Bottom Backscattering Research Articles

50 kHz bottom backscattering measurements from two types of artificially roughened sandy bottoms

Laboratory measurements of 50 kHz bottom backscattering strengths as a function of grazing angle were performed on the sandy bottom of a water tank; two types of bottom roughnesses, a relatively smooth interface and a rough interface, were created on the bottom surface. The roughness profiles of the two interface types were measured directly using an ultrasound arrival time difference of 5 MHz and then were Fourier transformed to obtain the roughness power spectra. The measured backscattering strengths increased from −29 to 0 dB with increasing grazing angle from 35 to 86°, which were compared to theoretical backscattering model predictions. The comparison results implied that bottom roughness is a key factor in accurately predicting bottom scattering for a sandy bottom.

A new time-domain model for bottom backscatter

Bottom backscatter is important for a number of underwater applications: it is a source of noise in target detection and a source of information for sediment classification and geoacoustic inversion. Bottom backscatter measures sound scattered from roughness and volume heterogeneity in different sediment layers. While current models can successfully handle scattering from such sediment layers, they cannot correctly predict the scattered intensity as a function of time. A new model for bottom backscatter is introduced which provides an exact solution for scattered intensity as a function of time under first order perturbation theory. Examples of backscatter from various sediment layers will be shown in contrast to previous models. Issues such as the “spherical wave effect” will be addressed, and applications to model shallow water reverberation will also be discussed.

서해 천해환경에서 단상태 해저면 후방산란강도 측정

ABSTRACT: Measurements of bottom backscattering strengths in a frequency range of 6-14 kHz were made on the shallow water off the southern Gyeonggi Bay in Yellow Sea in May 2013, as part of the KIOST-HYU joint acoustics experiment. Geological surveys for the experimental area were performed using multi-beam echo sounder, sparker system, and grab sampling to investigate the bottom topography, sub-bottom profile and composition of surficial sediment, respectively. In this paper, the backscattering strengths as a function of grazing angle (in range of 28° ~ 69°) were estimated and compared to the predictions obtained by Lambert’s law and APL-UW scattering model. Finally, the effects of geoacoustic parameters corresponding to the experimental area on the backscattering strengths are discussed.Keywords: Bottom backscattering, Reverberation, Bottom roughness, Seafloor volume scattering PACS numbers: 43.30.Gv, 43.30.Hw†Corresponding author: Jee Woong Choi (choijw@hanyang.ac.kr) Department of Marine Sciences and Convergent Technology, Hanyang University, 55 Hanyangdaehak-ro, Sangnok-gu, Ansan15588, Republic of Korea(Tel: 82-31-400-5531, Fax: 82-31-406-6255)

천해 지질환경에서의 음파전달 특성 연구를 위한 KIOST-한양대 공동실험 개요

본 논문은 "천해 지질환경과 음파전달환경과 상호 연계 연구"를 목적으로 한국해양과학기술원과 한양대학교가 2013년 4 ~ 5월에 경기만 태안반도 서쪽에 위치한 천해환경에서 공동으로 수행한 지질환경 조사 및 수중음향 실험에 대한 개요를 기술하였다. 실험지역은 강한 조류와 천해의 지형학적 특성에 의해 다양한 종류의 퇴적상과 베드폼이 형성되는 해역이다. 집중적인 지질환경 특성 조사를 위해 다중음향측심기, 천부지층탐사기, 중천부지층탐사기, 그랩을 이용하여 조사해역의 지질환경 특성을 파악하였으며, 저주파수, 중주파수 대역의 음원과 수직선배열청음기를 이용하여 주파수 200 ~ 16,000 Hz 대역에서 1) 저주파수 음파전달, 2) 중주파수 해저면 반사손실, 3) 주변소음의 공간 코히런스 분석, 4) 중주파수 해저면 후방산란에 대한 연구 주제로 해상실험이 수행되었다. 본 논문에서는 연구지역에서 음향실험을 실시한 동기, 음향실험 방법론과 측정된 지질자료를 기반으로 음향자료 해석과, 실험기간 동안 측정된 지질환경, 기상 해양물리 자료를 요약 정리하여 기술하였다. This paper presents an overview of the geological environment investigation and underwater acoustic measurements for the purpose of "Study on the Relationship between the Geological Environment and Acoustic Propagation in Shallow Water", which are jointly carried out by KIOST (Korea Institute of Ocean Science & Technology) and Hanyang University in the western shallow water off the Taean peninsula in the Yellow Sea in April-May 2013. The experimental site was made up of various sediment types and bedforms due to the strong tidal currents and coastal geomorphological characteristics. The geological characteristics of the study area were intensively investigated using multi-beam echo sounder, sub-bottom profiler, sparker system and grab sampler. Acoustic measurements with a wide range of research topics in a frequency range of 20~16,000 Hz: 1) low frequency sound propagation, 2) mid-frequency bottom loss, 3) spatial coherence analysis of ambient noise, and 4) mid- frequency bottom backscattering were performed using low- and mid-frequency sound sources and vertical line array. This paper summarizes the topics that motivated the experiment, methodologies of the acoustic measurements, and acoustic data analysis based on the measured geological characteristics, and describes summary results of the geological, meteorological, and oceanographic conditions found during the experiments.

