GB2137750A - Modular Seismic Cable Section - Google Patents

Abstract

The number of end connections and the weight and diameter of an oil floated modular marine seismic cable section (28) are reduced by reducing the number of both the local and through electric conduit pairs and the oil required for flotation thereof by employing the phantom pair technique for transmitting the hydrophone signals. The impedance matching transformers (A) of the hydrophones are used as parts of the bridge circuits used in the phantom pair technique, with similar transformers (B) at the receiving station (19) also being used as parts of the bridge circuits. Where the cables feed charge amplifiers similar reductions are effected by the use of capacitor bridges, see Fig. 7. The invention is also applicable to land seismic cables. <IMAGE>

Description

SPECIFICATION Modular Marine Seismic Cable Section This invention relates to seismic cables used in prospecting for underground minerals, especially marine seismic cables used offshore employing a plurality of hydrophones.

High resolution marine seismic surveying systems employ arrays of many hydrophone groups per cable. A practical limit on the number of hydrophone groups per cable has been reached due to the difficulty of ensuring that all connections between cable sections are operative. All connections must be good, but if there are too many connections it is a near certainty that one connection is bad. Consider the following example: 1 5 meters 1 hyd group=20 hyd. in parallel=1 conductor pair 105 meters 7 hyd. groups/cable section=7 local conductor pairs/section 3255 meters 31 cable section/cable=30 x7=2 10 through conductor pairs/cable Add on 30 pairs for auxiliary transducers such as depth sensors, water brakes, cable depth controls, digital compasses and the total is 240 conductor pairs/cable.This requires 480 connector pins and 480 connector sockets at the respective ends of each cable section.

If it is desired to double the number of hydrophone groups to increase resolution, nearly one thousand pins and sockets per cable section would be required. But that is too many connections for reliability. Bearing in mind that the cost of operating a marine seismic exploration system is of the order of $100,000/day, a lost day due to faulty connections is very uneconomical.

A further problem encountered in attempting to increase the number of wires in a cable is the matter of cable weight. As the number of conductor pairs increases, the weight increases due to the weight of the copper wires, unless the wire size is reduced with corresponding increase in resistance. In addition, marine seismic cable section have a positive (nearly neutral) buoyancy.

As the number of copper wires is increased, the volume and weight of flotation oil must be increased to achieve the desired buoyancy (with foam spacers for floats, not much added flotation weight would be required, but foam spacers tend to crack up on cable reels). It takes seven volumes of light oil (density 0.7-) to buoy 1 volume of dense material (density 3.1) such as conductor pairs.

Since the weight of the conductor pairs and the oil to float them represents perhaps 80% of the total weight and volume of a cable, an increase in the number of conductor paris effects a substantial increase in the weight and volume of a cable.

A further problem encountered in attempting to increase the number of wires in a cable section arises because of the difficulty in winding a large diameter cable on a reel; when the number of wires and flotation oil increase, the cable diameter increases. When a cable is wound on a reel as the cable is withdrawn from the water, the tension stress is taken on the outermost stress wire (or wires) and the innermost stress wires are placed in compression and tend to kink, which is deleterious to the cable structure. As the diameter of the cable increases, this problem is aggravated.

According to the invention the phantom pair technique heretofore known for multiplexing telephone circuits is employed to multiplex the conduit pairs that connect the hydrophones to the proximal end of the cable. Hydrophones are high impedance devices requiring the use of an impedance matching transformer for each conduit pair, and according to the invention the impedance matching transformers are put to the further use of providing line balancing means for the phantom pairs. Employment of the phantom pair technique can reduce the number of real pairs required by nearly 50%. The number of connections at the ends of each modular cable section is thereby reduced and it is possible to provide a practical cable with twice the number of hydrophone arrays presently practicable.In addition, the weight, volume, and diameter of the cable are reduced, making it easier to handle, especially when reeling it in and out for use or storage. All of this reduces the cost of manufacture and use of the cable.

