9. Echolocation in Dolphins
383
3.2 Target Detection in Reverberation
A second way in which a sonar system can be limited is by the presence of
reverberation. Reverberation differs from noise in several aspects. It is
caused by the sonar itself and is the total contribution of unwanted echoes
scattered back from objects and inhomogeneities in the medium and on its
boundaries. The spectral characteristics of reverberation are similar to those
of the projected signal and its intensity is directly proportional to the intensity of the projected signal. Therefore, in a reverberation-limited situation,
target detection cannot be improved by increasing the intensity of the projected signal. Target detection becomes dependent on the ability of the
system to discriminate between the target of interest and false targets and
clutter that contribute to the reverberation.
Titov (1972), as reported by Ayrapet'yants and Konstantinov (1974) and
Bel'kovich and Dubrovskiy (1976), investigated the capability of T. truncatus to detect targets in the presence of smooth rocks, varying in size from
5 to 30 mm in 5-mm increments forming two 40-cm circles on the bottom
of a tank; each circle containing 300 rocks, 50 of each size. The dolphin could
detect a 50-mm lead sphere lying on the bottom in the center of the clutter
75% of the time at a distance of 5 m. A 33-mm solid steel sphere had to be
raised 1.7 cm above the largest rock before the animal could detect it 75%
of the time at a distance of 5 m. The dolphin approached the target swimming close to the bottom rather than at the surface, presumably to minimize the reverberation. Murchison (1980) studied the effects of bottom
reverberation on the target detection capabilities of two T. truncatus in
Kaneohe Bay. A 6.35-cm diameter solid steel sphere was used at depths
varying from 1.2 to 6.3 m. At a depth of 6.3 m, the target was on the bottom.
The animals' 50% correct detection threshold ranges for the different target
depths are plotted in Figure 9.13. As the target depth increased, the animals'
detection ranges decreased, showing the effects of bottom reverberation.
Au (1992) used a simulated dolphin sonar signal to measure the scattering strength of the bottom where Murchison's (1980) experiment was
conducted. The target strength of the 6.35-cm diameter solid steel sphere
was also measured with a simulated dolphin sonar signal and found to
be approximately -33.4 dB. Inserting the appropriate values into the reverberation-limited form of the sonar equation, the echo-to-reverberation
ratio at the dolphins' detection threshold was estimated to be approximately 4 dB.
The target echo and the bottom reverberation corresponding to an Ee/R e
of 4dB are shown in Figure 9.14. Also include in the figure are the sum of
the target echo with the bottom reverberation and the spectra of the
summed echo, denoted by the solid line, and the target echo, denoted by
the dashed line. The effects of the bottom reverberation in masking the
signal in both the time and frequency domains can be seen in Figure 9.14.
The echo plus reverberation waveform does not contain the distinct high-
383
3.2 Target Detection in Reverberation
A second way in which a sonar system can be limited is by the presence of
reverberation. Reverberation differs from noise in several aspects. It is
caused by the sonar itself and is the total contribution of unwanted echoes
scattered back from objects and inhomogeneities in the medium and on its
boundaries. The spectral characteristics of reverberation are similar to those
of the projected signal and its intensity is directly proportional to the intensity of the projected signal. Therefore, in a reverberation-limited situation,
target detection cannot be improved by increasing the intensity of the projected signal. Target detection becomes dependent on the ability of the
system to discriminate between the target of interest and false targets and
clutter that contribute to the reverberation.
Titov (1972), as reported by Ayrapet'yants and Konstantinov (1974) and
Bel'kovich and Dubrovskiy (1976), investigated the capability of T. truncatus to detect targets in the presence of smooth rocks, varying in size from
5 to 30 mm in 5-mm increments forming two 40-cm circles on the bottom
of a tank; each circle containing 300 rocks, 50 of each size. The dolphin could
detect a 50-mm lead sphere lying on the bottom in the center of the clutter
75% of the time at a distance of 5 m. A 33-mm solid steel sphere had to be
raised 1.7 cm above the largest rock before the animal could detect it 75%
of the time at a distance of 5 m. The dolphin approached the target swimming close to the bottom rather than at the surface, presumably to minimize the reverberation. Murchison (1980) studied the effects of bottom
reverberation on the target detection capabilities of two T. truncatus in
Kaneohe Bay. A 6.35-cm diameter solid steel sphere was used at depths
varying from 1.2 to 6.3 m. At a depth of 6.3 m, the target was on the bottom.
The animals' 50% correct detection threshold ranges for the different target
depths are plotted in Figure 9.13. As the target depth increased, the animals'
detection ranges decreased, showing the effects of bottom reverberation.
Au (1992) used a simulated dolphin sonar signal to measure the scattering strength of the bottom where Murchison's (1980) experiment was
conducted. The target strength of the 6.35-cm diameter solid steel sphere
was also measured with a simulated dolphin sonar signal and found to
be approximately -33.4 dB. Inserting the appropriate values into the reverberation-limited form of the sonar equation, the echo-to-reverberation
ratio at the dolphins' detection threshold was estimated to be approximately 4 dB.
The target echo and the bottom reverberation corresponding to an Ee/R e
of 4dB are shown in Figure 9.14. Also include in the figure are the sum of
the target echo with the bottom reverberation and the spectra of the
summed echo, denoted by the solid line, and the target echo, denoted by
the dashed line. The effects of the bottom reverberation in masking the
signal in both the time and frequency domains can be seen in Figure 9.14.
The echo plus reverberation waveform does not contain the distinct high-