The Seabed Characterization Experiment was carried out from March 5 to April 10, 2017 (SBCEX17) on the New England Mud Patch, approximately 90 km south of Martha's Vineyard. The SBCEX17 experimental site covers an area of 11 km x 30 km with water depth in the range of 75–80 m. The Sediment Acoustic-speed Measurement System (SAMS) is designed to measure sediment sound speed and attenuation simultaneously over the surficial 3 m of sediments. During SBCEX17, SAMS was successfully deployed at 18 sites, which were chosen to coincide with coring locations, with the goal of developing a geoacoustic model for the study area. In this article, a summary of SAMS operation during SBCEX17 is presented, as well as preliminary results for sediment sound speed and its spatial variation in the frequency band of 2–10 kHz. It is found that in mud, the sound-speed ratio is in the range of 0.98–1. Little dispersion was observed in this frequency band. Using the preliminary SAMS sound-speed results measured at different depths, the sound-speed gradient in mud within the surficial 3 m favors an exponential rather than a linear dependence at SBCEX17 site. Large gradients are observed for depth shallower than 1.5 m. For the sandy basement beneath the mud layer, the sound-speed ratio is as high as 1.105.

We examine profiling float-based measurements of rainfall, wind speed, and near-surface salinity in the eastern tropical Pacific Ocean collected during the SPURS-2 field program. The data show large-scale meridional and zonal variability in these quantities, with considerable variability near the Intertropical Convergence Zone (ITCZ). The eastern tropical Pacific data show strong, intermittent, near-surface, low-salinity events driven by rainfall along the ITCZ that generally do not occur elsewhere in the tropical Pacific. The float salinity data suggest that low-salinity surface water can be entrained 50 m or more into the mixed layer from mid-summer to early in the following calendar year, although the annual periods of strong wind and high precipitation do not coincide. Many of the low-salinity anomalies observed during the SPURS-2 program appear to result from strong, transient storms generated by atmospheric convection along the ITCZ that move across the region.

The Target and Reverberation EXperiment 2013 (TREX13) included a comprehensive reverberation field project in the frequency band of 2–10 kHz, and was carried out off the coast of Panama City, FL, USA, from April 21 to May 17, 2013. A spatially fixed transmit and receive acoustic system was used to measure reverberation over time under diverse environmental conditions, allowing study of reverberation level (RL) dependence on bottom composition, sea surface conditions, and water column properties. Extensive in situ measurements, including a multibeam bathymetric survey, chirp sonar subbottom profiling, gravity/diver cores, sediment sound speed and attenuation, interface roughness, wind-generated sea surface waves, and water column properties, were made to support studies of environmental effects on RL. Beamformed RL data are categorized to facilitate studies emphasizing physical mechanisms of 1) bottom reverberation; 2) sea surface impact; and 3) biological impact. This paper is an overview of RL over the entire sea trial, intending to summarize major observations and provide both a road map and suitable data sets for follow-up efforts on model/data comparisons. Emphasis is placed on the dependence of RL on local geoacoustic properties and sea surface conditions.

The sediment acoustic-speed measurement system is designed to measure in situ sediment sound speed and attenuation within the surficial 3 m of sediments in the frequency band of 2–8 kHz. Measurements were carried out during the Target and Reverberation EXperiment 2013 (TREX13) off Panama City, FL, USA. During TREX13, nine deployments at five selected sites were made along the 20-m isobath, termed the main reverberation track. The sediment types at the five selected sites ranged from coarse sand to a mixture of soft mud over sand, and the measured results show a spread of 80 m/s in sediment sound speed among the different types of sediments for all frequencies. Between 2–8 kHz, about 3% dispersion was observed at the sandy sites, whereas little dispersion was observed at the sites with mud. Preliminary attenuation results show 0.5–3.3 dB/m at the sandy sites, and 0.5–1.0 dB/m at the sites with mud in the same frequency band.

Reverberation measurements obtained with towed arrays are a valuable tool to extract information about the ocean environment. By superimposing a polar plot of reverberation beam time series on bathymetry maps, bottom features (often uncharted) can be located. As part of Rapid Environmental Assessment exercises, Preston and Ellis used directional reverberation measurements to extract environmental information using model-data comparisons. This early work used range-independent (flat bottom) ray-based models for the model-data comparisons, while current work includes range-dependent models based on adiabatic normal modes. Here, we discuss a range-dependent shallow-water reverberation model using adiabatic normal modes that has been developed to handle bottom scattering and clutter echoes in a range-dependent environment. Beam time series similar to those measured on a horizontal line array can be produced. Comparisons can then directly be made with data, features identified, and estimates of the scattering obtained. Of particular interest will be data obtained on the triplet line array during the 2013 Target and Reverberation EXperiments in the Gulf of Mexico off Panama City, FL, USA, where interesting effects in sea bottom sand dunes were observed. Particular attention has been paid to calibration to get estimates of scattering strengths. In addition to the reverberation, a preliminary investigation of the target echo is presented.

