Showing papers on "Attenuation published in 1979"
TL;DR: In this article, correction factors for attenuation of beta radiation by spherical quartz grains embedded in ceramics were obtained for grain diameters ranging from 0.005 to 10.0 mm.
Abstract: Correction factors for attenuation of beta radiation by spherical quartz grains embedded in ceramics were obtained for grain diameters ranging from 0.005 to 10.0 mm. The calculation was based on the absorbed-dose distributions for point-isotopic beta sources listed by Berger, in conjunction with the scaling procedure developed by Cross, and included beta particles, internal conversion electrons, and Auger electrons emitted by /sup 40/K, and the /sup 232/Th and /sup 238/U series.
861 citations
TL;DR: In this article, the attenuation of compressional (P) and shear (S) waves in dry, saturated, and frozen rocks is measured in the laboratory at ultrasonic frequencies.
Abstract: The attenuation of compressional (P) and shear (S) waves in dry, saturated, and frozen rocks is measured in the laboratory at ultrasonic frequencies. A pulse transmission technique and spectral ratios are used to determine attenuation coefficients and quality factor (Q) values relative to a reference sample with very low attenuation. In the frequency range of about 0.1–1.0 MHz, the attenuation coefficient is linearly proportional to frequency (constant Q) both for P‐ and S‐waves. In dry rocks, Qp of compressional waves is slightly smaller than Qs of shear waves. In brine and water‐saturated rocks, Qp is larger than Qs. Attenuation decreases substantially (Q values increase) with increasing differential pressure for both P‐ and S‐waves.
637 citations
TL;DR: In this article, several hypothesized attenuation mechanisms are discussed in relation to published data on the effects of pressure and fluid saturation on attenuation, including friction, fluid flow, viscous relaxation, and scattering.
Abstract: Theoretical models based on several hypothesized attenuation mechanisms are discussed in relation to published data on the effects of pressure and fluid saturation on attenuation. These mechanisms include friction, fluid flow, viscous relaxation, and scattering. The application of these models to the ultrasonic data of Toksoz et al (1979, this issue) indicates that friction on thin cracks and grain boundaries is the dominant attenuation mechanism for consolidated rocks under most conditions in the earth’s upper crust. Increasing pressure decreases the number of cracks contributing to attenuation by friction, thus decreasing the attenuation. Water wetting of cracks and pores reduces the friction coefficient, facilitating sliding and thus increasing the attenuation. In saturated rocks, fluid flow plays a secondary role relative to friction. At ultrasonic frequencies in porous and permeable rocks, however, Biot‐type flow may be important at moderately high pressures. “Squirting” type flow of pore fluids from...
503 citations
TL;DR: In this paper, a model is presented to describe the attenuation of seismic waves in rocks with partially liquid-saturated flat cracks or pores, where the presence of at least a small fraction of a free gaseous phase permits the fluid to flow freely when the pore is compressed under wave excitation.
Abstract: A model is presented to describe the attenuation of seismic waves in rocks with partially liquid‐saturated flat cracks or pores. The presence of at least a small fraction of a free gaseous phase permits the fluid to flow freely when the pore is compressed under wave excitation. The resulting attenuation is much higher than with complete saturation as treated by Biot. In general, the attenuation increases with increasing liquid concentration, but is much more sensitive to the aspect ratios of the pores and the liquid droplets occupying the pores, with flatter pores resulting in higher attenuation. Details of pore shape other than aspect ratio appear to have little effect on the general behavior provided the crack width is slowly varying over the length of the liquid drop.
452 citations
TL;DR: In this paper, the authors measured attenuation and velocities in resonating bars of Massilon sandstone at various degrees of saturation and found that shear energy loss simply increases with degree of saturation, whereas bulk compressional energy loss increases to ∼95% saturation and then rapidly decreases as total saturation is achieved.
Abstract: Seismic attenuation and velocities were measured in resonating bars of Massilon sandstone at various degrees of saturation. Whereas shear energy loss simply increases with degree of saturation, bulk compressional energy loss increases to ∼95% saturation and then rapidly decreases as total saturation is achieved. This behavior is analogous to the behavior of shear and compressional velocities, but the effect on attenuation is larger by an order of magnitude. Our observations are in excellent agreement with the predictions of several models of energy loss involving partial or total saturation. Pore fluid attenuation mechanisms are expected to be dominant at least in the shallow crust.
