Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
ROT~TING ECCENTRIC WEIGHT ~PPARATUS
AND METHOD FOR GENERATING CODED SHEAR WAVE SIGNALS
Technical Field
This invention relates to the art of ~eophysical pros-
pecting using artificially induced seismic energy, andmore particularly -to apparatus and methods ~or gerleratin~
shear waves suitable for use in seismic exploration
methods.
Background ~rt
Two types of seismic signals have been used in the
seismic eY~ploration of earth strata. One type is the so-
called pressure (P) wave in which the earth particle mo-
tion is in the same direction as the wave propagation.
Pressure waves are sometimes also called compressional or
longitudinal waves. The other type i9 the shear wave in
which the earth particle motion is generally normal to
the direction of wave propagation. Shear waves in which
the particle motion is oriented normal to the incident
plane are called horizontal shear (SH) waves and shear
waves in which the particle motion is oriented within the
incident plane are called vertical shear (SV) waves.
Pressure waves are the most commonly used signals for
seismic exploration and may be generated in numerous ways,
such as the detonation of an explosive, the dropping of
weights or the use of a mechanical vibrator. U.S. Patents
; 4,143,747 and 4,234,053 disclose a rotating eccentric
weight seismic apparatus and method particularly suited to
,
.
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the generation of pressure waves in relatively inaccess-
ible areas.
Recently, however, there has been an increased interest
in the use of shear waves in seismic exploration. U.S.
Patents 3,286,783 to Cherry et al.; 3,302,164 to Waters e-t
al. 3,372,770 to Clynch; 3,835,954 to Layotte and
4,059,820 to Turpening disclose various apparatus and
methods for the generation and use of shear waves in seis-
mic exploration. Heiland, C.A., Geophysical Exploration,
Prentice-Hall, Inc., 1940, discloses the use of rotating
eccentric weight vibrators to generate shear waves.
In the search for petroleum and other valuable re-
sources, it has become the practice to transmit a desired
pressure or shear wave signal into the earth from a source-
point near the surface of the earth. The reflected and/orrefracted energy returning from within the earth to a
receiver location is sensed and raw seismic data is
recorded. The raw seismic data is ma-thematically pro-
cessed and then interpreted to provide an indication of
the structure of the underlying strata. In the explora-
tion of onshore regions which are relatively inaccessible
to vehicles, the weight of supplies and equipment required
determines the practicality of a particular exploration
system.
At the present time, a wide variety of seismic explor-
ation systems are available. In some of these systems, a
coded energy signal is transmitted into the earth and the
raw seismic data which is obtained is correlated with a
signature of the coded energy signal. The signature of
the coded energy signal must be of very good quality in
order to obtain a good quality correlated trace. These
coded energy signal systems can be generally classified,
according to the method by which the signature used to
correlate the raw trace is obtained, as either a master-
type or a slave-type source system. In the master-type
source systems, the signature used to correlate the raw
t
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seismic data is generally sensed as the coded energy
signal is transmitted. In the slave-type source systems,
the source signa~ure used to correlate the raw seismic
data is the predetermined signal which is used to drive
the "slave" source during the generation of the coded
energy signal.
In the master-type systems disclosed in the prior
art, the source signature is typically generated by an
acoustic sensor which is responsive to the outgoing seis-
mic signal, such as an accelerometer or a geophone locat-
ed on or near the seismic wave generator. These master
type systems have generally not been used successfully
because tbe acoustic sensors employed are inherently sen-
sitive to any acoustic energy, including acoustic energy
unrelated to the coded energy signal being transmitted.
The source signatures generated by these acoustic sensors
are often attenuated and/or phase-shifted usually contain
signi~icant background inter~erences. While numerous
methods have been proposed to extract the true source
code from the output signals generated by acoustic sen-
sors, these methods have been only moderately successful.
Accordingly, the quality of the processed traces produced
` with the prior art master-type coded energy signal system~
remains generally unacceptable for high resolution seis-
mic exploration.
On the other hand, the correlated traces obtained byuse of the "slave-type" source systems genera?ly have much
better resolution. Because the source used in these
methods can be made to transmit coded energy signals
according to a predetermined code, the code is known and
need not be detected by use of an acoustic sensor.
Furthermore, carefully preselected coded energy signals
which tend to yield high resolution seismic data can be
transmitted by precise control of the source. These sys-
tems, such as the well known VIBROSEIS* system developed
* Trademark of Continental Oil Company
i
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and licensed by Continental Oil Company, ~onca City,Oklahoma, have been relatively successful. However, the
weight of the equipment required, specifically the heavy
source control devices and vibrators employed, increases
markedly as the resolving power of these systems is en-
hanced. Moreover, since the VIBROSEIS* and similar sour~s
must be coupled to the ground, it has been determined
that the peak force to weight ratio must be less than 1.
These sources are normally vehicle mounted and weigh
between about 10 and about 20 tons. Due to this great
weight, these "slave-type" source systems are impractical
for use in regions not accessible to vehicles.
While the deficiencies o~ the prior art systems have
been largely overcome by the apparatus and method for
generatin~ pressure waves disclosed in U.S. Patents
4,143,737 and 4,234,053, the apparatus and method disclos-
ed therein are not directly applicable f~r use in seis-
mic exploration methods using shear waves. 1'hus, a need
exists for a portable apparatus and a method for generat-
ing shear waves and obtaining high resolution seismicdata with such shear waves. Accordingly, a primary object of this invention is
to provide a portable apparatus and a method for generat-
ing shear waves and obtaining high resolution seismic
data with such shear waves.
Another object of this invention is to provide an
apparatus and method for transmitting coded shear wave
signals into the earth and o~taining a code signal for
correlation of the resulting seismic data, which code
signal is free from the attenuation, phase-shifting and
background interferences exemplifying the comparable prior
art systems.
