GEOPHONE WITH MAGNETIC SELF-DAMPING SYSTEM
BACKGROUND
Various types of tools are used to explore and assess subsurface formations for
hydrocarbons. One such tool is the geophone, which is typically used in seismic applications
(e.g., reflection seismology). A geophone is a type of motion transducer that converts detected
seismic activity into electrical signals. Specifically, a typical geophone contains a springmounted
magnet positioned within a conductive coil. Seismic activity causes the springmounted
magnet to move, thereby generating an electrical signal in the coil that is proportional
to the magnet's velocity. Thus, the electrical signal reflects the degree of seismic activity acting
upon the geophone.
Geophones are sometimes deployed in seismic-while-drilling (SWD) applications. In a
common SWD application, a geophone is coupled to a drill string, and, as drilling begins, the
geophone is lowered into the subterranean formation. Once the geophone reaches the depth at
which the seismic sensing is to be performed, drilling is temporarily halted, a seismic source
(e.g., a controlled explosion at the Earth's surface) is induced, and the geophone measures the
resulting seismic activity from its position downhole. To reach this depth at which seismic
sensing is performed, the geophone experiences drilling and downhole conditions—such as
drilling vibrations—as it "rides" the drill string to the target depth.
BRIEF DESCRIPTION OF THE DRAWINGS
Accordingly, there are disclosed in the drawings and in the following description a
geophone with a magnetic self-damping system. In the drawings:
Figure 1 is a schematic diagram of a drilling environment.
Figure 2A is a schematic diagram of a self-damping geophone that includes external
damping magnets.
Figure 2B is a schematic diagram of a self-damping geophone that includes internal
damping magnets.
Figure 3 is a graph indicating a force between geophone magnets as a function of the
distance between the magnets.
Figure 4 is a flow diagram of a method for making and using the self-damping
geophone.
It should be understood, however, that the specific embodiments given in the drawings
and detailed description thereto do not limit the disclosure. On the contrary, they provide the
foundation for one of ordinary skill to discern the alternative forms, equivalents, and
modifications that are encompassed together with one or more of the given embodiments in the
scope of the appended claims.
DETAILED DESCRIPTION
Disclosed herein is a novel geophone with a magnetic self-damping system. The selfdamping
geophone generally includes a housing and a magnet suspended inside the housing
using multiple coils. The magnet is adjacent to an electrically conductive coil. In some
embodiments, the magnet may be disposed within—but not touching—the conductive coil,
although any embodiment that generates a variable current in the conductive coil based on the
movement of the magnet relative to the coil is contemplated and included within the scope of
this disclosure.
The geophone further includes damping magnets on opposing ends of the geophone—
that is, the ends of the geophone that align with the axis of the magnet's movement within the
geophone. In many cases, the spring-suspended magnet will move in a vertical direction as a
result of mechanical energy (e.g., seismic waves, drilling vibrations) acting upon the geophone.
Thus, in such embodiments, the damping magnets will be located on the vertical ends of the
geophone. In some embodiments, the damping magnets will be disposed outside of the
geophone housing—for example, they may be coupled to the outer surfaces of the geophone
housing. In some embodiments, the damping magnets will be disposed inside the geophone
housing—for instance, they may be coupled to the inner surfaces of the geophone housing.
Other embodiments are contemplated, such as those in which one of the damping magnets is
coupled to an inner surface of the geophone housing and another damping magnet is coupled
to an outer surface of the geophone housing, or such as those in which the damping magnets
are coupled to other structures outside or inside the geophone housing besides the surfaces of
the housing themselves. In any event, the damping magnets are oriented so that they repel the
spring-suspended magnet as the magnet moves back and forth due to mechanical energy acting
upon the geophone. For example, if the spring-suspended magnet has a north pole oriented
toward the top of the geophone housing and a south pole oriented toward the bottom of the
geophone housing, then a damping magnet coupled to the top of the housing will be oriented
so that its north pole faces downward (i.e., toward the spring-suspended magnet's north pole)
and a damping magnet coupled to the bottom of the housing will be oriented so that its south
pole faces upward (i.e., toward the spring suspended magnet's south pole).
