Geophone Coupling

8/11/2019 Geophone Coupling

1/5

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-

6

GEOPHYSICS: THE LEADING EDGE OF EXPLORATION APRIL 1985


2/5

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

ml

i?

Xl

kl

x2

k2

0

1 I

I

I

I

I I

0

50 100 150 200 250

300 350 4

frequency - Hz

A

200,

I I

I I I I I

I

c

0

-200

I

I

I

I

I

I

I

I

0

50

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


3/5

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-

I

I

I

I I

I

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-

58

GEOPHY SICS: THE LEADING EDGE OF EXPLORATION APRIL; 1985


4/5

Y

P

-20 -

E

11

.40 -

-00 -

I d I

I

I

I

I

I

-5

0 5

10

15

20 25

30 35

I

I

I

1

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

GEOPHYSICS THE LEADING EDGE OF EXPLORATION APRIL 1985 59


5/5

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

.

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

Documents

Geophone Coupling