GB2544544A

GB2544544A - Method and apparatus for acquiring geophysical data

Info

Publication number: GB2544544A
Application number: GB1520504.0A
Authority: GB
Inventors: Sjøen Pedersen Asmund; Thompson Mark
Original assignee: Statoil Petroleum ASA
Current assignee: Equinor Energy AS
Priority date: 2015-11-20
Filing date: 2015-11-20
Publication date: 2017-05-24
Anticipated expiration: 2035-11-20
Also published as: GB2544544B; GB201520504D0

Abstract

A method and system for acquiring geophysical data, where geophysical sensors 1 cover a first area 7, a first geophysical data record is collected by the sensors from wavefields generated by a source over a second area 8, where the second area 8 is larger than and includes the first area 7, a second geophysical data record is collected by the sensors from wavefields generated by a source over a third area 9 which is smaller than the second area 8, and where processing of data from the second geophysical data record uses data from the first geophysical data record. This process is commonly called time-lapse monitoring or 4D surveying. Optionally the geophysical source can be a marine seismic source and the geophysical sensors can be an array of seismic sensors located on the sea bed 2. The advantage of this invention is that the second survey can be carried out in a shorter period of time, as less data has to be recorded.

Description

Method and Apparatus for Acquiring Geophysical Data

The present invention relates to a method and apparatus for acquiring geophysical data.

In the prior art, it is known to permanently monitor a subterranean hydrocarbon reservoir by having a seismic sensor array permanently deployed on the seabed. This permanent monitoring may also be referred to as time-lapse monitoring or 4D seismic monitoring. Monitoring is achieved by taking seismic data records of the reservoir at different times, for instance during production from the reservoir, to monitor how the reservoir changes over time. A good knowledge of the reservoir is very important to optimise production.

The sensor array covers a first area on the seabed and generally covers the hydrocarbon reservoir that is desired to be monitored over time, i.e. the reservoir is vertically below the sensor array. The sensor array is typically in a grid formation that is permanently fixed to the seabed.

To monitor the reservoir over time, at certain time intervals seismic data is recorded using the sensor array. Each record of seismic data is achieved by producing shots of seismic energy using a seismic source over and throughout a second area that includes, and is larger than, the first area. For each record, the seismic source is moved over the same paths and fired at the same locations. For instance, when offshore, the seismic source is towed from a ship along a predefined path and fired at predefined locations. These locations are the same for each data record.

The first area and the second area overlap each other such that the second area completely surrounds the first area. The portion of the second area that is not overlapping with the first area (i.e. the difference between the second area and the first area) is described herein as a “fifth area”. The fifth area typically surrounds the first area, i.e. the fifth area may effectively form a border around the first area. (The term “fifth” is used here for consistency with the rest of the specification.)

In the prior art, when taking a seismic record, seismic shots are fired throughout the second area (i.e. including the fifth area). Shots are fired in the fifth area in order to aid data processing techniques (such as seismic migration calculations and to produce better-sampled data) in order to more accurately monitor the reservoir beneath the sensor array. The fifth area may be considered to be a migration aperture. Migration is a known data processing technique for relocating data that accounts for subsurface structure.

In the current state of the art, in each data record (which may be taken at time intervals of up to once every 6 months or once a year) seismic shots are generated over and throughout the whole of the second area. Seismic migration is performed for each data record using only the seismic data in each record. A more detailed explanation of the prior art methods is given in relation to Figures 1 and 3, discussed later.

There is a desire to improve the permanent monitoring of reservoirs.

In one aspect the invention provides a method of acquiring geophysical data using a first geophysical wavefield source and a geophysical energy sensor, wherein: the geophysical energy sensor covers a first area; there is provided a first geophysical energy data record generated by using a second geophysical wavefield source to generate a plurality of geophysical wavefields over a second area and by using the geophysical energy sensor to record the geophysical energy; and the second area includes, and is larger than, the first area, the method comprising: generating a second geophysical energy data record by using the first geophysical wavefield source to generate a plurality of geophysical wavefields over a third area and by using the geophysical energy sensor to record the geophysical energy, the method being characterised by processing the data of the second geophysical energy data record using at least some of the data of the first geophysical energy data record, and by the third area being smaller than the second area.

