Page 41 - Acoustics Today Summer 2011

Fig. 4. (a) The wide band backscatter spectrum predicted from a simple bar target model (arbitrary log intensity units). (b) The wide band backscatter measurement from a single fish measured in a test tank facility. (c) The inferred orientation of the bar target, derived from the data in (a). (d) The inferred orientation of the fish, derived from the data in (b).
Figure 4 shows pictures of the logarithm of the intensity of the fraction of backscattered sound, related to target strength, as a function of frequency and orientation for both a simple bar target model (a) and a live animal (b). Based on the insight that the data consist of radially spreading spokes, going from the bottom right to the top left, locally perpen- dicular to the animal’s orientation, an algorithm was formu- lated to compute the direction perpendicular to those spokes to estimate orientation. This fortuitously turned out to be a gradient operator; however, the application of the gradient to a noisy set of data like (b) is a challenge. The results whose details are describe in (Jaffe and Roberts, 2011), where the local estimate of orientation, as depicted by the direction of the arrow as a function of frequency and true orientation are shown for the simple bar target model (c) derived from (a) and the more complicated data (d) derived from (b). The results indicate that mostly, a reasonable estimate of orienta- tion can be derived. However, there are clearly some prob-
lem areas as well. Nevertheless, because most animals are orientated at nearly 180 degrees in the field, it is hoped the pragmatic application of this method to a next generation of field deployable systems, when coupled with better models and more advanced signal processing, will yield ever-more accurate estimates of animal orientation and hence, permit the measurement of the size spectrum of marine animal populations.
Identifying individual clicking whales acoustically
In the last decade many new signal processing algo- rithms have been developed to understand social organiza- tion, population dynamics, and behavior patterns of large marine mammals, such as sperm and beaked whales. Because acoustics is their primary exploration and communication tool, it has been suggested that click acoustic characteristics and time patterns carry attributes of individuals among a group. Success in identifying individual marine mammals
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