The strength of the damping magnets is selected based at least in part on the strength
of the spring-suspended magnet and other relevant factors, e.g., expected mechanical energy
stress on the geophone in the intended downhole application. Specifically, the damping
magnets are selected to be strong enough to resist unnecessary movement of the springsuspended
magnet toward the ends of the geophone housing at which the damping magnets are
disposed (e.g., to prevent impact of the spring-suspended magnet against the geophone
housing; to prevent unnecessary spring wear and tear). At the same time, the damping magnet
strengths are selected to permit proper movement of the spring-suspended magnet and a reliable
frequency response (e.g., voltage potential across the two ends of the conductive coil as a result
of various possible seismic wave frequencies).
In operation, the self-damping geophone is mounted on a drill string and the drill string
is used to drill a borehole in a target formation. During the drilling operation, the geophone is
subjected to harsh vibrations and mechanical impacts, but the damping magnets—each of
which repels the spring-suspended magnet—prevents the spring-suspended magnet from
colliding with the geophone housing and also reduces the wear and tear on the springs by
impeding unnecessary movement of the spring-suspended magnet. In this way, damage to the
geophone is reduced in comparison to the damage that would be sustained by a non-selfdamping
geophone. When the geophone within the drill string reaches a desired depth, drilling
is stopped and a seismic event (natural or induced) is recorded using the geophone.
Specifically, the seismic waves act upon the geophone, causing the spring-suspended magnet
within the geophone to move within the geophone housing. Although the damping magnets
have magnetic moments strong enough to preclude contact or excessive wear as described
above, they are not so strong that they prevent proper movement of the spring-suspended
magnet in response to seismic waves. (To facilitate proper seismic energy readings, the selfdamping
geophone may be tested to study its frequency response and the structure of the
geophone—for example, the strength of the damping magnets—may be adjusted to calibrate
the frequency response.)
Figure 1 is a schematic of an illustrative drilling environment 100. The drilling
environment 100 comprises a drilling platform 102 that supports a derrick 104 having a traveling
block 106 for raising and lowering a drill string 108. A top-drive motor 110 supports and turns
the drill string 108 as it is lowered into a borehole 112. The drill string's rotation, alone or in
combination with the operation of a downhole motor, drives the drill bit 114 to extend the
borehole 112. The drill bit 114 is one component of a bottomhole assembly (BHA) 116 that may
further include a rotary steering system (RSS) 118 and stabilizer 120 (or some other form of
steering assembly) along with drill collars and logging instruments, such as one or more of the
self-damping geophones described herein. A pump 122 circulates drilling fluid through a feed
pipe to the top drive 110, downhole through the interior of drill string 108, through orifices in the
drill bit 114, back to the surface via an annulus around the drill string 108, and into a retention
pit 124. The drilling fluid transports formation samples—i.e., drill cuttings—from the borehole
112 into the retention pit 124 and aids in maintaining the integrity of the borehole. Formation
samples may be extracted from the drilling fluid at any suitable time and location, such as from
the retention pit 126. The formation samples may then be analyzed at a suitable surface-level
laboratory or other facility (not specifically shown). While drilling, an upper portion of the
borehole 112 may be stabilized with a casing string 113 while a lower portion of the borehole
112 remains open (uncased).
The drill collars in the BHA 116 are typically thick-walled steel pipe sections that provide
weight and rigidity for the drilling process. The thick walls are also convenient sites for installing
the arrays of transmitters and receivers (as described in greater detail below) and logging
instruments that measure downhole conditions, various drilling parameters, and characteristics
of the formations penetrated by the borehole. The BHA 116 typically further includes a
navigation tool having instruments for measuring tool orientation (e.g., multi-component
magnetometers and accelerometers) and a control sub with a telemetry transmitter and receiver.
The control sub coordinates the operation of the various logging instruments, steering
mechanisms, and drilling motors, in accordance with commands received from the surface, and
provides a stream of telemetry data to the surface as needed to communicate relevant
measurements and status information. A corresponding telemetry receiver and transmitter is
located on or near the drilling platform 102 to complete the telemetry link. One type of telemetry
link is based on modulating the flow of drilling fluid to create pressure pulses that propagate
along the drill string ("mud-pulse telemetry or MPT"), but other known telemetry techniques are
suitable. Much of the data obtained by the control sub may be stored in memory for later retrieval,
e.g., when the BHA 116 physically returns to the surface.