: .
- * Trademark of Continental Oil Company
--5--
Ye-t another object of this invention is to provide
a shear wave generating apparatus and methoA in which the
required weight of the exploration equipment is reduced
without sacrificing seismic data quality.
Still another object of this invention is to provide
an apparatus and method or simultaneously generating
both a multiple impulse pressure wave and a sinusoidal
shear wave.
Other objects, advantages and features of this in-
vention will become apparent to those skilled in the art
from the following description when taken in conjunction
with the drawings.
Disclosure of Invention
-
Briefly, the invention provides a method Eor the
seismic exploration of earth strata underlying an earth
surface between a sourcepoint and a receiver location,
which comprises: (a) positioning on said earth surface
at said sourcepoint a seismic source having (1) an eccen-
tric element which is rotatable about an axis of rotation
and (2) a position sensor comprised of a first sensor
element and a second sensor element; (b) rotating said
eccentric element and said first sensor element about
said axis of rotation at varying speeds so as to transmit
into said earth strata a shear wave signal having a fre-
quency variable code; (c) positioning said second sensorele~.ent at a first selected position about said axis of
rotation such that saia first sensor element passes in
close proximity to said second sensor element at and only
at the instant at which the center of mass of said eccen-
tric element passes that angular position about said axisof rotation at which said seismic source develops the
peak shear force during each revolution of said eccentric
element; (d) causing one of said first or second sensor
elements to generate a shear code signal charac-terized by
a substantially interference-free background and a
plurality of discrete pulses, each of said pulses
--6-
' corresponding to one of the instants at which said first sensor element passes in close proximity to said second
sensor element; (e) sensing seismic shear wave energy
returning from said earth s-tra-ta to said receiver loca-
tion in order to obtain raw shear wave data which is pro-
portional to said seismic energy; a:nd (f) correlating said
raw shear wave data with said shear code signal to there-
by form a correlated shear wave trace which is indicative
of the structure of said earth strata.
The invention also provides a method for the seismic
exploration of earth strata underlying an earth surface
between a sourcepoint and a receiver location, which com-
prises: (a~ positioning on said earth surface at said
sourcepoint a seismic source having (1) a body s~ction
comprising a base plate, an eccentric element rotatabl~
supported above said base plate so as to be rotatable ab~
: an axis of rotation substantially perpanclicular to an
imaginary line between said sourcepoint and said receiver
location, and a position sensor comprised of :Eirst, second
2~ and third sensor elements, and (2) an outrigger section
pivotally coupling said body section to said earth surface
along a pivotal line parallel to said axis of rotation
. and spaced from said body section; (b) rotating said
eccentric element and said first sensor element about said
25 axis of rotation at varying speeds between 0.5 and 150
revolutions per second so as to simultaneously transmit a
coded pressure wave signal through said base plate into
said earth strata and a sinusoidal coded shear wave signal
through said outrigger section into said earth strata;
(c) positioning said second sensor element at a first
selected position about said axis of rotation such that
said first sensor element passes in close proximity to
said second sensor element at and only at the instant at
which the center of mass of said eccentric element passes
that angular position about said axis of rotation at which
said seismic source develops the peak shesr ~orce ~uring
--7--
each revolution of said eccentric element, and position-
ing said third sensor element at a second selected posi-
tion about said axis of rotation such that said first
sensor element passes in close proximity to said third
sensor element at and only at the instant at which the
center of mass of said eccentric element passes that
angular position about said axis of rotation at which
said seismic source develops the peak earthward force
during each revolution of said eccentric element; (d)
causing said second sensor element to generate a shear
code signal characterized by a substantially interference-
free bac~ground and a plurality of discrete pulses, each
of said pulses indicating the time break of one of the
instants at which said :Eirst sensor element passes in
close proximity to said second sensor element, and causing
said third sensor element to generate a pressur~ code
signal characteri2ed by a substantially inter:fere:nce-:Eree
background and a plurality of discrete pulses, each of
said pulses indicating the time break of one of the in-
stants at which said first sensor element passes in closeproximity to said third sensor element; (e) sensing seis-
mic shear wave energy and seismic pressure wave energy
returning from said earth strata to said receiver location
so as to obtain raw shear wave data and raw pressure wave
data; and (f) corxelating said raw shear wave data with
said shear code signal to thereby form a correlated shear
wave trace indicative of the structure of sald earth
strata and correlating said raw pressure wave data with
said pressure code signal to thereby form a correlated
pressure wave trace indicative of the structure of said
earth strata.
The invention further provides a seismic source
apparatus for generating coded shear wave signals, com-
prising: a rotatable element having a center of mass dis-
placed from and rotatable about an axis of rotation;support means for rotatably supporting said rotatable
3 i~
-8-
element; drive means for rotating said :rotatable element
about said axis of rotation; and position sensor means
comprised of first and second sensor elements, one of
said sensor elements being an actuator and the other of
said sensor elements being a first pulse generator which
generates a discrete electrical pulse each time said
first and second sensor elements pass in close proximity
to each otherl said first sensor element being rotatable
about said axis of rotation at the same speed as said
rotatable element and said second sensor element being
supported by said support means at a first preselected
position about said axis of rotation such that said first
sensor element passes in close proximity to said second
sensor element at and only at the instant at which the
center of mass of said rotatable element passes that
angular position about said axis of rotat.ion at wh.ich
said apparatus clevelops the peak shear force during each
revolution of said rotatable element.
