Ground coupling: the mechanical connection of a geophone to the earth.
The geophone ground coupling for vertical and horizontal geophones has
been measured in the laboratory and in the field. The data can be fit to a
calculated geophone response with a single coupling resonant frequency
and damping factor. For frequencies much lower than the coupling
resonant frequency, the geophone accurately follows the ground motion,
but for higher frequencies the coupling can alter both the amplitude and
phase of the seismic signal. The normal planting of vertical geophones in
the field results in coupling adequate for conventional recording that uses
frequencies less than 100 Hz. However, for very loose soils or for highfrequency seismic recording, I recommend that the geophones be buried to
place the geophones in firmer soil. The coupling resonant frequency for
vertical geophones is determined by the firmness of the soil, and I have
measured resonant frequencies ranging from 100 to 500 Hz at different
locations. The coupling resonant frequency is insensitive to changes in the
mass or base diameter of the vertical geophones. Because the firmness of
the soil increases with depth, the coupling resonant frequency can he
increased by burial of the geophones or by the use of longer spikes.
Adequate coupling is very important in shear-wave recording because the
rocking of horizontal geophones causes a low-frequency coupling
resonance. It is crucial that horizontal geophones be planted with their
bases firmly contacting the soil. Geophones so planted have a resonance
around 130 Hz, whereas those 1 cm off the ground can have a resonance of
30 Hz or lower. Soil conditions have little effect on the resonant frequency.
Horizontal geophones with 1-inch spikes are as well coupled as those with
longer spikes, but the best coupling is achieved by burial of the
geophones. ©1984 Society of Exploration Geophysicists
Resonance
Seismic signals are detected in land seismic surveys with geophones. The
essentials of a geophone are a magnet; a coil; and a spring. The magnet is
attached to the geophone case, so that when the ground moves up and down
with the seismic signal, the coil, attached to the case by a spring, tends to
remain in the same place. The movement of the magnetic field through the coil
induces an electric voltage in the coil, and this voltage is the signal recorded
for processing and interpretation. If the coil were totally unconnected to the
case, it would record this signal accurately. But the spring connection means
that when the case moves a force is applied to the coil proportional to the
displacement of the case relative to the coil. If movement changes direction
(from up to down) slowly, the coil can follow the movement of the case. If
changes in direction are very rapid, the coil has hardly started to move before
it is pulled in the opposite direction, so it hardly moves at all. In between,
there is a frequency of direction change where the reversal of direction joins
with the energy stored in the spring to pull the coil back to the rest position
and past it, with the cycle repeated in the other direction. At this frequency,
the geophone coil movement is very large. So is the signal. This is the resonant
frequency.
Damping: Loss of energy in wave motion due to transfer into heat by frictional forces.
Damping
If no energy is taken from the system, a weight on a spring (which is
what a geophone really is), once excited, will oscillate at the resonant
frequency indefinitely. This is obviously not what is needed for
recording a seismic signal, where we want to distinguish one reflection
from the next.
If energy is taken out of the oscillating system, the amplitude of
oscillation decreases. This is called "damping". "Critical damping"
occurs when the
system just fails to
oscillate.
The figure to the left
shows the response
of a geophone to a
transient signal
(such as a tap on the
top of the case). The
horizontal axis is
time measured in
units equal to periods at the resonant frequency. For example, if the
resonant frequency is 10 Hz, the units on the axis are 100 ms=1.0. The
response with no damping is continuous oscillation at the resonant
frequency with no change in amplitude.
With increased damping, the oscillation dies with time, until at critical
damping the response is never negative: it rises rapidly to a maximum
then decays exponentially. The problem with critical damping is that
the maximum is only about 36% of the response for an undamped
geophone.
The practical solution is to use geophones with a damping factor of
about 0.7 critical.This increases the response to 45% and improves the
frequency response.
The amplitude of the
geophone response
is theoretically
infinite at the
resonant frequency,
levelling off to a flat
response about 3
octaves above the
resonant frequency,
if the geophone is
undamped. As the
damping increases,
the peak at the
resonant frequency decreases in amplitude, and disappears completely
at 0.7 critical damping.Increased damping reduces the response at low
frequencies. At 0.7, the response is down 3 dB at the resonant frequency,
and drops at about 12 dB/octave. For 1.0 critical, the response is down 6
db at the resonant frequency.
Phase response is a
further problem. As
the figure to the
right shows , the
phase of the signal
from a geophone
rotates 180 degrees
as it goes through
the resonant
frequency. For little
damping, the phase
change is abrupt and
occurs at the
resonant frequency.
For large damping
factors, the phase change is more gradual, and is spread over several
octaves.
What causes damping
As mentioned above, damping is caused by some factor taking energy
out of the system. In practice, this means converting the mechanical
energy of the coil movement into heat. There are three practical
techniques, two of which are commonly used:



The coil can be immersed in a viscous fluid.
The coil can be wound on a conductive base, so that energy is
absorbed in eddy currents.
Electrical current can be drawn from the geophone by a load
resistance, so that resistive heating of the geophone coil and the
load dissipates energy.
Interpretation Considerations
The interpreter needs to consider what the variations in amplitude and
phase response around the resonant frequency might do to any
calculations using amplitudes or phase differences.
ON THE MINIMIZATION OF THE DISTORTION CAUSED BY THE
GEOPHONE-GROUND COUPLING
ABSTRACT
It is well known that the application of the "bright spot' technique has been
more successful in marine prospecting than in land prospecting. This is
due partly to the problem of distortion of the seismic signal caused by the
geophone-ground coupling, especially when carrying out high resolution,
shallow seismic surveys in swampy terrain.
