FIELD OF THE INVENTION

This invention is in the field of generating and transmitting into the earth seismic energy waves. More particularly the invention relates to a controlled-polarization marine seismic shear-wave source having a base adapted to be positioned on a seafloor, i.e., on the floor of a body of water, with controllable azimuthal orientation for predetermining the polarization-orientation of shear waves being generated and propagated into the body of the seafloor material.

BACKGROUND

Seismic shear waves are transverse elastic waves in which particle motion is perpendicular to the direction of propagation. Horizontally polarized seismic shear waves (SH-Waves) are particularly useful in subsurface exploration and may be generated by subjecting a localized area of a-surface of the earth (seafloor in this instance) to impulses which have a horizontal component of force perpendicular to the direction of propagation of the SH-Waves.

Also, vertically polarized seismic shear waves (SV-Waves) have utility in subsurface exploration and may be generated by subjecting a localized area of a surface of the earth (seafloor) to impulses which have a horizontal component of force in line with the direction of propagation of the SV-Waves.

In addition to generating SV-Waves, subjecting a localized area of a surface of the earth (seafloor) to impulses having a vertical component of force also will generate compressional seismic waves (P-Waves). P-Waves are waves wherein particle motion occurs in the direction of propagation.

It is not possible to transmit seismic shear waves (either SH-Waves or SV-Waves) within a body of water, because water does not provide nor carry shear elastic force. A water body will propagate compressional seismic waves (P-Waves).

SH-Waves, SV-Waves and P-Waves described herein are “body waves” which propagate within a body of seafloor material and are to be distinguished from “surface waves” which propagate along a surface of the seafloor.

SUMMARY OF THE DISCLOSURE

This controlled-polarization marine seismic shear-wave source is capable of generating and propagating down into the seafloor SH-Waves, SV-Waves and P-Waves.

The controlled-polarization marine seismic shear-wave source shown and described herein is adapted to be positioned on a seafloor, i.e., the floor of a body of water, in controlled azimuthal orientation for establishing the polarization-orientation of shear waves being generated and propagated into the body of the seafloor material.

In other words, the azimuthal orientation of the base of the source can be controllably changed by suitable pneumatic thrusters as illustrated and described. Moreover, azimuthal orientation of the base is shown being sensed by a gyroscopic compass for transmitting electrical orientation data signals to a ship by a suitable umbilical cable, or by remote acoustic (sonar) transmission.

Also, azimuthal orientation of the base of the marine seismic shear-wave source can be determined from a ship by using sonar transponders suitably positioned on the base.

The marine seismic shear-wave source embodying the invention includes a suitably robust base adapted to be positioned on a seafloor. This base has downwardly-projecting blade-like teeth elements protruding downwardly therefrom for penetrating into the seafloor surface sediments in mechanical engagement with such seafloor material for augmenting coupling with the seafloor of horizontal force components generated by the shear-wave source.

Mounted on the base is at least one (and preferably two) elongated water-discharge tube(s) aimed upwardly from the base at an acute angle relative to the base. An open discharge mouth is at the upper end of a discharge tube. The open mouth allows water to enter into and fill the tube when the source has been submerged into a body of water. A quick-firing, compressed-air discharger shown as a marine airgun is mounted in a lower portion of the tube. Upon firing a charge of compressed air from this discharger, a slug of water is blasted upwardly along the tube toward its mouth with water surging out of the tube's mouth, thereby generating a powerful reaction impulse acting in the direction opposite from the direction in which the slug of water is being blasted. This powerful reaction impulse is applied by the base to the seafloor thereby generating SH-Waves and SV-Waves in the seafloor having predetermined polarizations and which propagate in predetermined directions through the body of the seafloor. Also, P-Waves are generated and propagated down into the seafloor body.