Additionally, the invention provides a seismic
source apparatus for simultaneously generating coded
shear wave signals and coded pressure wave signals which
comprises: a base plate; a rotatable element having a
center of mass displaced from and rotatable about an
axis of rotation; support means attached to said base
; 25 plate for rotatably supporting said rotatable element;
drive means mounted on said base plate for rotating said
rotatable element about said axis of rotation; an actuator
mounted so as to be rotatable about said axis of rotation
at the same speed as said rotatable element; a first
pulse generator mounted on said support means at a first
preselected position such that said actuator will pass in
close proximity to said first pulse generator at and only
at the instant at which the center of mass of said rotat-
able element passes that angular position about said axis
of rotation at which said apparatus develops the peak
shear force during each revolution of said rotatable
_9_
element, said first pulse generator including means for
generatin~ a shear code signal characterized by a sub-
stantially interference free background and a plurality
of discrete pulses, each of said pulses corresponding to
one of the instants at which said actuator pas~es in
close proximity to said first pulse genera-tor; a second
pulse generator mounted on said support means at a second
preselected position such that said actuator will pass in
close proximity to said second pulse generator at and
only at the instant at which the center of mass of said
rotatable element passes that angular position about said
axis of rotation at which said apparatus develops the peak
earthward force during each revolution of said rotatable
element, said second pulse generator including means for
generating a pressure code signal characterized ~y a sub-
stantial.ly interference-free background and a plurality
of discrete puls~s, each o:E said pulses corresponding to
one of the instants at which said actuator passes iTI close
proximity to said second pulse generator; one or more out-
rigger support elements having a first end attached tosaid base plate and an opposed second end spaced outwardly
from said base plate; and a shovel element coupled to
said second end of said outrigger support elements and
adapted to be at least partially driven into the earth so
as to define a pivotal line parallel to said axis of
rotation and spaced from said base plate.
Brief Description of the Drawings
The invention will be more readily understood by re-
ference to the accompanying drawings, wherein like numerals
refer to like elements, and in which:
FIGS. 1 and 2 are elevational views of a preferred
embodiment of the shear wave generating apparatus of this
inven~ion; and
FIG. 3 is an elevational view of the coupling device
of the shear wave generating apparatus illustrated in
--10--
FIGS. 1 and 2.
Detailed Disclosure of the Invention and Best ~ode
The novel shear wave generating apparatus of this
invention includes one or more eccen-tric wei~hts mounted
for rotation about an axis oE rotation. If more than one
rotatable eccentric is employed, they may rotate in
unison or may be counter-rotating. A pair or pairs of
matched counter-rotating eccentrics are preferred when it
is desired to transmit only shear wave signals into the
earth. ~wo or more eccentrics rotating in the same direc-
tion about a common axis of rotation or parallel axes of
rotation, or a single eccentric rotating about its axis
of rotation, are preferred when it is desired to generate
both shear wave signals and pressure wave signals.
The shear wave generating apparatus of this inven-
tion may be coupled to the earth or only pivotally coupled
to the earth, or allowed to alternately decouple from and
impact against the earth. As used herein, the term
"coupled to the earth" means that the apparatus is held
in continuous contact with the earth surface, such as by
means of hold down weights or anchors. The term "pivotal-
ly coupled to the earth" means that the apparatus is in
continuous contact with the earth at a pivotal point or
along a pivotal line but that portions of the apparatus
are allowed to alternately decouple from and impact
against the earth. The term "decouple from the earth"
means to lift substantially all of the apparatus up out
of contact with the earth surface in a manner that in-
herently results in the subsequent impact of the apparatus
against the earth surface. Expressed another way, a
decoupling apparatus or a decoupling portion of the
apparatus bounces, i.e., its weight-supportive contact
with the earth is discontinuous during the period of
seismic energy transmission.
Irrespective of the mode of operation selected, a
position sensor mounted on the apparatus directly detects
the exact instants at which the center of mass of the
rotating eccentric passes that particular angular
position about its axis of rotation which corresponds to
the position at which -the peak shear force is developed
~y the apparatus during each revolution of the rotating
eccentric. Where both pressure wave signals and shear
wave signals are to be transmitted, the position sensor
preferably also directly detects the exact instants at
which the center of mass of the rotating eccentric passes
that particular angular position about its axis of rota-
tion which corresponds to the position at which the peak
earthward force is developed by the apparatus during each
revolution of the rotatin~ eccentric.
The force developed by a rotating eccentric is
generally perpendicular to its axis of rotation. The
force in any particular direction reaches a maxim-~ or
peak only once during each revolution of the rotating
eccentric. When used as a shear wave generator, a rotat-
ing eccentric generates maximum shear forces twice during
each revolution of the rotating eccentric but in opposite
directions. For the purposes of this invention, one of
the shear directions ~i.e., one of the two directions
which is perpendicular to both the axis of rotation of
the eccentric element and the direction of wave propaga-
tion) is arbitrarily selected as the "positive" shear
direction and the other shear direction is the "negative"
shear direction. The selection of one direction over the
other is a matter of choice, however the selection is
important to positioning of the sensor elements and the
processing of the shear wave data. The term "peak shear
force" as used herein means the maximum force in the
"positive" shear direction during each revolution of the
rotating eccentric. Similarly, the term "peak earthward
force" as used herein means ~he maximum force toward the
earth during each revolution of the rotating eccentric.
-12-
The peak shear force and the peak earthward force
developed by the apparatus will each occur once during
each revolution of -the rotating eccen-tric, with their
magnitude depending upon -the rotational speed and eccen-
tric moment of the rotating eccen-tr:ic. ~}oweverr for a
given rotating eccentric or a given set of rotatincJ
eccentrics, the angular position of the eccentric(s)
at the instant at which the peak shear force is developed
will always be the same, and the angular position of the
eccentric~s) at the instant at which the peak earthward
force is developed will also be the same. ~hen two or
more rotating eccentrics are employed it is possible to
shift these angular positions by phase-shifting the
eccentrics as described in U.S. Patent ~ 3,7~7, however
for any given arrangement the above~described critical
angular positions are easily determined.