A phantom seismic cable pair is created by connecting the secondary of one impedance matching transformer to the midtaps of the secondaries of two other impedance matching transformers rather than to another pair of conductors leading to the proximal end of the cable section. At the receiver on board ship, two, e.g. one-to-one, coupling transformers are provided which are connected to the two real pairs of conductors, and midtaps from the primaries of the two transformers are connected to the primary of a third, e.g. one-to-one, coupling transformer whose secondary provides the output from the phantom pair. Thus three outputs are provided from two real pairs of conductors.

If four real pairs of conductors are provided, two phantom pairs can be obtained by midtapping their impedance balancing transformers, and a third phantom pair can be obtained by midtapping the secondaries of the transformers of the first two phantom pairs. By thus cascading the transformers, providing additional stages of phantomizing, the total number of pairs approaches nearly twice the number of real pairs of conductors; it can be said that depending on the number of stages in the cascade, the number of phantom pairs will be between one-third one stage and nearly one-half many stages the total number of pairs.

As shown in Figure 9, if three real pairs of conductors are provided, one phantom pair can be obtained by midtapping the secondaries of the impedance balancing transformers of two of the real pairs, and a second phantom pair can be obtained by midtapping the secondary of the impedance matching transformer of the third real pair and midtapping the secondary of the transformer of the first phantom pair. As a matter of definition of terms, this example involving production of a phantom pair from the transformers of one real pair and one phantom pair may be considered as one and one-half stages of phantomizing.

Regardless of definition of terms, each time a real pair is added, an additional phantom pair can be obtained. The total number of conduit pairs can be up to 2(n)-1 where n is the number of real pairs.

Local conductor pairs, that is, conductor pairs connected to the group of hydrophones within the respective cable section, do not extend the full length of such section, going only from the hydrophones to the proximal end of the section.

But each cable section also includes many more through cable pairs for passing signals from hydrophone groups in sections nearer the distal end of the cable. If the cable sections are modular, that is, interchangeable, each section must have the number of through conduit pairs that are required for the section that is positioned at the proximal end of the cable. By employment of the phantom pair technique, the through conduct pairs are reduced in number the same as the local conduit pairs.

the conduit pair (copper) have a density of about 3.1. It takes about 7 volumes of light oil (density 3.1) to buoy 1 volume of conduit pairs.

Reduction in the number of conduit pairs reduces the cable weight and volume by both the weight and volume of the conduit pairs and that of the flotation oil. If the weight of the conduit pairs and flotation oil is about two-thirds of the total weight and volume of a cable section, reducing the number of conduit pairs by 50% in accordance with the invention effects a weight and volume reduction of about one-third. In addition, there is a corresponding reduction in cost of manufacture and use of the cable.

Summarizing, according to the invention, to solve the problems of increasing the number of hydrophone groups, a modular marine seismic cable includes a flexible sheath closed at each end by a bulkhead and having multiple electrical connectors at each end. Strain wires extend inside the sheath from the bulkhead at one end to the bulkhead at the other end. Spacers insides the sheath at intervals along its length hold the sheath round. Through pairs of electric conductors extend inside the sheath from some of the connectors at one end of the sheath to some of the connectors at the other end. Local pairs of electric conductors extend from others of the connectors at the one end of the sheath to impedance matching transformers, each local pair connects to the secondary of an impedance matching transfer whose primary is connected to a group of, e.g. 20 hydrophones in parallel.In addition, each of a number of extra impedance matching transformers may have its primary connected to a further group of hydrophones and the secondary connected to the midtaps of a pair of the other transformers or, in cascade, to the midtaps of a pair of the extra transformers, or, in semi-cascade, to one of the other transformers and to one of the extra transformers, as in Figure 9, in accordance with the phantom pair technique. Similarly connected shipboard transformers which may, e.g. be one-to-one transformers, are interposed on board the seismic exploration boat between the end of the cable and the seismic recording apparatus.The result is an increase in the number of hydrophone groups from fifty to nearly one hundred percent without increase in the number of conductor pairs and the corresponding increases in the number of connector pins and cable wire and oil weight and volume and outer diameter.

The invention is also applicable to transformerless seismic cable sections feeding charge amplifiers by employing capacitor bridges in the cable sections and at the receiver to produce phantom pairs for transmitting signals from additional hydrophone groups in each section, charge amplifiers being interposed between the receiver ends of all pairs, real and phantom, and the receiving means.