Reverberation modeling approximately accounting for three-dimensional forward scattering effects

A transport theory approach has been developed for modeling shallow water propagation and reverberation at mid-frequencies with emphasis at 1–3 kHz. With this approach, sea surface forward scattering can be taken into account in a 2-D (range-depth) approximation. While the effects of surface forward scattering on transmission loss are found to be modest (1–2 dB), the corresponding effects on reverberation level for typical conditions can be significant (~10 dB), even though bottom backscatter dominates reverberation in shallow water. An attempt to make data/model comparisons brought out the issue of the proper way of using the 2-D (in horizontal plane) surface roughness spectrum to model forward scattering with a 2-D (range-depth) propagation model. For reverberation modeling, we are investigating the approximation of preserving the distribution in vertical scattering angles in reducing the full 3-D problem to a 2-D (range-depth) problem when using the traditional N ' 2-D approach. A description of the method will be given, followed by data-model comparisons for TREX-13. [Work supported by ONR Ocean Acoustics.]

Development and Application of MB System Software for Bathymetry and Seabed Computation

MB-Systems was a useful software tool for mapping the seafloor by processing multibeam bathymetry and bottom backscattering. Multibeam echosounder system was the underwater acoustic technology that used to determine morphology and characteristics of the seabed. The objectives of these research were to produce and describe the map of bathymetry, distribution of seabed backscatter value, beam density distribution, standard deviation of the depth, standard deviation as a percentage of water depth, and also to produce and describe graph of ping interval for each depth. The maps were obtained by using MB-System software after correction of tide, amplitude against grazing angle, sound velocity profile, and 3D editing of swath bathymetry. Implementation of 30 0 as reference angle to both sides of the beam, low pass filtering, and mosaic amplitude were conducted to calculate the backscatter values of their absolute level. By these steps, nadir stripping and outlayer values in the map of seabed backscattering distribution could be eliminated. After computing the whole correction stages, we found depth value of the slope area were 813.59 meters to 4904.71 meters and at the basin area were 723.01 meters to 1065.21 meters depth. The distribution of backscatter values in slope area ranged from -42.37 dB to -4.47 dB and in the basin area were -41.59 dB to -16.63 dB.

Overview of the Target and Reverberation EXperiment 2013 (TREX13)

The Target and Reverberation Experiment (TREX13) is a shallow water reverberation experiment that endeavored to measure contemporaneously acoustic and adequate environmental data so detailed model/data comparison can be achieved and important environmental factors can be identified for different applications. TREX13 was sponsored by ONR Ocean Acoustics and the SERDP DoD programs. It was conducted during April to June of 2013 off the coast of Panama City, Florida, with participation from multiple institutions and involving three research vessels: The R/V Sharp and R/V Walton Smith from the United States, and the Canadian Force Auxiliary Vessel Quest. From a SONAR viewpoint, reverberation consists of two-way propagation and a single backscatter. Therefore, reverberation, transmission loss, and bottom backscatter were repeatedly measured over a time period of several weeks in the frequency band of 2–10 kHz, along with extensive environmental measurements. Discussed will be planning and execution of the field experiments, strategies and steps for data analysis, and modeling efforts.