352 citations
TL;DR: In this article, the attenuation coefficient of fixed and fresh bovine and human soft tissues for various temperatures in the range 5 − 65°C was determined experimentally in both fresh and fixed Bovine soft tissues.
Abstract: Ultrasonic attenuation in the frequency range 1–7 MHz, and the speed of sound, were determined experimentally in both fresh and fixed bovine and human soft tissues for various temperatures in the range 5–65°C. At temperatures below 40°C the attenuation coefficient behaves similarly for fixed and fresh tissues where, at high frequencies, it has a negative dependence on temperature, the value at 20°C being about 21% higher than that at 37°C. As the frequency is reduced, the temperature coefficient of attenuation progressively decreases until, after passing a transition frequency (this varies with the tissue specimens but is around 1–2 MHz), a positive dependence on temperature may be observed. At temperatures above about 40°C, the attenuation coefficient of freshly excised tissues increases with temperature, whereas for fixed tissues the attenuation coefficient continues to decrease. These observations help to resolve a possible discrepancy evident in previous reports of the temperature dependence of attenuation. The speed of sound in non-fatty tissues increases with temperature and exhibits a maximum at around 50°C, while for fatty tissues a negative dependence is observed. The implications of this result for improved diagnostic procedures is discussed.
344 citations
TL;DR: In this paper, an acoustic Fabry-Perot interferometer is used for measuring high-frequency acoustic losses in isotropic and anisotropic materials with high Q-factor.
Abstract: An acoustic Fabry-Perot interferometer is used for measuring high-frequency acoustic losses in isotropic and anisotropic materials with high Q-factor. Interferometer samples of the investigated materials were fabricated in the form of cubes. The reduction in interferometer contrast caused by acoustic losses is used for determining the attenuation coefficient. The spectral distribution of the acoustic field in the interferometer is measured by means of an acoustooptic method. The attenuation coefficient is determined from the ratio ImaX/Imin and the additional losses of acoustic energy causing a reduction of the contrast are taken into account. The results obtained in this way are used for a determination of the attenuation coefficient for waves propagating in fused quartz as well as in a-quartz.
272 citations
TL;DR: It was found that, due to the finite width of detector responses, compensation of scan measurements by measured attenuation correction factors does not completely correct for attenuation, and statistical noise in measured attenuations correction factors can significantly increase noise levels in images, resulting in lower effective total counts for the emission scan.
Abstract: Effects of inaccurate attenuation correction on quantitation in positron emission computed tomography were investigated by both computer simulation and theoretical analysis, followed by experimental verification with phantom measurements It was found that, due to the finite width of detector responses, compensation of scan measurements by measured attenuation correction factors does not completely correct for attenuation For FWHM greater than 18 cm, errors in object size and in activity levels of reconstructed images are more than 4% Mismatches between true and assumed attenuation media are found to give large errors These mismatches include object shift, incorrect size and shape, uniform and nonuniform mismatches in attenuation coefficients, and nonzero background levels Errors as large as 14% in the reconstructed activity level can be introduced by a 5 mm shift in object position A 5% error in the cylinder diameter can give differences between the total reconstructed activity and true value more than 12% Strong image distortions, which could lead to incorrect interpretations of images are created by object shift and by nonuniform mismatches in attenuation coefficient It was also found that statistical noise in measured attenuation correction factors can significantly increase noise levels in images, resulting in lower effective total counts for the emission scan Effective total counts can be estimated as Neff = kNxmNem/(Nem + kNxm), where Nem and Nxm are total counts in emission and transmission scans, and k is a scanner dependent factor, which accounts for configuration differences
268 citations
TL;DR: In this article, it was shown that seismic energy loss caused by grain boundary friction is important only at low confining pressure and at strains greater than about 10−6, since these conditions are generally not encountered in seismology, frictional attenuation is not important in situ.
Abstract: Precise experimental results, combined with theoretical predictions, indicate that seismic energy loss caused by grain boundary friction is important only at low confining pressure and at strains greater than about 10−6. Since these conditions are generally not encountered in seismology, frictional attenuation is not important in situ. Other mechanisms such as fluid flow must dominate seismic attenuation in the upper crust.
236 citations
TL;DR: In this paper, a relaxation spectrum is found which satisfies the frequency dependence of laboratory Q and the time dependence of transient creep data, which makes it possible to provide a physical interpretation of the parameters in Jeffrey's modified Lomnitz creep function.