For a detailed description of preferred embodiments of the invention reference will now be made to the accompanying drawings wherein: Figure 1 is a pictorial, schematic view of a marine seismic exploration system embodying the invention; Figure 2 is a schematic diagram of the electroacoustic components of the Figure 1 system; Figure 3 is a schematic elevation of a marine cable section employed in the apparatus of Figures 1 and 2; Figure 4 is an electrical circuit diagram for a portion of the marine seismic cable section of Figure 3; Figure 5 is an electrical circuit diagram for a portion of the on board ship seismic signal receiver of Figures 1 and 2; Figure 6 is a schematic electric circuit illustrating phantomizing by means of transformers; ; Figure 7 is a schematic electric circuit illustrating phantomizing by means of capacitor bridges, as suitable for a transformerless cable coupled to charge amplifiers; Figure 8 is a schematic elevation of a seismic cable suitable for use on land with the invention; and Figure 9 is a view similar to Figure 6 showing a method of phantomizing by half stages.

Referring now to Figure 1 of the accompanying drawings there is shown a marine seismic exploration system including a boat 11 towing seismic generator means 13 in the form of an airgun. The gun is connected by air/electric line 1 5 to compressed air source 1 7 and electric control and receiving means 1 9. Any other means of generating seismic impulses, e.g. sparkers, water guns, gas exploders, or the now taboo dynamite could be employed. See for example U.S. Patent No. 4040000 to Dwivedi. Plural guns, etc, could be employed for seismic generating means 13. Seismic source means 13, 15, 17 is conventional and need not be described in detail.

Gun hydrophone means 21, comprising one hydrophone per gun, is also towed by the boat, being connected to control and receiver means 1 9 on boat 11 by electric cable or leader 23.

Leader 23 may be an armored cable. Hydrophone means 21 and leader 23 are conventional and need not be described in detail.

Also towed by boat 11 is marine seismic cable or hydrophone streamer 27. Cable 27 is made up of a plurality of modular sections 28, connected together electrically and mechanically. Cable 27 is connected by electric cable or leader 29 to control and receiver means 19 on board boat 11.

Leader cable 29 is tyically an armored cable containing, e.g. 240 electrical conduit pairs.

Cable 27 is of slight positive buoyancy, e.g. ten or fifteen pounds per hundred meters, compared to a weight in air of 750 to 1000 pounds.

Hydroplane means or birds 31 are connected to the cable at intervals along the length thereof to keep the cable submerged when the boat is in motion pulling the cable through the water. Only one such hydroplane means 31 is shown in the drawing. When the boat stops the cable floats to the surface. Hydroplane means 31 is conventional and need not be described further.

Control and receiver means 1 9 will be described in greater detail hereinafter with respect to Figure 2. The more mechanical aspects of the construction of streamer cable 27 will be described in greater detail hereinafter with respect to Figure 3.

Drawing enlargements A and B represent employment of the phantom pair techique in the streamer cable and control and receiver means as will be described in greater detail with respect to Figures 4 and 5.

Referring now to Figure 2, compressed air source 1 7 on board boat 11 includes diesel engine 33 driving air compressor 35. Line 1 5 between air compressor 35 and gun 13 includes an air hose 36 and an electric conduit means 37.

Control and receiver means 19 on board boat 11 includes acoustic firing control means 39 to which electric conduit means 37 is connected.

Control means 39 controls the initiation of blasts from gun 13. Blast energy is picked up by gun hydrophone 21 and transmitted by leader 23 to control means 39 to reset the control means after each blast.

Control and receiver means 1 9 further includes receiver means 41 comprising analog to digital converter 43, galvanometer camera 45, and tape recorder 47. Blast signals from leader 23 are further transmitted from control means 39 to A D converters 43 via interconnecting electrical conduit 49, whereby the blast signals will be recorded by tape recorder 47 and by galvanometer camera 45.

Converter 43 primarily receives signals from streamer cable 27 via tow leader 29 and on board leader 51. Enlargements A and B represent the employment of the phantom pair technique for cable 27 and for converter 43 where it is connected to leader 51. Except for the phantom pair connection B between on-board leader 51 and converter 43, the apparatus on board boat 11 is conventional and need not be described further.

