Chirp Sonar System Research Articles

Morphological and Stratigraphic Investigation of a Holocene Subaqueous Shelf Fan, North of the İstanbul Strait in the Black Sea

In 2002, the Bosphorus outlet was mapped using an EM 300 multibeam echo-sounder together with a Chirp sonar system. This survey, carried out on board the Ifremer RV 'Le Suroit' in the frame of the BlaSON project, completes the data previously acquired directly at the mouth of the Bosphorus by Di Iorio et al. (1999) in the frame of a NATO SACLANT Undersea Research project using jointly the NATO RV Alliance, and the Turkish Navy Survey ship 'Çubuklu'. This acoustic imaging has identified a canyon system at the slope and a shallow marine fan, which contains shelf incisions extending the İstanbul Strait incision. Multibeam bathymetry, ultra-high resolution seismic profiling and coring correlations on this subaqueous fan area allowed reconstruction of morphology and patterns of sediment distribution indicative of high energy sediment transport processes. The discovery of a shallow water/shelf type fan directly offshore from the Bosphorus and connected to its outlet is consistent with the theories of sudden discharges of large volumes of water. Age dating obtained at the bottom of this subaqueous shelf fan yielded an age of 6700 yr C14 BP (uncorrected age) for the first marine mollusc encountered at the base. This is in accordance with a last and abrupt reconnection of the Marmara Sea to the Black Sea. A detailed morphological map of the shelf and slope along with seismic profile interpretation and core correlation is presented here. A synthesis is proposed to explain the formation of this subaqueous fan and its relationship with the last connection between Black and Marmara seas after the Last Glacial Maximum. This interpretation can be summarized as follows: stage A corresponds to the first erosion surface seen on the shelf related to the Last Glacial Maximum low stand; stage B is the ravine surface onlapping to ca. -30, -40 m; stage C is a second erosional surface related to a sea level fall and eroding most of underlying Unit 1B; and stage D corresponds to the onset of the fan deposit during a period of high water run-off from the Black Sea entering from the Bosphorus. Avulsion branches show that this fan has been active for a long time.

To obtain geo-acoustic parameters by the acoustic inversion technique using subbottom profiler

The fast sound speed at sea surface in the water surrounding Taiwan causes strong refraction. If the water depth is shallower than 5000 m, the sound ray will reflect at the bottom, causing severe bottom losses. Therefore, geo-acoustic parameters are important for acoustic studies in the water surrounding Taiwan. In the past, researchers used single-pulse sonar of 3.5 kHz to obtain geo-acoustic parameters by the spectral ratio technique. Now the sediment exploration has already advanced and used the Chirp Sonar system on Taiwanese Research Vessels such as Ocean Researcher I (OR I) and Ta-Kuans TOPAS system. This paper presents the results of attenuation coefficients in ocean bottoms using the acoustic inversion technique on the chirp sonar data collected by OR I. The results are compared with the core data, and the error is within 15%.

Ocean bottom scattering: characterization with chirp sonar

Shallow water acoustic propagation in the mid-frequency range (∼1–10 kHz) is strongly affected by scattering from the ocean bottom. In general, this scattering is a combination of rough surface scattering from the water-sediment interface and sediment layers, as well as volume scattering from embedded inhomogeneities. It is necessary to develop an understanding of the spatial and temporal characteristics of the acoustic field to optimize Navy sonar performance. Although there has been a growth of research activity in this area, the validity of different modeling approaches still remains an open research question. One promising tool for bottom sediment characterization is a chirp sonar. These sonars can produce high-quality data in which time of arrival is often associated with location of subbottom sediment layers to produce sediment profiles. However, features of the acoustic scattering such as multiple scattering or particle resonances can have complicated signatures that are not straightforward to interpret. In this work, the effect of these signatures on the chirp sonar data is studied through application of a numerical FDTD model applied to a variety of ocean bottom geometries. Results obtained from the chirp sonar system are presented and compared with ocean bottom scattering models.

