geophone coupling
TRANSCRIPT
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By CHRISTINE E. KROHN
Exxon Production Research Co.
Houston, Texas
G
eophysicistsften ascribehe cause f poordataquality
to geophone round oupling. o accuratelyecordground
motions for a seismicsurvey, he geophonesmust be
coupled irmly to the ground. Certainly, the geophone
hanging n a bu shor loosely placed n a crack will no t
accurately ecordgroundmotions.Even houg h he well-
planted geophon e an follow groun d motions at lower
frequencies,t may fail at higher requencies. enerally,
coupling s not a problem or conven tional ecording n
favorable errain, but it is a crucial actor n hig hresolu-
tion and shearwave recordings.
Having measuredground coupling for vertical and
horizontal geopho nesn both the laboratory and the
field, I have determinedhow coupling dependsupon
soil conditions,plusgeophone lacement, pike ength,
radius,and mass. n this paper, discuss ow coupling
is measured,how coupling affects the amp litude and
phaseof the seismic ignal,and how to plant the geo-
phon es or the bestresults.
In this investigationof geophoneground coupling,
laboratory measu rements ere made with a large shake
table, whichwasvibratedat different frequencies nd at
different amp litudes.A box wa s bolted onto the table,
soil was placed n the box, an d a geophoneplanted n
the soil. An accelerometermonitored able motion and
a feedback ircuitkept ablevelocityconstant s requency
waschanged.Both the voltageamplitude nd phaseof a
geophoneweremeasured sa functionof frequencywith
a gain/phasemeter.Alternatively,he amp litudesor two
separate eophon es ould be measu red imu ltaneously.
U
se
f the shake able had the advantage f carefully
controlling he vibration of the geophones.t wasespe-
ciallyuseful or measu ringhe effect of vibrational mp li-
tude and for comp aring he responseor two geopho nes
under the sameconditions. t had the disadvantage f
havingsoil confined o a b ox. I found that the table re-
sponse or sand n a box was airly flat except or a small
perturbationaround270 Hz. In add ition, he measu red
geophon e esponse a d extra high-frequencyesonances
whichwerenot seen n the field, but wereseenwith other
techniques henusedwith the box. I think that both of
theseeffectscould be caused y the couplingof the soil
to the box.
Two field techniques ere used o m easure ou pling.
In the first technique, two ge ophone swere fastened
together.One geophonewas driven with an oscillating
voltage, and the motion w as detectedwith the second
geopho ne. his method ielded cleangeophoneesponse
with little noise;however, he dual geophone asheavier
andmorecumbersomehan he original.The secondech-
nique waseasier o perform; the geophon ewasgiven a
small ap eitherby dropping steel all on it from a fixed
height or by us inga very small hamm er.The impulse
responsevoltage as a function of time) was measu red
with a spectrumanalyzer and the geophone esponse
was obtained from the F ourier transform (frequency
spectrum)of the impulse.
Typical data obtained n this investigation re shown
in Figures1 and 2. Figure 1 is the measured eophone
response or a vertical geophon e.The responses de-
fined o be the outputvoltageof the geopho ne, xpressed
as a functionof frequency, or constant elocitymotion
of the ground. At low frequencies, he phase s linear
and the amp litude s flat with a respo nse etermined y
the sensitivity f the geophon e.At about 50 Hz, the am-
plitudestarts o increase raduallyup to a peakof about
1.4 times he low-frequency alue. The peak in the am-
plitudeat 222 Hz correspondso a change n the phase.
The response or a horizontal geophon e s plotted in
Figure 2.
T
he geophone esponse hown n Figures 1 and 2 is
characteristic f d amp edharmonicoscillation.The in-
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ternal mechanism of the geophone, Itself, is also a har-
monic oscillator. Thus, a m odel of the planted geophone
can be constructed based on a system of two damped
springs as shown in Figure 3A. One spring represents he
real spring within the geophone; he other spring represents
the elastic coupling of the geophone to the ground. The
responseof each spring is defined by a resonant frequency
and a damping coefficient. I have shown (GEOPHYSICS,
June
1984) that this simple spring model is adequate to
describe geophone ground coupling.
The calculated amplitude and phase of the geophone
responsebased on the dual spring model is shown in Fig-
ure 3 for two choices of damping. As can be seen in this
example, the geophone resonancedominates the response
for frequencies less than 50 Hz, and the coupling reso-
nance determines he response or frequencies bove 50 H z.
Figure 1. Geophone response showing coupling of vertical
geophone measured at Friendswood, Texas with the dual
geophone field technique.
Figure 2. Geophone response showing coupling of hori-
zontal geophone measured in sand with a shake table.
0 dBV = I Vrms.
The basic effect of coupling on seismic data can be
describedusing Figure 3. If the damp ing is low, the ampli-
tude pea k is high and narrow as in the solid curve. In
this case, the coup ling will act as a low-pass filter and at-
tenuate the respo nse beyond th e resonant frequency.
