AU488331B2 - Method of initiating and collecting seismic data related to strata underlyingbodies of water using a continuously moving seismic exploration system located ona single boat - Google Patents

Description

5535 5/73 FIELD OF THE INVENTION This invention relates to marine seismic exploration and more particularly to a method of collecting, processing and displaying areal marine seismic data using a contin- uously moving marine exploration system aboard a single seismic boat. In one aspect of the present invention, seismic data is associated with a two-dimensional array of center points provided by keeping the trailing end of the marine streamer along a staight course line parallel but offset to the base course line while keeping the lead-in cable, attached between the streamer and boat, in a vertical plane through the base course line of the boat. In another aspect, the relative positions of hydro- phones of the streamer relative to the boat during each shooting and collection period are accurately established using/digital sonic ranging technique in which sonic pulses from a transmitter aboard the boat are detected 4ona"~4~oz* by a series ofre L along the streamer and retrans- mitted back to the boat for recording onto a magnetic tape in a digital format compatible with a special purpose or a programmed general purpose digital computer. BACKGROUND OF THE INVENTION In the art of seismic exploration, the desirability of increasing signal-to-noise ratio of marine seismic data is well known. A successful technique of producing a series of locational seismic traces associated with cross sets of center points of constant density is described inj ccninb pplioation of Aubra E. Tilley, R.J.S. Brown and Roger D. Judson for "Method of Collecting Seismic Data of Strata Underlying Bodies of Water", Wv~lwk4

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.6S06269 filed Decembert±, 1969. In the aforementioned applications locational traces related to a cons tan Ygrid of two-dimensional center points are obtained utilizing a plurality of shooting boats in which seismic sources aboard a first and second shooting boats are sequentially fired as the boats transverse flanking zigzag course lines relative to a streamer containing a series of hydrophones. After the locational traces, are generated, these traces can be associated with cross sets of center points substantially normal to the base course line of the streamer-and processed by a technique known as beam steering to produce a series of cross directional traces.* Each cross directional trace has an enhanced (increased) signal-to-noise ratio. Although sonar and radar techniques are known which could synch- ronize operations of the shooting and recording boats, most service companies do not have such equipment due to the limited call for three-dimensional subsurface collection efforts. Accordingly, there has been some difficulty in getting the technique into field usage. Anoth er difficulty of a more generic nature relates to the requirement of digital recording of seismic data in the field. Digital field recording of data is of recent origin due to the availability of general and special purpose digital computers for processing seismic data. In this type of recording, the analog seismic signals detected by the hydrophones isfirst multiplexed by appropriate circuitry, say, located aboard a recording boat and then converted to a digital format com!patible with computer processing. Normally, in digital field

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55,355/73 recording, 12 binary bits are needed to define a number representing the amplitude of the analog seismic signal at a given point along the time base. These binary bits of information are then recorded on the data record section of the seismic magnetic tape in a time sequence related to the sampling rate. The resulting digital field tapes are then transmitted to a digital computing center for processing. Among the mathematical and statistical processing techniques normally used at a computer center (in addition to the beam-steering process noted above) is a process known as "common depth point stacking" (CDPS). In CDPS, associative seismic signals in digital form binary bits associated with semicommoh subsurface reflection areas) are combined so as to enhance primary seismic events in the records while attenuating noise and secondary (multiple) events. The associative character of the processing requires that the data be collected in the field in a manner that provides subsurface coverage of the same area many times, as obtained using the so-called "rollalong" field collection technique.

(In the rollalong technique, a first record is made with the seismic source and hydrophone spread positioned at a first series of locations; then a second record is made with the source advanced a certain in-line distance relative to the hydrophone spread, The hydrophone spread is advanced a similar in-line distance relative to a line of survey. By moving the source and hydrophone spread the same in-line distance, say, the equivalent of twice the spacing between hydrophone stations, the resulting

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seismic information in digital form provides multiple coverage of the same subsurface area along the line of survey.) It is evident, whether or not two- or three-dimen- sional mapping is contemplated, that positional annotation between hydrophone stations and seismic source points must occur as the data is recorded. If the aforementioned annotation does not occur, later processing of such data using CDPS can provide erroneous results since the algebraically summed results related to "common" depth points are themselves in error. Data annotated as related to a particular geographic area may actually relate to another area displaced to a substantial distance from the former. Further, such positional errors can be retained in later produced records Pree paerve of the final display format of the data. OBJECTS OF THE INVENTION An object of the present invention is the provision of a continuously moving marine seismic exploration system utilizing a single.marine boat for operation and collection of seismic data in the form of locational traces associated with a swathe of uniform two-dimentional grid arrays of center points along the base course line of the boat.

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SUMIARY OF TIIE INVENTION

The present invention provides a seismic data collecting method for determining the three-dimensional configuration of an earth formation underlying a body of water by means of a continuously moving marine seismic exploration system including a repetitive seismic source means and a plurality of hydrophones arranged in a marine streamer spread, comprising the steps of:

moving, by single seismic boat means travelling a substantially straight base course line, said repetitive seismic source means and said streamer spread while firing said seismic source means to initiate travel of said seismic energy downward toward seismic discontinuities within said earth formations, so tnat center points between the source means and the respective hydrophones at the successive moments of firing of said seismic means form a two-dimensional grid of center points laterally offset to one side of said base course line including cross sets of center points aligned substantially perpendicular to said base course line, said movement being further characterized by requiring the trailing end of said streamer to trail behind said seismic boat offset relative to the base line;

providing a multibit digital positional data indication of the of at least tworeceivers/transmitters aboard said streamer relative to said seismic boat means, the receivers/transmitters being spread along the streamer so that

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such data may be utilised to determine the positions of the

respective hydrophones;

receiving at said plurality of-hydrophones after firing of said seismic source means, separate reflection signals

representative of the energy received as the hydrophones are

instantaneously positioned at a plurality of hydrophone .stations

so as to provide a series of multibit seismic data indications,

being associable with cross sets of center points of the grid

so as to provide three-dimensional seismic information as to

stratography in said earth fornation.

In accordance with a preferred application of the

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invention, a continuously moving marine seismic exploration

system includes a repetitive seismic source and a streamer

of hydrophones trailing behind a single seismic boat traversing the body of water along a straight base course

line. The seismic source is fired at a selected repetition

rate to generate a series of firing stations coincident

with.the base course line. The streamer includes a plurality of hydrophones connected to the seismic boat

by means of a lead-in cable extending from the rear of

the boat and also includes a paravane assembly connected to the trailing end of the seismic streamer. The paravane has a rudder assembly whose position can be preset or

adjusted, as by radio commands from the seismic boat or

under mechanically induced programmed commands, so that

the trailing end of the streamer is drawn through the

water along a straight line parallel to the base course line and a constant off-set distance from it. /Both the

terminus of the lead-in cable and the remainder of the

streamer (including all hydrophones) maintain the

same geometry with respect to the base course line of the seismic boat. When the seismic source is activated at a selected repetition rate, a swathe of seismic data

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thus can be generated related to positional locations

in form of a swathe of two-dimensional center point grid

arrayo(per half cycle of each survey operation) having

an outer boundary parallel to the base course line and

at a perpendicular distance from it equal to one-half

the offset distance provided by the trailing end of the

streamer. Since the seismic boat proceeds along the base

course line at a constant speed, there results a uniform

density of in-line seismic depth points. The density of cross-depth points depends on the distribution of hydro-

phones along the streamer. For equi-spaced hydrophones, the density of cross-depth points decreases away from

the base course line. In a preferred implementation of the method the hydrophones are spaced in a manner to

yield a uniform density of cross-depth points. In order for the swathe of data to be clearly identified with the series of center point arr ys, he stremer is provided 4ona#t-Ae a44 Ui4"4n^e. Aiddaes with a series ofl~eni trtndueer along its length.

Poi el are emitted from: a transmitter aboard the boat (preferably after/seismic source is activated but

before the reflections are received at the hydrophones)

and are subsequently detected by the streamer transducers

and retransferred back to the boat. In that way, identifying the instantaneous locations of the transducers

by means of digital ranging techniques can be achieved.

In one form of the invention, the binary indications of aeoudh the travel time of the eene wave, and hence the instant-

aneous positions of the transducers per each shot, are

directly encoded onto the field magnetic tape for later

use in mapping of the sub-surface under survey.

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Other advantages and objects of the present invention

will become more apparent from the following detailed

description taken in conjunction with the following

acccipanying drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIGURES 1 and 2 illustrate, in plan view, a chronologically related sequence of two instantaneous

positions of a maine streamer and repetitive seismic

source operated from and in conjunction with circuitry

aboard a single seismic boat; these Figures illustrating

-n-series of locational traces can be associated with individual center points aligned along a series of pro-

Jections as the trailing end of the marine streamer main- tains its perpendicular offset distance from -he base

course line. FIGURE 3 is a plot of a two-dimensional, grid of center points related to the instantaneous positions of

the hydrophone and source array of FIGURES 1 and 2 and

subsequent positions; FIGURE-4 is a schematic representation, in block

form, of circuitry for generating digitized locational information related to the relative positions of the

marine streamer during each shooting and collection period;

FIGURE 5 is a diagrammatic representation of a digital sonic detecting and recording circuitry for

collecting, recording and coding of seismic data including locational information related to the relative position

of the marine streamer during each shooting and collection

period in binary form, using the collection method of

FIGURES 1, 2 and 3; 488,33 1

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FIGURES 6 and 7 illustrate bit-word representation of the locational information onto magnetic tape in binary format as provided by the circuitry of FIGURE 4 and FIGURE 8 illustrates, in functional block form, the digital seismic field system of FIGURE 4 for con- trolling the collection method of FIGURES 1, 2 and 3. PREFERRED EMBODIMENTS Referring now to FIGURE 1, a marine seismic ex- ploration system 10 is shown at one point in time as single shooting and recording boat 11 traverses along a straight, base course line of traverse 12. Behind shoot- ing and recording boat 11 are seismic source 13 and marine streamer 14 including a series of hydrophones (functionally indicated at i5)and terminating in paravane assembly 16. At its leading end, the marine streamer 11F. is coupled to .the boat 11 by means of lead-in cable 17. During the collection process, the shooting and recording boat 11 and seismic source 13 travel a straight line, along course line 12. However, the travel of the marine streamer 14 is more complex. Paravane assembly 16 is provided with a rudder assembly (not shown) which can be programmed to maintain the cakn position of the trailing end of the streamer along a pathway 18 parallel to but a fixed distance D from the course line 12. Terminus 22 of lead-in cable 17, however, remains in a vertical plane coincident with course line 12. Accordingly, the more centrally located individual hydrophones 15 follow a family of parallel paths having different (and pro- gressively smaller) offset distances with respect to the 11 488.331

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line 12. Locationp of the source 13 as activation occurs is a point A in FIGURE 1. In FIGURE 1, the paravane assembly 16 is at its initial position relative to the course line 12. Accordingly, when source 13 is activated, a series of center points designated C -C24 located at 1 the midpoint between source point A and hydrophones of the streamer 14 are generated which form the first set of cross offset points of the grid array. As shown, each of the center points C1 C2 C 2 4 is midway alongW imaginary family of curve lines drawn between source point A and the instantaneous positions of respective hydrophone stations of the streamer 14. The curvature of the center points is seen to be parallel with that of the streamer, say in the form of a two- or three-dimensional catenary. The cross offset of the hydrophone positions forming respective hydrophone stations hl...h 2 4 is not constant, i.e. the distance from the course line 12 to station h1 is larger than that from the line 12 to the station h2. Accordingly, the cross offset of the center points C1 -C2 4 from line 12 is also variable. In FIGURE 2, marine exploration system 10 is shown at another point in time along base line 12. As shown, the paravane assembly 16 has moved from its initial position (FIGURE 1) to a more forward location, while maintaining its distance D with respect to line 12. As the seismic source 13 is activated, the instantaneous position of the seismic source generates a new source point B. At this instant, the instantaneous positions of the individual hydrophones generate a new array of center points C25-C48 12 488.331

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as shown. FIGURE 3 illustrates the two-dimensional grids of center points generated during a sequence of surveying operations comprising 7 shots. Typical spacing of the center points is indicated in feet across the bottom of the Figures. The offset spacing angle between the paravane assembly and the course line 12 is indleated a loft hand ri ot otae Figur as being about 13 degrees. It should be noted that the variation of instantaneous positions as the data is collected generates center point grids whose boundary parallels line 12. Although the center points of the grid are aligned along imaginary lines perpendicular to base course line 12, the density of points decreases away from this line. It is important to know the position of each center point of the grid. Accdrdingly, the relative instanteous positions of the hydrophones of the streamer relative to the seismic source must be accurately established during each period of shooting and collection of data cycle. That is, the relative position of the hydrophone stations hl-h2l of FIGURE 1 relative to source point A must be determined and indicated, as the seismic data is collected. FIGURE 4 is a system block diagram of digital sonar range indicating circuitry 25 for indication of relative positions of three target transducers 26, 27 and 28 positioned along the marine streamer during each shooting and collectien period. Before describing circuitry in detail, however, a brief discussion of digital field equipment useful in recording of field data in a digital format may be instructive, and will now be presented with 13 488,331

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specific reference to FIGURE As shown in FIGURE9 5 the heart of the digital field collection systems of the present invention is digital field control system 1.0 which coordinates operations of- all remaining circuitry. As shown, the remaining field circuitry includes: sonar range circuitry 25 interconnect- ing target transducers 26, 27 and 28 with the digital field control system 4.0 through conductor means 4+1.. Paralleling conductor means 1+1 and sonar range circuitry is field conductor means 1.2 connecting the hydrophone s of the streamer to the digital field control system In order to synchronize data collection operations, the digital field control system 1.0 is also seen to connect via conducting means 4~3a through firing system 4+3 to a seismic source 13. All system information is recorded on a field magnetic tape 441within system Although relative mapping of the streamer is established by sonar range indica ting circuitry.25 (to be explained in detail be low), there is also a need for determining the absolute geographic positions of the streamer. For this purpose, navigational circuitry is seen in FIGURE 5 to connect a through conducting means 4.6a to a separate digital recording means 1.6.- Timing logic for controlling the operation of the circuitry of FIGUREX 5 can be quite complex and a key to understanding the interrelated operations is by observing the digital format of the information recorded onto magnetic tape 1+4. Primary requisite of the format of tape 1.4: it must be, of course, compatible with later processing by digit al 14 488.331

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computer. Further, since the computer basically manipulates and processes data based on data character length divided

into "words" and "blocks", the magnetic tape format of

tape 44 must be likewise organized.

In FIGURE 6, for example, a conventional magnetic

tape format is seen to be arranged in a series of 21 tracks

across the width of the tape 44 and in a plurality of

channels along the length of the tape 44. Motion of the tape is in the direction of arrow 51. Transfer of data to the tape is accomplished by a 21-track head unit 52

positioned at the right-hand side of FIGURE 6. Indications of specific sections of data along the longitudinal length

of the tape divide each record 4nheader section 53, and

a data record section 54. Within the data record section 54, the data is further organized into a series of data

section as indicated.

Along the width,.the series of tracks include the

following: 18 data tracks and 3 control tracks. The control tracks comprise block track 55, clock track 56

and parity track 57. Further, one of the data tracks, say, the 0 track, can be utilized, sequentially, for

control purposes, viz, to indicate the sign of the channel.

The block and clock tracks 55 and 56 are used to

record block pulses and clock pulses, respectively. A block pulse is generated at the dividing point between

each two data blocks and thus distinguishes the data

blocks which follow. The block pulses are recorded by continuously magnetizing the block track 55 in one direction

by means of the head unit 52. To establish the ONE state, 488,331

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the block track 55 is magnetized continuously at the start of the record, the end of the record, and for each block address word. Where data words are inserted into the record after the block address, no pulses* are recorded within the block track 55 (ZERO state). A clock pulse is generated at clock track 56 at each channel for all data words. Where a gap appears in the record, say, at gaps 58a, 58b, 58c, the data bits on block track 55 and clock track 56 are in the ZERO state. At the start of each block of data, except for the zero block address (remaining channel tracks are in the ZERO state), the address is signified by means of 20 to2N orders of binary bits which can appear in the 18 data channels. The parity track 57 is used to record a parity pulse which serves as a check of the efficiency of the transfer of the seismic data to the magnetic tape. All information within one data channel should add up to either an odd or even number which can be checked with the parity signal provided within each parity track 57. For example, for the data tracks 1, a parity pulse ONE will appear if these data tracks contain an even number of ONE's. Gaps 58a, 58b and 58c have previously been indicated by means of ZERO states at the block and clock tracks and 56 for all channels comprising the gap. Gap 58b is seen to be positioned between header record sectiLon 53 and data record section 54+. After data has been transferred to the header record station, head unit 52 can be positioned at gap 58b to aw~ait reception of seismic signals at the hydrophones. When the tape is mot icaless, the head 52 preferably rests over the middle of the gap. 16 488,3 31

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One half of the length of the gap is usually required to accelerate the tape motion up to full recording speed. Within the data record section 54, each channel is divided into one 18-bit word and 3 control bits, as previously mentioned. The number of blocks required to provide a seismic data record varies with the amount of fixed information which is provided to the head unit 52. For example, a 6-second analog signal received at the hydrophone spread of FIGURE 1 and sampled at .002-second intervals, digitized and then recorded, will have 6/.002 3,000 blocks of data arranged along the longitudinal length of the magnetic tape. Within each data scan, the first channel of each block is referred to as the block word. The block word, as previously mentioned, specifies the number of the block and identifies the channel as a block word in the form previously mentioned. The remaining number of channels in each block are referred to as data words and, in length, can vary in accordance with the number of hydrophones within the hydrophones spread. The seismic data is located within each data channel in, say, the most significant 13- bit positions. of each 18-bit word. The remaining bits of each data channel provide for recording of binary amplifier gain change indications (4 bits) as well as the sign indication (1 bit). As will be explained in more detail below, binary gain amplifiers are used to amplify the signal provided each hydrophone. The intensity of these signals received at the streamer spread and amplified by the binary gain amplifiers varies over an extremely large dynamic range. 17. 488.3 31

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To avoid overloading the binary gain amplifier, the gain

must be varied in accordance with the amplitude of the

received signal. The incremental gain of each amplifier is indicated by the binary amplifier gain change indications

(4 bits) mentioned above.

FIGURE 7 depicts the header section 53 in more

detail. Along the left-hand side, as viewed in FIGURE

7, tracks 0 to 17 are illustrated related to the 18 data

channels (with four blanks) while the control channels

b, c and p relate to the block track, the clock track and

the parity track, respectively. At the start of the header record, block track and clock track b and c are coded

with a "ONE" stored as shown. Within the remaining data tracks, the sign number, reel number, geographic location

can be inserted in alphanumeric form. Beginning at block address various conventional instrument settings are

applied to the tape, such as the amount of presuppression

set into each binary gain amplifier (initial gain) as well

as the constant gain factor of each amplifier. Since modern seismic collection techniques employ a great number

of individual binary amplifiers, the binary data identifying characteristics as well as filter characteristics can

occupy a rather large block of data, say, from block "0"

to block of the header section 53. Following the eight blocks of amplifier information, in binary form,

there are set aside several blocks of data to indicate the

following data in binary form: serial number, instrument type, sampling rate, hydrophone spacing interval, record length, trip delay as well as a series of hydrophone

position marking codes. .At selected channels, say,

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identified with words 6o, 61, 62, of block 10 of the header record section 53, a three-word, 11+-bit digital code (four blanks) identification is set forth *As

explained below, each word 60, 61 or 62 identifies in binary form the time of sonic wave travel to trans- ducers 26, 27 and 28 of FIGURE 5 with respect to an origin point on the boat just prior to reception of the seismic energy. Each multibit digital code can be trans- posed as represented by a decimal number related to range. For example, bit''characters of each code XO...Xl 3 positioned on tracks 16 3 can represent several orders of ranging' information. Whether or not all bit characters are utilized depends on the travel time required for encoding onto the magnetic tape. In marine practice, it is not usual to have ultralong marine streamers. The travel time of the sonic wave can be represented by the quantity R/V where R is the slant range to a-certain streamer trans- ducer, and V is the velocity of sonic transmission equal to 5,000 feet/sec. Accordingly, assuming a maximum slant range of 3 miles (15)840O'), the maximum time of travel (to*a, trailing end of the streamer) would be about 3.17 secs. If the counting rate is,. say, 5,000 counts/sec. then 15,840 counts would be generated during each ranging cycle. In a digital code utilizing at least 14-bit positions X1),a time duration function of up to 16,382

counts can be designated per each 18-bit digital word (with three blank bits). (If greater range is to be indicated or a higher counting rate utilized the three blank bits of each 18-bit word can be used as a gain function equal to a multiplier function of 2, 4+or 8. 19 488.3 31

55.355/73 Otherwise, these three bits are blank.) The rema"i4 n Z bit is utilized to indicate whether or not theAede-4s- to the right or left flank of the seismic boat as viewed in FIGURE 1 a ONE, say, for the left, and a ZERO for

the right. The invention is not limited to a 21-track format, however. A 9-track format could also be used without departing from the intended scope of the present invention. In a 9-track format, 8 data tracks are utilized, number 0, 1, 2, 3, 4,5, 6 and 7, and one parity track is also utilized. Each value is contained, in each record channel, in one group of 8-bit characters called a byte. Accordingly, in a 9-track format, each range code, con- taining at least 1-bit characters, would require to be recorded as at least two separate bytes of information. As an example of the output of the binary code related to travel time, consider the binary codes for a range of 16,381 feet (for a transducer positioned, say, at the trailing end of the marine streamer); a mid- positioned transducer (at, say, 9,855 feet from the marine boat); and a leading edge transducer located at about 330 feet from the marine boat. The binary code provided at the bit characters on three adjacent channels of a 2 1-track tape would be as follows assuming a counting rate of 5,000 counts/sec. and a travel velocity of 5,000 ft;/s ec.: X0 x1 x2 x3 xF X5 x6 x 7 x 8 x9 x1 0 x11 x1 2 x1 3 Binar y 1 k 1 1 1 1 1 1 1 1 1 1 1 Dec'imal 16,381 (counts)

X0 X1 X2 X3 X8 X X1 6 X7 X8 X9 X10 XI XI2 X13 488.33 1

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.11 1 .1 10 1 Binary 0 .0 Z1 1 a Z 0 1 0 0 0 1 Decimal 9,855 (counts) 1 2 13 x 5 x6 x? x8 x9 xI 0 x11 x x -x0 x1 x2 x3 Binary 0 1 o 1 o 0 1 0 1 0 0 0 0 0 Decimal 330 (counts) The most significant bit of the code is, of course, 0 the X bit.

Since the streamer is assumed to be at a constant

depth, the travel time indication to each transducer

aboard the streamer establishes the relative locations of the hydrophones in the following manner: at the computer

center, the travel time binary code is converted into

slant range indications for each collection period.*

Since the streamer is at a common depth and all hydrophone

positions relative to the transducer positions are known,•

the relative locations of all hydrophones can be thus

established. Similar techniques at the center can be used to

establish the geodetic positions of the streamer trans-

ducers and then the hydrophones. Since the hydrophone

geodetic positions are derived from the geodetic positions

of the transducer, only the coordinate generation of the

former will be described in detail. At the computer center, the binary multiword range

indications with respect to the seismic boat, as provided by navigation circuitry 45 of FIGURE 5, le, utilized to

transform the coordinates established for the boat to

indicate hydrophone position. As to the transformation process, reference is

made hereinafter to the following example.

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55,355/73 iven: Slant ranges R1 R2 and R3 Depth of the streamer D and Xa' Ya' Za where Xa, Ya' Za are the three-dimensional coordinates of the transmitting transducer 5-of the digital sonar ranging circuitry 25 of FIGURE during a selected collection period;

Calculate: The individual target coordinates X1 Y1 Z1 X2 Y2 Z2 X3 Y3 3 of the transducers of the marine streamer to be computed using the following

equations:

Slant RI =I(X 1 Xa)2 (YY)2 (1-Za)2

Slant R2 (X 2 2 2-a (Z2 -Za)2.

Slant X) 2 (Y -Y )2 z )2 slant 3 3 a 2 3 a 3 a2

Assmkg a datum plane at the surface of the body of water, then the three-dimensional coordinates at the datum plane, namely, Xdl, Ydl Zd Xd2 Yd2' Zd2; and Xd3, Yd3 Zd3 for the transducers can be calculated using the following equations:

R 2 Xdl-X2 (dla)2 (Zd-Z) 2

2 D2 2 (Xd-Xa d2 a d2 (Zd2 Za)2 R-2 2 -X )2 2 2

3 A d3 (Y -Ya) 2 -a) 2

where R1 R2 and R3 are the slant ranges separating the transmitting and receiving transducers, D is the depth 22 488.331

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of the streamer and Xl, Ydl Zdl; Xd2 Yd2' Zd2; and

Xd3' Yd3, and Zd 3 are the three-dimensional coordinates

of .the target transducers transformed to the datum plane, It should be obvious that distribution (spacing)

of the hydrophones could be varied so that the seismic

information could be associated with positional locations

defining a grid of center points of uniform density, as,

for example, described in the aforementioned patent

applications assigned to the assignee of the present

application. Referring again to FIGURE 4, the digital sonar

ranging circuitry 25 for providing the aforementioned

binary travel time codes for encoding onto magnetic tape

will now be described in detail.

After the seismic disturbance has been initiated,

time of travel determinations to streamer transducers

26, 27 and 28 are carried out by the sonar ranging

circuitry 25 in the following manner: transmitter 29 is

energized by means of control circuit 30 to transmit a

pulse of sonic energy to transmitting transducer 31 at,

say, the rear of the boat and thence the energy travels

through the water until detected by receiving transducers

26, 27 and 28 within the marine streamer. Transducers

26, 27 and 28 are energized in the order of their location

with respect to origin transducer 31. Assume that trans-

ducer 26 is located at the leading position of the hydro-

phones of the streamer; that transducer 27 is located

at the midposition; and that transducer 28 is located

near the trailing end of the streamer. Accordingly, the transducer 26 will be energized first, followed by the

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the midposition transducer 27 and finally the trailing transducer 28 is actuated. These signals are passed by

way of conductors 32a, 32b and 32c through the marine

streamer to the seismic boat. Since transducer 26 at

the leading edge of the streamer receives the pulse first,

receiver 34a will be actuated first; receiver 34b will

be actuated next, followed by receiver 34c. Return

signals detected by the receivers 34a, 34b and 34c pass

through OR gates 35a, 35b and 35c to flip-flops (bistable

multivibrators) 36a, 36b and 36c. These flip-flops are

of the type where successive pulses reverse the flip-

flops between stable states of operation corresponding to "on" and "off" period modes of operation. Leads 37a, 37b and 37c of the flip-flops 36a, 36b and 37?econstitute

the output which will deliver an output signal when the

flip-flop is 3 an condition. Connected to the out- put leads 6a re3band 37c are AND gates 38a, 38b and 380

which are responsive to an enabling signal which is

provided by control circuit 30 for passing the signals

provided by the flip-flops to counters 39a, 39b and 39c.

In order that the counters' readings be indicative of the travel time of a sonic wave from the transmitting

transducers to the streamer transducers, oscillator 24

is operational to provide timing and counting pulses in

addition to providing pulses to the counters 39a, 39b and

39c through the AND gates 38a, 38b and 38c as shown,

In the sequence of operations, a reset signal from the signal control circuit 30 is first utilized to set

the counters 39a, 39b and 39c to zero as well as to. reset flip-flops 36a, 36b and 36c so that zero output

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signal to the AND gates 38a, 38b and 38c is provided. At this time, the AND gates are closed, and counters 39a, 39b and 39c do not receive the counting pulses from oscillator 24.. However, when a transmitting signal from the signal control circuit 30 triggers transmitter 29, each of the flip-flops 36a, 36b and 36c is triggered through OR gates 35a, 35b and 35c permitting a high enabling signal to AND gates 38a, 38b and 38c. Resulting counting-pulses are accumulated by counters 39a, 39b and 39c. After the.transmitted sonic signal (from transducer 31) is received by the transducers 26, 27 and-28 aboard the streamer, these signals pass via the conductors 32 a, 32b and 32c back to the boat, specifically to receivers 31+a, 34~b and 34c and thence through OR gates 35a, to flip-flops 36a, 36b, 36c. As the signals are received at OR gates 35a, 35b, 35c, the respective flip- flop is turned "off". Since-the flip-flop is turned off, the associated AND gate is blocked, and the associated counteri 39a, 39b and 39c stops counting. Connected to each counter is a register 23a, 23b and 23c which receives the count from each counter upon the occurrence of a register set signal from control circuit Such register holds the count until reset through an enabling signal from control circuit 30. In this regard, control circuit 30 includes a series of input channels generally indicated at 65 connected to the digital field control system of.FIGURE, 5. Logic signals pass from the control system 40O through input channels 65 in a predetermined enabling sequence so that binary indications of travel time can be transferred 488.331

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from the registers 23a, 23b, 23c in proper sequence. Logic circuitry for generating enabling signals to provide for such logic transfers as well as for digital recording of all binary information is depicted with reference to FIGURE 8, and will now be described in detail. As shown in FIGURE 8, during collection of seismic data signals generated by the hydrophones pass from the streamer to digital field system 40 aboard the boat via conductor means 42, thence through amplifiers 66, multiplexer 67, analog-to-digital converter 68, master copy logic circuit 69, format control circuit 70 to magnetic tape unit 71 and onto magnetic tape. Binary gain shifts of amplifiers 66 are indicated by binary gain feedback control circuit 72 through binary logic circuit 73 connected between the feedback control circuit 72 and the master copy logic circuit 69. To provide word lengths and block lengths of seismic data that are compatible with computer processing techniques, logic circuits 69 and 73 are carefully con- trolled for correct sequential operation utilizing a timing circuit generally indicated at 75. As indicated, timing circuit 75 includes a timing logic circuit 76 controlled by a master clock 77 and produces a series of timing (clock) pulses which are applied to logic circuits 69 and 73 through word counter 78 and block counter 79. The word and block counters 78 and 79 dictate, in conjunction with timing circuit 75, when the other groups of circuit elements must perform a preselected function. All operations are preferably performed 26 488,331

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in synchronism with the clock pulses (synchronous control). Each operation requires a certain number of clock pulses and, consequently, the timing to complete

any one of the various operations is an exact multiple

of the clock pulse. Thus, the readout of copy logic circuit 69 to magnetic tape unit 71 is accomplished at

specific intervals of time that are exact multiples of

the clock pulses. Further, the master copy.logic circuit 69 can also be used to actuate other circuits, or other

circuits can be used to terminate simultaneously with

its actuation.

Header Information Encoding on Magnetic Tape at Magnetic

Tape Unit 71: Initial binary gain settings are gated through master copy logic circuit 69, in correct time sequence, to permit digital recording onto the tape header section at magnetic tape unit 71. Timing logic circuit 75 in conjunction with word counter 78 and block counter 79 also provide enabling signals to a channel generally indicated at 65 of FIGURE 4 which correctly enables con- trol circuit 30 of FIGURE 4. As a result, binary range position data a three-word, 14-bit binary code is passed through master logic circuit 69 and thence to magnetic tape unit 71 through format control circuit

Format control unit 70 may be provided with manual

indexing circuitry in order to provide suitable binary

information to the master copy logic circuit 69 during

recording of header information. Although all header encoding activity is paced by signals from master timing

logic circuit 75, during the header encoding activity, 27 488.331

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it is preferable that the amplifiers, multiplexer, and analog-to-digital converter remain in an inactive state. Usually master clock 77 is interrupted after header information has been placed onto magnetic tape at tape unit 71. Consequently,.a gap is provided in the formatting of the tape record, in the manner previously discussed. Seismic Data Encoding: After header information has been encoded, processing steps are carried out in sequence to record in digital form the seismic data onto magnetic tape. In more detail, at amplifier 66, the amplitude of the data is determined using binary gain feedback control circuit 72. The binary gain of feedback control 72 is then gated through binary control logic circuit 73 to master copy logic circuit 69, in correct time sequence to permit its digital recording in the same channel as the binary seismic data. At multiplexer 67, the amplitude of each analog signal is electrically sampled in sequence over a plurality of very small time intervals say, .002-second intervals. These signals, after being sampled, are transferred to analog-to-digital converter 68 where the digital results of the multiplexing opera- tion are represented by a series of multibit binary code indications. The binary code information is electrically suited for storage on magnetic tape on the same channel as associated binary gain information. During all these steps, all activity is paced by regularly occurring clock signals from master clock 77. No event occurs within the entire system except at the occurrence of one of these clock signals or its multiple. In addition to block and word clock pulses, internal timing pulses are generated 28 488,331

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to cause transfer and manipulation of header and

seismic data information such as at timing logic cir-

cuit 76 and master copy logic circuit 69. Thus, the

output of copy logic circuit 69, for example, is

accomplished at specific intervals of time which are

exact multiples of the clock pulses produced by master

clock 77. Other operations are caused to terminate

simultaneously with the actuation of master copy logic circuit 69, while certain other circuits are being

turned to a new-state, signifying expiration of time for a preconceived, specified operation. As one subset of circuits is disabled, a new subset is enabled by timing pulses so as to perform new operational functions. The process (enabling some circuits, disabling others, in sequence) is repeated over and over. Format control unit 70 is capable of manual changes during the seismic data encoding activity. In that way the format of the seismic data can be varied to meet new application requirements. Magnetic tape unit 71 may be one of several commercially available types and should have the cap- ability of recording seismic data in binary form onto magnetic tape.

Although the system described in FIGURE 8 controls

multiplexing and conversion of analog seismic data to

digital data as well as to provide data in correct time

sequences, additional circuitry can be inserted and

combined with the system hereinbefore described to provide

additional data processing features, if desired.

The method of the present invention is preferably 29 488.331

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incorporated into digital seismic field systems and

digital firing systems conventionally used in collect-

ing marine data in digital form, such as provided by

the Texas Instrument Company, Inc., Dallas, Texas.

However, only portions of that system pertinent to the

present invention have been discussed in detail; e.g.,

the portion of the system related to timing logic for

incorporating binary code identification of marker range

positions onto the magnetic tape has been discussed in

detail above.. Coordination of operations between the

digital firing system and the digital seismic field

system can, of course, be achieved in many ways, as,

for example, shown in U.S. Patent 3,416,631 for "Digital

Remote Firing System", John D Patterson.

In establishing the true position of the streamer relative to true geodetic coordinates, information derived

from navigational circuit 45 of FIGURE 5 is used in the

following manner.

Navigational circuit 45 includes components for

accurately measuring time travel of a radio signal from

a transmitter to a receiver, and can be classified into

one of five general groups: hyperbolic, ranging azimuthal,

composite and satellite systems, with a subgeneric

classification of each being pulsed (time difference),

CW (phase comparison) and combined (time difference and

phase comparison). In general, they can be distinguished by the type of latice (network of positional lines)

generated by the transmitters as positional fixed occur. Source: H. W. Bigelow, "Electronic Surveying: Accuracy of Electronic Positioning Systems", Int. Hydrographic

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Bur. Radio Aids to Maritime Nay. Hydrographic Operational Reports 6 (Sept. 1965), pp. 77-112. Although only certain embodiments of the present invention have been illustrated and described, the

invention is not meant to be limited to these embodi-

ments, but rather to the scope of the following claims.

4 8 8,33 1

Claims (1)

1. 55,35 5/73

   THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

   1. A seismic data collecting method for determining the

   three-dimensional configuration of an earth formation underlying

   a 'body of water by means of a continuously moving marine

   seismic exploration system including a repetitive seismic

   source means and a plurality of hydrophones arranged in a marine

   streamer spread, comprising the steps of:

   moving, by single seismic boat means travelling a

   substantially straight base course line, said repetitive

   seismic source means and said streamer spread while firing

   said seismic source means to initiate travel of said seismic

   energy downward toward seismic discontinuities within said.

   earth formations, so that center points between the source

   means and the respective hydrophones at the successive moments

   of firing of said seismic means form a two-dimensional grid

   of center points laterally offset to one side of said base course

   line including cross sets of center points aligned substantially

   prependicular to said base course line, said movement being

   further characterized by requiring the trailing end of said

   streamer to trail behind said seismic boat offset-relative

   to the base line;

   providing a multibit digital positional data

   indication of the pit of at least twolreceivers/transmitters

   aboard said streamer relative to said seismic boat means, the

   receivers/transmitters being spread along the streamer so that

   32 488.331

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   the positions of the such data may be utilised to determine

   respective hydrophones; hydrophones after receiving at said plurality of reflection signals firing of said seismic source means, separate as the hydrophones are representative of the energy received of hydrophone instantaneously positioned at a plurality of multibit seismic data stations so as to provide a series cross sets of center points indications being associable with seismic of the grid so as to provide three-dimensional earth formation. information as to stratography in said

   multibit positional 2. The method of Claim 1 in which said onto seismic and seismic data indications are recorded

   magnetic tape.

   33 488.331

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   3. The method of claim 1 or 2 in which step'(b) is further characterised by the substep of:

   determining the geodetic position of the seismic boat means by measuring the time of travel of an electromagnetic wave between a known geodetic position and said boat means;

   whereby said multibit digital positional data indicates the geodetic positions of the receivers/transmitters aboard said streamers.

   4. The method of claim 1, 2 or 3 in which step includes digitally measuring travel time of an acoustic wave between said receivers/transmitters and said seismic boat means.

   34 4-88:3 31

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   The method of Claim 4 in which substep (iii) includes:

   initiating counting by counting means as said

   acoustic wave is generated;

   (ii) terminating counting when said wave is detected

   at said receivers/transmitters;.

   (iii) storing said counts in a register;

   (iv) generating a time-dependent enabling signal

   by means of word and block counters connected to said

   register so as to cause gating of said counts from said

   register;

   recording said counts as multibit positional

   digital data indications on a header section of a magnetic

   tape so as to identify the receiver/transmitter positions as

   seismic data is received.

   6. The method of Claim 5 in which said magnetic tape has

   at least 21 tracks.

   7. The method of claim 5 in which said magnetic tape

   has at least 9 tracks.

   8. The method of Claim 5 in which step is further

   characterized by generating enabling signals from bistable

   multivibrator means and oscillator means at AND gate means

   so as to initiate said counting means; and step (ii) is

   further characterized by disenabling said bistable multivibrator

   means as said wave is received at the receivers/transmitters

   thereby blocking the AND gate and terminating counting.

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   9. The method of Claim 4with the additional steps of:

   processing said multibit positional digital

   indications to determine the range from said known geodetic position to said receivers/transmitters; and (ii) transforming the three-dimensional coordinates

   of said known geodetic position to establish three- dimensional coordinates of said receivers/transmitters so as to enable mapping of said earth formation.

   A seismic data Collecting method for determining the three-dimensional configuration of an earth formation

   underlying a body of water by means of a continuously moving marine seismic exploration system including a lepetitive seismic source means and a plurality of. hydrophones arranged in a marine streamer spread, comprising the steps of: moving, by single seismic boat means travelling

   a substantially straight base course line, said repetitive seismic source means and said streamer spread while firing. said seismic source means at known times to initiate travel of said seismic energy downward toward seismic discontinuities within said earth formations so that center points between the source means and the respective hydrophones at the successive moments of firing of said seismic means form a two-dimensional grid of center points including cross sets of center points aligned substantially perpendicular to said base course line, the total offset length of said two-dimensional grid of center points in said cross direction remaining substantially constant

   36 488,3 31

   55.35 5 7

   during surveying operations, said movement being further

   characterized by requiring the trailing end of said streamer

   to trail behind said seismic boat at a substantially constant

   offset distance relative to but parallel with the base line,

   the leading edge of the streamer remaining substantially

   fixed at a point aligned with said base line and central sections

   of said streamer being arcuately disposed therebetween so

   that the streamer forms a modified catenary;

   providing a multibit digital positional data

   indication of the position of at least two/receivers/transmitters

   aboard said streamer relative to said seismic boat means, the

   receivers/transmitters being spread along the streamer so that

   such data may be utilised to determine the positions of the

   respective hydrophones;

   receiving at said plurality of hydrophones after

   firing of said seismic source means, separate reflective

   signals representative of the energy received as the

   hydrophones are instantaneously positioned at a plurality of

   hydrophone stations so as to provide a series of multibit

   seismic data indications, being associable with cross sets

   of center points of the grid so as to provide three-dimensional

   seismic information as to stratography in said earth formation.

   11. A seismic data collecting method for determining the

   three-dimensional configuration of an earth formation

   underlying a body of water by means of-a continuously moving

   marine seismic exploration system including a repetitive

   37 488.331

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   seismic source means and a plurality of hydrophones arranged in a marine streamer spread, the method being substantially

   as hereinbefore described with reference to the accompanying drawings.

   Dated this 26th day of July, 1977 CHEVRON RESEARCH COMPANY, By their Patent Attorneys, DAVIES COLLISON.

   488.3 31

   5"5.3 5 5 /7

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