The inventors have discovered that the data of the first data record can be used in the processing of the data of the second data record. From this discovery, the inventors have found that when taking a subsequent data record using the sensor array, geophysical wavefields can be generated over an area smaller than the area previously required. The inventors have found that this can be done because the data recorded from the “excess” area surrounding the sensor array of the first data record can effectively be used in the processing of subsequent data records.

Using the first geophysical energy data record to aid with the processing of the second geophysical energy data record allows the third area to be smaller than the second area, whilst still producing accurate monitoring. Having the third area smaller than the second area increases the efficiency of the data acquisition. Further, the time taken for the first source to generate the required wavefields for the second data record is reduced in comparison to the prior art (where the source must be moved over the entire second area). This reduces the time required to produce the second (and any further subsequent) data records. This is an important advantage because the production of a data record must typically be done in certain calm weather conditions, but can take a considerable amount of time. In order to produce a data record, a “weather time slot” must be identified and used to ensure all wavefields are generated by the source within the weather slot. The weather time slot is reduced for the present method in comparison to the prior art, and so allows the second data record to be recorded in shorter weather slots that previously could not have been used.

Processing the second geophysical energy data record using the first geophysical energy data record may comprise using only a portion of the data of the first geophysical energy data record (i.e. not at all of the data of the first geophysical energy data record).

The portion used for such processing may be the portion of first data record originating from source locations outside of the third area. The inventors have found that if the third area preferably at least covers the first area, then the data from the second data record may be processed using this portion of the data from the first data record. For instance, the data from the second data record can be merged with this portion of the data from the first data record, and the merged data can be successfully migrated. This is based on the assumption that over time the production from the well has only affected the data received from the source generating at locations within the third area (or indeed within the first area), and the production has not substantially affected the data originating from the portion of the second area outside of the third (or indeed first) area. Using this assumption, this portion of the first data record can be used in the processing of the second data record.

Processing the data of the second geophysical energy data record using at least some of the data of the first geophysical energy data record may comprise using the at least some of the data of the first geophysical energy data record and the second geophysical energy data record to construct a more fully-sampled geophysical energy data record. The second data may set may be incomplete or may appear to be irregularly sampled, since no/less data is recorded from source locations outside of the first area. It is, however, beneficial to construct a complete, fully-sampled geophysical energy data record, as a complete geophysical energy data record eases subsequent processing of the data. It should be understood that the constructed more fully-sampled geophysical data is merely a construction of a geophysical data record that appears to be more fully sampled than the recorded second data record. It is not actually a fully-sampled geophysical data record: it is constructed by merging the at least a portion of the first data record with the data of the second geophysical data record. A more fully-sampled data record may merely be one that is better sampled than the recorded data record, or may be one that is completely fully sampled. By completely fully-sampled, it is meant that the constructed data record is substantially as well sampled as the recorded data record would have been had the second data record been recorded by moving the first source over the entirety of the second area.

Additionally or alternatively, processing the data of the second geophysical energy data record using at least some of the data of the first geophysical energy data record may comprise migrating the data of the second geophysical energy data record using at least some of the data of the first geophysical energy data record. The difference between the second and first/third areas may be considered to be a migration aperture. The migration aperture of the first data record can be used for the second data record.

The second data record may first be combined with the at least a portion of the first data record to produce the constructed, better-sampled data record. This constructed data record may be used in migration. Prior to migration, some standard processing techniques may be performed, such as noise attenuation.

As an aside, no technical significance should be placed on the specific numbering of these areas - the terms “first”, “second”, “third”, “fourth” and “fifth” used in the specification merely label different areas (or sub-areas) of the Earth’s surface that are covered by the source(s) and/or sensor(s).

The areas defined in the present method may be areas of the Earth’s surface when viewed from above (or below).

The areas may be 2-dimensional areas. In this case, the sensor may be a grid of sensors covering a 2-dimensional area. The first and/or second source may be moved over a 2-dimensional area that is substantially in the same plane as the sensor.

