The power supply should be situated 
in a well ventilated area near the generator or 
other source of 110 or 220 line voltage. If the 
power supply must be placed at some distance 
from the generator, the thumper should be 
operated on 220 volts to reduce peak currents. 
To change over from 110 to 220 volt operation 
requires a simple change of transformer jumpers. 
The capacitor bank should be placed 
relatively close to the point where the output 
cable to the transducer leaves the ship in 
order to keep the high current cable as short 
as possible, High voltage D.C. from the power 
supply is brought into the capacitor bank and 
charges the 160 microfarad bank to approximately 
4 kv. A 110-115 volt a-c line is also plugged 
into the capacitor bank to power heater lamps, 
safety interlocks, the trigger circuit, and 
indicator lights. 
After installation, the unit can be 
checked by firing into a dummy transducer con- 
sisting of a 2-ohm, 200 watt resistor or a 
standard transducer with aluminum plate removed. 
The standard transducer should never be fired 
in the air without a damping water load. When 
fired, the force is so great the transducer 
plate of the standard thumper will leap 30 to 40 
feet vertically if it is fired in air without 
its retaining bolt. Testing at the dock can be 
carried out by lowering the transducer into the 
water alongside the ship. Proper grounding of 
all units should be checked before firing. 
For operation underway, a towing 
vehicle is necessary for the transducer. A 
number of vehicles have been used. The Naval 
Electronics Lab initially mounted the transducer 
on the hull of the ship and now uses an aluminum 
towing vane. The U.S. Coast and Geodetic Survey 
has used the transducer mounted in an internal 
water filled hull well to transmit pulses through 
the hull. Woods Hole Oceanographic Institute has 
a sled with heavy nose weight which they call the 
"flounder". The National Institute of Ocean- 
ography of England uses a fiberglass dinghy wi*1 
a reaction mass above the transducer of 200 lbs. 
of concrete. Two towing vehicles are now 
commercially available. One is a fiberglass 
"y'efin (1) designed with negative lift and the 
other is an arrow-like unit with a counterweight 
in Front and the transducer mounted in the 
tail . 
The size-of the boat used has varied 
widely from 30 ft or 40 ft fishing boats to large 
naval survey vessels. The only requirement is 
sufficient enclosed cabin space for the thumper 
and recorder components. Towing booms are easily 
rigged for support of the transducer fish. For 
small vessels, a 4" x 4" timber across the stern 
or a boat davit can be used. The transducer 
fish is towed by means of a rope or cable which 
OE —————E———————————— 
(1) Braincon Corporation Type 108 V-Fin 
(2) EG&G Model 261 Transducer Fish 
285 
should withstand at least 1,000 pounds for an 
adequate safety factor. Cable tension depends 
on towing speed and is considerably less than 
1,000 pounds unless fast speeds of over © knots 
are required. Ordinarily, water, and ship noises 
become excessive above 5 knots; thcrefore towing 
usually will be below this speed. 
The larger thumper units simply require 
a little more space for electronic componerts 
than the standard model. The symmetrical trans- 
ducer can be mounted horizontally or vertically 
because the frequency of the pulse is so low 
that it creates an essentially spherical wave 
front with equal signal intensity in all direc- 
tions, 
III. Receiving System 
A. Noise Problem 
Before reviewing possible hydrophone 
setups for a receiving system to use with the ; 
thumper, a consideration of noise problems is in 
order. The sources of troublesome noise which 
may partially or completely mask the seismic sig- 
nal are: 1) Ship screw noises from propeller 
cavitation and drive shaft vibration, 2) Engine 
vibration transmitted through the ship's hull, 
-) Cavitation or burble produced by the hydro- 
piione and towing element moving through the water. 
Each vessel has its own characteristic 
noise level at various speeds. The optimum opera- 
ting speed is the maximum speed possible at which 
useable records can be obtained. Ship noises can 
be partially eliminated by towing either at a 
considerable distance behind the ship or on large 
vessels a long boom off the bow is sometimes 
effective. Distance between the receiver and the 
sound source can be varied considerably. Separa 
tions of several hundred feet can be tolerated 
in relatively deep water. For shallow water re- 
flection work, the receiver should be relatively 
close to the thumper if an accurate representa- 
tion of shallow reflection layers is desired. 
B. Hydrophones 
A number of hydrophones are commercially 
available which are adaptable to the thumper 
system, The pressure type hydrophone with wide 
response is preferable. Preamplification of the 
signal may be necessary if a considerable length 
of cable is used. The preamplifier can be sealed 
jn with the hydrophone unit or it can be used on 
board the vessel before the signal is fed to the 
recorder amplifiers. 
One approach for increasing signal to 
noise ratio is the use of a multiple crystal 
arrangement of detectors hooked in parallel with 
a null along the axis to partially cancel noise. 