Further details are to be found in brochure entitled "Operations Manual System 29300, 120 Trace Hydrostreamer System for Teledyne Exploration and M/V China Seal Decoder System published by Teledyne Exploration Company. Receiver means 41 records a seismic events detected by the hydrophones in cable 27.

Referring now to Figure 3, there is shown an elevation of one of modular sections 28 of hydrophone streamer cable 27. The cable section is generally similar to that shown in US Patent No. 3885286 to Hill.

The cable section includes a tubular outer sheath 61 of polyurethane or other tough flexible, preferably transparent waterproof and oil proof material. Sheath 61 is closed at its ends in a fluid tight manner by cylindrical metal bulkheads 63 inserted therein and held in place by metal straps 65. The sheath is filled with light oil 67, having a low enough density to bring the cable section to nearly neutral (slightly positive) buoyancy. Three wire strain ropes 71, disposed 120 degrees apart, extend through the section, being connected at their ends to eyebolts 73, 75 screwed into the bulkheads. Pin and socket multiple connector means 77, 79 are provided at the ends of the cable section to make mechanical connection with other cable sections and to transmit electric signals from one cable section to another.

Local electrical conduit pairs 81 connect impedance matching transformers 83 connected to hydrophone groups 84, and auxiliary sensor and control means 85, to connector means 77 at the proximal end of the cable section. Through electrical conduit pairs 87 extend from electrical connector means 77 at the proximal end of the cable section to connector means 79 at the distal end of the section.

Along the length of the cable section at suitable intervals are spacers 109 which keep the sheath in round, and separate and position the strain ropes and electrical conduit pairs.

Referring now to Figure 4, there is shown the electrical circuit of one of modular cable sections 28. A number of through conduit pairs 87 extend from the connection means 77 at the proximal end of the section to the connection means 79 at the distal end of the section. The number X of through conduit pairs may be equal, e.g. to (N-1) (h+s) where N is the number of cable sections in the streamer and (h+s) is the number of local conduit pairs per section, which comprises h local pairs for seismic signals from hydrophones and s local pairs for auxiliary sensors or controls.

Assuming 60 cable sections, one local conduit pair per section for auxiliary sensors and controls, and four real local conduit pairs per section for seismic signals from hydrophones, there will be (60--1) (4+1 )=295 through conduit pairs included in through conduit pair line 87. This will require that connector means 77, 79 handle (295+5)=300 through and local conduit pairs and include 600 pins and sockets each. This is about maximum. As will be explained hereinafter, by two stages of phantomizing in cascade, there will be provided three phantom pairs per cable section for hydrophone seismic signals. There will therefore be provided (4+3+1) (60)=480 outputs for receiver means 41 on board ship, about double the number currently available.

Still referring to Figure 4, cable section 28 includes one or more auxiliary sensors and controls 85, a desired one (or more) of which is connected by a local conduit pair 111 to connector means 77 at the proximal end of the cable section; conduit pair 111 forms part of local conduit pair line 81 of Figure 1. A desired one of the sensors and controls 85 is connected to pair 111 by suitable switch means (not shown).

As shown in Figure 4, there are seven hydrophone groups 84. Each group includes in parallel a plurality of hydrophones 113, e.g. 20 hydrophones per group. Each group is connected to the primary of one of seven iron core, acoustic frequency, voltage step down, impedance matching transformers 83. Transformers 83 may for example have a ratio of ten to one.

The secondaries of four of the transformers 83 are connected to four local conduit pairs 1 15, 11 7, 119, 121 which form part of local conduit pair line 81 (Figure 3). The secondaries of two others of transformers 83 are connected by electric conduit 123, 125, 127, 129 to the midtaps of the first mentioned four transformers.

This is one stage of phantomizing and produces two phantom pairs. The secondary of the seventh transformer 83 is connected by electrical conductors 131,133 to the midtaps of the two blast mentioned transformers 83. This is a second stage of phantomizing and produces another phantom pair. Thus, by cascading two stages of phantomizing, three phantom pairs are products from four real pairs. Additional stages of phantomizing could be employed. Generally, the possible numbers of phantom pairs is one less than the number of real pairs. In other words, the total number of pairs T equals R, the number of real pairs, plug R-1, the number of phantom pairs.