However, the first area may also be considered to be in 1-dimension, e.g. a line. In this case, the sensor may be line of sensors extending linearly. The first and/or second source may be moved over a 1-dimensional line that is in substantially the same direction as the sensor line, and may directly vertically over the line. In this case, the fourth/fifth areas mentioned below may be seen as opposite end portions of the line over which the first and/or second source is moved, with the middle portion of the line overlapping with the line of the sensors.

Thus the “areas” mentioned in the present invention may be considered to be one or two dimensional regions of the Earth’s surface.

The sensor may be a permanent sensor. The sensor may be located on the Earth’s surface. The sensor may be located below the Earth’s surface. Such a permanent sensor is advantageous as the location of the sensor is unchanged over time, which aids the monitoring of the reservoir over time.

The first data record may be referred to as the baseline record. The first data record may be the initial data record recorded using the sensor array, or any previous data record that is recorded using the sensor array. As mentioned above, the inventors have discovered that all subsequent data records can use such an earlier data record to perform processing of the subsequent data record.

The method may be a method of time-lapse data acquisition. The method may be for permanently monitoring a reservoir. The method may be 4D data acquisition. The first/baseline/initial data record may be taken at a first time. The second data record may be taken at a second time. Any subsequent data records may be taken at subsequent times. The time between consecutive data records may be more than one month, more than six months or more than one year. It may take up to one, two, three, four, five, six or seven days to record each data record.

In the present method, it should be understood that the recorded geophysical energy that makes each geophysical data record originates from the geophysical wavefield generated by the source(s). The recorded geophysical energy may, of course, have been reflected and interfered with as it passes between the source(s) and the sensor through the Earth (and through the sea).

The first area may be positioned to cover a hydrocarbon reservoir below the Earth’s surface. Thus, the sensor may cover such a reservoir. By “cover” it is meant that, when viewed vertically with respect to the Earth’s surface, the first area at least partially overlaps with the horizontal area defined by the reservoir. Preferably, the reservoir is substantially completely covered by the first area. The first area may have substantially the same shape and area as the area defined by the reservoir. This minimises the size of the sensor, whilst still covering the reservoir.

The geophysical energy sensor may be located on the Earth’s surface. The geophysical energy sensor may be located beneath the Earth’s surface.

The first and/or second source may be at the same vertical height, with respect to the Earth’s surface, as the sensor. However, preferably, the first and/or second source may be located at a height above the sensor. The first and/or second source may be at or above the Earth’s surface.

The reservoir may be an onshore reservoir. In this case, the first and/or second source may be moved over the second and third areas on the Earth’s surface, thus generating geophysical wavefields directly on the Earth’s surface. Preferably, however, the reservoir may be an offshore reservoir. In this case, the first and/or second source may be above the Earth’s surface (the seabed), either at a certain depth or at/near the sea surface. Onshore and offshore geophysical wavefield generation is a well-known field of technology and need not be explained in any great detail in the present application.

For each data record, the source(s) may be moved over the same path and may generate wavefields at the same locations. For instance, when offshore, the seismic source may be towed from a ship along a predefined path and fired at predefined locations. These locations are the same for the second and third areas, where these areas overlap.

The source used to record the second data record may be the same as or may be different from the source used to record the first data record, i.e. the first and second sources may be the same source, or different sources. As an aside, no technical significance should be placed on the specific numbering of these sources.

The geophysical energy sensor may comprise an array of geophysical energy sensors. The geophysical energy sensor(s) may also be referred to as receiver(s). The array may take the form of a line of sensors, preferably extending in a straight line. This may the case in the “1 -dimensional” case, mentioned above. Preferably, however, the array may be 2-dimensional array of sensors. In this case, the sensors may be arranged in a grid. The receiver grid may comprise a plurality of straight lines of sensors, preferably substantially parallel to one another, and preferably substantially horizontally spaced. The spacing between each of the lines may be substantially constant. The array may cover a rectangular or square area of the Earth’s surface.