A surface interface 126 serves as a hub for communicating via the telemetry link and for
communicating with the various sensors and control mechanisms on the platform 102. A data
processing unit (shown in Fig. 1 as a tablet computer 128) communicates with the surface
interface 126 via a wired or wireless link 130, collecting and processing measurement data to
generate logs and other visual representations of the acquired data and the derived models to
facilitate analysis by a user. The data processing unit may take many suitable forms, including
one or more of: an embedded processor, a desktop computer, a laptop computer, a central
processing facility, and a virtual computer in the cloud. In each case, software on a non-transitory
information storage medium may configure the processing unit to carry out the desired
processing, modeling, and display generation. The data processing unit may also contain storage
to store, e.g., data received from tools in the BHA 116 via mud pulse telemetry or any other
suitable communication technique. The scope of disclosure is not limited to these particular
examples of data processing units.
Figure 1 further illustrates a seismic source 132, such as a controlled explosion, at the
surface of the Earth. Any suitable distance between the borehole 112 and the seismic source 132
may be selected by drilling personnel. The seismic source 132 emits seismic waves 134.
Although the seismic waves 134 shown in Figure 1 propagate in the direction of the wellbore
112, in practice, such waves may propagate in all directions from the location of the seismic
source 132. A geophone disposed within the drill string 108 receives these seismic waves 134
and records them as described below.
Figure 2A is a cross-sectional schematic diagram of a self-damping geophone 200.
Specifically, the geophone 200 includes a cylindrical housing 201, a cylindrical moving magnet
202, a cylindrical stationary mass 204, and a conductive coil 206 wrapped around the stationary
mass 204. The coil 206 has ends 206A and 206B, which electrically couple to leads 207A and
207B, respectively. The geophone 200 further includes springs (e.g., leaf springs) 208 that
couple the magnet 202 to the housing 201, thereby suspending the magnet 202 in the air within
the housing 201. As shown, the magnet 202 is disposed within the conductive coil 206, but it
does not couple to the conductive coil 206. Other embodiments are contemplated and included
within the scope of the disclosure, such as those in which the magnet 202 and conductive coil
206 are arranged in different configurations but that still keep the conductive coil 206 within
the magnetic field of the magnet 202.
The geophone 200 further includes cylindrical damping magnets 210 and 212 which,
at rest, are separated from the magnet 202 by distances 214 and 216, respectively. As shown,
the damping magnets 210, 212 are oriented to repel the magnet 202—that is, the north poles of
the damping magnet 210 and magnet 202 face each other, while the south poles of the damping
magnet 212 and the magnet 202 face each other. As a result, movement of the magnet 202 in
the vertical direction is restricted to an extent that depends on the strengths of the damping
magnets 210, 212 and the lengths of distances 214, 216. Each of these values may be set in any
suitable manner to facilitate a desired behavior of the magnet 202 in response to drilling
vibrations and seismic waves. (In some embodiments, a ferromagnetic material may be
positioned between the magnet 202 and each of the damping magnets 210, 212 to help calibrate
the force between the magnets.) In addition, after these values have been determined and the
self-damping geophone has been created, the frequency response of the geophone is studied
prior to sending the geophone downhole so that electrical signals received from the geophone
reflecting seismic activity may be properly interpreted.
In operation, the self-damping geophone 200—which is coupled to a drill string—
moves downhole as the drilling process progresses. The springs 208 suspend the magnet 202
and thus permit the magnet 202 to move up and down in a vertical direction as a result of
drilling vibrations. The damping magnet 210, however, repels the magnet 202 as the magnet
202 moves in the upward direction (i.e. toward the damping magnet 210), and the damping
magnet 212 repels the magnet 202 as the magnet 202 moves in the downward direction (i.e.,
toward the damping magnet 212). As explained, the repulsion between the damping magnets
210, 212 and the magnet 202 depends on a number of adjustable factors that are considered
during the design of the self-damping geophone, including, without limitation, the strengths of
the damping magnets 210, 212 and the magnet 202; the lengths of distances 214, 216; the
material with which the housing 201 is manufactured, etc. As the magnet 202 moves closer to
either of the damping magnets 210, 212, the repulsion force grows exponentially, thereby
allowing for generally unrestricted movement when the magnet 202 is at or near the center of
the housing 201, but resisting movement of the magnet 202 with increasing force as the magnet
202 approaches either of the damping magnets 210, 212. In this way, because the movement
of the magnet 202 is restricted, the wear and tear on the springs 208, magnet 202, and housing
201 is mitigated. As a result, the longevity of the geophone 200 is enhanced.