The effect of geophone-ground coupling on the response of a single
geophone to the incident compressional waves has been treated by several
authors. However, they have always neglected the influence of mutual
interaction between an array of geophones on the response of each
geophone forming the array. We show that mutual interaction, which
results from the re-radiation of the incident compressional waves by the
geophones forming the array, can have considerable effect on the
response of each geophone.
The effect of the geophone-ground coupling on the response of a seismic
channel is considered in the absence and presence of mutual interaction
between a group of geophones for the case when the shear wave velocity
of the soil varies by a factor of three
Determination of geophone coupling
Information about coupling of a seismic receiver is obtained from a power
spectrum for a record acquired at a seismic receiver. In one method, the
power spectrum for the record is compared with a reference power
spectrum, which may be known a priori or which may be determined from
the power spectra of records acquired by a group of receivers. In an
alternative method, one receiver of a group of receivers is designated as a
reference receiver, and the power spectra of records acquired by other
receivers in the group are compared with a power spectrum for the
reference receiver. The obtained information about the coupling of a
receiver may be used to determine a coupling correction operator for the
receiver, and this operator can be applied to seismic data acquired by the
receiver to correct for the effects of imperfect coupling of the receiver.
ELECTRONIC ACCELERATION-SENSITIVE GEOPHONE FOR SEISMIC
PROSPECTING
ABSTRACT
Previously ignored characteristics of the seismic recording instrument are
presently experienced as limitations as more sophisticated interpretive
methods using wider frequency ranges are developed to extract
stratigraphic information from seismic land data for hydrocarbon and
mineral exploration. Most of these limitations arise from inadequate
characteristics of the first element of the seismic instrument: the
geophone. A geophone does not faithfully follow the motion of the earth for
higher frequencies due to poor geophone-earth coupling. This filtering
effect brings about time shifts that are dependent on the frequency and the
soil type. A geophone can also produce spurious outputs, brought about
by the motion of the suspended part of the geophone, with a magnitude
comparable to that of the desired output. The suspension is made very
compliant to obtain the required sensitivity. A compliant suspension,
however, gives a large sag. The geophone can therefore only be used in
one position, tolerating little tilt. A compliant suspension also widens the
traveling range of the movable part. Minor sensitivity changes with travel
are then noticeable as nonlinearity, since the surface wave is large with
respect to the reflected wave. A compliant suspension is usually realized in
the form of thin, spirally shaped spring-spiders. Such suspensions exhibit
transverse or rotational resonances that are in or close to the seismic
frequency band. Excited by ground roll, they can produce considerable
undesirable output.
The novel geophone we describe is a light-weight (17 g) accelerationsensitive transducer which gives good ground coupling and partial
correction for the increasing damping in the earth with increasing
frequencies. It employs internal hybrid electronics for a magnetodynamic
velocity-nulling feedback system. Velocity nulling makes the movable part
of the geophone virtually rigid with respect to the housing. This makes the
geophone characteristics independent of the suspension. The springs
used are stiff in a transverse and rotational direction so that the
suspension resonances are well outside the useful frequency band. This
suspension also allows the geophone to be used in any orientation while
being only sensitive to the vibration component along the main axis. The
feedback system makes the sensitivity flat within 1 dB from 2 Hz to 500 Hz,
with a phase tolerance smaller than 5°. The geophone is robust, has no
moving internal wires, employs a current output [sensitivity 1 mA/(m s−2)]
and internal gain so that the signal-to-cable-noise ratio is improved. This
type of output allows parallel connection without any interaction between
the geophones.
EFFECT OF SOURCES AND GEOPHONE COUPLING ON MASW
SURVEYS
Abstract
We have used the multi-channel active surface wave (MASW) seismic
method to map
stratigraphy and bedrock at sites with differing soil and rock
characteristics. The purported advantages of using the shear wave velocity
field calculated from surface waves to detect, delineate, and/or map
anomalous subsurface materials include the insensitivity of MASW to
velocity inversions and cultural noise, ease of generating and propagating
surface wave energy in comparison to body wave energy, and sensitivity to
changes in velocity. The advantages of this method may be valid in theory
and controlled field experiments; however, they become less obvious when
the method is incorporated into a competitive world of geophysical
consulting. We have successfully used the MASW method with
landstreamers in a variety of applications and configurations to profile
lateritic overburden, bedrock to depths up to 100 feet, and delineate
shallow fill boundaries at a former sand and gravel quarry. Our experience
also shows that energy source, geophone coupling method, and coupling
medium are as important as survey geometry parameters in determining
the successful cost-effective application of MASW and other seismic
methods.
Accelerometer vs. Geophone Response: A Field Case History
Summary
A method is derived for the calculation of ground acceleration from
geophone data using a frequency-domain inverse filter and an empirical
scaling constant. Acceleration-domain spectra from geophones and MEMS
accelerometers from an oilfield survey at Violet Grove, Alberta, Canada are
compared. We find that the geophone and accelerometer data, over a band
of 5-200 Hz, are very similar. The accelerometer amplitudes are larger than
the geophones’ below 5 Hz and there are some differences at very high
frequencies. Significant events related to the first breaks are not observed
on the accelerometer records at some stations.
Conclusions
Generally, the two types of sensors appear to both record ground motion
similarly. If data from the two sensor types must be merged, a scaling
factor based on matching amplitude spectra should be found. The spectra
should be broadly similar once the appropriate scaling is found, especially
around the dominant frequency. We do observe some differences in the
data related to high frequencies, very low frequencies, and near the first
breaks.