Abstract: For most solids the ‘high temperature background’ attenuation dominates at low frequencies and temperatures greater than about one-half the melting temperature. It is likely to be important in the mantle at seismic frequencies. The same mechanism also contributes to transient creep at low stresses and low total strains. A relaxation spectrum is found which satisfies the frequency dependence of laboratory Q and the time dependence of transient creep data. This makes it possible to provide a physical interpretation of the parameters in Jeffrey's modified Lomnitz creep function.
Q is predicted to increase as ω^α in the lower Q regions of the mantle. At high and low frequencies Q should increase as ω and ω^(−1), respectively. The location of the ω^α band depends on temperature and therefore shifts with depth. At high temperatures, seismic waves are on the low-frequency side of the absorption band and Q decreases with frequency. Far from the melting point and at sufficiently high frequencies Q should increase linearly with frequency. We use Chandler wobble, tidal and free oscillation data to estimate that α is ∼ 1/5 to 1/3, consistent with laboratory measurements of transient creep and internal friction at high temperature. A preliminary attempt is made to estimate the transient creep response of the mantle from Q measurements. The inferred viscosity agrees well with direct measurements.
The effect of anelasticity is to lengthen the calculated period of the Chandler wobble by 5-20 days, depending on the Chandler wobble Q. A Q of 300 for the wobble, which is within the experimental uncertainty of recent determinations, gives the observed period after correcting for the effect of the oceans.
199 citations
TL;DR: In this paper, the propagation theory of multimode and single-mode optical waveguide fibers is reviewed, including basic propagation theory, influence of the glass medium on attenuation and pulse dispersion, and the effects of perturbations of the waveguide's geometry and index profile.
Abstract: The propagation theory of multimode and single-mode optical waveguide fibers is reviewed. The subjects reviewed include basic propagation theory, the influence of the glass medium on attenuation and pulse dispersion, and the effects of perturbations of the waveguide's geometry and index profile.
TL;DR: In this paper, it is shown that the important dynamic parameters can be derived from a knowledge of the permeability, grain size, and porosity of a fluid-saturated porous medium.
Abstract: Based on Biot’s theory for the propagation of sound in a fluid‐saturated porous medium, the viscous attenuation of sound has been studied both theoretically and experimentally. It is shown that the important dynamic parameters can be derived from a knowledge of the permeability, grain size, and porosity. Measurements in uniform sized beads show attenuation and velocity dispersion as a function of frequency in agreement with the theory. A comparison between theoretical viscous attenuation and earlier reported measurements indicates that viscous losses may be of significant importance for higher frequencies in sands of high permeability.
TL;DR: In this article, a cyclic subgradient projection (CSP) method was proposed to calculate simultaneously attenuation and activity concentration coefficients, and some experimental results in its favor are presented.
Abstract: In order to obtain truly quantitative reconstruction of gamma-emitter concentration in Emission Computerized Tomography (ECT) the attenuation within the region of interest has to be accounted for. In this report we propose to calculate simultaneously attenuation and activity concentration coefficients. Starting from the system of non-linear equations which describes the model of single photon gamma-ray emission in a discretized form we show how to reduce it to a mixed convex-concave feasibility problem and apply to it our method of Cyclic Subgradient Projections (CSP). This approach is discussed and some experimental results in its favor are presented.
TL;DR: In this article, Dutta and Ode' proposed an approximate theory for the calculation of attenuation and dispersion of compressional seismic waves in porous rocks filled mostly with brine but containing gas-filled regions.
Abstract: In two important papers, J.E. White and coauthors (White, 1975; White et al, 1976) have given an approximate theory for the calculation of attenuation and dispersion of compressional seismic waves in porous rocks filled mostly with brine but containing gas-filled regions. Modifications of White's formulas for k/sub E/ and Q in the case of gas-filled spheres brings the results into good agreement with the more exact calculations of Dutta and Ode' (1979 a, b, this issue), who used Biot's theory for porous solids. In particular, the modified formulas give the expected Gassman-Wood velocity at very low frequencies. Inclusion of the finite gas compressibility in numerical calculations for gas-filled spheres shows an interesting maximum of the attenuation at low gas saturations which is not seen if the gas is ignored. A comparison of the attenuation calculated for the same rock and fluids but for three different geometries of the gas-filled regions suggests that the configuration of the gas-filled zones does not have an important effect on the magnitude of the attenuation.