In order to control azimuthal orientation of the marine seismic shear-wave source on the seafloor for propagating SH-Waves of predetermined polarization and having a predetermined and known direction of propagation, two pneumatic thrusters are shown mounted on the base of the source. These thrusters are aimed outwardly from the source in opposite directions. They are positioned away from the center of the base so that simultaneous actuation of these two thrusters applies a torque to the base for changing its azimuthal orientation. A preferred way in which to accomplish such change in azimuthal orientation is to use a lifting sling connected to the base of the source. This sling is operated from a ship for raising the base up away from significant engagement with the seafloor. After thus lifting the source, the pneumatic thrusters are actuated for turning the azimuthal orientation of the source while it is supported away from significant contact with the seafloor by the lifting sling.

A swivel is shown provided in a lifting cable line connected to the sling for facilitating turning of azimuthal orientation of the source while it is being supported by the sling and lifting cable.

A gyro compass is shown mounted on the source for providing electric signals to the ship indicating azimuthal orientation of the source. Also, sonar transponders are shown mounted on the base for use in determining azimuthal orientation of the shear-wave source.

In the preferred embodiment of the controlled-polarization marine seismic shear-wave source as shown, its base has a twin pair of elongated water-discharge tubes aimed upwardly and outwardly at equal angles from a vertical centerline.

Thus, a side elevational view of this source is characterized by a symmetrical arrangement of the two water-discharge tubes each extending diagonally upwardly and defining a V-shape between them. The vertical centerline of the source bisects the vertex of this V-shape configuration.

Among advantages of having the twin pair of symmetrically positioned, water-discharge blast tubes are those arising from the capability of alternately discharging them for producing sequences of SH-Waves of alternately opposite polarity.

In addition to the downwardly-projecting blade-teeth elements, the base of the source has a multiplicity of apertures therein for allowing water to flow vertically through the base as it is being lowered through the water suspended by its sling and cable line from a surface ship. These water-flow apertures serve to avoid significant sideways “kiting” or “sailing” through the water as the source is being lowered.

Various other patentably novel features, aspects, advantages and objectives of the controlled-polarization marine seismic shear-wave source embodying the invention will be pointed out and described in the specification with reference to the accompanying drawings and/or will become understood by those skilled in the art from their consideration of the specification and drawings.

The terms “horizontal” and “vertical” are used herein for illustrative convenience in making reference to various Figures. These terms “horizontal” and “vertical” are not intended to be restrictive, because in use the base of the source may be seated down in a seismic exploration or surveying site at a localized region of a seafloor which happens to be inclined relative to a geographically horizontal plane at the site of operation. In such a situation a “vertical symmetrical centerline CL” (

FIG. 2

) of the source will be tipped relative to geographical vertical and the base seated down upon such inclined local region correspondingly will be slanted relative to geographic horizontal.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objectives, features, advantages and aspects thereof, will be more clearly understood from the following detailed description considered in conjunction with the accompanying drawings. Like reference numerals indicate like elements or like components throughout the different views.

The accompanying drawings, which are referenced to and which we thereby consider as being an integral part of the specification, illustrate a presently preferred embodiment and a presently preferred modified embodiment of the invention. The drawings, together with the general description set forth above and the detailed description of the preferred and modified embodiments set forth below, serve to explain the principles of the invention. In these drawings:

FIG. 1

is a perspective view of a controlled-polarization marine seismic shear-wave source embodying the invention. This shear-wave source includes a twin pair of discharge tubes symmetrically mounted on a suitably robust, strong base and aimed upwardly diagonally in a V-shape configuration. A multiplicity of sediment-penetrating blade-like teeth are shown protruding downwardly below the base.

FIG. 2

is a side elevational sectional view of the marine shear-source shown in

FIG. 1

illustrating an arrangement of an array of the downwardly-projecting, sediment-penetrating blade-like teeth.

FIG. 3

is an elevational sectional view taken along the plane

3

—

3

in

FIG. 2

looking toward the left in

FIG. 2

for showing a modified embodiment of the invention wherein the blade-like teeth are removably mounted on a modified base structure. The components shown in

FIG. 3

are enlarged by slightly more than twice compared with similar components as seen in FIG.

2

.

