WO2010119077A2

WO2010119077A2 - Method of and apparatus for surveying a region of the earth

Info

Publication number: WO2010119077A2
Application number: PCT/EP2010/054909
Authority: WO
Inventors: Sascha Bussat; Peter Hanssen; Simone Kugler
Original assignee: Statoil Asa
Priority date: 2009-04-17
Filing date: 2010-04-14
Publication date: 2010-10-21
Also published as: GB0906635D0; GB2469518A; WO2010119077A3

Abstract

In order to perform a passive survey, sensors (5, 8) for obtaining data relating to the structure of the region (1) are deployed on ice bodies (4) which move as a result of natural drift. The sensors (5, 8) are used to obtain data at different locations resulting from movement of the ice bodies (4) with respect to the region (1). The sensors (5, 8) may be seismometers which respond to ambient noise, for example resulting from cracking or colliding of the ice bodies (4, 24).

Description

Method of and Apparatus for Surveying a Region of the Earth

The present invention relates to a method of and an apparatus for surveying a region of the earth. Such a surveying method may be used for offshore exploration, for example to locate possible hydrocarbon reservoirs or to determine the subsurface structure, in regions with ice coverage.

Manned stations disposed on drifting ice bodies have been used for many years. In particular, such stations have been used for meteorical investigations in environments where it is only feasible to dispose such stations on drifting ice, for example in polar regions. Seismological studies have also been performed on moving floes to record earthquakes as close as possible to an offshore event. Positioning or knowledge of the positions of such stations is important but the movement of such stations caused by drifting of the ice is not desired and, in most cases, causes an undesirable disturbance.

Seismological surveys have been performed on ice floes. For example, Schlindwein, V., Mϋller, C, Jokat, W. (2007). Microseismicity of the ultraslow-spreading Gakkel ridge, Arctic Ocean: a pilot study, geophysical journal international, 169(1 ), 100-1 12 disclose the deployment of seismometers on ice floes for localisation of micro- earthquakes. Also, Rogenhagen, J. & Jokat, W., 2000. The sedimentary structure in the western Weddell Sea, Marine Geology., 128, 45-60 disclose deploying geophones on ice floes for active reflection seismic surveying. The motivation for locating such sensors on ice floes is to allow data to be obtained in regions where ice coverage makes it impossible or impractical to dispose instruments on the seabed and/or recover such instruments. However, the drifting of such ice floes is a problem because such known techniques have relied on the sensors remaining fixed or within a relatively small area. Thus, drifting of the ice has been considered to be a major disadvantage, for example when the sensors drift away from the studied area of micro-earthquakes.

RU2076342 discloses an active seismic arrangement in which a seismic source and seismic receivers are fixed to a body of drift ice. The source and receivers are suspended in the water column beneath the ice and seismic data are acquired as the ice drifts.

According to a first aspect of the invention, there is provided a method of surveying a region of the earth, comprising the steps of: deploying at least one sensor, for obtaining data relating to the structure of the region, to move with at least one ice body which moves, or is predicated to move, with respect to the region; and using the at least one sensor to obtain the data at a plurality of different locations, with respect to the region, resulting from movement of the at least one ice body with respect to the region, the at least one sensor being arranged to perform passive surveying.

The using step may comprise recording the data obtained at the plurality of locations.

The at least one ice body may be disposed above the region during the deploying step. The at least one ice body may cover the region during the deploying step.

The at least one ice body may be a floating ice body. The at least one body may comprise at least one of an iceberg, an ice floe, and an ice field and an ice sheet.

The deploying step may comprise deploying a plurality of the sensors as a substantially horizontal array.

The at least one sensor may be sensitive to gravity or gravity gradient. The at least one sensor may be sensitive to magnetic field or magnetic field gradient. The at least one sensor may comprise at least one magnetotelluric sensor.

The at least one sensor may comprise at least one electromagnetic sensor.