Techniques for estimating sediment properties from chirp sonar data

The chirp sonar is a towed wideband FM reflection profiler developed to collect normal incidence reflection data suitable for sediment property inversions; the sonar also generates detailed reflection profiles of the seabed. The chirp sonar system transmits FM pulses with high time-bandwidth products to attain high pulse energy; consequently, the acoustic data have a high signal-to-ambient-noise ratio after correlation processing. The vertical resolution of the images is about 1 cm when transducer bandwidth is approximately 40 kHz. Data processing methods have been developed to automatically map the locations of sediment layer interfaces, and to estimate compressional wave attenuation, acoustic impedance, and compressional wave velocity of sediment layers using normal incidence FM reflection data. The height of sand ripples and the phase dispersion of compressional waves can also be measured from the acoustic imagery. Field data collected by other investigators during the High Frequency Acoustics DRI experiments off Fort Walton Beach were used to verify the acoustic property estimates. Comparisons show that chirp sonar estimates of acoustic properties agree with direct measurements of the properties based on in situ acoustic probe data and sediment core data collected by investigators from Naval Research Laboratory, Stennis Space Center, and APL, Washington State.

A design study of an acoustic system suitable for differentiating between orange roughy and other New Zealand deep-water species

Using the simple slab-cylinder acoustic model for fish, developed by Clay and Horne [J. Acoust. Soc. Am. 96, 1661–1668 (1994)], the target strengths of three of New Zealand’s deep-water fish species, namely orange roughy, black oreos, and smooth oreos, have been derived. The target strengths derived for the model fish have been found to be in reasonable agreement with currently accepted target strength values. These three model fish were used in a study to test the species discrimination of a chirp sonar system. Chirps of center frequencies 40, 80, and 160 kHz and bandwidth of 10, 20, and 40 kHz have been used to acoustically illuminate the three fish species listed above and the matched, filtered responses to the chirps determined. The effect of the response of transducer or system bandwidth has also been investigated. It has been found that the bandwidth of the chirp is much more important for resolving detail in a fish target than the chirp center frequency. A bandwidth of at least 20 kHz, and preferably 40 kHz, produces matched filtered responses for black and smooth oreos and orange roughy which are quite clearly species specific. Results suggest that with orange roughy it may be possible to size and even sex the targets acoustically.

Acoustic monitoring of generation and evolution of shallow-water solitary internal waves

The feasibility of acoustic monitoring of generation and evolution of shallow-water solitary internal waves is investigated by using the acoustic and environmental data collected during the 1995 Shallow Water Acoustics in a Random Medium (SWARM) experiment. Spatial and temporal variability of acoustic signals (60 to 450 Hz) were measured by using two well-populated vertical receiving arrays at 30- and 40-km ranges from two moored sources and at 20-km cross-range from two ship-deployed sources. Environmental data were collected by using CTDs, thermistor strings, a high-frequency acoustic backscattering imaging system, and a chirp sonar system. Correlation between cross-range propagation and oceanographic measurements during the generation and evolution of a solitary internal wave packet indicates the possibility of acoustic monitoring of this oceanographic process. Results of a parametric study using broadband numerical simulations are also presented to develop the best monitoring strategies for future experiments. [Work supported by ONR.]

First high-resolution chirp sonar profiles from the central Arctic Ocean reveal erosion of Lomonosov Ridge sediments