In the case of a sensor (array) of a certain shape, the shape of the second area may be substantially similar to the sensor (array) but may be larger, and the shape of the third area may be substantially similar to the sensor (array) but may be substantially the same size as the sensor (array), or between the size of the sensor (array) and the second area. The first, second and third areas may be concentric. For example, when the sensor (array) is rectangular, the shape of the second area may be a substantially rectangle but may be larger, and the shape of the third area may be a substantially similar rectangle but may be substantially the same size as the sensor (array), or between the size of the sensor (array) and the second area. The rectangle may be a square.

The source(s) may be any geophysical wavefield source suitable for use in monitoring the reservoir. The first and/or second source may be a seismic source. The first and/or second source may be formed of a plurality or an array of sources. The first and/or second source may be located on the Earth’s surface or in the sea. The first and/or second source may be towed or moved by a vehicle or a boat.

The first, second and third areas in the present method are overlapping areas. When viewed from above, at least a part of each of these areas overlaps with at least a part of the other areas. This overlap is overlap when viewed from vertically above (or below) with respect to the Earth’s surface.

The third area may include the first area. The second area may include and may be larger than the third area. The third area may be completely within the second area. A fourth area may be defined as the portion of the second area that does not overlap with the third area. The fourth area may surround the third area, i.e. it may be considered to be a border around the portion of the second area that does overlap with the third area. The data recorded from the geophysical wavefields generated in the fourth area may be used as a migration aperture with the second data record (and with all subsequent data records). The portion of the first data record originating from source locations within the third area may be combined/merged with the second data record. The merged data may be migrated. The merged data may be considered to be reconstructed (more) fully-sampled data. Migration can be achieved using any standard technique. The same migration technique can be used to migrate the second data record (merged with the appropriate portion of the first data record) as was used for the first data record. The migration may be seismic migration.

The width of the fourth area (i.e. the distance from the edge of the third area to the edge of the second area) may be up to 10m, up to 100m, up to 500m, up to 1km, up to 5km or up to 10 km.

The shape of the second area may be substantially similar to the shape of the third area, except for the second area being larger. The shape of the third area may be substantially similar to the shape of the first area. The first, second and/or third areas may be substantially concentric. Thus, the width of the fourth area may be substantially constant.

Thus, the geophysical energy of the first data record originating from geophysical wavefields generated in the fourth area may be merged with the second data record. The geophysical energy of the first data record originating from geophysical wavefields generated in the fourth area may be used to reconstruct a more fully-sampled (i.e. better-sampled) data record from the second data record and/or in the migration of the second data record.

For instance, the first area may be completely within the second area. A fifth area may be defined as the portion of the second area that does not overlap with the first area. The fifth area may as previously noted (completely) surround the first area, i.e. it may be considered to be a border around the portion of the second area that does overlap with the first area. As mentioned above, this fifth area may be used as a migration aperture (not only with the first data record, but with the second and all subsequent data records), i.e. the portion of the first data record originating from source locations within the fifth area may be combined/merged with the second data record. The merged data may be migrated. The merged data may be considered to be reconstructed better-sampled data. The reason for generating geophysical wavefields in the fifth area (and not just in an area that overlaps with the first area) is that the data acquired from the fifth area may be used for data processing of subsequent data records, to ensure an accurate monitoring of the reservoir beneath the sensor.

The width of the fifth area (i.e. the distance from the edge of the first area to the edge of the second area) may be up to 10m, up to 100m, up to 500m, up to 1km, up to 5km or up to 10km.

The shape of the second area may be substantially similar to the shape of the first area, but the second area may be larger. The first and second areas may be substantially concentric. Thus, the width of the fifth area may be substantially constant.

Thus, the geophysical energy of the first data record originating from geophysical wavefields generated in the fifth area may be merged with the second data record. The geophysical energy of the first data record originating from geophysical wavefields generated in the fifth area may be used to reconstruct a more fully-sampled (i.e. better-sampled) data record from the second data record and/or in the migration of the second data record.