FIG. 4

is an enlarged partial elevational sectional view of a lower portion of one of the elongated, water-discharge tubes showing a long-life marine seismic airgun mounted therein serving as a pneumatic discharger for blasting water upwardly out of the tube.

FIG. 5

is a top plan view of the marine shear-wave source shown in

FIGS. 1 and 2

.

FIG. 5

shows simultaneous actuation of two pneumatic-discharge thrusters mounted near diagonally opposite corners of the base plate. These thrusters may be used to reorient azimuthal orientation of the whole shear-wave source.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The best mode for carrying out the invention is shown in

FIG. 1

with reference to

FIGS. 2

,

4

and

5

.

FIG. 3

shows a modified embodiment of this best mode wherein blade teeth are conveniently removable and replaceable.

FIG. 1

shows a controlled-polarization marine seismic shear-wave source, generally indicated at

10

. This source is adapted to be lowered into a body of water

11

(

FIGS. 2

,

4

and

5

), for example such as a sea, pond, lake, river, estuary, bay, channel, etc. It includes a strong, robust base structure

12

having a multiplicity of downwardly projecting blade-like teeth

14

for augmenting mechanical coupling of horizontal components of force with a seafloor below the body of water

11

where source

10

has been positioned.

The base structure

12

shown in

FIGS. 1 and 2

is rectangular seen in plan view in

FIG. 5

constructed of suitable structural steel. The base includes a rectangular top plate

16

having a plurality of anti-kiting, i.e., anti-sailing, apertures

18

. As shown in

FIG. 1

, these anti-kiting apertures are elongated, extending lengthwise relative to length of rectangular base

12

, and they are staggered in their respective positions in top plate

16

. These apertures

18

allow water to flow vertically through the base as the source

10

is being lowered through the water from a surface ship by means of a sling

20

and lifting cable

22

for preventing significant sideways kiting (sailing) of the source

10

through the water as the source is being lowered. These anti-kiting apertures thereby enable the source to be lowered substantially vertically to an intended location on a seafloor. The sling as shown comprises four lengths of steel cable or chains extending diagonally downwardly from a suitable common fitting

24

, such as a ring, connected by a suitable swivel

26

to lifting cable

22

. Lower ends of the four sling lines

20

are shown in

FIG. 2

connected by shackles to pad eyes

28

welded to respective corners of the base structure

12

.

The sectional view seen in

FIG. 2

is taken through the lengthwise centerline of the base structure

12

. There are no apertures

18

along this lengthwise centerline seen in FIG.

2

. Thus, for clarity of illustration, twelve nearby apertures

18

located in the half of the base structure behind this sectional view are indicated by dashed lead lines and dashed outlines.

A strong perimeter frame

30

extends around the rectangular base

12

, and there are eight spaced, parallel, stiffening beams

32

(only one beam is seen in

FIG. 2

) extending lengthwise in the base structure welded onto the lower surface of top plate

16

. These beams

32

are positioned between respective rows of anti-kiting apertures

18

and are located above a bottom plate

34

which also has anti-kiting apertures

36

. This rectangular base structure

12

shown in

FIGS. 1

,

2

,

3

and

5

has a length in a range between about 130 inches (about 330 cm) and about 150 inches (about 380 cm), and as seen in these Figures has a length of about 141 inches (about 358 cm). The rectangular base structure

12

as shown has a width in a range of about 90 inches (about 230 cm) to about 100 inches (about 250 cm), and as seen has a width of about 96 inches (about 244 cm).

Blade teeth

14

extend down through tightly fitting slots

38

in bottom plate

34

. The upper end of each tooth is rigidly secured to a suitably stiff durable top plate

16

, for example as shown in

FIG. 1

, by an assembly

40

of a top strap with triple machine screws extending down through the strap and through top plate

16

and threaded into upper ends of the blade teeth

14

as shown in FIG.

2

. It is to be noted that blade teeth

14

are spaced apart, being arranged in parallel rows extending transversely of the rectangular base. Their surface areas are parallel with each other in their respective rows. Blade teeth in alternate rows are positioned in staggered relationship, as is indicated by alternate sectioning and outlining of the teeth seen in edge view in thirteen rows in FIG.