When the geophone 200 reaches its intended depth, drilling is stopped so that drilling
vibrations do not interfere with the geophone's seismic measurements. As seismic waves—
whether induced or natural—approach and affect the geophone 200, the magnet 202 moves
vertically in an up-and-down motion with a particular velocity. The magnet 202 generates a
magnetic field, and the motion of the magnet 202 relative to the conductive coil 206 affects the
current flowing through the coil 206. The current in the coil 206 directly corresponds to the
motion of the magnet 202. Thus, measuring the current in the coil 206 (or the voltage across
the ends 206A, 206B of the coil, measured using leads 207A, 207B) provides information
regarding the motion of the magnet 202, which, in turn, provides information about the seismic
activity affecting the geophone 200. (A resistor or other load, not specifically shown, may be
placed along the coil 206 to provide a potential across coil ends 206A, 206B.) Because the
frequency response of the geophone 200 is known, the signals received from the geophone 200
may be used to determine properties of the seismic waves affecting the geophone 200.
Figure 2B is a schematic diagram describing other embodiments of the self-damping
geophone 200. Specifically, in such embodiments, the geophone 200 is virtually identical to
the geophone 200 shown in Figure 2A. The damping magnets 210, 212, however, are disposed
within the housing 201 . Specifically, the damping magnet 210 is coupled to an inner surface of
the housing 201 so that it opposes the upward motion of the magnet 202, and the damping
magnet 212 is coupled to an inner surface of the housing 201 so that it opposes the downward
motion of the magnet 202. Incorporating the damping magnets inside the housing 201 improves
the profile of the geophone 200 and reduces the amount of space required to accommodate the
geophone 200 in the drill string. In addition, the damping magnets are protected by the housing
201.
Figure 3 is a graph 300 indicating a force between geophone magnets as a function of
the distance between the magnets. The graph 300 includes an x-axis representing distance
between any two cylindrical magnets (e.g., distances 214, 216 shown in Figures 2A and 2B)
and a y-axis representing normalized force between those magnets. Thus, for example, the
curve 302 on graph 300 represents the behavior of the force between, e.g., damping magnet
210 and magnet 202 as the distance between them varies. The curve 302 is based on the
following equation for the force between the magnet 202 and a damping magnet:
F z ,m1,m2 = - (1)
where z is the distance between the magnetic poles, mi and , are the dipole moments of the
magnets, and m o is the permeability of the material between the magnets. This equation and the
curve 302 may be used to select strengths of the damping magnets 210, 212 and distances 214,
216 to achieve a desired behavior of the magnet 202 in response to drilling vibrations and
seismic waves.
The curve 302 includes a portion 304, which represents a heavy damping zone. In the
heavy damping zone, the damping magnet is located close to the magnet 202 and the force
between the two magnets is relatively strong, thus preventing or at least impeding the magnet
202 from making contact with any other object (e.g., the housing 201 or the damping magnet,
depending on the location of the damping magnet inside or outside the housing). As the distance
between the magnets increases, the force between the magnets drops exponentially (i.e., at the
rate of z4, where z is the distance between the repelling poles of the magnets), and in the no
damping zone 306, movement of the magnet 202 is minimally restricted, if at all. As explained,
the behavior of the curve 302—that is, the interaction between the magnet 202 and the damping
magnets 210, 212—may be calibrated as desired by manipulating the strengths of these
magnets, the distances between the magnets, and/or the permeability of the material between
the magnets.
Figure 4 is a flow diagram of a method 400 for making and using the self-damping
geophone 200. The method 400 begins with selecting the damping magnets' strengths based
on the force calculations described above (step 402). The method 400 next includes coupling
the selected damping magnets to the geophone (step 404). The distance between the damping
magnets and the magnet 202 is selected as desired, at least in part based on the force calculation
described above. As described with respect to Figures 2A and 2B, the damping magnets may
be coupled on the outside or the inside of the housing of the geophone. In some embodiments,
one damping magnet is coupled on the outside of the housing and another damping magnet is
coupled on the inside of the housing. Any and all such permutations are included within the
scope of the disclosure.
