Reduction of deep sea refraction data : technical report no. 1

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Lamont Geological Observatory
Palisades, New York

Columbia Auiversity

Technical Report No. 1
(Contract NO bsr 43355)

MBLWHOI Librai

0030110000844

Lamont Geological Observatory
(Columbia University)
Palisades, New York

Reduction of Deep Sea Refraction Data
Technical Report No. 1

by
Charles B. Officer
Paul C. Wuenschel

The research reported in this document has been made
possible through support and sponsonship extended by
the U. S. Navy, Bureau of Ships under Contract NObsr
45555. It is published for technical information only
and does not represent recommendations or conclusions
of the sponsoring agency.

August 1951

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Table of Contents

Introduction

Shot Instant Correction

Sinking Rates of Explosive Charges
Reduction to Surface of Reference
Topographic Correction

Curved Ray Paths

Sound Velocities in the Ocean
Check Plots

Ry - D versus Ry Graphs

Check List of Data Necessary for

Complete Reduction

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SiO ts Figures

Shot and Shooting Ship

Bottom and Surface=bottom Reflections

inking
Sinking
Sinking
Sinking
Sinking
Sinking
Sinking
Sinking
Sinking
Sinking

Rate
Rate
Rate
Rate
Rate
Rate
Rate
Rate
Rate
Rate

Ray path for

Ray paths for D, Ry»
Graph for Correction
Graph for Correction
Graph for Correction
Graph for Correction

Graph of Topographic

Graph of Ry

Graph
Graph
Graph
Graph
Graph
Graph
Graph
Graph
Graph
Graph

Ry

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for
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Z - 1 # Charges of 4 # Blocks

3 - 5 # Charges of = # Blocks

8 - 10 # Charges of 3 # Blocks
18 - 25 # Charges of = # Blocks
50 # Charges of = # Blocks

25 # Canadian Depth Charges

Mk 42T 100 # Aerial Bombs

Mk 30 100 # Aerial Eombs

Mk VI 300 # Depth Charges

Mk 54 350 # Aerial Depth Charges

and G

of Ry to Reference Surface
of Ro to Reference Surface
of Rz to Reference Surface
of G to Reference Surface
Corrections

- D versus Ry

Reduction of Deep Sea Refraction Data
Charles B. Officer and Paul C- Wuenschel

Ie Introduction

During the summer of 1950 an extensive two ship
deep sea refraction operation was made on Atlantis 164 and
Caryn 17 cruises. A total of seventy refraction stations
were made on tracks from Bermuda to Charleston, Norfolk to
Bermuda, Bermuda to the Nares Deep, Bermuda to Halifax, and
Halifax to Woods Hole. Again during the spring of 1951 the
opportunity arose to make another two ship refraction
operation on Atlantis 172 and Caryn 22 cruises. Thirty-
three profiles were made covering the Caribbean, the Puerto
Rico Trough, and a track from Puerto Rico to Bermuda to
Woods Holee

It has been necessary to make several corrections
and reductions to the raw data in order to obtain the final
values for the travel time plotse The method of reduction
of the deep sea refraction data is not found in any of the
standard geophysical texts, and that used in shallow water
seismic work is not applicable. It has been necessary to
devise new methods as the occasion arosee These methods of
reduction are, in general, quite simple; but it has been
thought advantageous to present them in this form for the
benefit of and use in future investigations. Further it

is hoped that this report results in a uniform method of

reducing deep sea refraction data. The several graphs
that have been found helpful in carrying out the
reductions have also been included in such a manner that

they may be used directly from the reporte

II. Shot Instant Correction

In a two ship refraction operation one ship, say
the Atlantis, heaves to and lowers her hydrophones prepara-
tory to receiving. The Caryn then proceeds on course shooting
the necessary shots to make the profile. After completing the
shooting, the Caryn heaves to and prepares to receive, as the
Atlantis gets underway on course toward the Caryn, firing
shots to complete the reverse profile.

The shot instant is picked up on the shooting ship
and sent over the air to the re cei ineten ike where it is
recorded on the refraction records Simultaneously, a shot
record is made on the shooting ship. Several identifiable
buzzes and signals are sent over the air after the shot and
recorded on both the shot and refraction record in order to
allow correlation of the two records and location of the
shot instant on the refraction record when, due to poor
radio contact or other causes, the shot instant is not
recorded on the refraction record. FEesides allowing this
correlation the shot record is useful in determining the
depth of the shote

After locating the shot instant on the refraction

record either by direct reading or by correlation with the

- shot record, it is necessary to make a correction for the
time it took the shot instant to travel from the shot point
to the shooting ship (see Figure 1). In order to do this
it is necessary to know the depth of the shot and the dis-
tance that the ship has traveled from the time the shot was
thrown over the side to the time it detonated.

