International Journal of Oceanography

Volume 2015 (2015), Article ID 589463, 11 pages

http://dx.doi.org/10.1155/2015/589463

## Effects of Noise and Absorption on High Frequency Measurements of Acoustic-Backscatter from Fish

Tokyo University of Marine Science and Technology, 4-5-7 Kounan, Minato, Tokyo 108-8477, Japan

Received 7 June 2015; Accepted 12 October 2015

Academic Editor: Renzo Perissinotto

Copyright © 2015 Masahiko Furusawa. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

Quantitative echosounders operating at multiple frequencies (e.g., 18, 38, 70, 120, 200, 333, and 710 kHz) are often used to observe fish and zooplankton and identify their species. At frequencies above 100 kHz, the absorption attenuation increases rapidly and decreases the signal-to-noise ratio (SNR). Also, incomplete compensation for the attenuation may result in measurement error. This paper addresses the effects of the attenuation and noise on high frequency measurements of acoustic backscatter from fish. It is shown that measurements of a fish with target strength of −40 dB at 200 m depth are limited by SNR to frequencies up to about 100 kHz. Above 100 kHz, absorption coefficients must be matched to local environmental conditions.

#### 1. Introduction

Quantitative echosounders (QES) or scientific echosounders have evolved for routine use in acoustic surveys of fisheries resources [1]. Traditionally, QES operate at 38 kHz with low attenuation, a high signal-to-noise ratio (SNR), and small measurement errors [2]. Recently, however, QES are often operated at multiple frequencies including higher frequencies, for example, 18, 38, 70, 120, 200, 333, and 400 kHz [3]; 70, 120, 200, 333, and 710 kHz [4]; and 125, 200, 455, and 770 kHz [5], to classify and study fish and zooplankton. Measurements at the higher frequencies, particularly those above 100 kHz, are affected by increasingly high and uncertain absorption attenuation and therefore increasingly low SNR and measurement accuracy. This paper highlights some considerations when measuring with high frequencies, estimates associated errors, and proposes mitigation measures to obtain precise and accurate data.

The frequency dependence of backscatter provides useful information for fish species identification [6, 7]. To evaluate the accuracy that can be achieved by multifrequency QES, the SNR is quantified, considering the limitation of transmit power due to nonlinear phenomenon, and acoustic noise from various sources. Then, effective measurement ranges are identified for various frequencies using a modified diagram of QES performance [2]. Also, error from absorption attenuation at high frequencies is quantified and discussed.

#### 2. Materials and Methods

##### 2.1. Noise

There are several sources of acoustic noise received by a QES (Figure 1), such as rain, wind-induced wave, snapping shrimp, self-vessel, and heat (thermal noise). Similar to [2], the noise power spectral level of the vessel self-noise is modeled here aswhere is the noise power spectral level for frequency projected to 1 Hz. In the case of the upper vessel self-noise [8] in Figure 1, . (Throughout this paper, the MKS system of units is kept in equations, but actual values are expressed sometimes in convenient units as in this case.) The lower vessel self-noise in Figure 1 was obtained by fitting (1) to the noise spectral level data for a noise-reduced vessel [9], and the value of is 142 dB. The thermal noise spectrum was deduced from the formula in [10] and expressed aswhere dB re 1 *μ*Pa^{2} Hz^{−3}. Although noise from rain or shrimp may occasionally dominate the noise level for short periods or small regions, noise below 100 kHz is generally dominated by the vessel self-noise. Electrical noise in the QES transducer and receiver may be significant at higher frequencies and will be considered later.