The Swedish `Arctic Ocean-96' expedition to the central Arctic Ocean with the icebreaker Oden carried out a geological/geophysical program focused mainly on Lomonosov Ridge sediments. Approximately 195 km of high-resolution seismic data from the ridge crest and slope were collected with a chirp sonar system providing new insights into the sediment stratigraphy and the accompanying depositional history. These data reveal evidence of substantial erosion that acted in laterally limited areas on shallow parts of the investigated ridge segment. More than 50 m of sediment stratigraphy is missing in the most eroded parts. Moreover, the chirp sonar records indicate that, below the erosional surface, the ridge crest and slope is characterized by a low-energy sedimentary environment. The termination of the erosional phase is indicated by a prominent reflector. The subsequent depositional phase resulted in an approximately 3 m thick stratigraphic unit. The cause of the erosion is attributed to either ice grounding or current erosion. Furthermore, Oden echo sounding data has been used together with data from the US Navy SCICEX program in order to update existing bathymetric charts in the `Arctic Ocean-96' working area. The update shows a narrower ridge crest with a different outline than in previously published charts with a minimum depth of 607 m which is the shallowest depth of the Lomonosov Ridge recorded (in the public domain) in the central Arctic Ocean.

High-resolution, three-dimensional measurements of low-frequency sound propagation in shallow water

An overview is presented of the modal mapping experiment (MOMAX), which was conducted in March 1997 in the vicinity of the East Coast STRATAFORM site. Both fixed and moving source configurations were used to transmit several pure tones in the frequency range 50–300 Hz. The magnitudes and phases of these signals were recorded on several freely drifting buoys, each containing a hydrophone, GPS and acoustic navigation, and radio telemetry. High-resolution, three-dimensional measurements of the sound field were made out to ranges of 10 km and illustrate the influence of the laterally varying seabed, measured with a chirp sonar system. The precision navigation also enabled the creation of a two-dimensional, synthetic aperture planar array, parallel to the ocean surface. The pressure field data measured on the array were transformed into the wave number domain, where the lateral variability of the waveguide manifests itself in the spatially evolving spectral content of the modal field. Finally, the phases of the measured signals show remarkable stability and regularity, even in the context of complex, multimodal fields. This behavior can be exploited to make accurate estimates of the relative source/receiver speed from measurements of the time rate-of-change of the phase. [Work supported by ONR.]

Analysis of internal wave and bottom effects on the acoustic field generated by a towed source during the SWARM95 experiment

Thirty-second duration 250- to 375-Hz LFM pulses were transmitted from a towed acoustic transducer to study the effects of internal waves and the sea bottom on an acoustic signal propagating along a 30-km transmission path. The data were taken on the New Jersey continental shelf as a part of the multi-institutional SWARM95 experiment. Environmental data were collected by using CTDs, thermistor strings, a high-frequency acoustic flow visualization system, and a chirp sonar system. Geoacoustic properties of the bottom were inverted from the chirp sonar data with a 1.5-m vertical resolution and 40-m penetration. A deterministic sound speed field in the water column was constructed by imposing the high-frequency visualization measurements on both CTD and thermistor string data. Environmental data indicated that two distinct geological provinces and two major internal wave packets were crossed during the 30-km towed transmission. Using a full-wave broadband PE model, vertical structures of sound field were calculated for comparison to data obtained by a 32-element vertical line receiver array. Preliminary model/data comparisons indicate the dominance of water column effects over the bottom effects. [Work supported by ONR.]

Three-dimensional chirp sonar surveying of SWARM-95 experimental site

A full-spectrum chirp sonar system [LeBlanc et al., J. Acoust. Soc. Am. 91(1) (1992)] was used to invert three-dimensional density and sound-speed structure of the subbottom at the SWARM-95 (shallow water acoustics in random media) experimental site on the New Jersey Shelf. Biot’s theory was used in a forward model to calculate normal-incident reflection seismograms for layered marine sediments. Efficient inversion algorithms based on global optimization techniques were used to survey large areas for characterization of sediment properties and their spatial distributions. Chirp sonar inversion results were compared with ground-truth core data at the AMCOR-6010 site and good agreement was observed. Three-dimensional images of sediment density and sound-speed structure were constructed from several parallel runs along the SWARM acoustic track 1003 at the outer-shelf sediment wedge area. Corresponding 3-D wave-number spectra of density and sound-speed variability were also calculated.