The at least one sensor may comprise at least one seismic sensor. The at least one seismic sensor may be sensitive to ambient noise. Ambient noise typically comprises any type of waveform directly produced or caused by the earth and not by humans.

The method may comprise the further step of performing at least one of ambient noise amplitude analysis, ambient noise surface-wave tomography, ambient noise array measurements, HA/ analysis and traveltime tomography, for example using earthquakes.

The at least one sensor may be sensitive to seismic reflections using noise generated by ice as a seismic source. The noise may be generated by cracking and/or colliding of further ice bodies. The method may comprise the further step of performing interferometry to obtain reflection seismic data. The at least one sensor may provide, or may be associated with means for providing, an indication of the position of each of the locations and/or an indication of the sensor orientation at each of the locations, and/or an indication of the sensor tilt at each of the locations.

The data may be transmitted from the at least one sensor and/or stored at the at least one sensor for subsequent retrieval.

The method may comprise the further steps of deploying a base station on the seabed or on a coast and performing at least partial processing of the data in the base station. The processing step may comprise at least one of correlation, normalisation and analysis of the data.

According to a second aspect of the invention, there is provided a method of surveying a region of the earth, comprising the steps of: deploying at least one seismic reflection sensor for receiving seismic reflections from the region; and using the at least one sensor to obtain seismic reflection data using noise generated by ice as a seismic source.

The noise may be generated by cracking and/or colliding of ice bodies. The ice bodies may be floating bodies, for example using the technique disclosed in Claerbout, J. F., 1968, Synthesis of a Layered medium from its acoustic transmission response: Geophysics, 33, 264 (269).

The method may comprise the further step of performing interferometry to obtain the seismic reflection data.

According to a third of the invention, there is provided an apparatus for performing a method according to the first or second aspect of the invention.

According to a fourth aspect of the invention, there is provided use of the movement of an ice body, on which at least one sensor arranged to perform passive surveying for obtaining data relating to the structure of a region of the earth to be surveyed is deployed, to obtain data at a plurality of different locations with respect to the region. It is thus possible to make use of the natural ice drift of ice bodies to change the position of the or each sensor with time. This allows passive surveying of regions with ice coverage or where conventional techniques are impractical or impossible. Information about the structure of a region, for example below the seabed, may be obtained in regions which would otherwise be difficult or impossible to survey so that, for example, exploration for hydrocarbon reservoirs may be extended to regions which would otherwise be inaccessible or would otherwise be difficult or far more expensive to explore. Also, by using passive techniques, surveying and exploration may be extended into protected areas or areas which are not normally available for survey, for example by active seismic techniques.

The invention will be further described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a diagrammatic side view of an arrangement for performing a method constituting an embodiment of the invention;

Figure 2 is a plan view of a first example of an arrangement for performing a method constituting an embodiment of the invention; and

Figure 3 is a plan view of a second example of an arrangement for performing a method constituting an embodiment of the invention.

Figure 1 illustrates, to a highly exaggerated scale, part of an apparatus for performing a method of surveying a region 1 of the earth to be surveyed. The region 1 is below the seabed 2, which is covered by a column of seawater 3. The seawater 3 is, for example, partially or totally covered by one or more ice bodies, one of which is shown at 4. The or each ice body 4 may comprise an iceberg, an ice floe, an ice field or an ice sheet. The region 1 may, for example, be located in the Arctic or Antarctic polar regions, or in any other region with permanent or temporary ice cover.

One or more instrument packages are deployed so as to move with the ice body 4 and a typical example of such an instrument package is shown at 5. The or each package 5 is shown as being fixed to the upper surface of the ice body 4 by resting on a horizontal plane upper surface thereof. The package may be fixed or anchored to the ice body by any suitable means and such that it travels with the ice body. Alternative arrangements may be used for constraining the package, or at least a sensor thereof, to move or travel with the ice body. For example, where the sensors are of the in-water type such as hydrophones, they may be suspended below the ice body by means of one or more cables. In another example, the package or sensor(s) may float in a hole in the ice body.

