Page 122 - Whole Earth Geophysics An Introductory Textbook For Geologists And Geophysicists

are illustrated in Fig. 5.7a. Fig 5.7b shows
Common midpoint (CMP)
traces for a common midpoint,
1
method. The six seismic traces for Shot
wavelength of the groundroll,
105
The
high
down
geo-
unison; their electrical signals
method
(see
of seismic
com-
midpoint
on
progresses, left.
first
the
S,/R,,
respectively.
bold raypaths.
point
of other arrivals (Fig. 5.5c).
attenuated. Reflections commonly arrive 3).
at
ie
the
ratio
term
Chapter
moving
Acquisition
(S,/R2,
The CMP
by
During
common
common
determination
(The
sent
signal-to-noise
highlighted by the
3,
are
concept).
and
combinations:
see
survey
signals
groups.
5.6).
5.6
half
a
i
three
2
velocity;
have
the
FIGURE
shots
(Fig.
while
eleceiéal
the
the
receiver
in
the
shots
for
pairs
As
conducted
(S2/Rg, S3/Ry, S4/Rg).
apparent
enhances
down in
up,
same
facilitates
the

ee F
expense
right.
S/R,
other
different
i
to
beative
receiver
moving
six
the
proportioned
interpretation.
"t
up and
of
the
and
for
high
express
the
that
commonly
arrays
and
are
from
toward
S,/R,
and
midpoints
at
the groundroll is with
positive oe
r
move
below)
signals
information
array
traces
ae
between-source
into
to
is,
as
are
be
used
r
array
;
and
(that
the
midpoint
and
is
shot
reflected
array can
discussion
seismic
a a
receivers
common
in
(S2/Ryy S3/R3, Sy/Rs);
costs
survey
the
is
processing
that surface,
f
is
Method
CDP,
geophones
of
in
source
whereby
that geophones
same
Common
M. jdpoin t
phase, enhancing
(Fig. 3.12). The
:
redundancy
reflection
Waveform”
and
ni
are
midway
ground
or
a
the
point,
Midpoint
in
phones cancel, so
source
there
5.6
the
i
used
(Fig. 5.5b). The
mode,
is
the
Fig.
Shot
surface,
half
(R,)
depth
seismic
velocities
Likewise,
“Seismic
to
provides
the
In
is
(CMP)
S3/Rg);
surface
that
angles
add in
group
result
Common
shot,
:
mon
the
so
A
c:
« oTC
CE SA
RY ROARED QAR LEC HTES
, the toward interval one of steps in “pulled” are positions receiver and source ? <0 receiver first the and (S,) source the between midpoint the shot, first the During we IRE OE PLE 2 TERR IYO, ALOIS ADE AER et phe R, Rs R, Ay Re Ryi Shot 2 CZ 7 At ifr r r r r Shot 4 S, WMiddé SESS ASST LALA LETTS VIITALTTT
- “fired called
Analysis airgun) is commonly electron- (or ie reflecte also of the ground fired into to the is equal to the
Waveform station receivers, connected an enhancing ay are Receiver Station 3 "Groundroll" responses for a group add length negative signals from P-wave at close distance
And (explosive; Vibroseis; receiver . while they signals, noise; up-and-down, rolling motion Wave 1s moving up cancel arrival.
Processing, Each este several per unwanted ' =o of AAA geophone The reflected unison, enhancing the
Acquisition, ; the es m an ee unwanted certain example an produce Receiver Arrays --- er al * seismic is thus attenuated. c) A in
, source xe of array mi of Receiver Station 2 Length = Wavelength Direction Fayleigh Propagation f ~Particle Motion array. a) Source waves by Response of one geophone group. If the group
Reflection: so that rather but a ee cancel an are the waves 1 “2 aoe se vs g BS Ls, 4 / i enhancement of receiver stations of connected geophones (‘“arrays” or “groups”). geophone up or down
/
Seismic designed * receivers of a single instrument, soe designed waves to Rayleigh because Recelver Station nace fF SO \\ and ion seismic trace. b) wavelength of a Rayleigh wave, positive signals from geophones Rayleigh wave
Chapters Arrays surveys Most cee into not ee ically is group) events. groundroll, A FIGURE5.S together as a single moving down; the source moves the group of geophones
104 Receiver 1s a those
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