Research Letter | Open Access

# Bearing and Range Estimation Algorithm for Buried Object in Underwater Acoustics

**Academic Editor:**Miguel Lagunas

#### Abstract

A new algorithm which associates (Multiple Signal Classification) MUSIC with acoustic scattering model for bearing and range estimation is proposed. This algorithm takes into account the reflection and the refraction of wave in the interface of water-sediment in underwater acoustics. A new directional vector, which contains the Direction-Of-Arrival (DOA) of objects and objects-sensors distances, is used in MUSIC algorithm instead of classical model. The influence of the depth of buried objects is discussed. Finally, the numerical results are given in the case of buried cylindrical shells.

#### 1. Introduction

The main target of Array processing is to estimate the bearing and range of sources or objects radiating in a medium of propagation [1]. MUSIC is one of the commonly used high resolution algorithms for DOA estimation. It uses the orthogonality property between the signal subspace and the noise subspace to localize sources [2].

In this letter, we propose a new algorithm for bearing and range estimation of buried objects in underwater acoustics. The approach is based on MUSIC combined with the acoustic scattering model [3, 4]. We consider the reflection and the refraction of wave at water-sediment interface. This method develops a new source steering vector including the information of bearing and range of buried objects. The vector is used in MUSIC algorithm instead of the classical plane wave model [5]. The attenuation in the sediment is distinct for the objects buried deep, so we discuss the influence of the depth of buried objects. The proposed algorithm is evaluated by numerical simulations in the case of buried cylindrical shells.

The remainder of the letter is organized as follows. Section 2 summarizes the problem formulation. In Section 3, the scattering acoustic model of generating the received signals is discussed. An algorithm is proposed in Section 4. Next, the influence of the depth of buried objects is presented in Section 5. Finally some numerical results are addressed in Sections 5 and 6 conclude the paper.

Throughout the paper, lowercase boldface letters represent vectors, uppercase boldface letters represent matrices, and lower and uppercase letters represent scalars. The symbol โโ is used for transpose operation. The superscript โ+โ is used to denote complex conjugate transpose and denotes the norm for complex vectors.

#### 2. Problem Formulation

Consider a linear array of sensors receives the signals scattered from objects (). The received signal can be grouped into a vector written as where is the transfer matrix, is the vector of signal, and is the vector of additive Gaussian noise.

The wavefront is assumed to be plane when the objects are far from the array. We use MUSIC algorithm to estimate the angle of the plane wave associated with the objects where is the steering vector and , is the the matrix of eigenvectors spanned by the noise subspace, is the sound speed, is the distance of sensors, and is the complex operator.

#### 3. Scattering Acoustic Model: To Generate the Received Signals

We assume an object is buried in the sediment associated to the first sensor of the array. An incident plane wave propagating in the water reaches the interface with (see Figure 1). A reflected plane wave is generated in the water and a refracted plane wave is propagated in the sediment. So the array receives three components [6]:

The pressures in the water and the sediment are given by five unknown parameters , , , , and the depth of buried object based on and (see Figure 1): where denotes

We consider the case of infinitely long elastic cylinder shell. The first sensor of the array receives the acoustic pressure components as follows: where is the pressure incident in the water: is the pressure reflected by the sediment-water interface: where is the reflection coefficient of the interface.

is the diffused acoustic pressure wave transmitted in the water [7]: where is the identity matrix, is a linear operator, is the transition diagonal matrix, is the vector of transmitted wave, and is defined by where is the transmission coefficient.

#### 4. Algorithm for Bearing and Range Estimation of Buried Objects

(1)Find an initial estimation of , , and the number of objects by the beam forming method.(2)Fill the matrice and the components are filled with cylindrical scattering model for . The first vector is given by (4). The other for associated with the th sensor can be formed by (see Figure 1) (3)Estimate the spectral matrix .(4)Calculate sources spectral matrices by where is the identity matrix and is the noise variance.(5)Compute the average of the spectral matrices where represents the number of frequencies and is the center frequency of the spectrum of the received signals.(6)Caltulate .(7)Use the eigenvectors to obtain the focusing operator, where and are, respectively, the eigenvector matrices of and .(8)Calculate the focused spectral matrix: (9)Estimate the number of objects by AIC or MDL [8].Calculate the spatial spectrum of MUSIC method for estimating bearing and range of buried objects: where is the eigenvector matrix of noise subspace.

#### 5. Influence of the Depth of Buried Objects

In sandy sediment, the attenuating effect of suspended material is negligible. Conversely, the attenuating effect of the sediment is significant. It can be reported in dB/cm/kHz since the examination of attenuation yielded a linear dependency with frequency. The attenuation coefficient of common sand [9] is โdB/cm/kHz.

#### 6. Numerical Results

The parameters of the simulations are defined as follows: is 0.002โm, is 10, the frequency band is [200, 300]โkHz, and the signal frequency is 250โkHz. The wave speed in the water is 1500โm/s and in the sediment is 1700โm/s. The water density is 1000โkg/, the sediment density is 1500โkg/. The incidence angle is 60ยฐ.

The array is placed in the water with the hight โm. The variance of the noise is 100 and SNR is 30โdB. As shown in Figure 2, the white points coordinate two cylindrical shells (30ยฐ, 0.35โm) and (49ยฐ, 0.16โm).

When the cylindrical shell is deeply buried (20ยฐ, 0.3โm), we vary the interface of water-sediment for each . The signal may be unworkable if the object is buried deep. Furthermore, we evaluate statistically the influence of the depth of buried objects by Standard Deviation: where is the bearing or the range and .

The results obtained (see Figures 3 and 4) show that the algorithm is not efficient when the object is buried deeper than 0.25โm.

#### 7. Conclusion

In this paper, we propose a new algorithm based on MUSIC associated with acoustic scattering model for bearing and range estimation of buried objects. There is an analogy of the water-sediment interface by combining with the reflection and the refraction of wave in the model. A new directional vector, including the information for bearing and range estimation, is employed instead of the plane wave model in the MUSIC algorithm. The results of buried cylindrical shells are significantly accurate. Then we study the influence of the depth. The results show that beyond a certain depth, the attenuation becomes too large and therefore the objects cannot be detected or located neither.

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#### Copyright

Copyright © 2009 Dong Han et al. 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.