The method 400 then includes determining the geophone's frequency response (step
406). The frequency response may vary based on a variety of factors, including, without
limitation, the strength of the springs 208 used to suspend the magnet 202, the distances
between the damping magnets and the magnet 202, the strengths of all magnets, the
permeability of material between the damping magnets and the magnet 202, the characteristics
of the conductive coil 206, etc. The geophone's frequency response is studied so that seismic
measurements captured later may be properly interpreted. The method 400 then includes
mounting the self-damping geophone on the drill string (step 408). In at least some
embodiments, the geophone is housed within the bottomhole assembly (BHA), although other
possibilities are contemplated. The drill string is then used to drill into the target formation
(step 410), and the method 400 concludes by stopping the drilling and performing seismic
measurements using the self-damping geophone (step 412). Measurements obtained from the
self-damping geophone may be recorded downhole for later extraction at the surface,
transported using a telemetry technique (e.g., mud-pulse telemetry), or conveyed using
slickline services. Other known communication techniques also may be used as suitable.
Although the foregoing embodiments and figures describe the springs 208 as
suspending a magnet 202 within the geophone while the mass 204 and coil 206 remain
stationary, in some embodiments, the springs 208 may couple to the coil-wrapped mass 204
while the magnet 202 remains stationary. In such embodiments, drilling vibrations and seismic
waves cause the mass 204 and the coil 206 to move in a vertical direction, and the magnetic
field generated by the movement of the coil 206 is repelled using the damping magnets, as
described above. Any such variations may be appropriate and included within the scope of this
disclosure as long as the moving member within the geophone can be repelled using the
damping magnets, the movement of that member results in relative movement between the
magnet 202 and the coil 206, and the frequency response of the completed geophone is studied
prior to sending the geophone downhole.
Numerous other variations and modifications will become apparent to those skilled in
the art once the above disclosure is fully appreciated. It is intended that the following claims
be interpreted to embrace all such variations, modifications and equivalents. In addition, the
term "or" should be interpreted in an inclusive sense.
At least some of the embodiments in the present disclosure are directed to a selfdamping
geophone, comprising: a housing containing a conductive coil and one or more
springs; a first magnet suspended within said housing by the one or more springs, said
conductive coil located within a magnetic field of the first magnet; and damping magnets
disposed outside of said housing, each of the damping magnets oriented to repel a pole of the
first magnet closest to that damping magnet. Such embodiments may be supplemented in a
variety of ways, including by adding any or all of the following concepts, in any sequence and
in any combination: wherein the damping magnets repel the first magnet such that they impede
contact between the first magnet and the housing; wherein a force between the first magnet and
at least one of the damping magnets varies based on a distance between the first magnet and
the at least one of the damping magnets; wherein, between a first value of said distance and a
second value of said distance, said force decreases, and between a third value of said distance
and a fourth value of said distance, said force is constant; wherein the geophone is housed
within a drill string; wherein at least one of the one or more springs is a leaf spring; wherein
the first magnet is disposed within the conductive coil and is electrically independent of the
conductive coil; wherein the damping magnets are disposed on outer surfaces of the housing.
At least some of the embodiments in the present disclosure are directed a self-damping
geophone, comprising: a housing containing a conductive coil and one or more springs; a first
magnet suspended within the housing by said one or more springs, said conductive coil located
within a magnetic field of the first magnet; and damping magnets disposed inside said housing,
each of the damping magnets oriented to repel a pole of the first magnet closest to the damping
magnet. Such embodiments may be supplemented in a variety of ways, including by adding
any or all of the following concepts, in any sequence and in any combination: wherein the
damping magnets repel the first magnet such that they impede contact between the first magnet
and either of the damping magnets; wherein a force between the first magnet and at least one
of the damping magnets varies based on a distance between the first magnet and the at least
one of the damping magnets; wherein, between a first value of said distance and a second value
of said distance, said force changes by a rate of z4, and wherein z is the difference between the
first and second values of said distance; wherein the geophone is housed within a drill string;
wherein the first magnet is disposed within the conductive coil but does not contact the
conductive coil; wherein the damping magnets are disposed on inner surfaces of the housing.