2

. Thus, in

FIG. 1

a viewer sees only six blade teeth

14

, which are those teeth positioned at the ends of the alternate six rows in

FIG. 2

identified by sectioning of blade teeth.

Twin water-blast barrel tubes

44

(seismic-impulse-generation thrusters) are shown aimed upwardly and outwardly at equal acute angles &bgr; (

FIG. 2

) relative to a horizontal plane H of a blast-tube mounting plate

42

. These blast tubes

44

are positioned symmetrically on opposite sides of a vertical centerline CL of the source

10

. Mounting plate

42

is a centrally located, strong, thick, horizontal, transversely extending rectangular plate, for example being about 1.0 inch (about 2.5 cm) thick, with its length extending transversely across the full width of base structure

12

and with its own width, as seen edgewise in

FIG. 2

, being about 36 inches (about 92 cms).

Mounting plate

42

is shown in

FIGS. 1 and 2

welded onto upper edges of eight spaced, parallel, gusset braces

43

which are welded onto top plate

16

. These parallel gusset braces

43

extend lengthwise of the base

12

and are aligned respectively directly above the eight parallel stiffening beams

32

(

FIG. 2

) welded to the lower surface of top plate

16

. These gusset braces, for example have a height of about 8 inches (about 20 cms) and preferably extend at least about 50% of the overall length of base structure

12

. As shown in

FIGS. 1 and 2

, gusset braces

43

extend, for example about 80 inches (about 203 cms), which is about 56% of overall length of base

12

. Their triangular ends (

FIG. 2

) are tapered at an angle “z” shown, for example, as about 15°.

Water-blast tubes, i.e., seismic-impulse-generation thrusters,

44

are circular cylindrical tubes each having an axis “A” and an inner diameter (I.D.) “D” in a range of about 12 to about 24 inches (about 30 cms to about 60 cms). In the source

10

, for example, D is shown as about 19 inches (about 50 cms). These blast tubes

44

are shown constructed of lower and upper axially-aligned tube sections

44

-

1

and

44

-

2

. These tube sections are axially connected by a ring of bolts

46

passed through mating reinforced connection flanges

48

.

As shown in

FIG. 1

, pad eyes

50

are welded onto the upper surface of lower and upper tube sections

44

-

1

and

44

-

2

. These pad eyes

50

are positioned near an upper end of each tube section for convenience in supporting it by a hook and cable line from a lift crane, while these tube sections are being assembled. During storage on shipboard or on land, the mouth of each tube is covered by a removable, resilient, protective, skirted cap

52

snugly fitting over the upper end (mouth) of each upper tube section

44

-

2

.

Downward extensions (not shown) of the axes A of blast tubes

44

meet at an alignment point “O”, i.e., at an origin, which is located at a point where the centerline CL of seismic energy source

12

passes through the mounting plate

42

. Lower ends of lower tube sections

44

-

1

are welded to mounting flanges

54

which are strengthened by reinforcing gussets

56

(

FIG. 4

) welded to the flange

54

and also welded to the outside of tube sections

44

-

1

. A strengthening band

57

is shown in

FIG. 4

encircling the lower tube section

44

-

1

opposite discharge ports

88

of an airgun

80

. Alternatively, the tube wall of lower section

44

-

1

is suitably thicker such that an encircling band

57

is not used.

To mount blast tubes

44

removably and securely onto mounting plate

42

and to provide a removable closure for the lower end of each lower tube section

44

-

1

, as is shown enlarged in

FIG. 4

, there is a pedestal assembly

58

for each blast tube. Each pedestal assembly

58

(Please see also

FIG. 1.

) comprises a truncated circular cylindrical pedestal wall

60

concentric with axis A and aligned with the wall of tube section

44

-

1

. Each pedestal wall

60

is welded to its twin pedestal wall

60

along a central seam

62

. This welded seam

62

is located in a plane aligned with centerline CL; this plane of seam

62

extends transversely of the base

12

. Each pedestal wall

60

is welded at

64

to mounting plate

42

and is welded at

66

to an annular tube-end wall

68

of blast tube

44

. A plurality of stiffening gussets

70

are welded to mounting plate

42

, to pedestal wall

60

and to annular tube-end wall

68

. A strong plate member

71

extends perpendicular to axis A and is welded to mounting plate

42

, to pedestal wall

60

and to its twin plate member

71

.