Although the axis of rotation of -the rotating eccen-
tric o~ the apparatus of this invention can be arranged
in any plane, the energy of the seismic pressure waves
transmitted by the apparatus will be maximized by posi-
tioning the source such that the axis of rotation isperpendicular to the desired direction of wave propaga-
tion. Also the energy of the horizontal shear waves
generated will be maximized by positioning the source
such that the axis of rotation is perpendicular to both
the desired direction of wave propagation and an imagin-
ary line drawn from the sourcepoint (i.e., the location
of the source) and the receiver location (i.e., the
location of the geophones or other receivers used to
detect the reflect~d.. and/or refracted seismic waves).
To maximize the energy of the vertical shear waves gener-
ated, the axis of rotation should be perpendicular to the
direction of wave propagation and parallel to an imaginary
line between the sourcepoint and the receiver location.
It is preferred to maximize the energy of the pressure
waves and the horizontal shear waves. Accordingly, for
13-
the exploration of earth strata underlying a horizontal
earth surface the axis of rotation of the rotatable
eccentric is preEerably substantially horizontal and
perpendicular to an imaginary line drawn from the source-
point to the receiver location.
The position sensors used in this invention include
magnetic, optical and/or electrical clevices which are
known in the sensin~ art. The position sensor is com-
prised of at least two sensor elements, one being an
"actuator" and the other being a "pulse generator" which
generates a discrete pulse at each instant that the
actuator passes in close proximity to the pulse generator.
The actuator and the pulse generator are mounted on the
apparatus so that one of these sensor elements, normally
the actuator, is rotated about an axis oE rotation at the
same speed as the rotatable eccentric is rotated about its
axis of rotation and the second sensor element, normally
the pulse ~enerator, is mounted on the apparatus at a
selected position so that the rotating sensor element
passes in close proximity to the second sensor element at
and only at the instant at which the center of mass of
the rotating eccentric passes that particular angular
position about its axis of rotation at which the peak
shear force is developed by the sourca during each revo-
lution of the rotating eccentric. The rotating elementcan be mounted on the rotating eccentric or, for example,
on a wheel, gear, disk or arm which is rotated at the
same speed as the eccentric element. Since rotating the
rotatable sensor element about an axis other than the
axis of rotation of the ro-tatable eccentric is the prac-
tical equivalent of rotating both the sensor element and
the eccentric about a single axis of rotation, it is in-
tended to include all such practical equivalents wherever
in this specification and the appended claims the rota-
tion of both the eccentric and the rotatable sensorelement about a common axis of rotation is described.
: -14-
.. Where it is desired to use both pressure wave
signals and sheax wave signals to produce pressure wave
traces and shear wave -traces, it is pre:Eerred that the
. position sensor include at least a third sensor element,
generally another pulse generator, which cooperates with
~`~ either the first or the second sensor elements jwst des-
cribed to generate a pressure wave code signal character-
ized by a substantially interference-free background
and a plurality of discrete pulses, each o~ which pulses
~ 10 corresponds to one of the instants at which the third
`~ sensor element is in close proximity to the cooperatingone of the first or second sensor elements. Preferably,
the third sensor element is a pulse generator which is
. mounted on the apparatus so tha~. the rotating actuator.~; 15 passes in close proximity to the third sensor element at
and only at the instant at which the center of mass of
the rotating eccentric passes that particular angular
position about its axis of rotation at which the peak
~ earthward ~orce is developed by the source during each
.~ 2~ ravolution of the rotating eccentric.
Position sensors suitable for use in this invention
are those which emit a pulse or small wavelet in response
~ to a desired stimulus, but which are relatively insensi- -
'~ tive to background interferences including vibrations,
. 25 sounds, radio signals and ground noises. Suitable
position sensors include: optical sensors, comprising a
light emitting or transmitting actuator and a photocell
pulse generator; electrical sensors, comprising for
~ example a metal contact actuator which completes the
r ~ 30 otherwise open circuit of the electrical pulse generator,. thereby allowing a current to flow; and magnetic sensors,
comprising a metal protrusion, or preferably a magnet,
actuator and a pulse generator comprising an electric
.~ wire coiled around either a magnet or a metal pole piece
in a magnetic field, in which coil an electric current is
induced by the movement of the actuator past the pulse
.,
.
-15-
generator. Optical and magnetic position sensors are
preferred because they are more durable and require less
maintenance. A wide variety of suitable magnetic sensors
are available from the Electro Corporation of Sarasota,
Florida, and others. Sui-table optical position sensors
are available from Datametrics, Inc. of Wilmington,
Massachusetts, and others. Several exemplary position
sensors are disclosed in U.S. Patents 4,143,737 and
4,234,053.
As used herein, the term "master-type source system"
means a seismic exploration system in which the seismic
data obtained is correlated with a source signature ob-
tained by sensing or monitoring the outgoing seismic
signal or some other characteristic of the seismic
source, as opposed to "slave-type source systems" in
which the seismic data obtained is correlated with the
predetermined source code by which the source was driven
to generate the coded energy signal. The preferred
sources for exploration of relatively inaccessible
regions are manually controlled sources, although sources
which generate energy in response to a master controller,
such as a programmed minicomputer, can be used.
