Acoustical Physics, Vol. 46, No. 5, 2000, pp. 505–510. Translated from Akusticheskiœ Zhurnal, Vol. 46, No. 5, 2000, pp. 581–587. Original Russian Text Copyright © 2000 by Andreeva, Galybin, Tarasov. Vertical Structure of the Acoustic Characteristics of Deep Scattering Layers in the Ocean I. B. Andreeva, N. N. Galybin, and L. L. Tarasov Andreev Acoustics Institute, Russian Academy of Sciences, ul. Shvernika 4, Moscow, 117036 Russia e-mail: [email protected] Received December 27, 1999 Abstract—Quantitative data on the vertical structure of the acoustic characteristics of deep scattering layers in the ocean are obtained for the first time from multiple full-scale measurements. The measurement procedure used to obtain the acoustic data for thousands of geographic points of the World Ocean in the frequency range from 3 to 20 kHz is briefly described. A simplified model of the vertical structure under study is suggested in the form of several scattering sheets that are split in depth. The parameters of the model are the depths of the sheets, the contribution of every sheet to the total column strength, and the mean backscattering coefficient cor- responding to every sheet. The model is illustrated by the description of the daytime structure along five sections in the Atlantic Ocean; the lengths of these sections measure up to several thousand kilometers. The parameters of the model are shown to depend on the oceanic conditions and the sounding frequencies. The latter is evidence that, depending on frequency, the scattering is governed by different populations of the scattering layer inhab- itants differently responding to variations in hydrology. In particular, for sections crossing powerful currents, the changes in the vertical structure and the variations the model parameters essentially depend on the frequency of sound. © 2000 MAIK “Nauka/Interperiodica”. The properties of acoustic inhomogeneities of bio- This paper describes our data (obtained during the logical nature in the bulk of ocean waters continue to expeditions organized by the Andreev Acoustics Insti- attract the interest of researchers (see, e.g., [1]). The tute, Russian Academy of Sciences) on the vertical horizontally extended accumulations of small living structure of the acoustic characteristics of DSL for a creatures called deep scattering layers (DSL) represent number of areas of the Atlantic ocean. The data include one of the most widespread type of inhomogeneities. the depth-dependent coefficient of volume backscatter- They were investigated in many papers; however, this ing m and the column strength M, which is the integral of phenomenon is far from being exhaustively understood m over the depth z. The information was obtained in day- today. In particular, the vertical structure of the acoustic time for sounding frequencies from 3 to 20 kHz. The characteristics of DSL and its dependence on various experimental records are stored in the Andreev Acous- factors are poorly known. tics Institute’s database on the acoustic properties of DSL [2]. In a deep ocean, the scattering layers are character- ized by a fairly complicated structure in the vertical During these expeditions of the Andreev Acoustics direction. This structure depends on the ocean area and Institute, we used several procedures for studying the the hydrological and biological features characteristic vertical structure of DSL. On the one hand, these pro- of this area. The acoustical structure of DSL is poorly cedures differ in their errors and the reliability of the understood; at least, as far as we know, there is no results, and, on the other hand, they are of different exhaustive consideration of this problem in the litera- complexity and require different time for carrying out ture. The features of this structure are known only qual- the full-scale measurements. The most reliable data itatively; namely, by daytime, the scattering layers were obtained either by sounding the scattering accu- descend to depths of hundreds of meters, and, by night- mulations with directional multifrequency cw/pulsed time, they rise closer to the surface. This phenomenon lowered systems of transmission and reception or by is called diurnal vertical migrations of DSL. It is known alternative acoustic sounding of these accumulations to be less pronounced in polar regions where other tem- with explosive sound sources from above and from poral scales govern the daytime duration. The vast below. The first procedure requires the vessel to be majority of researchers who carried out acoustic mea- equipped with complicated stationary equipment. Both surements published only the results concerning the these procedures are time-consuming; the measure- total column strength at a given point in the ocean and ments at a single point can take several hours. Some give no information on the interval of depths where the results obtained with the second procedure are