6 kHz ranged from 0.12 to 0.96 The choice of acoustic frequency depends on the pardB cm 1 kHz 1. Lower attenuation tended to be in loose dry samples. ticular media being examined. To obtain significant resCorrelation coefficients were 0.35 (P 0.01) and 0.31 (P 0.03) olution of objects at depths less than a meter in soils, between attenuation and soil water content and soil bulk density, acoustic frequencies on the order of 0.5 to 6 kHz should respectively. Propagation speeds ranged from 86 to 260 m s 1. The be used. correlation coefficient with speed was 0.28 (P 0.05) for soil water The behavior of sound propagation in porous media content and 0.42 (P 0.002) for total porosity. Given the acoustic is described by the Biot theory (Biot, 1956a,b), which properties, it is theoretically possible to detect an object down to 40 predicts the propagation of two compressional waves cm below the soil surface. and a shear wave in a porous medium. The existence of these waves in a porous material was confirmed in the experiments of Plona (1980). The first compressional L buried artifacts is a concern to large landwave is characterized by particle motion in phase with owners such as the defense department because the fluid motion. The second compressional wave is they are required to protect archaeological and cultural characterized by particle motion out of phase with the sites in their vast landholdings. Once a cultural or arfluid motion. The first wave is often referred to as the chaeological resource site is identified, it must then be fast compressional wave because it generally has a assessed to determine its significance and eligibility for higher speed than the wave of the second type, or slow National Registry of Historic Places (Executive Order compressional wave. Also, the slow compressional wave 11593). A Phase II eligibility assessment currently costs generally has a much higher attenuation than the fast about $10 000 to $30 000 per site. Given that there are compressional wave. The shear wave has the slowest 120 000 archaeological sites in the U.S. Army alone, speed and greatest attenuation. the cost of complete Phase II assessments is prohibitive. Saturation levels in a porous material have been Therefore, there is an urgent need to significantly reduce shown to affect the speed and attenuation of compresthe cost of data recovery at sites with an unknown probasional and shear waves (Tittmann et al., 1980; Velea bility of containing significant cultural or archaeological et al., 2000). The slow wave is difficult to observe in resources. A method that would detect buried artifacts sediments or soils saturated with water because of the from the surface would avoid the expense and complicahigh attenuation and the way in which the sound is tions that excavation causes. We propose an acoustic coupled at the soil or sediment interface (Stoll and Kan, system that could detect and classify buried artifacts 1981). Claims of observing the propagation of a slow (Frazier et al., 2000). As a necessary prelude to the wave in sediments have been made by Chitiros (1995). development of such a system, basic acoustic properties Recent studies by Thorsos et al. (2000) have indicated for the production, detection, and processing of acoustic that the supposed slow wave measurements by Chitiros signals in soils need to be determined. may actually be because of scattering from roughness Before an acoustic imaging system can be designed, at the water-sediment interface. Whether or not the slow an understanding of basic properties of acoustic propawave can be measured in sediments is still questionable, gation in various soils and in different soil conditions is however, if the slow wave does exist in water-saturated soils or sediments its effect is small. M.L. Oelze and W.D. O’Brien, Jr., Dep. of Electrical and Computer As the fluid in the pore space becomes less viscous, Engineering, Univ. of Illinois, Urbana, IL 61801; and R.G. Darmody, Dep. of Natural Resources and Environmental Sciences, Univ. of Abbreviations: ADA, Adrian soil; CAB, Catlin soil; COLE, coeffiIllinois, 1102 S. Goodwin Ave., Urbana, IL 61801. Received 4 Jan. cient of linear extensibility; DRA, Drummer soil; MEA, Medway soil; 2001. *Corresponding author ([email protected]). NRL, Naval Research Laboratory; PLA, Plainfield soil; SAC, Sable soil; TOF, time of flight. Published in Soil Sci. Soc. Am. J. 66:788–796 (2002).