The distance that the ship has moved is obtained
by multiplying the ship's speed by the time over the side.
The time over the side is the interval from the instant the
charge is thrown over the side to the instant of detonation.

The depth of shot is read directly from the shot record. It

SHOOTING SHIP

SHOOTING ship

SHOT

Fige 1 Shot and Shooting Fige 2 Bottom and Surface-bottom
Ship Reflections

is equal to half the time difference between the bottom and
the surface=bottom reflections for depths of water greater
than 1000 fathoms (see Figure 2). It is then an easy matter
to calculate, knowing the velocity of sound in the water
from the bathythermograph observations, the time it took the
snot instant to travel from the shot point to the shooting

ship, 1. e., the shot instant correction.

III. Sinking Rates of Explosive Charges

Unless the shot record is recorded photographically,
it is not possible to read the bottom-surface bottom reflection
interval with consistency; and even on some of the photographic
records it is not possible to read this interval due either to
blocking of the amplifier or excessive noise level. For these
shots it is necessary to use the data from those shots on
which this interval could be reade This is most easily done
by referring to the sinking rate graphs (see Figures 3 - 12).
These are plots of twice the depth of the explosion, i. 6.,
the bottom-surface bottom interval, versus the time over the
side.e Knowing the time over the side the depth of tie explosion
can then be read off the graphs. ‘The scatter of the plotted
points about the line that is drawn is + .01 second for the
depth of the explosion. It can safely be assumed that any
readings taken from these graphs will be accurate to that
figure, ire Ye fanqge of% whe plbserved = porns,
All the charges that were used in plotting these

points were fired by safety fuse. The charges consisting of

= pound demolition blocks were prepared by taping or tying
the blocks together. Sometime s 3z pound demolition blocks
were used alsoe The 50 pound charges of demolition blocks
were prepared by replacing the top and bottom of the 50 pound
boxes by slats and removing all the wax paper. The Mk 4eT
charges were fired without the associated tail assembly, and
the Mk 50 charges were fired with the tail assembly. The

Mk 54 charges were fired in their carrying casese

IV. Reduction to Surface of Reference

It is usually desirable to reduce the travel time
data to a surface of reference from which to measure seismic
depthse in the case of deep sea refraction profiles sea
level is chosen as the most convenient surface of reference.
The refraction, reflection, and direct wave travel times are
corrected to bring them to this surface of reference. The
correction to the first reflection is the time if would t ake
sound in water to travel from A to B plus the time from C to D
(see Figure 13). This is obtained by multiplying the depth
of the shot plus the depth of the hydrophone, expressed in
seconds, by cos 86, where © is the angle of incidence of the
reflected wave on the bottom. The correction for the second
and third reflections is obtained in a similar manner. The
correction for the refracted waves is equal to the product of
the sum of these depths by the cosine of the angle () whose
sine is the ratio of the waimam water velocity rea to the

velocity in the refracted layer (cy). The correction for the

direct wave is negligable for most of the shots. (The ray

paths for these various arrivals are shown in Figure 14.)

/

B SEA LEVEL rn)

OCEAN GOTTOM

Fige 13 Ray path for Ry Fige 14 Ray paths for D, Rj, and G

Figures 15 - 17 are graphs of cos ® verus direct
wave travel time, i. e-, range,for the first, second, and
third reflections.e The factor that is needed to multiply
the sum of the depths of the shot and hydrophone can be read
directly off this graph for the particular range involved.
Figure 18 is a graph of cos O versus the ratio cy, / Bye From

this graph the same factor can be read off forthe refraction

arrivals. For all basement velocities that are obtained
in deep sea refraction work, i.e e., Cn /ty greater than

4.0, this factor may be taken equal to unity.

Ve. Topographic Correction

Areas of moderate to small topography were chosen
for most of the deep sea refraction profiles; but before
any calculations of basement velocity or depth can be made,
it is necessary to remove the small effects of the irregu-
larities in the bottom topography on the refraction travel
timese A mean depth of water is chosen, and corrections are
made plus and minus about this depth for the travel times of
refraction arrivals. Difficulty arises in choosing what
material the bottom topography takes place, whether it
represents directly topography in the sediments or is a
representation of the basement topography. The choice that
is made depends on the type of t¥pography and structure
involved.

Figure 19 is a set of graphs of the corrections
to be applied to the ground arrivals versus the difference
in elevation of the bottom topography from the mean for
various velocity contrasts between the material forming the

bottom topography and the ocean.