A geoacoustic estimation scheme based on bottom backscatter signals from multiple angles

Aiming at the disadvantages of current methods to estimate sediment geoacoustic parameters, a geoacoustic estimation scheme is presented, in which used are the bottom back reflection signals from multiple angles sampled with a multi-beam parametric array sub-bottom profiler. Firstly, the sediment thickness and sound speed are estimated with difference frequency backscatter signals separately from upper and lower sediment surfaces with vertical and outboard direction. Then, the sediment attenuation coefficient is estimated by use of the two backscatter signals with different difference frequencies from upper and lower sediment surfaces with vertical direction. Finally, the sediment property impedance is estimated with primary frequency backscatter signals from upper sediment surface with vertical direction and followed by the computation of the sediment density, thereby solving its coupling with sound speed. The efficiency is tested through pool experiment.

Assessing grain size as a predictor of mid-frequency bottom backscattering strengths

Scattering from the seabed can be a complex mix of surface roughness and volume heterogeneity contributions. A series of mid-frequency (MF; 1.5–4.5 kHz) bottom backscattering strength data collected by the Naval Research Laboratory at a number of shallow-water locations (Stanton Banks, Malta Plateau, Heceta Bank) is first used to demonstrate the inadequacies of using Lambert’s Law to model bottom backscattering strengths, and that more general empirical power laws, where not only the strength but the angular exponent can vary, are needed to match the data at a given frequency. The Stanton Banks data, where sediment types range from mud to gravel, are then used to explore the extent to which easy-to-access geophysical data (such as surficial grain size distributions from bottom grab samples) may be capable of providing suitable estimates of key model inputs (such as sediment sound speeds/attenuations, density and roughness/volume spectral strengths/exponents). These results show that both grain size and “bottom type” are in general unreliable predictors of the measured MF bottom backscattering strengths, and that a physics-based modeling approach coupled with in-situ environmental characterization is required. [Work supported by ONR.]

High-Resolution Integrated Detection of Underwater Topography and Geomorphology Based on Multibeam Interferometric Echo Sounder

During the construction of hydraulic engineering, it usually needs high-resolution and integrated investigation of underwater topography and geomorphology, and shape and position of underwater construction or obstacle. Based on the multibeam interferometric echo sounder, this paper analyzes high-resolution bathymetric topography method of multibeam interferometric principle, and applies the time of arrival (TOA) and direction of arrival (DOA) estimated by interferometric technology to achieve bottom backscatter geomorphology imaging. This method obtains high-resolution bottom backscatter imagery, and its space position realizes the one-to-one with bathymetric data, which makes bathymetric and backscatter imaging data merged accurately in integrated detection of topography and geomorphology. Moreover, the processing results of experiment data from Qingjiang River, in Hubei province of China, show the validity of the method.

Low frequency seabed scattering at low grazing angles

Low-frequency (LF) seabed scattering at low grazing angles (LGA) is almost impossible to directly measure in shallow water (SW), except through inversion from reverberation. The energy flux method for SW reverberation is briefly introduced in this paper. The closed-form expressions of reverberation in an isovelocity waveguide, derived from this method, indicate that in the three-halves law range interval multimode/ray sea bottom scattering with different incident and scattering angles in forming the reverberation may equivalently be represented by the bottom backscattering at a single range-dependent angle. This equivalent relationship is used to derive the bottom backscattering strength (BBS) as a function of angle and frequency. The LF&LGA BBS is derived in a frequency band of 200-2500 Hz and in a grazing angle range of 1.1°-14.0° from reverberation measurements at three sites with sandy bottoms. This is based on three previous works: (1) The closed-form expressions of SW reverberation [Zhou, (Chinese) Acta Acustica 5, 86-99 (1980)]; (2) the effective geo-acoustic model of sandy bottoms that follows the Biot model [Zhou et al., J. Acoust. Soc. Am. 125, 2847-2866 (2009)] and (3) A quality database of wideband reverberation level normalized to source level [Zhou and Zhang, IEEE J. Oceanic Eng. 30, 832-842 (2005)].