The tube-mounting flange

54

is removably secured to a rim

72

of annular tube-end wall

68

by a ring of machine screws

74

threaded into sockets in the rim

72

. A pneumatic blaster

80

, shown for example in

FIG. 4

as a marine air gun manufactured and sold by the Assignee of the present patent application and of the invention disclosed herein, is removably mounted in shock-absorbing relation onto annular tube-end wall

68

. This air gun blaster

80

is shown in

FIG. 4

having an encircling flange

92

secured to tube-end wall

68

by a clamp ring

76

and a pair of resilient, annular, squishy, shock-absorbing elements

78

, for example made of suitable polyurethane elastomer. There is annular clearance

79

for allowing the shock-absorbing elements

78

to squish.

The air gun

80

, as is known in the art, has an operating chamber

82

with a solenoid-type firing valve

84

mounted onto the operating chamber. There is a main housing

86

for example, having four discharge ports

88

(only two are seen in FIGS.

2

and

4

). A firing chamber

90

contains a charge of compressed air, for example having a volume in a range from about 100 cubic inches (cu. in.) to about 400 cu. in. (about 1,635 cubic cms. to about 6,550 cu. cms.). The pressure of the compressed air in firing chamber

90

is preferred to be at least about 2,000 psi above hydrostatic pressure of ambient water

11

at the depth of submergence of the SH-Wave source

10

. For example, this air gun blaster

80

is shown as a Model 1500LL Long Life Air Gun having a firing chamber volume of about 300 cubic inches (about 4,920 cu. cms.) manufactured and sold by Assignee.

The air gun

80

is adapted for mounting in the blast tube

44

by welding an encircling annular flange

92

to the operating chamber

82

. This flange

92

is clamped between the shock-absorbers

78

. A lower end of operating chamber

82

extends down through a large central opening

94

in the annular tube-end wall

68

. For retaining shock-absorbers

78

in their respective positions encircling chamber

82

and extending radially outwardly above and below flange

92

, there are retainers

96

welded to tube-end wall

68

near its central opening

94

and welded to an inner rim of clamp ring

76

.

Each airgun

80

is supplied with compressed air through a high-pressure hose line

91

(

FIG. 4

) as well known in the art. An umbilical cable (not shown) of a type well known in marine and borehole seismic operations extends along and is suitably attached to lifting cable

22

for connecting seismic source

10

to a surface vessel or barge and provides necessary electrical and pneumatic connections.

Upon triggering operation of firing valve

84

through a firing cable line

85

, the charge of compressed air contained in firing chamber

90

blasts out through ports

88

and hurls a powerful slug of water upwardly out of the mouth

100

of the tube

44

. A powerful reaction impulse thrusts downwardly in an axial direction opposite from the direction in which the slug of water is being hurled upwardly along tube

44

. This powerful reaction impulse thrusts downwardly, directed generally parallel with axis A toward mounting plate

42

. This reaction impulse is applied by base

12

to the localized area of the seafloor located beneath the base. The blade teeth

14

augment coupling with the seafloor of horizontal force components of the powerful reaction impulse, thereby generating and propagating into the body of the seafloor SH-Waves and SV-Waves whose two directions of propagation are perpendicular and parallel (respectively) to the horizontal force component of the reaction impulse. Thus, propagation of the SH-Waves and SV-Waves occurs in two orthogonal directions, one parallel with and one in line with planes of face areas of blade teeth

14

. In other words, for a viewer looking at

FIG. 2

, the two directions of propagation of the SH-Waves and SV-Waves are directly in line with and perpendicular to a plane encompassing the viewer and the source.

Polarization of these SH-Waves is parallel with the horizontal component of force, i.e., is parallel with a plane containing centerline CL and axis A.