FIGS. 1 and 2 illustrate one preferred embodiment of
the apparatus of this invention employing a single rotat-
able eccentric. The apparatus comprises a major bodysection, indicated generally as 10, and, optionally, an
outrigger section, indicated generally as 12. ~ody sec-
tion 10 includes rotatable eccentric 14 comprised of a
solid semicylindrical weight fixedly attached to axle 16
which is rotatably supported above and parallel to the
bottom surface of base plate 18 by means of bearings or
the like, not shown, mounted in a housing, the outline of
which is indicated by dashed line 20. The center of mass
of eccentric 14 is spaced from and rotatable about an
- 35 axis of rotation, indicated by line A-A, coincident with
the center line of axle 16.
- -16-
.~
Prime mover 22 is mounted on base plate 18 and is
operably connected to axle 16 so as to rotate eccentric
14 and axle 16 about axis A-A at varying speeds. Prime
mover 22 can be a self-contained power source, such as a
gasoline-powered engine, or can be a motor, such as an
electric or hydraulic motor, driven by a po~er source
independently supported a distance away from the shear
wave generator of this invention. Where the minimization
` of total equipment weight is desired, it is preferred
that prime mover 22 be a self-contained power source. As
illustrated in FIG. 2, prime mover 22 rotates eccentric
14 in a clockwise direction.
Body section 10 also includes a position sensor,
indicated generally as 24, mounted on base plate 18.
lS Position sensor 24 includes first sensor element 26
fixedly attached to and rotatable with axle 16 about axis
A-A, second sensor element 28 fixedly attached to base
plate 18 and supported by means o~ bracket 3~ at a ~irst
critical position about axis A-A, and, optionally, third
i 20 sensor element 30 fixedly attached to base plate 18 and
supported by means of bracket 34 at a second critical
position about axis A-A. As best seen in FIG. 2, sensor
element 26 is an opaque disk having a long narrow actua-
tor slot 32. Sensor elements 28 and 30 comprise light
sources 28a and 30a, respectively, mounted on the inboard
side of sensor element 26, and photocell pulse generators
28b and 30b, respectively,mounted on the outboard side of
sensor element 26. Sensor element 26 prevents the trans-
mission of light from light sources 28a and 30a to pulse
generators 28b and 30b, respectively, except for each
brief instant that actuator slot 32 passes in close prox-
imity to th~ respective sensor element 28 and 30, that
is, actuator slot 32 allows a discrete pulse of light
from light sources 28a and 30a through sensor element 26
to pulse generators 28b and 30b, respectively, at and
only at the one instant during each revolution of eccen~ic
-
3~
-17
.
14 at which actuator slot 32 passes in close proximity
` to the respective sensor elements 28 and 30.
. As illustrated in FIG. 2, actuator slot 32 is
located about axis A-A in the same radial plane from
. 5 axis A-A that passes through the center of mass of
`~ eccentric 14. Sensor element 28 is supported by bracket
.~ 34 at a position about axis A-A such that actuator slot
32 passes in close proximity to sensor element 28 once
.. ~ during each revolution of eccentric 14 at exactly the
~ 10 instant when the center of mass of eccentric 14 is in
that angular position about axis A-A at which the appa-
ratus develops the peak shear force during each revolu-
:,.
~: tion of eccentric 14. In the illustrated embodiment,
. the direction from outrigger section 12 to bod~ section
.:~ 15 10 has been selected as the "positive" shear direction
`.~ and the peak shear force occurs when the center of mass
`~ of eccentric 14 is traveling parallel to the bottom o~
~ base plate 18 and toward outrigger section 12. Accord-
.:~ ingly, sensor element 28 is mounted directly above axis
: 20 A-A at a radial distance from axis A-A corresponding to
~` ~
'~ the distance that slot 32 is spaced from axis A-A. Of
course, as discussed above with respect to the definition
of the peak shear force, if the "positive" shear direc-
tion is selected to be the direction from body section 10
~` 25 to outrigger section 12, position sensor 28 would be moun-
.~ ted in a similar position but directly below axis A-A.
; When it is desired to obtain both shear and pressure
wave seismic data, sensor element 30 is employed in co-
~: operation with sensor element 26 to generate a code
.~ 30 signal with which to correlate the raw pressure wave data.
Sensor element 30 is supported by bracket 34 at a posi-
:~ tion about axis A-A such that actuator slot 32 passes in
.. close proximity to sensor element 30 once during each
revolution of eccentric 14 at exactly the instant when
the center of mass of eccentric 14 is in that angular
position about axis A-A at which the apparatus develops
!.
f
, .
.'
'~
'',
.
the peak earthward force during each revolutlon of eccen-
tric 14, i.e., in this embodiment when the center of mass
of eccentric element is traveling perpendicular to and
away from the lower surface of base plate 18. Accord-
5 ingly, sensor element 30 is positioned in the same hori-
zontal plane as axis A-A on the side of axis A-A farthes-t
from outrigger section 12.
It should be understood of course that the exact
angular position of sensor element 26 with respect to the
10 center of mass of eccentric 14 and the angular positions
of sensor elements 28 and 30 with respect to axis A-A
are not critical as long as actuator slot 32 passes in
close proximity to sensor elements 28 and 30 at the
above-described critical times.
Body section 10 can be allowed to decouple Erom the
earth to generate multiple impulse pressure and shear
waves, however body section 10 is preEexably at least
pivotally coupled to the earth, such as by means of
OUtriggQr section 12. Outrigger section 12 comprises
20 a shovel element 40 mechanically coupled through coupling
devices 42 (only one of which is shown) to one end of
outrigger supporting rods 44 which are fixedly attached
at their opposed second ends to base plate 18 by means of
attachments 46. As illustrated in FIGS. 2 and 3/ element
25 40 includes a substantially vertically extending end 40a
adapted to be driven into the earth so as to couple the
apparatus to the earth along the length of end 40a.
