1. Introduction
A rollalong switch is a special type of switch used in
Common Depth Point (also known as CDP, or
Common Midpoint,
or
CMP) seismic surveys. An
array of geophones is placed on the ground before
starting the survey. The number of geophones in the
array is substantially larger than the number of
channels on the seismograph. The rollalong switch
is used to select a group of geophones (usually the
same as the number of channels in the seis-
mograph) from the array on the ground.
As the switch is rotated, the active array will move
along the ground, adding new geophones on one
end and disconnecting those on the other. The
shotpoint is normally moved by the same amount,
so that the geometric relationships in the seismic
survey remain constant, but they roll along the
ground (hence the name).
This manual covers the operation of Geostuff
M o d e l s RS-96/24 a n d RS-120/48 R o l l a l o n g
Switches. Model RS-96/24 is designed to select 24
geophones from an array of up to 96 geophones (or
geophone groups). Model RS-120/48 will select 48
geophones from an array of up to 120 geophones
(or geophone groups).
Model RS-120/48 also provides for recording with a
“gap” between channels 24 and 25 for “split-spread”
surveys.
2. Operation
Operation is quite simple, although the arrange-
ment and type of geophone cables may vary. The
switch has 96 positions, labeled 1 through 96 on the
dial. In position 1, the first 24 geophone groups are
connected to channels 1 through 24 on the seis-
mograph. When the switch is set to position 2,
geophone groups 2 through 25 are connected to the
seismograph. The sequence continues until switch
position 73, which connects groups 73 through 96 to
the seismograph.
The switch will not rotate as far a s position 96.
Depending on the number of geophones connected,
the last geophones on the end of the line will be
connected at a much lower switch position number
and the switch should not be moved further. For ex-
ample, with an RS-96/24 with 96 geophones on the
ground, switch position 73 will connect stations 73
through 96 to the seismograph. When the cables are
re-positioned, those which were formerly 73-96 will
now be l-24. If you start in position 1, you will be
connecting the same geophones that were con-
nected at the end of the last
solun.
So. make your
last shot at posilion 72 and re-star; at position 1, or
finish at position 73 and restart at position 2.
There is a mechanical stop to prcvcnt
the
switch
from making a full revolution. Do
not force the
switch beyond this stop.
If the survey is to be continued, the seismograph
and rollalong switch are re-positioned
IO
the end of
the line, the cables re-arranged, and
the
process
resumes with the switch in
the
initial position.
Model RS-120/48 operates in a similar manner.
When the switch is in position 73, then the switch
connects the geophones at station 73 to 120 to the
seismograph.
The RS-120/48 system can also record with a gap
(an extra space between groups of gcophone).
There is a pointer which indicates the gap bctwccn
the two groups of channels. When in position “O”,
the space between channels 24 and 25 is equal to
one standard group interval. This is a gap of “zero”,
or no gap at all. The effect is the same as a con-
tinuuus, equally-spaced line of gcophoncs.
To set a gap, pull up on the large knob, and turn it.
The gap indicator will rotate to a diffcrcnt position
on the dial. When the dial indicates the dcsircd
amount of gap, lower the knob. it may be necessary
to gently move the knob back and forth to properly
mesh the
gears. Do not try to turn the small knob
which serves as the gap indicator.
When the gap is
set to “l”, then the space between gcophoncs 24 and
25 will be equal to two normal group intervals. The
pattern continues for larger gaps.
The RS-120/48 may also be used with a 24-channel
seismograph to record with a gap hctwccn channels
12 and 13. The output adaptor cahlcs must he wired
so that output channels 13 through 24 of the rol-
lalong switch are directed to channels 1-12 on the
seismograph, and output channels 25 through 36 of
the rollalong switch arc dircctcd
IO
channels 13-24
of the seismograph. This is done to position the gap
between channels 12 and 13. A diffcrcnt wiring
scheme is also required for the input adaptor
cables.
3. Cable Configurations
There are at least three suitable combinations of
cable types usable with the RS-96/24 rollalong
switches:
l
Two or more 24-channel refraction cables
Geostuff
RS-96/24 & RS-120/48
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