Milestones in naval through-the-sensor environmental acoustic measurements

This paper describes recent developments in the Bottom Loss and Scattering Strength Measurement System (BLOSSMS). The objectives of the BLOSSMS project are to (1) create technology from which measurements of several types of active sonar scattering phenomena may be made using a single unifying measurement and analysis scheme, (2) integrate measurements from Fleet active sonar systems with seafloor bathymetry and modeled acoustic data into a single root database, (3) develop algorithms that are based on sound physics and oceanographic principles to identify salient information in the root database for analysis, and (4) produce acoustic scattering parameters that are useful in sonar system performance analysis and the optimization of sonar operations. In this paper, the methods used to measure bottom backscatter strength, bottom forward scatter (bottom loss), and volume scatter strength are discussed. Examples of the resulting scattering parameters are provided. Also discussed are the advantages, disadvantages, and challenges that are associated with this approach to measuring fundamental acoustic scattering phenomena.

Geoacoustic modeling based on sediment particle analysis.

Geological underwater processes are affected by physical properties of sediment particles, their size, shape, and spatial variability. In geological modeling, these parameters are normally provided by analysis of sediment cores and documented in certain geologically relevant terms. What is the set of parameters most relevant from acoustics standpoint, or in “acoustically relevant” terms, is an open question. It is addressed using an acoustic scattering model directly based on sediment particle analysis, geo‐acoustic model of bottom interaction taking into account the sediment discrete heterogeneity. The particle size distribution in this model is comprised of central and coarse parts. The central part describes the sediment matrix and its large scale variability, or continuous heterogeneity, critical for modeling of acoustic propagation in the sediment. To account for discrete component of heterogeneity, the coarse part of size/shape distributions is attributed to “inclusions” in the sediment matrix and provides direct inputs for evaluation of volume scattering in the sediment. An example of such analysis and modeling based on the SAX04 geo‐acoustic data set is presented and used for calculating the frequency‐angular dependences of bottom backscattering strength. Comparisons with acoustic backscatter data measured at this site and possibilities for geoacoustic inversions are discussed.

The impact of bottom backscatter strength modeling on active sonar optimal frequency calculations.

For many active sonar problems, there exists an optimal frequency—one at which detection range maximized. The optimal frequency can be calculated using a frequency dependent analytical sonar equation to solve for the detection range, defined as the largest range with positive signal excess. The frequency with the maximum detection range is the optimal frequency. An advantage of this approach is that it combines all of the sonar equation terms into a single quantity. It might seem that such a frequency will not exist because propagation loss tends to increase with increasing frequencies, so performance might decrease monotonically with frequency. However, other terms in the sonar equation are also frequency dependent. For example, Knudsen or Wenz curves show that noise levels generally decrease with frequency, while source level and target strength may be complicated functions of frequency. In shallow water, active sonar may be reverberation limited rather than noise limited, but standard Lambert scattering strength models are frequency independent. New, more accurate bottom backscatter strength models have frequency dependence, which will impact the optimal frequency calculation. This approach demonstrates one operational impact of improved reverberation modeling, and quantifies the performance degradation caused by a suboptimal frequency selection.

A geoacoustic bottom interaction model (GABIM)

The geoacoustic bottom interaction model (GABIM) has been developed for application over the low-frequency and midfrequency range (100 Hz to 10 kHz). It yields values for bottom backscattering strength and bottom loss for stratified seafloors. The model input parameters are first defined, after which the zeroth-order, nonrandom problem is discussed. Standard codes are used to obtain bottom loss, uncorrected for scattering, and as the first step in computation of scattering. The kernel for interface scattering employs a combination of the Kirchhoff approximation, first-order perturbation theory, and an empirical expression for very rough seafloors. The kernel for sediment volume scattering can be chosen as empirical or physical, the latter based on first-order perturbation theory. Examples are provided to illustrate the various scattering kernels and to show the behavior predicted by the full model for layered seafloors. Suggestions are made for improvements and generalizations of the model.

8-kHz bottom backscattering measurements at low grazing angles in shallow water

8-kHz bottom backscattering measurements at low grazing angles (6 degrees -31 degrees ) are presented. The experiment was performed at a very shallow water site with a silty bottom on the south coast of Korea. Backscattering strengths between -42 and -30 dB were obtained and were compared to a theoretical backscattering model and Lambert's law. The fit of the theoretical model to the measurements suggests that sediment volume scattering is dominant over scattering from bottom interface roughness. Combining these results with previous measurements found in the published literature implies that backscattering strengths from silty sediment increase slightly as the frequency increases.