VI.- ‘Curved Ray Paths

The time intercept for a refraction line in the

simplest case of a single refracting layer is given by the

(1)

approximate formula,
24
eS
C, Cy

where G, is the "time average" of the sound velocity in

water talcen from sea level to the bottom. It is defined

by the relation,

Cy= rr) vy (2)
La
and is that velocity which when multiplied by the vertical
reflection time will give the true depth of water. Equation
1 is approximate because it takes an average vertical

velocity for the water. It does not give the exact expres-
sion for the change in the intercept ray path due to the
velocity structure in the water nor the change in time over
this path. Calculations by Tolstoy and separately by Worzel
and Officer show that the approximate formula is valid to
0.002 seconds for refraction velccities greater than 20,000
feet per second and is valid to 0.01 for velocities greater
than 5600 feet per second at a depth of water of 2700 fathoms.
Thus, it is concluded that the approximate formula can be

used with negligable error for any deep sea basement cal-

culations.e

VII. Sound Velocities in the Ocean

For refraction calculations it is necessary to know
surface sound velocity in water for the determination of
range from the @irect wave and the average vertical velocity
for the determination of refraction depths. The surface
sound velocity is determined from the bathythermographic

observations taken along the shooting track and the surface

- 9

salinity quoted in various oceanographic reports for the
particular area and season involved. Sound velocity is

not critically dependent on the variations observed in
salinity so that with the bathythermographic observations
that have been taken the surface sound velocity can usually
be quoted to t 2 feet per seconde The average vertical
velocity is obtained from "Tables of the Velocity of Sound
in Pure Water and Sea Water," published by the Hydrographic
Department, Admiralty, London (1939). A check on this
value is obtained from the R® versus D® plots. These two
values usually agree to ¢ 10 feet per second, but for

the sake of uniformity the British Admiralty Table value

is used in all refraction calculations.

VIII. Check Plots

In order to insure confidence in the data used in
the travel time graphs, two check graphs are made. These
are the R© versus D© and the navigation plots.
gs7--aplotted against De will produce
straight lines except for changes in depth or slope of the
bottome These graphs will then bring out any obvious
errors that have been made in R or in D. The slope of this
line is the square of the ratio c,/@,, which gives a check
on the value oLedned from the Eritish Admiralty Tables for
Cy» The value for Cy can not be read from this plot to

better than # 10 feet per second.

The navigation plot is a graph of time of day that

the charge was thrown over the side versus the direct wave
travel time. This will also plot as straight line depending
on the constancy of the ship's speed and course. Besides
checking the value of D, this graph is useful in giving

the velocity of the shooting ship when there is no pit

log on that ship. Further, the navigation plot in con-
junction with the ship's log gives the range of the reverse

point when no shot was fired at this station.

IX. Ry - D Versus Ry Graphs

Depending on the thickness of the isothermal surface
water layer, the direct wave is recorded from six to forty
miles. On those refraction records for which there is no

D present, R, is used to determine the range. In the

2
a

range at short distances (less than 40 miles), and curved

at
laboratory the R

versus p° plots are used to find the

ray path calculations are used for the longer shots. However,
at sea it is desirable to have a quicker method for the

rough travel time plots. For this purpose the BS D versus
Ry curves are included (see Figure 20). Knowing the value

of Ry and the depth of the water, the expected time interval
between R, and D can te read off the graph. These curves

are not accurate for the longer shots because the velocity
structure in the water becomes important, and the simple

straight line, average velocity calculations are in errore

These curves are plotted from the equation,

Xe Check List of Data Necessary for

Complete Reduction

In general -

For each

1. Ship's navigation.

Ze) chip's) Logie

5- Miscellaneous cata not appearing in ship's log
such as distance and azimuth when shooting ship

is abeam at beginning and end of reversed profile,
time shooting ship is abeam, and propeller revolutions
at time of each shot when no pit log or taffrail
log is available.

4. Lathythermograph observations.

shot -

1. Charge size and fuse length.

2e Time of day the charge was thrown over the side.
Se Amount of time charge was over the side before
detonation.

4. Depth of water under shot. (Also indicate shot
on fathometer tape.)

5e Pit log speed and mileage.

12 =

Symbols used

surface sound velocity in the ocean

average vertical velocity of sound in the ocean
sound velocity at the bottom of the ocean

velocity of compressional waves in the first
refraction layer

velocity of compressional waves in the nth refraction
layer

travel time of the first reflected wave

travel time of the second reflected wave

travel time of the third reflected wave

travel time of the mth reflected wave

travel time of the direct wave

travel time of refraction arrival

bottom reflection time from the shot record
surface-bottom reflection time from the shot record
topographic correction

difference in elevation ofthe bottom from the mean
depth of the ocean

angle of incidence of R, on bottom

angle of incidence of G on bottom

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