The instrument package comprises a controller 6, such as a microcontroller or microcomputer, which is arranged to control operations of the package in accordance with the requirements of the survey. The controller has a memory 7 which, among other things, may be used for temporary storage of survey data. A sensor 8 for passive surveying is connected to the controller and obtains data relating to the structure of the region 1. Various types of sensor may be used as described hereinafter.

The package 5 further comprises a position-determining arrangement, such as a global positioning by satellite (GPS) receiver 9 having an output connected to the controller 6 and an input connected to an aerial 10. The receiver receives GPS signals which allow the controller 6 to determine the location of the package 5 and therefore of the sensor 8. The package 5 also (optionally) comprises a compass 11 which supplies signals to the controller 6 allowing the controller to determine the orientation of the sensor. For sensors whose orientation is not required, the compass 11 may be omitted. Also, if the angle of inclination of the sensor 8 is required, a tilt sensor may be provided for supplying the controller 6 with information to allow the sensor inclination or tilt to be determined.

The package 5 comprises a transmitter/receiver 12, which allows the package 5 to communicate with a base station (not shown). If such communication is not required for the specific application, then the transmitter/receiver 12 may be omitted. The transmitter/receiver 12 is shown as sharing the aerial 10 with the GPS receiver 9 but separate aerials may be provided if necessary or desirable.

The instrument package 5 is deployed on the ice body 4 by any suitable technique for the survey area and/or for the prevailing conditions during deployment. For example, the package 5 may be placed on the ice body 4 manually, for example from one or more vessels, hovercraft, helicopters or base stations on ice or nearby land. Alternatively or additionally, the instrument package 5 may be deployed from the air, for instance by landing or by being parachuted on to the ice body 4, where other deployment techniques are impossible or inappropriate. Where convenient, when the survey of the region 1 has been completed, the instrument package 5 may be retrieved for reuse.

The instrument package 5 may be fixed to the ice body 4 by any suitable means in order to ensure that it remains in place during the survey. Any suitable anchoring system may be used for this purpose. Alternatively, where the instrument package 5 is such that it is most unlikely to move on the ice flow 4 or where the environmental conditions are such that the package 5 is unlikely to move, the package 5 may simply rest on the ice body 4. For retrieval purposes, the controller 6 may cause the transmitter/receiver 12 to transmit data indicating the location of the package 5 based on information supplied by the GPS receiver 9. Alternatively or additionally, the instrument package 5 may include a radio or other beacon allowing the package to be located for retrieval when the survey is complete. In environments where the ice body 4 may melt or break apart, the package 5 may be provided with flotation means to permit recovery.

One or more base stations 40 may be provided for cooperating with the package 5. Such base stations may be provided on the seabed 2 as shown in Figure 1 and/or along a nearby coast and may be used to correlate, normalise or analyse the data jointly for increased accuracy.

Figure 2 illustrates an example of a survey where a plurality of instrument packages is deployed on an ice body 4 in the form of an ice floe or ice field which, at the time of deployment, covers the region 1 to be surveyed. The ice body 4 is expected to, and normally will, drift in a direction indicated by the arrow 20. The instrument packages 5 are deployed in any arrangement which is appropriate to the survey. In the example shown in Figure 2, the sensors 8 of the packages 5 are seismic sensors for measuring seismic waves in a passive survey using ambient or naturally occurring noise as the seismic source. In this example, the sensors are arranged as a substantially horizontal array which may extend in a line or may cover a two-dimensional area according to the needs of the survey.

In the example shown in Figure 2, the sensors 8 of the packages 5 are arranged in a line extending substantially perpendicularly to the expected direction 20 of drift of the ice body 4 relative to the region 1. As an alternative, for example if the movement of the sensors is too fast for convenient measurement, it is possible to deploy more sensors such that each "main sensor" has one or more "brother sensors" which pass the same or nearly the same position at a later time. The main and brother sensors are deployed parallel to the drift direction 20 with a distance between the brother sensors larger than the expected resolution of the applied method. By using these brother sensors, it is possible to increase the data quality (by averaging the measurments of main and brother sensors for nearly the same position) or the resolution. In a further alternative, the sensors are arranged in a circle, for example so as to mitigate the effect of rotation of the ice body 4.