For complete phantomizing without the employment at half stage or inter stage phantomizing as per Figure 9, the number of real pairs R must equal (2)C i.e. 2 to the power C, where C is the number of stages of cascading.

To unscramble or decode the signals coming from the real conductor pairs of the streamer cable, which have been phantomized according to Figure 4, there is employed on board ship the circuitry shown in Figure 5. This circuitry includes seven one-to-one iron core acoustic frequency transformers 1 51 per cable section, e.g.

(7x 60)=420 transformers for the illustrated 60 section streamer cable. Figure 5 shows one group of seven on-board-ship transfers for unscrambling the signals from one cable section. Four of transformers 151 have their primaries connected to conduit pairs 1 53, 1 55, 1 57, 1 59 which are connected to pairs 115,117,119,121 from one of the cable sections. Two others of transformers 1 51 have their primaries connected to midtaps of the primaries of the first mentioned four of transformers 151 by electrical conductors 157, 159, 161,163. The seventh of transformers 151 has its primary connected by electrical conductors 1 65, 167 to the midtaps of the last mentioned two of transformers 1 51.The secondaries of the seven transformers 1 51 are connected by seven conduit pairs 171,173,175,177,179, 181, 183, to onboard-ship receiver means 41. Thus, seven real conduit pairs are retrieved from the seven groups of seismic hydrophones of the cable section even through in the cable section only four real conduit pairs were employed.

The theory behind the phantom pair technique may be described as follows, having reference to Figure 6.

The normal method used to transmit a multiplicity of signals down a cable requires a pair of wires for each signal channel, one wire carrying current in the forward direction and the other wire carrying the return current. If each of these wires carries current in the same direction there will be no voltage developed between the wires of the pair. Therefore the pair of wires can be used as if it is a single wire and two pairs of wires can be used as if they are a third pair. One method of doing this is shown in Figure 6.

This circuit consists of two wheatstone bridges (No. 1 and 2) fed by signal No. 3. As long as the bridges are balanced no signal No. 3 can be developed on circuit Nos. 1 or 2. Similarly, no signals on No. 1 or No. 2 can be developed on circuit No. 3.

As an alternative any one or more of the transformers can be replaced by pairs of resistors of equal value so as to maintain the bridge balance.

In a multiple pair cable the circuit above can be replicated resulting in an increase of channels to 1.5 times the number of pairs providing the number of pairs is even. As an extension of this method channel No. 3 can be used as one wire of another pair and if there are four actual pairs of wires connected as above Channels Nos. 4, 5 and 6 are added and channel No. 6 can be used as the return wire giving an added channel No. 7.

This method can be extended using group of 8 pairs as another signal pair. Thus 2 actual pairs can provide 3 channels, 4 actual pairs can provide 7 channels, 8 actual pairs can provide 15 channels etc. (i.e. twice the number of actual pairs less 1).

For a discussion of the phantom pair technique see: The New Encyclopaedia Britannica Volume 18, page 86 Telephone and Telecommunication Systems Multiplexing published by Helen Hemingway Benton.

The phantom pair technique can be adapted to use with transformerless cables feeding charge amplifiers by employment of capacitor bridges. As shown in Figure 7 three hydrophones 201,203, 205 are connected by two conduit pairs 207, 209 to three real outputs 211,213,215 by employment of two capacitor bridges 21 7, 219, 221,223 and 225, 227, 229, 231 interposed between hydrophones 201,205 and two charge amplifiers 233, 235 across pairs 207, 209.

Hydrophone 203 is connected to the inter connections between capacitor 219 and between capacitors 225, 227. Similarly, third charge amplifier 237 is connected to the inter connections between capacitors 221,223 and between capacitors 229, 231. Each of charge amplifiers 233, 235, 237 is provided with a capacitive feedback which can be made variable, as shown, e.g. at 239, 241, 243 to adjust the signal outputs, enhancing or depressing the signals as desired.