Furthermore, the amplitude near the resonant frequency
will be enhanced, causing the pulse to ring with this fre-
quency. If the damping is high, the amplitude peak is low
and b road as in the dashed curve. In this case, the ampli-
*2
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kl
x2
k2
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1 I
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50 100 150 200 250
300 350 4
frequency - Hz
A
200,
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loo 150 200 250
Jo0 350 400
FREQUENCY. Hz
B
Figure 3. Calculated geophone amplitude A) and phase B)
for a geophone with an internal resonant frequency of
8 Hz and a coupling resonant frequency of 200 Hz. The
solid curves have a damping of 70 percent of critical for the
internal resonance and a damping of 10 percent for the
coupling resonance. The dashed curves have a damping of
30 percent of critical for the internal resonance and a
damping of 50 percent for the coupling resonance.
GEOPHYSICS: THE LEADING EDGE OF EXPLORATION APRIL 1985 57
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tude s not drastically ffected,but there would be phase
distortion ver a broadbandof frequencies, hichcould
influencemeasu rements f traveltimes.As long as the
survey s done with frequenciesmuch ess han the reso-
nant frequency less han 50 Hz for a 200-Hz resonant
frequency) n the regimewhere h e amplitude s flat and
the phase s linear, the data will not be influencedby
coupling.
T
e geophone esponsen Figure 3 was calculatedas-
suming UIO-Hzcoupling esonant requency. he actual
effect of co uplin gon the seismic ata will dependon the
responseor the geophones t the locationof th e survey.
In the field, 1 havemeasu red oupling esonan t requen-
cies or verticalgeophones anging rom 100 to 500 Hz
with damping an ging rom 20 to 60 percentof critical.
(Critical damping s that dam pingat which a harmonic
systemwill not o scillatebut returnssmoothly o its rest-
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Figure 4. Response of a vertical geophone in firm and
loose sand measured with the shake table.
ing positionafter a disturbance.) orizontal geopho nes
have a couplingresonancewhich is sim ilar to vertical
geopho nes xcept hat it is lower in frequency nd hasa
smallervalueof damping.Typical values or horizontal
geopho nes re 170 Hz and 20 percentof critical.
Actually, he couplingphenomenons not assimpleas
described bove n tha t it is nonlinear; he resonant re-
quency nd dampingdepend pon he vibrationalampli-
tudesof the ground. Using a sh ake ab le with a d rive
amplitudeof 0.01 cm/set, I measured coupling eso-
nance frequencyof 310 Hz for a vertical geophon e n
sand. As the drive amp litude was increased, he reso-
nant frequencydecreased. t 0.25 cm/set, a resonant
frequencyof 230 Hz w asmeasu red. imilar resultswere
obtained or horizontalgeoph ones. t d rive amp litudes
below0.01 cm/set, little nonlinearity asobserved. on-
linearitywasalsoseen n the field; the resonantrequency
increased s the force used o tap a geopho newas de-
creaseduntil a level was reachedwhere there w ere no
further chang esn reson ant requency.
Most seismicsignalsoccur in the regime where the
coupling s linear. However, some irst breakscan have
velocitiesof 0.1 cm/set, and even arger velocities re
found near he source.Nonlinearcoupling oulddistort
the waveformsand attenuate h e high frequenciesor
geopho nes ear the source.
G.
ven the fact that the seism ic requencies hou ldbe
much ess han the coupling esonant requency, t is m-
portant
to
know what factors determine he co upling
behavior. found that the coupling f verticalgeophones
depends tronglyon the firmnessof the soil. Data for
a vertical geophon eplanted n sandwhich was poured
loosely n a box on a shake ab le and data for the geo-
phoneplanted n sandwhichwas horoughly om pacted
by vigorous haking re shown n Figure4. In the field,
I haveobservedhe resonantrequency h ift from 3 40 Hz
to 120 Hz, and the dampingshift from 60 percent o 44
percentof critical by moving he geophone rom a firm
lawn to a plowedgardennearby. n general,highercou-
pling resonant requencieswere associated ith higher
Figure 5. Response of a horizontal geophone for different
positions measured in sand with a shake table.
dampingand broader, ower peaks.
The couplingof vertical geopho nes asnot sensitive
to any parameter xcept he firmness f the soil. For ex-
ample, in the laboratory with uniform sandor clay, I
found that the resonan t requencywas he same or geo-
phoneswith different spike eng ths,different d iameter
flat bases,different m asses, r internal geophon e re-
quencies.Geophonesrom different manu facturers ad
the samecoupling esponse. he reson ant requency or
a buriedgeophone as he same sone normallyplanted,
but the dampingwas ncreased y burying.
In the field, the firmnessof the soil increaseswith
depth, and I found that the resonant requencyof the
geophone n creasedwith burial or w ith a longer spike.
