Tide gage readings were recorded continu- 
ously throughout the day at a shore station, and 
these readings were radioed to the ship and then 
recorded directly on the thumper seismic pro- 
files. Fathometer readings were taken at one- 
minute intervals corresponding to sextant po- 
sition fixes on major lines and 1/2-minute 
fixes used for detail work. Corrected mean-low- 
water depths were recorded for each station. 
A lead sounding line was used to calibrate the 
fathometer and a 2-1/2-foot correction was 
added to each fathometer depth. These fathom- 
eter readings were not absolutely necessary. 
However, they simplified the correction factor 
for the depth of the thumper transducer, depth 
of the hydrophone receiver, and distance be- 
tween the transducer and receiver. A constant 
correction factor was readily determinable. 
by comparison of the fathometer depth to tue 
seismic profile recorded depth. This was ap- 
proximately 5-1/2 feet for the major portion 
of the survey. The transducer and hydrophone 
were towed at depths of approximately 3 feet 
each. The sediment velocity was estimated to 
be approximately that of sea water in the upper 
section of interest. This assumption is based 
on previous velocity studies in the general 
Narragansett Bay area by Woods Hole Oceanograph- 
ic Institution. 
Figure 7. Thumper record from Hydrographer's 
Canyon Area. The 200 fathom recording sweep 
was used, The depth scale is on the basis of 
4800 ft/sec water velocity. 
A number of areas were located in the c.an- 
nel where bedrock removal or boulder removal 
appears to be necessary if deepening is to be to 
45 feet below mean low water. The best quality 
records were obtained in the southern portion of 
the channel where the probable bedrock surface 
can be delineated at depths of over 150 feet be- 
low mean low water. Some portions of the channel 
are characterized by very strong initial reflec- 
tions and subsequent multiples. This highly re- 
flective channel floor condition often results 
in a serious loss of subsequent sub-bottom re- 
flections at these locations. The high reflec- 
tivity of this initial layer may be caused by 
above-average consolidation of the bottom 
sediments. 
The bedrock surface in this area generally 
appears to be extremely irregular, and it is 
often difficult to determine whether an irregu- 
lar reflecting surface represents bedrock or 
large and irregular boulders. A number of 
boulder areas are suspected in the channel area 
288 
on the basis of point-source parabolic patterns 
on the record, 
Because of the lack of usable core or 
probe information in this area, a probing study 
is recommended to corroborate the existence of 
boulders or bedrock in questionable areas. 
During the probing survey to be conducted, a 
close check on seismic records and correlations 
to probing will give considerably more informa- 
tion than was extracted from the preliminary 
study. 
B. Thumper Field Test by the U. S. Coast 
and Geodetic Survey 
A short familiarization and test run was 
made of the Sonar Thumper on board the U. S. 
Coast and Geodetic Survey vessel, the "Explorer", 
July 20 through July 24, 1961. Comparisons were 
made between the transducer mounted inside the 
ship in a well situated below the water line and 
on an externally towed transducer. The pulses 
from the internal well mounted transducer suf- 
fered considerable transmission losses through 
the hull of the ship. However, usable results 
can be obtained with an internal well mounted 
transducer, although depth of penetration is 
reduced, 
A number of thumper profiles were obtained 
across Hydrographer's Canyon. One of these is 
shown in Figure 7. The depth of penetration on 
these tests varied somewhat but was usually on 
the order of 400 to 600 feet and in some cases 
weak, but continuous events were detected at 
depths exceeding 1,000 feet. Water depth gener- 
ally varied from 50 to 300 fathoms. The stand- 
ard thumper was used for these tests and the EGG 
Sonar Recorder was used for recording. The ex- 
ternal transducer was mounted on a V-fin fish. 
A V-fin hydrophone and the EG&G Rocket Hydro- 
phone were used alternatively as receivers. 
VIII. CONCLUSION 
Many off-shore areas of the world have 
never been explored for possible oil structures 
because of the tremendous expense of maintaining 
conventional seismic crews and the expense and 
difficulty of obtaining explosives. Some locali- 
ties also forbid the use of explosives in off- 
shore seismic work because of possible damage to 
fish and navigational hazards to shipping in har- 
bors. Large thumper units of 5000 watt-seconds 
on up can of ten give sufficient penetration to 
be of great value as a reconnaissance tool in 
off-shore oil prospecting. The Sonar Thumper 
is simple, reliable and rugged, and requires 
uothing more than 110 or 220 volts a.c. for 
power. The effectiveness of the Sonar Thumper 
and high powers available, along with continuing 
improvements in receiving and recording equip- 
ment, are all factors which point toward the 
increasing future use of the thumper as an off- 
shcre oil prospecting tool. 