TL;DR: In this paper, a new analysis of the propagation of acoustic pulses produced by local heating of liquids due to ionization by charged particles is presented, and it is shown that the wave equations with loss dominate the pulse shape after small distances and that a net observed pulse is just the time derivative of the received density of pulses from individual heating centers.
Abstract: A new analysis of the propagation of acoustic pulses produced by local heating of liquids due to ionization by charged particles is presented. It is shown that the wave equations with loss dominate the pulse shape after small distances and that, due to the bipolar $\ensuremath{\delta}$-function behavior of the individual pulses, a net observed pulse is just the time derivative of the received density of pulses from individual heating centers. Angular distributions, signal-to-noise ratios, detectable volume, and numerical examples are discussed. One important observation is that the effect of attenuation upon this type of radiation is to produce power-law rather than exponential cutoff with distance. For example, in the thermal-noise-limited case the signal-to-noise ratio defined herein only steepens by one-half power in falloff with distance due to attenuation.
TL;DR: In this article, a general description of internal surfaces is considered which allows for irregularities and partially closed cracks, leading to a Q−1 that increases with strain amplitude and suggests that under in situ (small strain) conditions, frictional attenuation becomes secondary to linear loss mechanisms.
Abstract: Frictional sliding on crack surfaces and grain boundaries is examined as a mechanism of wave attenuation. In contrast to previous work based on idealized elliptic crack models a general description of internal surfaces is considered which allows for irregularities and partially closed cracks. This leads to Q−1 that increases with strain amplitude. Such an amplitude dependence is often observed in large-strain laboratory measurements. This suggests that under in situ (small strain) conditions, frictional attenuation becomes secondary to linear loss mechanisms, either disappearing or becoming masked.
15 Jan 1979
TL;DR: A sound-propagation model based on normal mode theory is described, designed to give a realistic treatment of the ocean environment, including arbitrary sound-speed profiles in both water column and bottom, compressional and shear wave attenuation, scattering at rough boundaries, and range dependence.
Abstract: : A sound-propagation model based on normal mode theory is described. The model is designed to give a realistic treatment of the ocean environment, including arbitrary sound-speed profiles in both water column and bottom, compressional and shear wave attenuation, scattering at rough boundaries, and range dependence. Furthermore, the model has a flexible input/output structure that facilitates model handling and provides users with a wide choice of computational (output) options, ranging from plots of sound-speed profiles and individual mode functions to contoured transmission loss versus depth and range or versus frequency and range. The computer code is written in FORTRAN V with a few routines in NUALGOL. The version documented here runs on a UNIVAC 1106.
TL;DR: In this article, a new approach for deriving a simple relation between mass attenuation coefficient and X-ray energy or wavelength is described. But the accuracy is generally much better than ±5% with respect to the average of well-established data.
Abstract: This paper describes a new approach for deriving a simple relation between mass attenuation coefficient and X-ray energy or wavelength. Parameters of the latter have been adjusted to fit all up-to-date experimental, interpolated and extrapolated data thus providing means for computing a set of tables covering all values of coefficients usually encountered in applied X-ray spectroscopy. The accuracy is generally much better than ±5% with respect to the average of well-established data.
TL;DR: In this article, it was shown that the attenuation information is derived from frequency shifts rather than the signal amplitudes, which is somewhat insensitive to a partial loss of signal caused by beam refraction.
TL;DR: In this paper, an algorithm is developed to compensate for the effect of attenuation of?-rays within the body using a constant attenuation coefficient and a known body contour.
Abstract: Correction for internal ?-ray absorption is an important aspect in single photon emission computed tomography. An algorithm has been developed to compensate for the effect of attenuation of ?-rays within the body. This algorithm consists of two correction steps and assumes a constant attenuation coefficient and a known body contour. The correction steps are applicable for both parallel beam and fan beam geometries. Computer simulations and experimental results have shown that this method provides a simple and effective correction for attenuation. The incomplete projection problem arises in whole body emission CT imaging, especially when fan beam collimators are used. It has been shown that fan beam geometry provides better spatial resolution than parallel beam geometry. For small organs inside the thoracic and abdominal cavities, however, fan beam geometry often produces incomplete projections which are caused by a decreased field of view of gamma cameras equipped with fan beam collimators. Two methods are described to reduce the artifacts generated from reconstruction of incomplete (truncated) projections.