For example,a vertical geophon ewith a one-inchspike
had a reson ant requencyof 387 Hz and a dampingof
51 percent of critical. At the sam e ocation, the reso-
nant frequencyand dampingchanged o 440 Hz and 60
percentof critical with a three-inch pikeand to 650 Hz
and 66 percentof critical with a five-inch spike.
T
e coupling f ho rizontalgeophoness strongly epen-
dent on the placement s show n n Figure 5. In this ex-
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Figure 6. Tap test on a vertical geophone. A) time
Figure 7. Tap test on a horizontal geophone. A) time
response. B) Frequency spectrum. response. B) Frequency spectrum.
ample, he buriedgeophone ad a resonance round260
Hz and a dampingof 19 percentof critical, but a geo -
phone with its base firmly resting on the so il had a
resonance f 170 Hz and a dampingof eight percentof
critical. If the geophonewas lifted only a few milli-
metersso that the basewasno longer o uching he soil,
there wasa drastic oweringof the resonanceo 90 Hz.
If it was raised one centimeteroff the soil, the peak
shifted o 30 Hz, and at higherpositions he geophone
did no t respond o so il motion at all.
The measurementsor horizontalgeophones howed
that the couplingdoesnot dependon the manu facturer,
the internal geophone requency, the firmnessof the
soil, or the lengthof the spike. n the lab with sand , he
resonancerequencywas he samewith a spikeand with
a flat base;however, n the field, the flat baseswere n-
ferior to sp ikeswith resonancesf 30 to 40 Hz.
I have shown GEOPHYSICS, June 1984) hat the hori-
zontal geophonecoupling resonance s causedby the
tendencyof the geophon e o rock insteadof moving
horizontally with the ground . Such rocking was elimi-
natedby using dual spike ined up parallel o the direc-
tion of motion. This dual spikeeliminated he decrease
in resonant requency as the geophonewas raised off
0
100 200 303
400
the ground;however, f the baseof the geophone ested
firmly on the ground , he response as h e sameas hat
for a geophonewith a singlespike.
T*is resultshow s hat it is crucial h at horizontalgeo-
phonesbe plantedwith their bases irmly touch ing he
ground . Planting the geopho nes orrectly and leveling
the geophones easierwith a shorterspike.My measure-
ments indicate one-inch spikes should be used on
horizontalgeopho nes ince hey are aseffective n coup-
ling the geophoneso the groundas longerspikes.
The existence f coupling roblems an be determined
in the field from a tap test suchas conventionally er-
formed to check or polarity. Figures6 and 7 show he
impulse esponse o a tap test for a vertical and hori-
zontal geophone.Both the coupling eso nancerequency
and the dampingcan be determined rom the impulse.
Becausehe geophone oltage s a measu re f the velocity
responseo a stepaccelerationf the geophone,he oscil-
lations n the voltageare at the coupling e sonancere-
quency. hus, the resonant requency s he frequency t
which the voltage crosses ero an d at which there is a
peak n the Fourier transformof the data. The dam ping
can be determined rom the impulseby measuring he
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distance A, from peak to trough and the distance A2
from the next trough to peak, as in Figure 5. Then,
damping = In (AI/A,) / [n + In (A,/A2)*]. The
geophone response, tself, is equal to the time integral of
the Fourier transform of the tap test.
I
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domg the tap test, care must be taken that the tap is
not too ha rd s o that the coupling is in the linear regim e.
A procedure which can be used to check for nonlinearity
is to hit the geophon e repeatedly with decreasing force;
the coupling resonance should increase until it reach esa
limiting value, the coupling resonant frequency.
Because he seismicamplitude and ph ase s undistorted
by coupling only at freque ncies muc h less han the cou-
pling resonant frequency , it is important that the fre-
quencies used in the survey be much lower than the
coupling frequency. Measuring the coupling with a tap
test in the field w ill ind icate if th ere is a problem . To in-
crease the coupling frequency for vertical geopho nes,
longer spikescan be used, or the geophonecan be buried.
It is imperative that the horizontal geophonesbe planted
with their bases firmly resting on the soil. My conclu-
sion is that one-inch sp ikes should be used with h orizon-
tal geophones because they are as effective as longer
ones and eas ier to plant correctly. To increase the coup-
ling frequency for horizontal geophones, the geophones
can be buried. IE
An extensive technical presentation of this material, with
supporting mathematics, appeared in the June 1984 issue of
GEOPHYSICS.)
Christine E. Krohn received a B.S. degree in physicsfrom Emory
University in 1973 and a Ph.D. degree in physics from the Uni-
versity of Taas at Austin in 1978. She was a Welch post-doctoral
research fellow at the University of Texas during 1978-79 and
did research in the area of amorphous and liquid materials.
Since 1979, she has been employed by Exxon Production Re-
search Co. in their Long Range Research Division. Upon join-
ing Exxon, she initially worked in the seismic field research
group studying geophone ground coupling. Currently she is
working in the area of rock physics.
60 GEOPHY SICS: THE LEADING EDGE OF EXPLORATION APRIL 1985