Preferably, the third area may be substantially equal to the first area. Thus, the second (i.e. subsequent) data record may be generated using geophysical wavefields that are generated only within the first area (i.e. the area of the sensor). In this case, it should be understood that the fifth area is substantially the same as the fourth area discussed above. This is the preferable case as it minimises the area over which subsequent geophysical wavefields are generated, and hence increases the efficiency of the data acquisition.

The first area may be up to 1 km2, up to 2 km2, up to 5 km2, up to 10 km2, up to 50 km2, up to 100 km2, up to 300 km2 or up to 500 km2. The second and/or third area may be up to 1 km2, up to 2 km2, up to 5 km2, up to 10 km2, up to 50 km2, up to 100 km2, up to 300 km2, up to 500 km2 or up to 1000 km2. The second area may be larger than the first and/or third area by a factor of up to around 2, 1.5, 1.3, 1.2 or 1.1. The degree to which the second area is required to be larger than first/third area depends on the processing (e.g. merging and/or migration) calculation to be performed.

In the case where the second area is 1.5 times greater than the first area, and the third area is substantially equal to the first area, there will be an efficiency and time saving of around 33% when generating/recording the second data record.

The method may comprise generating the first geophysical energy data record by using the second geophysical wavefield source to generate a plurality of geophysical wavefields over the second area and by using the geophysical energy sensor to record the geophysical energy.

The method may comprise generating any number of subsequent geophysical energy data records (e.g. third, fourth, fifth.....nth records) by using a geophysical wavefield source to generate a plurality of geophysical wavefields over the third area (or over another area that is within the second area, but is smaller than the second area) and by using the geophysical energy sensor to record the geophysical energy. The method may comprise using (a portion of) the first data record to merge with the subsequent data record, similarly to the method described above for the second data record. The method may comprise using (a portion of) the first data record in the migration of the subsequent data, similarly to the method described above for the second data record. The subsequent geophysical energy data records may be generated using the same source as the first and/or second data records, or one or more different sources.

Preferably, the geophysical wavefield may be a seismic wavefield and the geophysical data may be seismic data.

The geophysical wavefield may be generated over the second and/or third areas as a plurality of shots at various locations over the second and/or third areas. The geophysical wavefields generated for the first data record and the second data record (and any subsequent data record) may be generated at substantially the same locations within the overlapping second and third areas. The shots may be generated in the same order for each data record.

In another aspect, the invention provides a system for acquiring geophysical data comprising: a first geophysical wavefield source; a second geophysical wavefield source; a geophysical energy sensor covering a first area; and a processor, the second geophysical wavefield source and the geophysical energy sensor being configured to generate a first geophysical energy data record by using the second geophysical wavefield source to generate a plurality of geophysical wavefields over a second area and by using the geophysical energy sensor to record the geophysical energy, wherein the second area includes, and is larger than, the first area; and the first geophysical wavefield source and the geophysical energy sensor being configured to generate a second geophysical energy data record by using the first geophysical wavefield source to generate a plurality of geophysical wavefields over a third area and by using the geophysical energy sensor to record the geophysical energy, characterised in that the processor is configured to use at least some of the data of the first geophysical energy data record to process the second geophysical energy data record, and in that the third area is smaller than the second area.

The system may comprise any of the above-discussed features and may be used or configured to perform any of the above-discussed methods. The first and second sources may be the same source, or different sources.

In another aspect, the invention provides a computer program product comprising computer readable instructions, wherein: there is provided a first geophysical energy data record generated by using a second geophysical wavefield source to generate a plurality of geophysical wavefields over a second area and by using a geophysical energy sensor that covers a first area to record the geophysical energy, the second area including and being larger than the first area; and there is provided a second geophysical energy data record generated by using a first geophysical wavefield source to generate a plurality of geophysical wavefields over a third area and by using the geophysical energy sensor to record the geophysical energy, the third area being smaller than the second area, the computer program product, when run a computer, being configured to cause a processor to use at least some of the first geophysical energy data record to process the second geophysical energy data record.

The computer program product may be configured to perform any of the above discussed methods.