Preferably, element 40 also has lip 40b adapted to facil-
itate the driving of end 4Oa into the earth. Coupling
30 device 42 can be a fixed mechanical connection, such as a
weld. Preferably, however, coupling device 42 is a
pivoting device selected to allow some vertical movement
of body section 10 without twisting element 40 while still
transmitting shear waves developed by the apparatus to
35 the earth via element 40. Ball and socket joints or
simple hinges are exemplary pivotal devices.
. .
3~
, . . .
-19-
Body section 10 can also be fully coupled to -the
earth by means of any conventional coupling device, such
as holddown weights or anchors, not shown, alone or in
combination with outrigger section 12 or the like.
In operation, prime mover 22 rotates axle 16 about
axis A-A to thereby rotate eccentric 14 and sensor
element 26 about axis A-A. When eccentric 14 has been
rotated clockwise from the illustrated position until
the flat face of eccentric 14 is perpendicular to the
lower surface of base plate 18 and facing outrigger
section 12/ the center of mass of eccentric 14 will be
traveling perpendicular to and away from the lower sur-
face of base plate 18, and actuator slot 32 will be
aligned with and in elose proximity to sensor element 30.
This is the angular position of eccentric 14 at the in-
stant of peak earthward force during each revolution of
eccentric 14, and light source 30a transmits li~ht
through slot 32 to thereby cause pulse generator 30b to
emit a discrete eleetrical pulse representing the time
break of the peak earthward force. When eccentric 14 has
been rotated clockwise until the flat face of eccentric
14 is parallel to the lower surface of base plate 18 and
facing base plate 18, the center of mass of eccentrie 14
will be traveling parallel to the lower surface of base
plate 18 and toward outrigger seetion 12 and actuator
slot 32 will be aligned with and in close proximity to
sensor element 28. This is the angular position of eceen-
tric 14 at the instant of peak shear force during each
revolution of eccentrie 14, and light source 28a trans-
mits light through slot 32 to thereby eause pulse genera-
tor to emit a discrete electrical pulse representing the
time break of the peak shear force. The magnitude of the
peak shear force and the peak earthward force will in-
erease as the rotational speed of eccentrie 14 is inereas-
ed and will decrease as its rotational speed is deereased.The code signals generated by pulse generators 28b
-20-
and 3~b are preferably transmitted, such as by electrical
conductors, not shown, or by radio signal to a recording
device or other device for use in correlating the raw
seismic data obtained.
In the method of this invention, a rotating eccen-
tric wei~ht seismic source of this invention is position-
ed at a sourcepoint on the earth surface and the rotat-
able eccentric and a first sensor element are rotated at
varying speeds about an axis of rotation so as to trans-
mit into the underlying earth strata a shear wave signal
having a frequency variable code. When the source is
fully or pivotally coupled to the earth, the shear wave
signal will be a sinusoidal sweep signal, whereas if the
source is caused to alternately decouple from and impact
against the earth surface a multiple impulse shear wave
signal will be transmitted. The source can be situatecl
so as to butt against a vertical earth surface, such as
the side of a hole, or a vertically extendin~ abutment
coupled to the earth to maximi~e the shear wave energy
transmitted into the earth when the source is allowed to
decouple.
; A second sensor element is positioned at a selected
position about the axis of rotation such that the first
sensor element will pass in close proximity to the second
sensor element at and only at the instant at which the
center of mass of the rotating eccentric passes that
angular position about its axis of rotation at which the
source develops the peak shear force during each revo-
lution of the rotating eccentric. One of the first or
second sensor elements, preferably the second sensor ele-
ment, is caused to generate a shear wave code signal
; characterized by a substantially interference-free back-
ground and a plurality of discrete pulses, each of which
pulses corresponds to one of the instants at which said
first sensor element passes in close proximity to said
second sensor element.
;
Seismic shear wave energy returning from the under-
lying earth strata to receiver location is sensed by
means of a conventional shear wave seismometer, such as a
single horizontally oriented geophone to detect horizon-
S tal shear waves or vertical shear waves, or, alternati~y,a pair of orthogonally arranged horizontal geophones to
detect both vertical and horizontal shear waves, so as to
obtain raw shear wave data. The raw shear wave data is
correlated with the shear wave code signal to thereby
form a correlated shear wave trace indicative of the
struc~ure of the earth strata.
In one preferred embodiment of -~his invention, a
rotating eccentric weight seismic source of the type
illustrated in FIGS. 1-3 is employed to simultaneously
transmit both a coded shear wave signal and a coded pres-
sure wave signal into the earth. The source is position-
ed at a desired sourcepoint and the rotatab~e eccentric
and a ~irst sensor element are rotated at varying speeds
about an axis o rotation so as to transmit both a pres-
sure wave signal and a shear wave signal having frequencyvariable codes. Where the source is fully coupled to the
earth, both the pressure wave signal and the shear wave
signal will be sinusoidal sweep signals. Where the source
is allowed to alternately decouple from and impact
against the earth without any coupling, both the pressure
wave signal and the shear wave signal will be multiple
impulse signals. In a particularly preferred embodiment
of this invention, the seismic source is only pivotally
coupled to the earth as illustrated in FIG. 2 so that the
shear wave signal generated by the rotating eccentric
will be transmitted to the earth through outrigger sec-
tion 12 as a sinusoidal sweep signal while eccentric 14
is rotated at speeds sufficient to alternately decouple
body section 10 of the source from earth surface 50 and
impact body section 10 against earth surface 50 so as to
transmit a multiple impulse pressure wave signal into the
-~2-
earth.