pub- scattering occurs. lished in [3] and demonstrate the unique potentiality of 1063-7710/00/4605-0505 $20.00 © 2000 MAIK “Nauka/Interperiodica” 506 ANDREEVA et al. the procedure for describing the acoustic structure of sheets were numbered. For every ith sheet, we calcu- DSL during both daytime and night. lated the depths of its upper and lower boundaries, the The well known procedure using explosions of backscattering coefficient m determined as the mean value of the backscattering coefficient between the small charges in the upper water layer as the sound ∆ source (see, e.g., [4]) offers a “prompt” method for upper and lower boundaries, and the percentage Mi determining the acoustic characteristics of DSL. This is corresponding to the contribution of the sheet to the not a precision procedure; however, it appears simple total column strength M0 of the layer. These character- and convenient for sounding the DSL in a wide range istics were considered as the parameters of the DSL of frequencies and allows one to estimate the funda- model. The sheets of the DSL model could either con- mental characteristics of DSL, including their vertical tact the adjacent sheets or be separated from them by structure. We obtained the data given below according layers of clean water. We considered the clean water to this very procedure. layer as significant when its width exceeded 20 m. For a narrower clean water layer, the sheets were combined We used a small trinitrotoluene charge (0.2 kg), in the course of processing. The data given below for M which was detonated in water near the surface, and a and m are measured in decibels relative to unity and to broadband omnidirectional receiver located near the 1 m–1, respectively. charge to record the pressure p of the scattered signal as a function of time t. Within the first 0.2–0.3 s after the Each experiment consisted of 5–12 explosions fol- explosion, the signal scattered by the surface could pre- lowing one after another every 3–5 min with subse- vail; however, later, the sound scattering in the water quent averaging of the measured results. In a single bulk predominated. The surface scattering determined experiment, the scatter in the measured column the skip depth of about 150–200 m and provided no strength and scattering coefficient did not exceed 1.0– reliable information on the vertical structure of DSL 1.5 dB, from one explosion to another; the correspond- during hours of darkness. The signal processing started ing scatter in the depths of the sheet boundaries was from six 1/3-octave filters in the range from 3 to below 10–15 m. Such a scatter can be considered as 20 kHz. In every frequency band, the dependence p(t) characteristic of random errors inherent in the measure- was squared and detected. At this point, we introduced ment procedure. For different experiments separated in corrections for the sound divergence and absorption time by several hours or days, the scattering in the during the propagation, as well as the factors describing experimental data was more significant; however, this the energy characteristics of the source and the receiver, scatter characterizes the instability of the phenomenon to calculate the column strength M as a function of rather than the measurement error. All experiments time t. Then, we recalculated the function M(t) for were carried out with the drifting vessel, which did not every frequency into M(z) assuming that the accumula- allow one to reliably distinguish between the spatial tion of scatterers is a plane-layered one. The total column and temporal variability of the results. strength is determined as M0 = M(zmax), where zmax is the Figure 1 shows the map with sections 1–5 along maximum depth of DSL recorded in the experiment. which the acoustic characteristics described in this At this point, the standard processing of experimen- paper were measured. The measurements were carried tal records is usually terminated (see, e.g., [5]); how- out from 1960 to 1974 aboard the research vessels Petr ever, we were able to advance the processing. In princi- Lebedev and Mikhail Lomonosov. ple, one can find the volume backscattering coefficient Figure 2 shows the model of the vertical acoustic as a function of depth m(z) by differentiating the mea- structure of DSL typical of a deep ocean and obtained sured function M(z) with respect to z. In our processing, from the data measured along section 1. The depths of we used a simplified procedure for calculating the the boundaries of the model sheets are slightly derivative. Namely, we approximated the curve M(z) smoothed along the section. The curves correspond to by several intersecting straight-line segments whose frequencies 5 and 20 kHz characteristic of DSL. For slopes were considered, to a certain scale, as the mean these frequencies, scatterers of different types predom- values of m over the approximation intervals (between inate in DSL.