At least some of the embodiments in the present disclosure are directed to a method for
protecting a geophone from vibration-induced damage, comprising: selecting a damping
magnet based on a strength of the damping magnet and a strength of a first magnet suspended
by springs within a geophone housing; coupling the damping magnet to the geophone housing,
the damping magnet oriented such that the damping magnet resists movement of the first
magnet toward the damping magnet; coupling the geophone to a drill string; using the drill
string to drill a borehole such that the geophone is disposed within the borehole; and using the
geophone to perform a seismic measurement. Such embodiments may be supplemented in a
variety of ways, including by adding any or all of the following concepts, in any sequence and
in any combination: wherein the sides of the damping magnet and the first magnet that are
closest to each other share a common pole type; wherein, during the drilling of said borehole,
the damping magnet precludes contact between the first magnet and either the damping magnet
or the geophone housing; wherein the damping magnet couples to an outer surface of the
geophone housing; wherein the damping magnet couples to an inner surface of the geophone
housing.

CLAIMS
The following is claimed:
1. A self-damping geophone, comprising:
a housing containing a conductive coil and one or more springs;
a first magnet suspended within said housing by the one or more springs, said
conductive coil located within a magnetic field of the first magnet; and
damping magnets disposed outside of said housing, each of the damping magnets
oriented to repel a pole of the first magnet closest to that damping magnet.
2. The geophone of claim 1, wherein the damping magnets repel the first magnet such that
they impede contact between the first magnet and the housing.
3. The geophone of claim 1, wherein a force between the first magnet and at least one of
the damping magnets varies based on a distance between the first magnet and the at least one
of the damping magnets.
4. The geophone of claim 3, wherein, between a first value of said distance and a second
value of said distance, said force decreases, and between a third value of said distance and a
fourth value of said distance, said force is constant.
5. The geophone of claim 1, wherein the geophone is housed within a drill string.
6. The geophones of claims 1-5, wherein at least one of the one or more springs is a leaf
spring.
7. The geophones of claims 1-5, wherein the first magnet is disposed within the
conductive coil and is electrically independent of the conductive coil.
8. The geophones of claims 1-5, wherein the damping magnets are disposed on outer
surfaces of the housing.
9. A self-damping geophone, comprising:
a housing containing a conductive coil and one or more springs;
a first magnet suspended within the housing by said one or more springs, said
conductive coil located within a magnetic field of the first magnet; and
damping magnets disposed inside said housing, each of the damping magnets oriented
to repel a pole of the first magnet closest to the damping magnet.
10. The geophone of claim 9, wherein the damping magnets repel the first magnet such that
they impede contact between the first magnet and either of the damping magnets.
11. The geophone of claim 9, wherein a force between the first magnet and at least one of
the damping magnets varies based on a distance between the first magnet and the at least one
of the damping magnets.
12. The geophone of claim 11, wherein, between a first value of said distance and a second
value of said distance, said force changes by a rate of z4, and wherein z is the difference between
the first and second values of said distance.
13. The geophones of claims 9-12, wherein the geophone is housed within a drill string.
14. The geophones of claims 9-12, wherein the first magnet is disposed within the
conductive coil but does not contact the conductive coil.
15. The geophones of claims 9-12, wherein the damping magnets are disposed on inner
surfaces of the housing.
16. A method for protecting a geophone from vibration-induced damage, comprising:
selecting a damping magnet based on a strength of the damping magnet and a strength
of a first magnet suspended by springs within a geophone housing;
coupling the damping magnet to the geophone housing, the damping magnet oriented
such that the damping magnet resists movement of the first magnet toward the
damping magnet;
coupling the geophone to a drill string;
using the drill string to drill a borehole such that the geophone is disposed within the
borehole; and
using the geophone to perform a seismic measurement.
17. The method of claim 16, wherein the sides of the damping magnet and the first magnet
that are closest to each other share a common pole type.
18. The method of claim 16, wherein, during the drilling of said borehole, the damping
magnet precludes contact between the first magnet and either the damping magnet or the
geophone housing.
19. The method of claim 16, wherein the damping magnet couples to an outer surface of
the geophone housing.
20. The method of claim 16, wherein the damping magnet couples to an inner surface of
the geophone housing.