The duration of the survey is selected according to the needs of the survey. Depending on the application, the duration may be days or weeks or months. For example, in the example shown in Figure 2, the duration depends on the speed of drift of the ice body 4, the size of the region 1 and hence the area to be covered by the sensors, and the starting position of the sensors illustrated in Figure 2. The area swept by the line array of sensors is required to cover the region 1 and preferably to cover an area which is larger that the region 1 and which overlaps the region on all of its sides. The survey therefore continues until the line of sensors has passed over and sufficiently beyond the region 1 in the direction 20 for sufficient data about the region to have been compiled.

The instrument package 5 is arranged to make repeated or continuous measurements by the sensor 8 at a series of locations. The controller 6 may take the measurement at regular intervals of time, which may be as short as a fraction of a second. As an alternative, the controller 6 may monitor the position of the package by means of information supplied by the GPS receiver 9 and may take measurements at predetermined positions or with predetermined separations of positions. As another alternative, the base station may instruct the instrument package to take measurements via the radio connection by means of the transmitter/receiver 12. For example, the drift of the ice body 4 may not be wholly predictable and the base station may therefore instruct the controller to take measurements as appropriate according to the path relative to the region 1 which the instrument package 5 actually takes, for example based on location measurement obtained from the receiver 9 by the controller 6 and transmitted by the transmitter/receiver 12 back to the base station. Thus, the instrument package 5 may take measurements autonomously or may be controlled at least partially by external intervention from the base station. The measurements made by each sensor 8 may be relayed substantially immediately to the base station by the controller by means of the transmitter/receiver 12.

Alternatively, measurements made by the sensor 8 may be stored in the memory 7 for subsequent retrieval. For example, the instrument packages 5 may be "polled" or interrogated at suitable intervals and may respond by sending the stored measurements, together with information about the location of the sensor 8 where each measurement was made, to the base station. A "hand-shaking" protocol may be used between the transmitter/receiver 12 of each instrument package 5 and the base station so as to ensure that measurements are not lost.

Alternatively, where it is possible to ensure with reasonable certainty that the instrument package 5 will be retrieved, downloading of the measurements and positions may be delayed until after retrieval. Thus, when the instrument package 5 has been retrieved, the information and position contents of the memory 7 may be downloaded for processing, for example at a land-based processing centre.

By using the natural ice drift to change the positions of the sensors 8 relative to the region 1 to be surveyed, it is possible to use relatively few instrument packages 5 to cover relatively large regions to be surveyed at relatively low cost. This is achieved at the expense of a survey which may be extended in time. However, for many applications, this compromise is acceptable and may well be the best, or even only, compromise available. For example, where there is extensive ice cover and/or in harsh environments, this technique may be the best or only technique available to allow the region 1 to be surveyed.

The sensors may be in the form of broadband seismometers for making measurements based on ambient noise as a seismic source. Such sensors are well-known and will not be described further. In one example, the packages 5 and hence the sensors 8 are arranged so as to permit ambient noise amplitude analysis, which is performed on the data obtained by the survey following completion of the survey.

In another example, the broadband seismic data obtained by the survey are used for ambient noise surface wave tomography. An example of this technique is disclosed in Bensen, G. D., M. H. Ritzwoller, M. P. Barmin, A. L. Levshin, F. Lin, M. P. Moschetti, N.

M. Shapiro, and Y. Yang, Processing seismic ambient noise data to obtain reliable broad-band surface wave dispersion measurments, Geophys. J. Int., 169, 1239 — 1260, doi: 10.11 11/i. 1365-246X.2007.03374.X, 2007.