Quantification of rainfall over the ocean is critical in understanding the global hydrological cycle. However, oceanic rain has proven difficult to measure due to problems associated with platform motion and flow distortion combined with the spatial and temporal variability of rainfall itself. Passive acoustic rain gauges avoid these issues by using the underwater sound generated by raindrops on the ocean surface to detect and quantify rainfall. In this paper, the operating principles for and data from the Passive Aquatic Listener (PAL), which uses underwater ambient sound to measure rainfall rate and wind speed, are presented. PAL was incorporated onto thirteen Argo profilers that were deployed in September, 2012 as part of the US National Aeronautics and Space Administration-sponsored Salinity Processes in the Upper ocean Regional Studies (NASA SPURS) field experiment in the North Atlantic Ocean. PAL-Argo was initially deployed within a 200 km x 200 km box, PAL-Argos now cover a 1600-km x 600-km region, and continue to telemeter rain rate and wind speed data. Comparisons of these PAL data with in situ and satellite measurements show good agreement for both rain rate and wind speed. Seasonal and inter-annual variability of wind and rain fields in the region are also presented.

Knowledge of the intensity and spatial-temporal distribution of rainfall over the ocean is critical in understanding the global hydrological cycle. However, rain has proven difficult to measure over the ocean due to problems associated with platform motion and flow distortion combined with the spatial and temporal variability of rainfall itself. Underwater acoustical rain gauges avoid these issues by using the loud and distinctive underwater sound generated by raindrops on the ocean surface to detect and quantify rainfall. Here, the physics and operation of and results from an instrument that uses underwater ambient sound to measure rainfall rate and wind speed are presented. Passive Aquatic Listener (PAL) instruments were mounted on a buoy deployed at Ocean Station P and on 13 Argo profilers that were deployed as part of the US National Aeronautics and Space Administration-sponsored Salinity Processes in the Upper-ocean Regional Study (SPURS) field experiment in the North Atlantic Ocean. The PALs provide near-continuous measurements of rain rate and wind speed during the two-year period over the SPURS study region defined by the Argo profilers. Comparisons of PAL data with rain and wind measured by other techniques, including direct in situ observations and satellite measurements, show good agreement for both rain rate and wind speed.

The Target and Reverberation EXperiment (TREX13) took place in the Gulf of Mexico just off the coast of Panama City, Florida. The reverberation experiments were conducted, weather permitting, between 22 April and 16 May. Both source and receiver, ITC2015 and FORA, were fixed in location and were 1.8 and 2.1 m above the seabed respectively. Of particular interest here are various pulses between 1800 Hz and 3600 Hz, with the latter frequency being near the design frequency of FORA. During TREX13, reverberation data were taken during all hours of the day, allowing study of reverberation variation over time and sea surface conditions. In addition to the time and weather dependence, the directional dependence of reverberation level (RL) could be determined using the FORA triplet array. The focus of the experiment was on RL returning from a track of relatively uniform water depth of 20 m, extending about 10 km to the southeast of the source and receiver. The most interesting observation was the effect of the bottom sand waves or dunes, roughly 1 m peak to trough and spaced about 300 m apart, on RL. This correlation between the two had been noted in the pre-TREX experiment in 2012 [Ellis and Preston, UA2013] but not analysed in detail. Predictions from an adiabatic normal mode reverberation model [Ellis et al., ISURC 2008] were used to compare with measurements, using the detailed bathymetry, but otherwise with inputs independent of range. As expected, the model predicts a peak in the reverberation at the peak of the sand dunes. However, counterintuitively, the peaks from the data are, more often than not, anti-correlated with the peaks of the bathymetry, i.e., high RL correlated with the troughs of the sand dunes. Clearly, some mechanisms other than depth effects are responsible for the changes in RL. Extensive bathymetric and bottom measurements have been made along this track; these are being investigated by other researchers to facilitate understanding of the reverberation mechanisms.