Reverberation clutter from combined internal wave refraction and bottom backscatter.

A hypothesis is proposed that one mechanism for clutter observed in shallow water reverberation measurements is due to the combined effect of forward scatter and subsequent backscatter. The forward scatter refracts part of the sound from low grazing angle to high grazing angle, then being backscattered by bottom roughness. The refracted sound impinges onto the bottom at higher grazing angle, resulting in higher backscatter because of elevated bottom scattering cross section as compared to that at lower grazing angles. The resultant reverberation will stand out as a target-like clutter. An example is presented as a non-linear internal wave propagates in a shallow water channel, resembling conditions found on the New Jersey Shelf. The effect on reverberation of the internal wave as clutter is investigated using a time-domain numerical model. The model uses parabolic equation for the two-way propagation and first order perturbation approximation for bottom backscatter. [Work supported by ONR.]

Acoustic Backscattering From a Sand and a Sand/Mud Environment: Experiments and Data/Model Comparisons

The results from two bottom backscattering experiments are described in this paper. These experiments occurred within about 1 km of each other but were separated by approximately five years (1999 and 2004). The experimental methods used in the second experiment were changed based on lessons learned in the first experiment. These changes and the motivation for them are discussed. The sediment at each experiment site would generally be classified as the same (as a well-sorted medium sand sediment) before the weather events (Hurricane Ivan and Tropical Storm Matthew) that occurred in late September and early October 2004. As a result of these weather events, the sediment present during the October 18, 2004 experiments was much more complicated than that in 1999 and in many places had a mud/sand surface layer. The environmental measurements in both experiments were sufficient to separate physical mechanisms responsible for scattering. For shallow grazing angles (less than 45deg), backscattering at frequencies between 20 and 150 kHz was attributable to sediment interface roughness in 1999, whereas volume scattering dominated in 2004. Furthermore, in 2004, volume heterogeneity within the mud/sand surface layer is a probable mechanism for the scattering feature seen in the data in the 20deg-30deg region. Above 200 kHz, the frequency dependence of both the 1999 data and the 2004 data indicates that a new scattering mechanism is coming into play. Other results within this issue [Ivakin, IEEE J. Ocean. Eng., vol. 34, no. 4, Oct. 2009] indicate that scattering from shells is a viable candidate for explaining the data above 200 kHz.

Model and measurement of high-frequency bottom backscattering intensity envelope.

High-frequency bottom scattering, generally, can be attributed to bottom interface roughness and volume inhomogeneities. The full grazing angle (0–90 deg) intensity envelope model of high-frequency bottom backscattering generated by these two kinds of mechanism was presented. Small-slope approximation method was used to explain bottom roughness backscattering. Bottom volume scattering coefficient was used to calculate bottom volume scattering assuming the volume scattering is homogeneous. The model incorporates the shape of transmitted pulse, directionality of transducer, scattering area, bottom characteristic, and so on. Tank experiment result shows the bottom roughness is the key factor to fine sand bottom backscattering. This backscattering intensity envelope model is useful for the application of bottom backscattering signal, such as sediment classification and geoacoustic inversion. [Work supported by the NSFC.]

High frequency scattering from inclusions in marine sediments: Data/model comparisons.

Recent theoretical modeling has shown that the size-distribution of inclusions in marine sediments, such as gravel-sized grains and shells embedded in sandy bottoms, is critical for high-frequency scattering. Analysis of existing data on the bottom scattering strength and sediment ground truth also shows that the backscatter data are primarily responsive to the coarse contents of the sediment. Previous ground truth data analysis, usually based on the sediment sieving, assumed that particles are nearly spherical, so that the sieve size is approximately equal to the equivalent diameter (diameter of sphere with the same volume). Coarse particles, such as shell fragments, however, appear to be very nonspherical. Therefore, a relationship between the particle sieve size and equivalent diameter, defined by the particle shape, is needed to provide required size-distributions for acoustic scattering models. Recent statistical analysis of sediment samples collected during the SAX04 experiment resulted in a set of such empiric relationships, and also provided estimating the particle shape factor, known to be an important parameter in the sediment transport processes. Obtained size distributions were used for calculations of frequency-angular dependencies of the bottom backscattering strength and provided a good agreement in the SAX04 scattering data/model comparisons. [Work supported by ONR.]