~ hen a pressure wave signal is transmitted, ik is
preferred that a second sensor element be positioned at
a selected position about the axis of rotation such that
5 the first sensor element passes in close proximi-ty to -the
second sensor element at and only at the instant at which
the center of mass of the rotating eccentric passes that
angular position about its axis of rotation at which the
source develops its peak shear force during each revolu-
tion of the rotating eccentric. Where the rotating firstelement is a pulse generator and the stationary second
element is the actuator, a third sensor element comprised
of a second pulse generator is rotated with the first
sensor element and rotatable eccentric about the axis o~
rotation. The position of the third sensor element i5
selected in view of the positions of first and second
sensor elements such that the third sensor element passes
in close pro~imiky to the stationary actuator at and onLy
at the instant at which the center of mass of the rotat-
ing eccentric passes that angular position at which thepeak earthward force is developed during each revolution
of the rotating eccentric. Preferably, however, the
rotating first sensor element is the actuator, the sta-
tionary second sensor element is a pulse generator and
a third sensor element comprised of a second pulse gener-
ator is positioned at a selected position about the axis
of rotation such that the rotating actuator passes in
close proximity to the third sensor element at and only
at the instant at which the center of mass of the rotat-
ing eccentric passes that angular position at which thepeak earthward force is developed during each revolution
of the rotating eccentric.
The two pulse generators are each caused to generate
a code signal characterized by a substantially interfer-
ence-free background and a plurality of discrete pulses.
The pulses of the shear code signal generated by the
first pulse generator (first or second sensor element)
will each correspond to one of the instants at which the
first sensor element passes in close proximity to the
second sensor element, and the pulses of the pressure
5 code signal generated by the third sensor element will
each correspond to one of the instants at which the third
sensor element is in close proximity to the cooperating
one of the first or second sensor elements.
Seismic shear wave and pressure wave energy return-
ing from the earth strata to a receiver location is
sensed by means of conventional multiaxial seismometers,
such as ~ne vertically oriented geophone and one or two
horizontally oriented geophones, so as to ob-tain raw
shear wave data and raw pressure wa~e data. The raw
shear wave data is correlated with the shear wave trace,
and the raw pressure wave data is pre~era~1~ correlated
with the pressure wave code signal to thereby form a
correlated pressure wave trace.
The correlation technique employed can be the con-
ventional integration method Ecf. Sheriff, R.E.,
~ncyclopedic Dictionary of Exploration Geophysics,
Society of Exploration Geophysicists, Tulsa, Oklahoma,
1973] or can be the type of correlation method disclosed
in U.S. Patent 3,698,009 to Barbier, which method is
herein termed the "shift-summing" method of correlation.
When the seismic signal transmitted into the earth is a
sinusoidal signal, the integration method of correlation
is preferred. On the other hand the shift-summing method
is particularly well suited to the method of this inven-
tion when a multiple impulse signal is transmitted intothe earth. The correlation step can be performed in real
time, i.e., the data can be automatically correlated as
it is being received, or the raw data and the code signal
can be recorded and processed at a later time either in a
computer located on site or a computer located elsewhere.
The pulses of the code signals generatea by the
-24-
position sensors of this invention indicate the exact
time break of each occurrence of the peak shear force or
the peak earthward force. Where the coded energy signal
transmitted is an impulse train, the code signal can be
used directly to correlate the raw seismic data. ~lthough
not usually required, the code signal can be "shaped" by
making each pulse the same amplitude and hreadth prior to
integration with the raw seismic data. Tn the shift-
summing method of correlation, of course, only the time
break of each pulse is used, not its amplitude. If the
coded energy signal transmitted is a sinusoidal signal,
then the code signal may be modified to resemble a sinus-
oidal function by modeling a facsimile signal having the
same frequency as the pulses of the code signal and an
amplitude which is calculated from the speed of rotation
of the rotating eccentric. This modification oE the code
signal to fo~m a facsimile signal is within the skill o~
the art.
~he raw seismic data obtained can be processed and/
or recorded in analog or digital form. As a practical
matter, computer processing is required to correlate the
raw data and accordingly the raw seismic data is prefer-
ably converted to a plurality of digitized samples. The-e
digitized samples contain either the polarity and the
amplitude of the raw seismic data, or, in the case of
"sign-bit" samples, contain only the polarity of the
sample. The processing of "sign-bit only" data is known,
for example U.S. Patent 4,058,791 to Martin et al. dis-
closes the use of "sign-bit only" data throughout the
correlation and stacking phases of the data processing
method.
The use of "sign-bit only" recording and data
processing is especially well suited to the method of
this invention where the impulses of the coded energy
signal transmitted into the ground have low and/or
irregular seismic energies, because the sign~bit only
l:L~
processing places greater emphasis on the number of im-
pulses transmitted from each sourcepoint, rather than the
1 magnitude and uniformity of the impulsive energy.
~ The method of this invention can be employed in any
5 onshore location, it is however particu].arly useful in
exploration of rela~ively inaccessib~e regions wherein
the prior art methods employing slave-type sources are
precluded due to the weight of th~ required equipment.
The various steps and preferred equipment of the method
of this invention combine to significantly reduce the
wei~ht of the equipment required.
~ The weight of the seismic source required is reduced,
as compared to the prior art systems, because: (1) the
use of a master-type decoupling source eliminates the
~ 15 need for heavy source control equipment; (2) by t:he pre-
: ferred method of processing and/or recording only the
si~n-bit of the sensed energy, the force magnitucle o
the individual impulses is less important than the number
of impulses transmitted; and (3) while prior art multiple
; 20 impulse methods, in which the sensed energy was auto-
matically shift-summed as it is recorded, require seis-
~ mic sources which transmit substantially identical im-
: pulses, the preferred processing of only the sign-bit in
the method of this invention makes it possible to employ
even the variable-force seismic sources of this inven-
tion, such as the devices illustrated in FIG. 1 which are
generally less sophisticated and lighter weight.