Transport theory has been developed for modeling shallow water propagation and reverberation at mid frequencies (1-10 kHz) where forward scattering from a rough sea surface is taken into account in a computationally efficient manner. The method is based on a decomposition of the field in terms of unperturbed modes, and forward scattering at the sea surface leads to mode coupling that is treated with perturbation theory. Reverberation measurements made during ASIAEX in 2001 provide a useful test of transport theory predictions. Modeling indicates that the measured reverberation was dominated by bottom reverberation, and the reverberation level at 1 and 2 kHz was observed to decrease as the sea surface conditions increased from a low sea state to a higher sea state. This suggests that surface forward scattering was responsible for the change in reverberation level. By modeling the difference in reverberation as the sea state changes, the sensitivity to environmental conditions other than the sea surface roughness is much reduced. Transport theory predictions for the reverberation difference are found to be in good agreement with measurements.

Transport theory has been developed for modeling shallow water propagation at mid frequencies (1-10 kHz) where forward scattering from a rough sea surface is taken into account in a computationally efficient manner. The method is based on a decomposition of the field in terms of unperturbed modes, and forward scattering at the sea surface leads to mode coupling that is treated with perturbation theory. Transport theory has recently been extended to model shallow water reverberation, including the effect of forward scattering from the sea surface. Transport theory results will be compared with other solutions for reverberation examples taken from ONR Reverberation Modeling Workshop problems. These comparisons show the importance of properly accounting for multiple forward scattering in shallow water reverberation modeling.

Geoacoustic inversion work has typically been carried out at frequencies below 1 kHz, assuming flat, horizontally stratified bottom models. Despite the relevance to Navy sonar systems many of which operate at mid-frequencies (1–10 kHz), limited inversion work has been carried out in this frequency band. This paper is an effort to demonstrate the viability of geoacoustic inversion using bottom loss data between 2 and 5 kHz. The acoustic measurements were taken during the Shallow Water 2006 Experiment off the coast of New Jersey. A half-space bottom model, with three parameters density, compressional wave speed, and attenuation, was used for inversion by fitting the model to data in the least-square sense. Inverted sediment sound speed and attenuation were compared with direct measurements and with inversion results using different techniques carried out in SW06. Inverted results of the present work are consistent with other measurements, considering the known spatial variability in this area. The observations and modeling results demonstrate that forward scattering from topographical changes is important at mid-frequencies and should be taken into account in sound propagation predictions and geoacoustic inversion. To cope with fine-scale topographic variability, measurement technique such as averaging over tracks may be necessary.

Geoacoustic inversion work has typically been carried out at frequencies below 1 kHz, assuming flat, horizontally stratified bottom models. Despite the relevance to Navy sonar systems, many of which operate at mid-frequencies (1-10 kHz), limited inversion work has been carried out in this frequency band. This paper is an effort to demonstrate the viability of geoacoustic inversion using bottom loss data in the frequency band of 2-5 kHz. The acoustic measurements were taken during the Shallow Water 2006 Experiment off the coast of New Jersey. A half-space bottom model, with three parameters, density, compressional wave speed, and attenuation, was used for geoacoustic inversion by fitting the model to data in the least-squares sense. Inverted sediment sound speed was compared with direct measurements and inversion results using different techniques in the same area. The comparison shows that bottom loss can be used to infer sediment geoacoustic parameters at mid-frequencies. In addition, observations and modeling results demonstrate that forward scattering from topographical changes is important at mid-frequencies and should be taken into account in sound propagation predictions and geoacoustic inversion. To cope with fine-scale topographic variability, measurement technique such as averaging over tracks may be necessary.

At frequencies of about 1 kHz and higher, forward scattering from a rough sea surface (and/or a rough bottom) can strongly affect shallow water propagation and reverberation. The need exists for a fast, yet accurate method for modeling such propagation where multiple forward scattering occurs. A transport theory method based on mode coupling is described that yields the first and second moments of the field. This approach shows promise for accurately treating multiple forward scattering in one-way propagation. The method is presently formulated in two space dimensions, and Monte–Carlo rough surface PE simulations are used for assessing the accuracy of transport theory results.

During the 2006 Shallow Water (SW06) experiment, simultaneous measurements were made of the sound-speed field as a function of range and depth associated with nonlinear internal waves and acoustic propagation at frequencies of 2–10 kHz over a 1-km path. The internal waves were measured by a towed conductivity-temperature-depth (CTD) chain to get high resolution. These measurements were coordinated so that the nonlinear waves could be interpolated onto the acoustic path, allowing predictions of their effects on the acoustics. Using the measured sound-speed field, the acoustic arrivals under the influence of the internal waves are modeled and compared to data. The largest impact of measured moderate amplitude internal waves on acoustics is that they alter the arrival time of the rays which turn at the thermocline.

