uncluttered by the direct reception of the pulse 
and its reverberation, Programs are based 
upon a count of integral recorder sweeps. 
They require the sweep period to be several 
times less than the travel times in deep water. 
The reflecting horizons below the sea- 
floor are often the boundaries of lenses of 
differing materials; they may be pinched out 
at the sea floor and more ancient formations 
exposed. The PGR gives a permanent display 
of their echo structures, which may be used 
for later study or for immediate use in 
coordinating coring operations. Since 12 kc 
attenuates rapidly in the sediments these 
reflections are most often only tens of feet 
deep, but they are within the reach of deep-sea 
corers, 
It might be asked why an oscilloscope 
isn't used. A scope is usually a part of the 
instrumentation, but scope photographs, are 
not easily compared with one another ata 
rate of several thousand an hour, Although 
they give needed amplitude information it is 
often difficult to obtain representative 
amplitude differences within the echo structure 
from only a few samples, because of the fluc- 
tuation found in such measurements, A visual 
integration of the returns on the PGR record 
is a useful guide in these bottom reflectivity 
studies. 
We also use precision soundings for 
navigation, 7,8 and often moor deep sea buoys 
on a particular bottom contour, a line of 
position easily followed with the PGR. 
Seismic Reflection and Refraction 
Profiling 
We record seismic reflections ina 
system made up of a broad spectrum sound 
source actuated by the triggering mechanism 
of the PGR which in turn records the signals 
received by a broadband hydrophone suspended 
from the ship, Adjustable bandwidth filters 
are used to filter the signal in differing 
frequency bands for each channel of the PGR. 
This system is known as the Continuous 
Seismic Profiler 29,10 Fig, 4, In its 
development at Woods Hole the sources most 
used have been the underwater spark and the 
E. G. and G. 'Thumper''. Both have higher 
peak pressures than most sounders, but in 
narrow bandwidths at the necessary low 
frequencies their output is generally below 
100 db above a microbar, Yet we obtain good 
253 
penetration of the sea-floor from these 
relatively weak sources, and attribute a great 
part of this success to the correlation 
capabilities of the PGR, These electrical 
sources are designed to follow the moderately 
high repetition rates of the PGR, which are at 
least an order of magnitude greater than those 
used in conventional explosive methods, The 
signal is coherent and generally remains so 
after sea-surface, sea-floor, and sub-sea- 
floor reflections, see Fig. 5. The wave 
length in water at these low frequencies is 
great enough and the sampling rate fast enough, 
that changes in geometry or sub-sea- floor 
structure do not erratically interrupt the 
correlation of the wave forms in the returning 
signals. Weak signals are traced through noise 
with unusual ease, (Fig. 6) for the PGR is in 
fact an autocorrelation device when it can 
control the timing of events, 
The PGR secord from vertical seismic 
reflections gives us a cross-section profile 
of the structure below the bottom, but the 
depth scale must be determined and corrected 
by propagation velocities obtained from oblique 
reflection and refraction data taken with the 
same system. Refraction arrivals, when 
present, can be immediately identified (Fig. 7) 
and compressional wave velocities readily 
computed. In their absence there is an 
overabundance of reflection data for a more 
complicated velocity determination 12,13, 
An oscilloscope and tape recorder are necessary 
accessories to this instrument, however; the 
scope for instantaneous observations and 
photography, the tape for storage and future 
analysis. There are simply not enough PGR 
channels available at one time, even if ''master 
and slave'' techniques are used. 
Location of Instruments and other 
Objects 
In addition to high resolution echo sound- 
ing studies, the PGR is used for echo location, 
the finding and identifying of objects suspended 
in the water. These objects may be animal, 
as in the case of fish, porpoises, or scattering 
layer**; they may be instruments, buoys which 
can be tracked and recovered, cameras!9, 
corers!®, nets and dredges!7, whose lowering, 
placing and raising can be accurately monitored 
with this system, 
Echo location techniques employ not only 
the shipboard transducers of sounders or 