In yet another example, ambient noise array measurements are obtained, for example as disclosed in Capon, J. (1969). High-resolution frequency-wavenumber spectrum analysis. Proceedings of the IEEE, 57(8), 1408-1419 Wathelet, M. (2005). Array Recordings of Ambient Vibrations: Surface Wave Inversion. PhD. Thesis, University of Liege, Belgium, 177 pp.

However, another technique may be used to provide seismic energy for reflection from the subterranean interfaces of the region 1 to the sensors 8 in the instrument packages 5. In particular, noise generated by ice may be used as the seismic source (or sources), lnterferometry may be used to obtain reflection seismic images from such noise.

In Figure 2, two possible sources of ice noise are illustrated. Another ice body 24 is shown colliding with the ice body 4 and such a collision generates a sufficient amplitude and bandwidth of noise to act as a seismic source, whose position may be accurately determined using the seismic array. Also, cracking of the ice body 24, as illustrated by a fracture line 25, produces a sufficient amplitude and bandwidth of noise to act as a seismic source. Thus, the natural processes occurring with floating ice may be used to provide seismic source energy so that a passive seismic surveying technique which is more like an active seismic technique may be performed without having to deploy and use active seismic sources. This avoids the expense of deploying and using active seismic sources, permits improved seismic surveying in areas where active seismic sources are impossible to use or are not allowed, and permits improved information about the subterranean structure of the region 1 to be obtained.

Figure 3 illustrates another survey arrangement, which differs from the arrangement shown in Figure 2 in that the instrument packages 5 and their sensors 8 are deployed on several floating and drifting ice bodies 4. The arrows indicate the predicted or expected directions of drift of the ice floes 4 and, again, the instrument packages 5 are deployed so as to sweep an area over and extending beyond the region 1 to be surveyed. Although the instrument packages 5 are arranged in a more random and two-dimensional array, they extend in a direction substantially perpendicular to the drift direction 20 of the ice bodies.

In this example, the sensors 8 of the instrument packages 5 are also seismometers or geophones. A passive survey technique is used and Figure 3 shows an ice sheet 30 from which a portion 31 connected by a fracture line 25 is about to split so as to act as a source of seismic energy. However, this may be augmented by active sources and two examples are shown in Figure 3.

A further ice body 34, such as an iceberg or an ice floe, has deployed on it a seismic source 35. The seismic source 35 may be of any suitable type and may comprise a single source or an array of individual sources forming a compound or array source. For example, the source 35 may comprise an airgun array deployed on the ice body 34.

Alternatively or additionally, a towed source, such as airgun array, may also be deployed. Figure 3 illustrates such a source 36 being towed behind a vessel 37. Where it is possible to tow a source behind a vessel, this has an advantage that the seismic source may be actuated in more desirable positions relative to the sensors of the instrument packages 5. Thus, active sources such as 35 and 36 may be used to augment the results of passive survey techniques using ice noise or other ambient (natural or artificial) sources of seismic energy.

The sensors 8 of the instrument packages 5 are not limited to seismic sensors and other types may be used alternatively or additionally. For example, gravimeters or gradiometers may be used to perform measurements based on local variations in gravity so as to provide information about the subterranean structure of the region 1 below the seabed. Gravimeters are sensor devices which measure the magnitude of the local gravitational field whereas gradiometers are sensor devices which measure the local gravitational field gradient. Such devices are well-known and will not be described further.

Alternatively or additionally, electromagnetic sensor devices may be used to obtain information about the structure of the region. Sensors of this type are also well-known and will not be described further. Passive surveying may be based on magnetotellurics so that, again, passive surveying may be performed where it is inconvenient, forbidden, or impossible to perform active electromagnetic surveying.

