ferric ions from the printing blade, In general, 
full-wave rectification is used in studies where 
signal envelopes are to be correlated, and half- 
wave for effectively correlating wave forms 
within envelopes. Signal phase may be deter- 
mined by selecting either the positive or the 
negative excursions of the signal. Scale or 
timing lines generated directly from the fork 
signal serve two purposes, measurement and 
indication of recorder synchronization. 
USES: Bathymetry 
The first use of this recorder in 
bathymetry still remains an important part 
of its job, During research cruises, the 
PGR's are routinely put to this task 24 hours 
a day. Consider what information is to be 
gained from echo-sounding, Travel time is 
the most obvious and is now measured with 
great accuracy. For the most part, resolution 
of about 30 milliseconds per inch is adequate. 
This resolution presents a 200-fathom depth 
interval across eighteen inch paper, In 
addition to its obvious use in hydrographic 
charts, this accurate information has been 
used by physical oceanographers at Woods Hole 
to guide their Nansen bottle lowerings to within 
a few feet of the bottom. More recently, 
however, they have used the acoustic pinger 
technique described later. Mooring deep-sea 
buoys also requires this knowledge of depth, 
for in one method used here, moorings are 
cut to match the water depth. 
The PGR's fastest sweeps (€ 10 milli- 
seconds per inch) are useful in the detection 
of subtle changes in travel time caused by 
gradual slopes found in the abyssal plains and 
elsewhere. The PGR can make these measure- 
ments because of the constancy of its timing. 
It is easily seen that cumulative sweep-time 
errors resulting from inconstant timing would 
raise havoc on a 50-millisecond sweep when 
displaying travel times several seconds long, 
The high resolution of which the PGR 
is capable has led to the discovery of small 
rough features of the bottom and also to areas 
of sedime nt ponding®, The vertical dimensions 
of these features seen with sounders are 
252 
relatively small. The in-the-water length of 
the pulse cannot be much greater than the 
dimensions of the features for the pulse alone 
will cause the detail we seek to be hidden, 
There is a part of the Blake Plateau, 
for instance, where, (see Fig. la and b) in 
spite of many crossings, several canyons and 
many peculiar bumps having only a few fathoms 
relief had not been detected with older sounders 
These were discovered when we surveyed the 
area with a PGR, using resolution of less than 
30 milliseconds per inch and short pulses of 
less than one millisecond®, Some indications 
of these features can be found on the old records, 
but the changes in the travel times defining the 
features are of the same order or less than 
the expected errors of the older equipment. 
In the sediment ponds, where a long 
pulse hides,Fig. 2 (from Hersey”), the 
existence of reflecting horizons below the sea- 
floor, the PGR controls pulse length to in-the- 
water dimensions which are hopefully less than 
twice the distance between the reflecting hori- 
zons, The structure of the echo-train reveals 
the presence of the horizons which is most 
important to display. Much of this work has 
been done with 0. 2 millisecond pulses. The 
PGR provides the bandwidth, 5000 cycles, 
necessary to permit these short pulses to 
pass to the recorder without distortion, 
Echoes! structures often change rapidly, 
Fig. 3, especially at ship's speeds of ten knots 
or more, yet the validity of these sub-sea- 
floor reflections depends upon the continuous 
correlation of particular returns over 
considerable distances. Strong correlation is 
obtained because the samples are taken closely 
enough in space and time so that substantially 
the same reflection recurs, because changes 
in transmission path geometry occurring 
between samples are less than the dimensions 
of the irregularities in the bottom, and because 
each of these samples is initiated on a precise 
time schedule. To achieve the necessary high 
sampling rate, we cannot wait to receive the 
echo from one pulse before sending another, 
but must have as many pulses as possible 
simultaneously in transit to and from the 
bottom. The PGR offers such a program, It 
is designed to have ''windows"' in the pulse 
schedule during which echoes may be received 