The weight of the required sampling and recording
equipment for use in the method of this invention is less
~ 30 than that of the high quality prior art equipment,
- because: (1) fewer memory positions are required since the
seismic data is automatically shift-summed; and (2) since
preferably only the sign-bit is sampled, shift-summed and
recorded, a less sophisticated sampling device, smaller
capacity memory positions and less sophisticated compu-
tational capacity are required.
~ ~.3~
-26-
Furthermore, these weight reductions are not
achieved at the expense of seismic data quallty. Due
to the precise record of the impulse time breaks emitted
by the position sensors of this invention, the raw seis-
S mic data is more accurately correlated, consequently thecorrelated trace obtained is superior iII quality to a
trace obtained by correlating the raw seismic data accor~
ing to the attenuated output signals emitted by the prior
art acoustic sensors. In the correlation step, the
amplitude of the seismic events on the correlated trace
is effectively rebuilt by the shift-summing of the sign-
bit samples as the randomly oriented background noise is
e~fectively cancelled and the energy due to seismic
events adds in phase.
The coded energy signals which can be transmitted
into the earth using a decoupling eccentric weight seis-
mic source comprise a plurality of impulses separated by
; discrete time intervals. Since the spe~d of the rotating
eccentric cannot be changed instantaneously from one
speed to another, the time intervals between successive
impulses necessarily define an ascending pat~ern, a des-
cending pattern, a constant pattern or a combination of
these patterns. The frequency of the impulses can vary
~ between about 0.5 and about 150 impulses per second and
: 25 more preferably between about 5 and about 75 impulses per
second. The minimum time interval between successive im-
pulses should be at least about 10 milliseconds and pre-
ferably at least about 15 milliseconds in order to avoid
overlap and therefore distortion of the outgoing coded
energy signal. The number of impulses in a single coded
energy signal can vary from about 10 to about 400 or more.
Preferably the coded energy signal consis~s of a ramp of
either steadily increasing or alternatively steadily de-
creasing frequency, although other codes may be success-
fully employed. The coded energy signals vary betweenabout 1 and about 20 seconds in length. The preferred
3~
-~7-
coded energy signals are those whose auto-correlation
function ~cf. Sheriff, R.~., op.cit.] has a central,
largest absolute amplitude to next-largest absolute
amplitude maximum ratio of at least about 3. This ratio
is preferably at least about 10 and more preferably at
least about 20. This ratio can be controlled by proper
selection of the pattern of time intervals between im-
pulses as disclosed in U.S. Patents 3,622,970 to Sayous
- et al., 3,698,009 to Barbier; and 3,326,320 to Forester.
10In another preferred embodiment of the method of
this invention, the seismic source of this invention is
employed to separately transmit a plurality of differ-
ently coded energy signals into the earth from the same
sourcepoint. A corresponding plurality of code signals
are generated by the position sensor~ The raw seismic
data detected at each geophone location is then corre-
; lated with the corresponding code signal to produce a
plurality of correlated traces, each of which corres-
ponds to one of the coded energy signals. The correlated
traces are then vertically stacked to form a stacked
trace. The use of differently coded energy signals serves
to attenuate undesirable correlation residuals, thereby
ef~ectively improving the resolution of the stacked trace.
In a particularly preferred embodiment of the method
of this invention, multiple coverage of a section of
earth strata is obtained by separately transmitting a
plurality of differently coded impulse trains, such as
from 10 to 20 impulse trains, into the earth from a
single sourcepoint. The sensor will generate a corres-
ponding plurality of code signals, a plurality of rawseismic data signals will be generated by the geophones
or other energy sensing device, and sign-bit samples
extracted from the raw signals will be automatically
shift-summed in response to the corresponding code sig-
nal. The shift-summed samples are then added to the
previously recorded shift-summed samples for the same
-28-
receiver location. A vertically stacked shift-summed
trace is thereby automatically obtained in real time.
eliminating the need to separately record each of the
.~ shift-summe~ tracers before vertically stacking them.
The use of differently coded impulse trains results in
~: smaller correlation residuals than repeated use of a
single code.
The correlated and/or vertically stacked corre-
lated traces obtained in the method of this invention
can be further processed by conventional seismic data
processing methods which are well known in the art. Such
~ methods include: common depth point stacking, moveout
: corrections, fre~uency filtering, and correlation resid-
ual reduction processing, such as the predictive sub-
traction deconvolution processing disclosed in U.S.
Patent 4,223l399.
The use of the position senso.rs of this inv~ntion
results in substantially superior resolution in the pro
~ cessed aorrelated traces obtained due to the much more
.~ 20 precise reco~d of the code of the transmitted signals
obtained with these position sensors as compared to the
; known acoustic sensors. Reference is made to the Example
and FIGS. 6A-6D of U.S. Patent 4,143,737 for a direct
comparison of the code signals generated by a magnetic
position sensor of this invention to the code signal
. generated by acoustic sensors under the same conditions.
:
Industrial ap~licability
As should be apparent from the foregoing, the method
and apparatus of this invention are suited for use in the
geophysical exploration industry, particularly in the
- seismic exploration of earth strata to identify potential
oil-, gas- an~/or mineral-bearing earth formations.
Other uses contemplated include engineering studies of
the vibrational characteristics of earth strata and/or
engineering structures, such as buildings.
-29-
While particular embodiments of the invention have
been described, it will be understood, of course, that
the invention is not limited thereto since many obvious
modifications can be made, and it is intended to include
within this invention any such modifications as will fall
within the scope of the claims.
Havin~ now described the invention, I claim:
