GB2132762A

GB2132762A - Seismic exploration

Info

Publication number: GB2132762A
Application number: GB08331664A
Authority: GB
Inventors: Muhammed Shafiqul Alam
Original assignee: Individual
Current assignee: Individual
Priority date: 1982-12-30
Filing date: 1983-11-28
Publication date: 1984-07-11
Also published as: GB8331664D0; GB2132762B

Abstract

Existing cables, analogue or digital, used in marine seismic surveys have only one hydrophone group at each offset. The data quality so recorded is not always useful. The quality may be improved with increased hydrophones per group but the improvement is limited because only a limited number of hydrophones can be used in a group. The present invention provides a seismic cable with more than one hydrophone group, eg. 1, 2 and 3, at each offset and the the maximum number of hydrophones in each group. It is so assembled that when used, all its groups are substantially at the same water depth and it is towed by a ship substantially as one compact body. The multiple traces of data so recorded at each offset from each shot are mixed before gathering by common depth point fashion for stacking. <IMAGE>

Description

SPECIFICATION A marine seismic super cable This discovery relates to the designing of a new cable for recording seismic reflection data for exploration for oil and gas in offshore areas.

The existing method of marine seismic reflection survey is shown in a sketch in Fig.

1. An artificial earthquake is initiated in the water at a predetermined nominal depth from the sea surface by firing dynamite or air guns or other material which is called the seismic shot. The point or the location where the shot is fired is called the shot point. In the diagram, the shot and the shot point are shown by a small arrow near the tail end of a ship.

The ship carries an instrument for recording seismic data, and also from it the shots are fired. The ship also tows a cable in which groups of hydrophones are fitted. During shooting and recording of data the cable is kept immersed under water at a predetermined nominal depth.

In Fig. 1 the cable is shown to consist of a number of alternately shaded and unshaded parts numbered 1, 2, 3... etc. All shaded and unshaded parts are identical and each one is meant to represent a group of hydrophones. The distance of the shot from the centre of a group of hydrophones is called the offset of that group. The hydrophones of all groups are located on one line which is the longitudinal axis of the cable.

Before starting recording data in any area of survey, the offsets of the hydrophone groups and the total number of hydrophone groups to use in the cable are decided and accordingly the cabel is assembled with only one group of hydrophones at each offset. The offsets increase by a regular distance, called the group interval. The group interval i.e. the distance between the centres of any two consecutive groups of hydrophones is the same in a cable used in a survey.

The hydrophones respond to the vibration of the earth caused by the shot and generate and transmit electrical energy to the recording instrument for recording by its recording channels.

In the conventional hardwired cables which have been in use since the beginning of marine seismic exploration, the hydrophones of each group are connected to the recording instrument through a separate transmission line. There are as many co-axial transmission lines in the cable and as many recording channels in the instrument as the number of groups of hydrophones in the cable. For each group there is a separate transmission line for its connection to a separate channel, and the outputs of the hydrophones of each group are summed and transmitted through the transmission line to the recording channel for recording.

The outputs of the various hydrophone groups are thus recorded by different recording channels of the instrument. The data so recorded by each channel corresponding to each shot is called a trace of data. If there are N hydrophone groups in the cable which are connected to the recording instrument by N different transmission lines then N different traces of data are recorded per shot. In the diagram 48 hydrophone groups are shown to be used i.e. N = 48 in this case.

In the past two years or so a new cable, called the digital cable has been introduced in the industry. In this cable, the hydrophone groups are not connected physically to the recording instrument by transmission lines.

Instead, there are a few electronic modules in it, each module being fitted in the cable before or after a definite number of groups of hydrophones for handling the data (outputs) of these hydrophone groups. If 4 such modules are used, for example, in a cable of 48 hydrophone groups, then each module is fitted after or before every 1 2 groups and it handles the data of these 1 2 groups.

Each electronic module fitted in the digital cable consists of a few digitizing and transmitting units, besides its other parts.

There are as many digitizing and transmitting units in it as the number of groups of hydrophones it is connected to. The hydrophones of each of these groups are connected by transmission lines to these units of the module, and their outputs are fed into these units for digitization i.e. sampling at a particular time interval, say 1 millisecond, and for wireless transmission to the recording instrument.

The modern recording instrument has receivers in addition to the recording channels to receive and record the data transmitted from the electronic modules. Also on board the ship there is a device for remote controlling the function of the electronic modules.

It is to be noted here that in the digital cable also, only one group of hydrophones is used at each offset like the conventional one, and the hydrophones of all groups are on one line i.e. the central longitudinal axis of the cable. It may be mentioned here also that the conventional cable is also called the analogue cable and it will be called so hereinafter in this specification. In the analogue cable the data from the groups of hydrophones are not digitized i.e. sampled before transmission to the recording instrument.

During the survey of an area for exploration for oil and gas, the ship moves along various predetermined lines in the area---called seismic lines of survey-in predetermined directions (indicated in the diagram by the arrow, A), and the shots are fired at regular distances called the shot point intervals, and the seismic data is recorded i.e. the response of the earth to these shots is recorded for each shot through the cable, digital or analogue, towed by the boat.

As already mentioned, in the existing method, only one group of hydrophones at each offset is used in the cable--analogue or digital-for recording data, the hydrophones being always placed on one line i.e. the central longitudinal axis of the cable. The quality of the data so recorded is not always useful. Sometmes the quality is improved by increasing the number of hydrophones in the group. But it is not quite practicable to use too many hydrophones per group. A solution of the problem is suggested here.

According to the discovery, a cable, either digital or analogue, is designed with more than one group of hydrophones substantially at each offset. It is so designed that all hydrophone groups of the cable are substantially at the same predetermined depth from the sea surface when in use, the hydrophones of the groups at each offset being placed along the length of the cable on as many lines as the number of groups per offset. If M is the number of hydrophone groups used at each offset of this cablM M being an integer and greater than unity-then this new cable is as good as M cables of the existing type whether digital or analogue, and the hydrophones at each offset are on M lines along the length of the cable, the M lines being substantially parallel to each other. The traces recorded per shot are M times more i.e. a total of M times N i.e.MN number of traces per shot are recorded if N offsets are used with M groups at each offset. Like the existing analogue cable, all the MN groups are connected to MN channels of the recording instrument by MN co-axial transmission lines.

Alternatively, according to the discovery, all the hydrophones of all the M groups of each offset in the analogue cable are connected to only one recording channel through one transmission line, instead of M channels, thereby recording only one trace of data per offset, instead of M traces per offset. In this case, N traces instead of MN traces are altogether recorded from each shot via N recording channels instead of MN channels, although the cable is still as good as M cables of the existing analogue type, the recorded trace of data of each offset of each shot being the sum total of the individual outputs of Mn hydrophones, instead of n hydrophones, n being the number of hydrophones per group.

In the digital cable of this discovery also, there are MN groups of hydrophones, the outputs of which are recorded by MN channels of the instrument, M being the number of hydrophone groups per offset. Like the existing digital type, also there are no transmission lines in it. Instead, it has a few electronic modules, one module being fitted before or after a fixed number of hydrophone groups and consequently before or after a fixed number of offsets. If L number of modules, for example are used in a cable of MN groups with M groups per offset, then one module is used before or after every N + L offsets and consequently before or after every MN . L groups to digitize and transmit the outputs of these MN . L groups to the recording instrument.Each module has MN + L digitizing and transmitting units to which the hydrophones of these groups are connected and into which their data (outputs) are fed for digitization and wireless transmission to the recording instrument. MN traces of data are recorded per shot with such a cable.

Alternatively, in the digital cable of this discovery like the analogue type of this discovery, the hydrophones of all groups of one offset are all connected by one transmission line to one digitizing and transmitting unit only of an electronic module. If such a cable has MN groups of hydrophones at N offset, M groups per offset and L electronic modules, one module being fitted before or after every N . L offsets and consequently before or after every MN + L groups of hydrophones, then each module has only N . L digitizing and transmitting units to each one of which all the hydrophones of all groups of each of these N . L offsets are connected for their outputs to be digitized and transmitted to the recording instrument.In this case N traces are altogether recorded per shot, instead of MN traces the cable is still as good as M cables of the existing digital type, each of the N traces of each shot being the sum total of the outputs of Mn hydrophones, instead of the sum total of the outputs of n hydrophones, n being the number of hydrophones per group used.

Before starting a survey the user decides the number, N of offsets, the offset increment, the number, M of hydrophone groups per offset, the number, n of hydrophones per group to use and the number, L of electronic modules for the digital cable, and accordingly the cable, analogue or digital is assembled.

The number, n is the maximum number of hydrophones that can be used in a group length and that produces according to the user-the best quality of data.

Figure 2 shows a sketch of an analogue calbe of this discovery with more than one hydrophone group per offset. In this case, 3 hydrophone groups are shown to be used per offset, i.e. in this case, M = 3. Also 48 offsets are shown to be used. Hence N = 48 in this case and the total number of hydrophone groups in this cable is 144. Also 144 traces are recorded per shot by 144 channels. Alternatively, 48 traces per shot are recorded, one trace from all the 3 groups of hydrophones at each of the 48 offsets. The horizontal spacing of the hydrophone groups at each offset is also decided by the user.The groups of hydrophones at a few offsets and preferably not at too many offsets of the cable are fastened together at each of these offsets so that during shooting and recording of data all groups of the cable are immersed together under water at the same depth and towed together by the ship as one compact body.

The number of offsets at which the groups are fastened is minimised in order not to make.

the cable too heavy. The hydrophone groups at the first offset are fastened by a fastener preceding them and those at the final offset are fastened by one following them. Each of the other fasteners used at their offsets fastens groups of hydrophones immediately following and preceding it.

In the example depicted by Fig. 2, groups 1, 2 and 3 are at the first offset, groups 4, 5 and 6 are at the second offset and so on. Also in this example groups 1 to 3 are shown fastened together by one fastener preceding them, groups 7 to 1 2 are shown fastened together by another fastener following groups 7 to 9 and preceding 10 to 1 2 and so are the groups at some other offsets fastened together by other fasteners. The groups 142 to 144 at the final offset are fastened by one fastener following them.

Figure 3 shows a sketch of a digital cable of this discovery. The ship, the shot and the shot point are not shown in this diagram for the sake of simplicity, but their location with respect to the cable during shooting and recording of data is same as in the case of the analogue cable sketched in Fig. 2. In this cable also, like the analogue cable, 3 groups of hydrophones are shown to be usd at each offset. Also there are altogether 144 groups of hydrophones at the 48 offsets of the cable and 1 44 traces of data are recorded per shot by 144 channels of the recording instrument or alternatively 48 traces are recorded per shot by 48 channels, one trace of data being generated from the 3 hydrophone groups of each offset and dititized and transmitted to the recording instrument.In this cable also like the analogue cable the groups of hydrophones at a few offsets including the first and last offsets are fastened together at each of these offsets and also with the fasteners the electronic modules are fitted at these offsets.

In the example of Fig. 3, groups 1 to 3 are at the first offset, groups 4 to 6 are at the second offset and so on. Also in this example groups 1 to 3 are fastened together by one fastener and an electronic module fitted before them for digitization and transmission to the recording instrument of the data of the groups 1 to 1 2. Similarly groups 10 to 1 5 are fastened by a fastener preceding groups 1 3 to 1 5 and following 10 to 1 2 and with the fastener an electronic module fitted for digitization and transmission of the data of groups 1 3 to 24. Other hydrophone groups of the cable are also similarly fastened by other fasteners and with them the electronic modules are fitted.

There is only a fastener at the end of this cable i.e. after groups 142 to 144 at offset 48 to fasten these groups. No electronic module is needed there. The data of these groups and those of groups 133 to 141 are digitized and transmitted by the module preceding the groups 1 33 to 1 35. The fasteners and the modules are so fitted in the cable that when it is in use all its groups of hydrophones are immersed at the same depth of water and towed together by the ship as one compact body.

In the existing method in which a cable is used with only one group of hydrophones per offset, the recorded traces of the various shots of a line of survey are sorted (gathered) in a data processing centre in a common depth point fashion, the process being called CDP sort (gather), and the gathered traces that belong to the same depth point are stacked (summed) for each depth point along the line---called CDP stacking. In this new system where data is recorded with M hydrophone groups per offset, the M traces that are recorded per shot per offset from the M groups of each offset instead of recording one trace per shot per offset from the M groups of each offset are summed (mixed) first, and the output traces are then CDP sorted and stacked.

When the outputs of the individual hydrophones of a group are summed the quality of data is improved. Similarly when the outputs of the groups of each offset are summed, the results are further improved.

The cable is used in both two-dimensional and three dimensional seismic surveys. It is useful to improve data quality in areas of very weak signal and generally, in areas where useful deep data is not produced by the present system.

Claims

1. A method of designing a marine seismic cable, either conventional i.e. analogue or digital with more than one group of hydrophones substantially at each of its various offsets from which the seismic data is recorded from each shot at each shot point of each line of a marine seismic survey in any area for exploration for oil and gas, the groups of hydrophones at each offset being each on a line along the length of the cable, the lines being substantially parallel to each other and their number being the same as the number of groups of hydrophones at each offset, and from these groups of hydrophones at each offset either one trace of data or as many traces of data as the number of groups of hydrophones being recorded per shot of each line of the survey.

2. A method as claimed in claim 1 in which the cable is so designed that when it is in use i.e. during the shooting and recording of data all its groups of hydrophones are kept immersed under sea surface at the same depth and it is towed by the ship substantially as one compact body.

3. A method as claimed in claims 1 and 2 in which the number of hydrophones to use per group of the cable designed is that the user thinks produces the best quality of data in the area of survey.

4. A method as claimed in claims 1 to 3 wherein the number of groups of hydrophones per offset, the number of offsets, the offset increment, the number of fasteners, and the number of electronic modules are decided by the user and accordingly the cable is assembled.

5. A method as claimed in claims 1 to 4 wherein the horizontal spacing of the groups of hydrophones at each of the offsets of the cable is decided by the user and accordingly the cable is assembled.

6. A method of designing a marine seismic cable, either analogue or digital, for collecting seismic data, substantially as hereinafter described and as illustrated in the accompanying drawings, 2 and 3.