Referring now to Figure 8, there is shown a section of geophone cable for use in seismic exploration on land, with which the invention can be employed. The cable comprises a plurality of geophones 301,303, 305, 304, 306. These may be connected by real conduit pairs to real outputs by employment of the phantom pair technique with interstage phantomizing e.g. as shown in Figure 9. Section 310 will include local conduit pairs 311,313,315 connected to connector means 314 at the proximal end of the cable section and through conduit pairs (not shown) connected to connector means 31 4 at the proximal end of the section and to connector means 31 8 at the distal end of the section.

Although reference has been made to geophones 301, 303, 305, 304, 306, it will be understood that these are actually group os geophones in parallel.

Since geophone cables must be carried and laid in terrain which is often rough, hilly and covered with vegetation, a reduction in weight by reduction in the number of local and through conduit pairs is very advantageous.

It will be understood that the transformers 331-335 will be disposed with receiver means in a recording truck and will be equal in number to the geophone groups and phantomized in as many stages as are the geophones, similar to the onboard ship transformers 151 shown in Figure 5.

While preferred embodiments of the invention have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit of the invention.

Claims (9)

1. Section of an N section seismic cable for transmitting seismic signals from seismic detection transducers to seismic signal receiving apparatus, said section having a proximal end and a distal end, said section comprising: multiple connector means at each end of the section for making connection with other correlative connection means, e.g. on the ends of like sections; a plurality (h) of local real conduit pairs extending from the proximal end of the section varying distances toward the other end of the section; a plurality (p) of groups of seismic detectors where p=(2h-1 )-2(b-1) and c is the maximum possible number of phantomizing stage with h rear conduit pair and (c-b) is the actual number of stages employed;; a plurality of voltage divider means disposed in phantom cascade connecting the groups of seismic detector means to the local conduit pairs; and a plurality (x) of through conduit pairs each connected at one end to said connector means at the proximal end of the section and at another end to said connector means at the distal end-of the section, where x=(N+1) (h+s) and s equals the number of extra through conduit pairs provided for auxiliary equipment.

2. Section according to Claim 1 , wherein said detector means are hydrophones and said divider means are impedance matching transformers.

3. Section according to Claim 1, wherein said divider means are capacitors in series.

4. A plurality N of section according to Claim 1, connected together to form a cable and decoder means connected to the proximal end of the cable comprising a plurality of voltage divider means.

5. A plurality N of section according to Claim 2, connected together to form a cable, and decoding means connected to the proximal end of the cable comprise a plurality of one to one transformers.

6. A plurality N of sections according to Claim 3 connected together to form a cable, and decoding means connected to the proximal end of the cable comprising a plurality of voltage divider means in the form of capacitors in series.

7. Apparatus according to Claim 6, said decoder means including a plurality of charge amplifiers connected in cascade to the voltage divider mid points.

8. Modular marine seismic cable section comprising: a tubular sheath; means closing each end of the sheath; signal means inside said sheath; multiple electric connector means at each end of the sheath; liquid having a density less than one inside said sheath adjacent said signal means providing a neutral buoying for said cable section; said signal means comprising:: a plurality of through electric conduit pairs extending inside the sheath connecting a portion of the electrical connector means at one end of the sheath to a portion of the electrical connector means at the other end of the sheath; at least three hydrophone means in said sheath; each hydrophone means including a hydrophone and an impedance matching transformer having a primary and a secondary with its primary connected to the hydrophone; at least two local electric conduit pairs each connected at one end to one of said transformer secondaries and at its other end to another portion of said electric connector means at said one end of said sheath;; said plurality of through electric conduit pairs having a multiplicity m equal to (n-1 )x where n is the number of modular sections which can be connected in series and x is the number of local conduit pairs per cable section; midpoints of the secondaries of the transformers that are connected to said two local electric conduit pairs being connected to the secondary of another transformer of said at least hydrophone means; whereby the weight and volume of said cable section is reduced by the weight and volume of at least a local conduit pair and the liquid of positive buoyancy required to neutralize the negative buoyancy of the local conduit pair and the weight and volume of a through conduit pair and the liquid of positive buoyancy required to neutralize the negative buoyancy of the through conduit pair both multiplied by m/3.

9. Seismic cables used in prospecting for underground minerals substantially as described herein and with reference to the accompanying drawings.