Preferred embodiments of the invention will now be discussed, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows a schematic view of a prior art method for time-lapse permanent monitoring of a hydrocarbon reservoir;

Figure 2 shows a schematic view of an embodiment of the present method for time-lapse permanent monitoring of a hydrocarbon reservoir;

Figures 3 and 4 show another schematic view of the methods shown in Figures 1 and 2; and

Figure 5 illustrates an issue with acquiring data when moving the source over a smaller area than was used in the prior art.

Regarding Figure 1, a prior art method is shown for time-lapse monitoring of a subterranean hydrocarbon reservoir (not shown). A permanent seismic sensor array 1 is deployed on the seabed 2. A seismic energy source 3 is located at a depth within the sea 4 and is moved to different locations by a ship 5. The source 3 is moved to various predetermined locations where the source 3 is then shot producing propagating seismic energy 12 that propagates from the source 3, through the sea 4, through the Earth 6 and the reservoir, and may be reflected to the sensor 1. The sensor 1 detects the reflected propagating seismic energy. The sensor covers a first area of the Earth’s surface 7.

Figure 1(a) shows the recording of a first data record. The first data record may be the initial or baseline data record. To record the first data record, the source 3 is moved in a predetermined path 11 over a second area 8 and is shot at predefined locations throughout the second area 8. The seismic data recorded by the sensor 1 from all of the shots produced over the second area 8 are used in the migration of the seismic data of the first data record. Due to the large area covered by the source, the data record recorded by each of the sensors of the sensor array 1 is a fully-sampled data record 13 (see Figure 5(a)).

Figure 1(b) shows the recording of a second data record. The second data record may be produced over 6 months or over a year after the first data record. The second data record is produced just like the first data record. Thus, the source 3 is moved in the same predetermined path 11 over the second area 8 and is shot at the same predefined locations throughout the second area 8. The seismic data recorded by the sensor 1 from all of the shots produced over the second area 8 are used in the migration of the seismic data of the second data record. Due to the large area covered by the source, the data record recorded by each of the sensors of the sensor array 1 is a fully-sampled data record 13 (see Figure 5(a)).

Thus, by repeating the method of Figure 1(a) at various times, migrated seismic data can be produced for the reservoir for each of said times. Further, since each recorded data record is fully sampled 13, there is no need to construct any constructed fully-sampled data record. The present method aims to improve this prior method.

Regarding Figure 2, shown is an embodiment of the present method for time-lapse monitoring of a subterranean hydrocarbon reservoir (not shown). The apparatus used is the same as for the prior art method shown in Figure 1. Thus, a permanent seismic sensor array 1 is deployed on the seabed 2. A seismic energy source 3 is located at a depth within the sea 4 and is moved to different locations by a ship 5. The source 3 is moved to various predetermined locations where the source 3 is then shot producing propagating seismic energy 12 that propagates from the source 3, through the sea 4, through the Earth 6 and the reservoir, and may be reflected to the sensor 1. The sensor 1 detects the reflected propagating seismic energy. The sensor covers a first area of the Earth’s surface 7.

Figure 2(a) shows the recording of a first data record. The first data record may be the initial or baseline data record. To record the first data record, the source 3 is moved in a predetermined path 11 over a second area 8 and is shot at predefined locations throughout the second area 8. The seismic data recorded by the sensor 1 from all of the shots produced over the second area 8 are used in the migration of the seismic data of the first data record. Thus, the method shown in Figure 2(a) is the same as that shown in Figure 1 (a). Due to the large area covered by the source, the data record recorded by each of the sensors of the sensor array 1 is a fully-sampled data record 13 (see Figure 5(a)).

Where the present method differs from the prior art is shown in Figure 2(b). Figure 2(b) shows the recording of a second data record. The second data record may be produced over 6 months or over a year after the first data record. The second data record is produced just like the first data record; however, the source 3 is not moved over the entirety of the second area 8. Instead, it is only moved over a third area 9, which is smaller than the second area 8 and, in this embodiment, corresponds to the first area 7.

Whilst the source 3 is not moved over the entirety of the second area 8, the source 3 is still moved in the same predetermined path 11 and is shot at the same predefined locations in comparison to the first data record, but is limited to those paths and shots of the first data record within the third area 9. Thus, there is a portion of the second area 8 (the fourth area 10) over which the source 3 is not moved and hence in which no seismic shots are fired.