Nonlinear internal waves are a common event on the continental shelf. The waves depress the high-gradient region of the thermocline and thicken the surface mixed layer with consequent effect on acoustic propagation. After the waves have passed, it may take several hours for the thermocline to rise to its prewave level.

To examine the effect of the rising thermocline, oceanographic and acoustic data collected during the 2006 Shallow Water Experiment (SW06) are analyzed. Midfrequency acoustic data (1.5-10.5 kHz) taken for several hours at both fixed range (550 m) and along a tow track (0.1-8.1 km) are studied. At the fixed range, the rising thermocline is shown to increase acoustic intensity by approximately 5 dB. Along the tow track, the transmission loss changes 2 dB for a source-receiver pair that straddles the thermocline. Using oceanographic moorings up to 2.2 km away from the acoustic receiver, a model for the rising thermocline is developed. This ocean model is used as input to a broadband acoustic model. Results from the combined model are shown to be in good agreement with experimental observation. The effects on acoustic signals are shown to be observable, significant, and predictable.

Several of the problems for the first Reverberation Modeling Workshop yielded interesting differences between solutions obtained with ray and normal mode methods. These particular problems were defined with high boundary scattering loss. A bottom reverberation case at 3.5 kHz with a down-refracting sound speed profile (Problem VI) will be considered as a case in point. The ray solutions show a "direct path" contribution unaffected by the bottom scattering loss as long as a direct path can reach the bottom, while the mode solutions obtained to date show a lower reverberation level during this period due to modal attenuation. These differences occur in both incoherent and coherent reverberation solutions for both rays and modes. Arguments will be presented that indicate the correctness of the ray solutions for this case. Suggestions will also be made on how the mode approach can be used to obtain solutions in agreement with the ray method.

In "sound transmission through a fluctuating ocean," Flattxe et al. described saturation of a single acoustic path as that path becoming a number of interfering uncorrelated micropaths due to refraction by internal waves. The probability density function of intensity becomes exponential with a scintillation index of 1.0. In deep water, however, full saturation is not achieved due to weak scattering and absorption.

Mid-frequency (1–10 kHz) data from the Shallow Water 2006 Experiment are used to determine single-path intensity statistics. At a range of 1 km in water 80 m deep, an acoustic path is isolated that went through two upper turning points separated by a single bottom reflection. The data were collected during a period when large nonlinear internal waves were absent. The scintillation index calculated from the data increases with frequency until reaching a maximum of 1.2 around 6 kHz. It then decreases to 1.0, suggesting that single-path saturation can be achieved at mid-frequencies in shallow water. The probability density functions of intensity at various frequencies show a trend toward exponential. Because shallow water internal waves are dominated by the first mode, uncorrelated micropaths are an unlikely mechanism for producing the observed saturation.

Knowledge of sediment sound speed is crucial for predicting sound propagation. During the Shallow Water '06 experiment, in situ sediment sound speed was measured using the Sediment Acoustic-speed Measurement System (SAMS). SAMS consists of ten fixed sources and one receiver that can reach a maximal sediment depth of 3 m. Measurements were made in the frequency range 2&$150;35 kHz. Signal arrival times and propagation distances were recorded, from which sediment sound speed was determined. Preliminary results from three deployments show that SAMS was capable of determining sediment sound speed with uncertainties less than 1.6%. Little dispersion in sediment sound speed was observed.

A unique Sediment Acoustic-speed Measurement System (SAMS) was developed to directly measure sediment sound speed. The system consists of ten fixed sources and one receiver. In a typical deployment, the SAMS is deployed from a ship that is dynamically positioned. The sources are arranged just above the sea bottom and the receiver is drilled into the sediment with controlled steps by a vibro-core. The maximal sediment penetration depth is 3 meters. At each receiver depth, the 10 sources transmit to the receiver at different angles in the frequency range of 2–35 kHz, providing 10 estimates of sound speed through time-of-flight measurements from the known source-to-receiver geometry. SAMS was deployed three times during the recent Shallow Water Experiment 2006 (SW06) on the New Jersey shelf at 80 m water depth. Preliminary results of sediment sound speed are 1618 ± 11, 1598 ± 10, and 1600 ± 20 m/s at three separate deployment locations. Little dispersion in sediment sound speed was observed.