It is important to note:—For simultaneously generating and simultaneously propagating into the body of the seafloor SH-Waves and SV-Waves of first predetermined polarities, one of the blast tubes

44

is fired. Then, for simultaneously generating and simultaneously propagating into the body of the seafloor SH-Waves and SV-Waves of second predetermined polarities, each respectively opposite to the first predetermined polarities, the other of the blast tubes

44

is fired. Thus, for providing a sequence of:—simultaneous SH-Waves and SV-Waves of first predetermined polarities alternating with simultaneous SH-Waves and SV-Waves of second predetermined polarities each respectively opposite to the first predetermined polarities, the two blast tubes

44

are alternately fired, and they are not simultaneously fired.

Also, it is important to note:—for generating P-Waves, the two blast tubes

44

are fired simultaneously.

For clarity of illustration in sectional view (FIG.

2

), a few of the detailed elements described with reference to perspective view

FIG. 1

are not shown in FIG.

2

.

In

FIG. 2

the angle &bgr; between horizontal plane H and an axis A of a blast tube

44

are both shown as about 45°. By decreasing both angles &bgr;, horizontal force components are increased, but vertical components are decreased. Conversely, by increasing angles &bgr;, vertical force components are increased, but horizontal components are decreased. In our present view, it does not appear advisable to decrease angles &bgr; below about 30° nor to increase angle &bgr; above about 60°.

To help support the inclined blast tubes

44

, there are vertical steel struts

97

(

FIG. 1

) extending between the top base plate

16

and the upper end of the lower tube section

44

-

1

. The lower ends of these struts

97

are welded to suitable load-distributing mounts

98

which are welded to base plate

16

. Upper ends of these struts are welded to saddles

99

positioned immediately below connection flanges

48

on lower tube sections

44

-

1

.

The blast tubes

44

each have the same overall length “L” from tube-end wall

68

up to the mouth

100

, for example being in a range of about 70 inches to about 105 inches (about 175 cms to about 270 cms). In

FIG. 2

as shown length L is about 97 inches (about 245 cms).

In order to change and control azimuthal orientation of base

12

, four pneumatic thrusters

110

are shown mounted near respective corners of base

12

. These thrusters are aimed outwardly generally perpendicular to the length of the base

12

. Adjacent to these thrusters are mounted respective containers

112

which are charged with pressurized air fed down through a high pressure hose line (not shown) in the umbilical cable previously referred to. An electrically-controlled valve is used to release compressed air from a container

112

through its associated thruster

110

. These valves are individually actuated through electrical lines in the umbilical cable.

To change azimuthal orientation of source

12

, it is lifted sufficiently away from the seafloor such that simultaneous discharge

114

(

FIG. 5

) of two diagonally opposite thrusters

110

will change azimuthal orientation as indicated by curved arrows

116

. A gyro compass

118

is shown mounted in shock-absorbing relationship between the blast tubes

44

. Through an electrical line (not shown) this gyro compass suitably signals the azimuthal orientation of the base

12

.

Also,

FIG. 5

shows two sonic transponders

120

mounted near diagonally opposite corners of the base

12

. Time differences between sonic responses received from these transponders by a remote sensor are used by a surface vessel or barge to verify or check on azimuthal orientation of the base

12

.

In the modified embodiment of base

12

A shown in

FIG. 3

, conveniently removable and replaceable blade teeth

14

′ are mounted by machine screws

121

to a top plate

16

A welded to gussets

43

welded to mounting plate

42

. These blade teeth

14

′ are captured in snugly fitting slots

124

(shown dashed) in a bottom plate

34

A which is welded to stiffening ribs

32

A. Suitable anti-skating apertures (not shown) are provided in the base structure.

Since other changes and modifications varied to fit particular operating requirements and environments will be recognized by those skilled in the art, the invention is not considered limited to the examples chosen for purposes of illustration, and includes all changes and modifications which do not constitute a departure from the true spirit and scope of this invention as claimed in the following claims and equivalents thereto.