Since the source 3 produces shots over a smaller area for the second data record in comparison to the prior art, the second data record is not a fully sampled data record; rather it is a reduced-sampled data record. As can be seen in Figure 5(b), this can mean that whilst sensor(s) toward the middle of the sensor array 1 may still show a full sampling 13, the sensor(s) toward the edge of the sensor array 1 may not be fully sampled 14. This incomplete, reduced-sampled data record is less useful for subsequent processing than a complete, fully-sampled data record. For example, if one were to migrate the data of the second data record (alone) then a less accurate result would be obtained. However, the inventors have discovered that the data obtained from the fourth area 10 from the first data record can be merged with the data of the second data record to effectively create or construct a complete, fully sampled data record (i.e. a data record that is as complete as one that would have been recorded had the source been moved over the entirety of the second area). This merged data can then be migrated, or otherwise processed, using conventional migration techniques, producing an accurate migration of the data of the second data record, without the need for producing seismic shots in the fourth area 9 for the second data record.

Thus, by repeating the method of Figure 2(b) at various times, processed and migrated seismic data can be produced for the reservoir for each of said times. However, in contrast to the prior art, it is not necessary to record seismic data produced at locations throughout the entire second area, rather the seismic shots need only be produced over a smaller area (i.e. the third area 9).

Although Figures 1 and 2 only show one dimension, it should be understood that the sensor 1, the path 11 of the source 3, the first area 7, the second area 8, the third area 9 and the fourth area 10 may be two dimensional, i.e. Figures 1 and 2 may merely be showing cross-sections of the set-up.

Such a two dimensional set up is shown in Figures 3 and 4. In this embodiment, the sensor array 1 is a receiver grid that comprises a plurality of parallel linear sensor arrays.

The sensor array 1 forms a rectangle. Thus, the first area 7 is rectangular.

Figure 3 shows a schematic plan view of the prior art method shown in Figure 1. Figure 3(a) corresponds to Figure 1(a), where the first data record is produced by moving the source over the entire second area 8, the second area 8 being larger than and completely overlapping with the first area 7 defined by the sensor array 1. The second area 8 is a similar shape to the first area 7. Figure 3(b) corresponds to Figure 1(b), where the method of Figure 1(a) is repeated at a subsequent time.

Figure 4 shows a schematic plan view of the present method shown in Figure 2. Figure 4(a) corresponds to Figure 2(a), where the first data record is produced by moving the source over the entire second area 8. Figure 4(a) is thus identical to Figure 3(a) (and Figure 3(b)).

However, Figure 4(b) is different and corresponds to Figure 2(b) and hence shows that the second data record is recorded by only producing seismic shots within the third area 9. The data of the first data record arising from the shots fired in the fourth area 10 are merged with the data of the second data record to form a constructed complete, fully sampled data record. This merged data can then be migrated.

Claims

Claims:

1. A method of acquiring geophysical data using a first geophysical wavefield source and a geophysical energy sensor, wherein: the geophysical energy sensor covers a first area; there is provided a first geophysical energy data record generated by using a second geophysical wavefield source to generate a plurality of geophysical wavefields over a second area and by using the geophysical energy sensor to record the geophysical energy; and the second area includes, and is larger than, the first area, the method comprising: generating a second geophysical energy data record by using the first geophysical wavefield source to generate a plurality of geophysical wavefields over a third area and by using the geophysical energy sensor to record the geophysical energy, the method being characterised by: processing the data of the second geophysical energy data record using at least some of the data of the first geophysical energy data record; and the third area being smaller than the second area.

2. A method as claimed in claim 1, wherein processing the data of the second geophysical energy data record using at least some of the data of the first geophysical energy data record comprises migrating the data of the second geophysical energy data record using at least some of the data of the first geophysical energy data record.

3. A method as claimed in claim 1 or 2, wherein processing the data of the second geophysical energy data record using at least some of the data of the first geophysical energy data record comprises using the at least some of the data of the first geophysical energy data record and the second geophysical energy data record to construct a more fully-sampled geophysical energy data record.