TL;DR: In this paper, a blue-green pulsed propagation through fog has been analyzed and three distinct regions for energy transport have been identified: Region I small number of attenuation lengths τ in the path (0 32), the direct beam and the forwardscattered beam have decayed to the point where the diffusion type multiple-scattered radiation is the dominant energy received.
Abstract: Measurements and analysis of a blue-green pulsed propagation through fog have identified three distinct regions for energy transport. Region I small number of attenuation lengths τ in the path (0 32): the direct beam and the forwardscattered beam have decayed to the point where the diffusion type multiple-scattered radiation is the dominant energy received. This component does not decay exponentially but results in large spatial, angular, and temporal spreading. This paper presents quantitative data on Region II.
TL;DR: In this article, preliminary results on attenuation and splitting derived from International Deployment of Accelerometers (IDA) recordings of a recent large earthquake are presented. But the results are limited to a single event.
Abstract: The project for the International Deployment of Accelerometers (IDA) is intended to provide very low frequency seismic data for the study of Earth structure and source mechanisms. This article outlines preliminary results on attenuation and splitting derived from IDA recordings of a recent large earthquake.
TL;DR: Results indicate that collagen appears to be the principal determinant of the elevated attenuation detected in regions of myocardial infarction, but is apparently not the primary determinants of the attenuation of normal myocardium.
Abstract: The relationship between ultrasonic attenuation and collagen content is examined in hearts from normal dogs and in hearts from dogs subjected to ischemic injury by coronary occlusion as an approach toward elucidating the physical mechanisms responsible for the attenuation of soft tissue. Increased ultrasonic attenuation is shown to correlate well with increased collagen concentration determined biochemically in regions of ischemic injury studied 2, 4, and 6 weeks following occlusion. Extrapolation of the experimentally determined relationship between attenuation and collagen concentration suggests that collagen is responsible for not more than 15% of the attenuation observed in normal myocardium. These results indicate that collagen appears to be the principal determinant of the elevated attenuation detected in regions of myocardial infarction, but is apparently not the primary determinant of the attenuation of normal myocardium.
TL;DR: The daily integral rate of photosynthesis attained by planktonic algae in Lake Minnetonka, Minnesota, conforms to an equation for a rectangular hyperbola proposed by Bannister.
Abstract: The daily integral rate of photosynthesis attained by planktonic algae in Lake Minnetonka, Minnesota, conforms to an equation for a rectangular hyperbola proposed by Bannister. The rate depends on the concentration of chlorophyll a in the mixed layer and also upon two kinetic parameters that are analogous to the kinetic constants in equations for reactions catalyzed by enzymes. One parameter is an upper limit, $, that would be attained by very dense populations, and the other is a concentration of chlorophyll, c’, at which attenuation of photosynthetically active radiation (PhAR) by the algae equals background attenuation by the water. The parameters were evaluated with the same statistical procedures used to evaluate the kinetic constants for enzyme reactions. The limit, 9, for the daily integral rate in this lake would be about 0.5 mol 0,. rnp2* d-l. Observed daily rates are usually ~0.7 9, because chlorophyll in the water usually intercepts <70% of the radiant energy. Coefficients for the attenuation of PhAR by chlorophyll, E,, and by the water, Q, were estimated from the linear regression of the total attenuation coefficient, E, on chlorophyll concentration. The estimate of E, is 0.022 + 0.005. m-‘*(mg Chl* m-3)-1. Background attenuation, E,, affects integral photosynthesis in the same way as a competitive inhibitor affects enzyme reactions: it is a constituent of c’ = E,/+ which, together with chlorophyll concentration, determines the fraction of underwater PhAR intercepted by planktonic algae.
TL;DR: Focussing properties were highly dependent on tissue type, however for all tissues, optimum focussing was achieved below a focal-number of 3.
Abstract: The focussing of ultrasound through human tissue has been measured for a range of transducer diameters (11–50 mm). The defocussing effects appear to be due to two major mechanisms: (1) attenuation and (2) phase distortion of the converging wavefront due to refractive index variations in the tissue. Attenuation studies show that the first mechanism is responsible for a large proportion of defocussing and can be controlled by the frequency bandwidth of the system. The second mechanism is important for heterogeneous tissues such as breast, where large fractions of the ultrasound energy can be deflected off axis. Focussing properties were highly dependent on tissue type, however for all tissues, optimum focussing was achieved below a focal-number of 3.
TL;DR: In this article, the authors presented a graphical method for the estimation of epicentral intensities, for different earthquakes, from the intensity-distance plot, using an iterative least squares fit procedure.
Abstract: The isoseismal maps for twelve earthquakes in different parts of Iran were analyzed to study the attenuation of intensities with distance. The attenuation is quite sensitive to the selection of epicentral intensities, I , for the earthquakes considered. A graphical method presented in this paper provides a consistent basis for the estimation of epicentral intensities, for different earthquakes, from the intensity-distance plot. The attenuation relations were derived by using an iterative least squares fit procedure, wherein an initial approximate estimate of epicentral intensity for each earthquake is successively improved. The isoseismal maps for a number of earthquakes are elongated in the direction of local structural trend/causative faults. Therefore, three different attenuation relations were derived. I ( R ) = I 0 + 6.453 − 0.00121 R − 4.960 log ( R + 20 ) R 120 km average attenuation I ( R ) = I 0 + 4.824 − 0.00548 R − 3.708 log ( R + 20 ) R 160 km parallel to the isoseismals I ( R ) = I 0 + 8.729 − 0.01158 R − 6.709 log ( R + 20 ) R 110 km transverse to the isoseismals
where I(R) is the intensity at a distance R (km) from the epicenter.
The average attenuation of intensities in Iran is slightly more rapid than the San Andreas province attenuation.
TL;DR: Ultrasonic attenuation in fresh and 5% formalin fixed beef skeletal muscle has been measured, as a continuous function of frequency, in the range 1-8 MHz, for muscle fibre orientations both parallel and normal to the direction of propagation.
TL;DR: In this article, the authors used pulsed dye laser and immersed piezoelectric transducers to measure the absorption spectrum of pure water at 21 °C in the 450-700 nm region.
Abstract: THE absorption spectrum of water in the visible region has been widely investigated1–9 because of the basic, technological and environmental importance of liquid water. Despite these efforts, there are significant disagreements between experimental results: discrepancies of factors of ∼2 in the absorption coefficients are not uncommon between various data. Possible reasons for the disagreements among the various studies are: (1) a lack of a reliable, sensitive technique for measuring small absorpton coefficients in liquids; (2) the presence of a significant amount of light scattering by particles (note that the amount of Rayleigh scattering by pure water is quite small and predictable in the visible8; and (3) measurements are often not done for pure distilled water stored in a noncontaminating vessel. We present here the first accurate measurement of the absorption coefficient of pure water at 21 °C in the 450–700-nm region. We have utilised a recently developed opto–acoustic (OA) technique10, using pulsed dye lasers and immersed piezoelectric transducers. This technique is ideally suitable for measuring weak linear or nonlinear absorptions in nonfluorescing neat liquids or solutions11,12. Our present absorption spectra for water, having typical accuracies of ±5%, should be useful not only for a basic understanding of the properties of water, but also for practical applications like underwater light propagation or intracavity dye laser measurement of dissolved materials in aqueous solutions.
TL;DR: In this article, the authors measured the exponential attenuation of electron currents in a straight line collision chamber using methane gas as the scattering medium and derived the total cross sections for the energy range 0−16 eV.
Abstract: Measurements of the exponential attenuation of electron currents in a straight line collision chamber are reported using methane gas as the scattering medium. From the transmission spectra obtained at different methane pressure in the scattering chamber, the total cross sections for the energy range 0–16 eV are derived and compared to previous measurements. A small structure in the region of the Ramsauer–Townsend minimum is observed, confirming earlier interpretations of electron swarm experiments. A possible mechanism of excitation is discussed.
TL;DR: Ultrasonic attenuation over the frequency range of 1.5–10 MHz has been measured as a function of temperature for porcine liver, backfat, kidney and spleen as well as for a single specimen of human liver.
Abstract: Ultrasonic attenuation over the frequency range of 1.5-10 MHz has been measured as a function of temperature for porcine liver, backfat, kidney and spleen as well as for a single specimen of human liver. The attenuation in these excised specimens increases nearly linearly with frequency. Over the temperature range of approximately 4-37 C the attenuation decreases with increasing temperature for most soft tissue studied.