tal program should be carried out initially to 
optimize filter settings and field procedure. 
The initial program may require several days, 
particularly if the survey area is extensive. 
An experimental program should be carried 
out in a geologically known area if possible. 
If cores or probe information is available in 
an area, initial thumping should be done over 
these areas. Filter settings should be opti- 
mized in a known area to best delineate the 
features of greatest interest, such as bedrock 
depth. If any known structural features near 
the prospect area, such as faults, can be 
initially delineated, it will be of great help 
in recognizing similar features in the unknown 
area. 
Optimum boat speed can be determined 
during initial work. The filter settings, gain, 
paper speed, positive, negative, or full-wave 
recording, and the most efficient procedures 
can also be optimized. 
If radio positioning techniques are used, 
calibration and position-checking can be done 
during the initial program. Whenever a posi- 
tion fix is made, a remotely controlled marker 
can mark the record. For rough reconnaissance 
work, celestial navigation and dead reckoning 
may be sufficiently accurate. If the survey is 
carried out near shore, it may be possible to 
obtain accurate fixes from charted shore land- 
marks. 
It is desirable to keep one man stationed at 
the recorder to record station numbers continu- 
ously and monitor recording results as the survey 
progresses. Any features of particular interest 
can be immediately marked on a map for future 
reference. The surveyor, navigator, or radio 
positioning operator can automatically mark the 
thumper record with the remote event marker when 
a position fix is made. The Sonar Recorder op- 
erator marks the corresponding time or station 
number directly on the record to correspond to 
the event mark. 
VII. APPLICATIONS AND RESULTS 
A. Providence River Channel Survey for the 
U. S. Corps of Engineers 
The Providence River Channel thumper survey 
was done to delineate portions of the channel 
where bedrock or boulder removal will be neces- 
sary for a dredging grade level of 45 feet below 
mean low water. The field survey was commenced 
8 May 1961 and completed 19 May 1961. The first 
day was spent in setting up equipment and the 
second day was used for experimental work. 
The vessel used for the survey was a modern 
40-foot fishing boat with flying bridge. The 
boat was equipped with two-way radio and a 
fathometer. 
287 
Boat positioning was done by sextant angle 
to charted landmarks on the shore. Sextant 
augle charts covering most of the Providence 
River Channel area enabled the surveyors to plot 
station positions continuously as the survey 
proceeded. 
Equipment used consisted of the standard 
Sonar Thumper unit, the Sonar Recorder, a BC50 
Atlantic Research hydrophone, and a Minneapolis- 
Honeywell lollipop hydrophone, and "V"'-Fin 
towing fishes for the transducer and the re- 
ceiver hydrophone. Two electrical generators 
were used, one supplying 2 kilowatts for the 
Sonar Thumper and a small auxiliary generator 
to supply the recorder power. 
Three main longitudinal lines were run 
along the channel, one down the middle, and the 
other two toward the sides of the channel. In 
areas where shallow bedrock or boulders were 
suspected, additional cross lines were run for 
added detail. Preliminary thumping was done 
near the area of previous core information. 
However, the only cores to bedrock in this area 
were in shallow water which was not navigable 
for the vessel. Good core information was avail- 
able south of the channel area west of Patience 
Island. A thumper traverse was run in this area 
to correlate to the core information. A portion 
of this thumper record is illustrated in Fig. 6. 
The Sonar Thumper was fired at a half-second 
repetition rate and half power. This faster 
rate gives greater resolution and is recommended 
for seismic studies which do not involve great 
depths. 
Figure 6. Thumper record of a traverse west of 
Patience Island, Narragansett Bay. The 50 
fathom sweep of the Sonar Recorder was used. 
The channel surface is shown underlain by an 
irregular reflection surface at 100 to 120 ft. 
Core information in this area shows bedrock at 
a depth of 111 ft corresponding to the irregular 
reflection surface. 
The optimum filter setting for this survey 
appeared to be 300 cps on the low end and 1200 
cps on the high end. Recording was done on the 
positive half-wave form. Paper speed was nor- 
mally 2.2 inches per minute, The gain attenu- 
ation setting normally used on this survey was 
OQ db, which represents full gain without a pre- 
amplifier. The recorder has a wide range of 
attenuation settings, from O db to 80 db. 