4. A method as claimed in claim 1,2 or 3, wherein the third area is completely within the second area, and wherein the second and third areas define a fourth area, the fourth area being defined as the portion of the second area that does not overlap with the third area.

5. A method as claimed in claim 4, wherein the fourth area surrounds the third area.

6. A method as claimed in claim 4 or 5, comprising merging the data of the first geophysical data record originating from source locations in the fourth area with the data of the second geophysical data record.

7. A method as claimed in claim 4, 5 or 6, wherein the data of the first geophysical data record originating from the fourth area are used in the processing of the data of the second geophysical data record.

8. A method as claimed in any preceding claim, wherein the third area is substantially equal to the first area.

9. A method as claimed in any preceding claim, wherein the first, second, third and fourth areas are 2-dimensional areas.

10. A method as claimed in any preceding claim, wherein the geophysical energy sensor is a permanent sensor located on the Earth’s surface.

11. A method as claimed in any preceding claim, wherein the geophysical energy sensor comprises an array of geophysical energy sensors.

12. A method as claimed in any preceding claim, wherein the first area is positioned to cover a hydrocarbon reservoir below the Earth’s surface.

13. A method as claimed in any preceding claim, wherein the method is performed offshore.

14. A method as claimed in any preceding claim, wherein the first and/or second source is a seismic source, the geophysical wavefield is a seismic wavefield, the geophysical data is seismic data, and the sensor is a seismic energy sensor.

15. A method as claimed in any preceding claim, wherein the geophysical wavefield is generated over the second and/or third areas as a plurality of shots at various predefined locations over the second and/or third areas.

16. A method as claimed in claim 15, wherein the geophysical wavefields generated for the first data record and the second data record are generated at substantially the same locations within the overlapping second and third areas.

17. A method of time-lapse data acquisition comprising the method of any of the preceding claims.

18. A method as claimed in any preceding claim, comprising generating the first geophysical energy data record by using the second geophysical wavefield source to generate a plurality of geophysical wavefields over the second area and by using the geophysical energy sensor to record the geophysical energy.

19. A method as claimed in any preceding claim, wherein the first and second geophysical wavefield source is the same geophysical wavefield source.

20. A system for acquiring geophysical data comprising: a first geophysical wavefield source; a second geophysical wavefield source; a geophysical energy sensor covering a first area; and a processor, the second geophysical wavefield source and the geophysical energy sensor being configured to record a first geophysical energy data record by using the second geophysical wavefield source to generate a plurality of geophysical wavefields over a second area and by using the geophysical energy sensor to record the geophysical energy, wherein the second area includes, and is larger than, the first area; and the first geophysical wavefield source and the geophysical energy sensor being configured to record a second geophysical energy data record by using the first geophysical wavefield source to generate a plurality of geophysical wavefields over a third area and by using the geophysical energy sensor to record the geophysical energy, characterised in that: the processor is configured to use at least some of the first geophysical energy data record to process the second geophysical energy data record, and the third area is smaller than the second area.

21. A system as claimed in claim 20, configured to perform any of the methods of claims 2 to 19.

22. A system as claimed in claim 20 or 21, wherein the first and second geophysical wavefield source is the same geophysical wavefield source.

23. A computer program product comprising computer readable instructions, wherein: there is provided a first geophysical energy data record generated by using a second geophysical wavefield source to generate a plurality of geophysical wavefields over a second area and by using a geophysical energy sensor that covers a first area to record the geophysical energy, the second area including and being larger than the first area; and there is provided a second geophysical energy data record generated by using a first geophysical wavefield source to generate a plurality of geophysical wavefields over a third area and by using the geophysical energy sensor to record the geophysical energy, the third area being smaller than the second area, the computer program product, when run a computer, being configured to cause a processor to use at least some of the first geophysical energy data record to process the second geophysical energy data record.

24. A computer program product as claimed in claim 23 that is configured to perform any of the methods of claims 2 to 19.

25. A system or method substantially as described herein with reference to Figures 2 and 4.