238 
Fishery Bulletin 96(2), 1998 
Figure 1 
Cruise tracks of NOAA ship David Starr Jordan during Population of Delphinus Stocks (PODS) cruises (28 July- 
2 November 1992) and (28 July-6 November 1993). 
dB // l\xPa at 1 m) to the echo level (EL), integrated 
and recorded after 201ogR TVG amplification in the 
transceiver, was formulated as follows (Simmonds, 
1990): 
S v = EL - ( SL + RR) - 10 log \\i - 10 log ct/2, (1) 
where EL = 20 log V o (dB) (V o =“gain-controlled en- 
velope” signal, volts); 
SL = source level (dB); 
RR - receiver response (including receiver 
gain and transducer on-axis voltage re- 
sponse), dB; 
10 log \| / = equivalent beam angle; 
c = speed of sound (m/s); 
t = pulse length ( 1 x 1/1 ,000 per millisecond ). 
Transmission loss does not appear in this equa- 
tion because it was corrected by a 201ogR time-var- 
ied gain (TVG) function applied to V o in the trans- 
ceiver. The TVG was checked with a transceiver test 
mode that input a constant voltage signal. The re- 
corded 201ogR TVG output did not differ significantly 
from the theoretical function (TVG = 20 log/? + 2a R, 
where a is the nominal sound absorption coefficient 
of 0.008 dB/m and R is range in meters). The range 
(R) was based on a sound velocity entered into the 
control unit at the start of the cruise. 
On-axis sensitivity ( SL+RR ) was estimated by 
measuring the system response (EL) to a standard 
calibration sphere (38.1-mm diameter tungsten car- 
bide with 6% cobalt binder, target strength TS = - 
42.3 dB; Foote, 1990) suspended 7 m below the trans- 
ducer. In this case, Equation 1 reduces to 
EL = TS + (SL + RR), (2) 
where EL = 20 log V o + 20 log R. 
The “gain-controlled envelope” signal (V o ) represents 
a peak voltage and was converted to a root-mean- 
square (rms) voltage by multiplying by 1/V2. The 
additional 201ogR term was added to the system’s 
201ogR TVG, used during the echo-integration sur- 
veys, to correct for the spherical spreading of the re- 
turn from the point calibration target. Solving Equa- 