It thus possible to provide a technique which specifically makes use of the natural movement or drift of ice bodies so as to change the positions of sensors with time to allow measurements to be taken at different locations relative to the region to be surveyed. This contrasts with known techniques for obtaining data on ice bodies where the movement of the ice body is a nuisance and the intention is to obtain measurements at the same location. Making use of the natural ice drift allows a few instruments to cover very large areas at very low cost and makes it possible to explore areas with ice coverage. Thus, structural information for huge areas may be obtained to assist in the identification of potential hydrocarbon reservoirs. For example, in the case of seismic sensors, an S-wave velocity cube of the region to be surveyed may be obtained at low cost and in areas where other techniques would be inconvenient, impossible or forbidden. For example, the technique may be used to record ambient noise on ice floes or other bodies to analyse ambient noise surface waves. For sensors which are sensitive to gravity or gravity gradient, high resolution gravity data of the region may be obtained. For electromagnetic sensors, passive recording based on magnetotellurics may be done on ice floes or the like. Three dimensional information about the subsurface structure and properties may be obtained in very harsh environments and at relatively low cost.

Claims

CLAIMS:

1. A method of surveying a region of the earth, comprising the steps of: deploying at least one sensor, for obtaining data relating to the structure of the region, to move with at least one ice body which moves, or is predicted to move, with respect to the region; and using the at least one sensor to obtain the data at a plurality of different locations, with respect to the region, resulting from movement of the at least one ice body with respect to the region, the at least one sensor being arranged to perform passive surveying.

2. A method as claimed in claim 1 , in which the using step comprises recording the data obtained at the plurality of locations.

3. A method as claimed in claim 1 or 2, in which the at least one ice body is disposed above the region during the deploying step.

4. A method as claimed in claim 3, in which the at least one ice body covers the region during the deploying step.

5. A method as claimed in any one of the preceding claims, in which the at least one ice body is a floating ice body.

6. A method as claimed in claim 5, in which the at least one ice body comprise at least one of an iceberg, an ice floe, an ice field and an ice sheet.

7. A method as claimed in any one of the preceding claims, in which the deploying step comprises deploying a plurality of the sensors as a substantially horizontal array.

8. A method as claimed in any one of the preceding claims, in which the at least one sensor is sensitive to gravity or gravity gradient.

9. A method as claimed in claim 8, in which the at least one sensor is sensitive to magnetic field or magnetic field gradient.

10. A method as claimed in claim 9, in which the at least one sensor comprises at least one magnetotelluric sensor.

1 1. A method as claimed in any one of the preceding claims, in which the at least one sensor comprises at least one electromagnetic sensor.

12. A method as claimed in any one of the preceding claims, in which the at least one sensor comprises at least one seismic sensor.

13. A method as claimed in claim 12, in which the at least one seismic sensor is sensitive to ambient noise.

14. A method as claimed in claim 13, comprising the further step of performing at least one of ambient noise amplitude analysis, ambient noise surface-wave tomography, ambient noise array measurements, HA/ analysis and traveltime tomography.

15. A method as claimed in claim 12, in which the at least one sensor is sensitive to seismic reflections using noise generated by ice as a seismic source.

16. A method as claimed in claim 15, in which the noise is generated by cracking and/or colliding of further ice bodies.

17. A method as claimed in claim 15 or 16, comprising the further step of performing interferometry to obtain reflection seismic data.

18. A method as claimed in any one of the preceding claims, in which the at least one sensor provides, or is associated with means for providing, an indication of the position of each of the locations and/or an indication of sensor orientation at each of the locations, and/or an indication of the sensor tilt at each of the locations.

19. A method as claimed in any one of the preceding claims, in which the data are transmitted from the at least one sensor and/or stored at the at least one sensor for subsequent retrieval.

20. A method as claimed in any one of the preceding claims, comprising the further steps of deploying a base station on the seabed or on a coast and performing at least partial processing of the data in the base station.

21. A method as claimed in claim 20, in which the processing step comprises at least one of correlation, normalisation and analysis of the data.

22. A method of surveying a region of the earth, comprising the steps of: deploying at least one seismic reflection sensor for receiving seismic reflections from the region; and using the at least one sensor to obtain seismic reflection data using noise generated by ice as a seismic source.

23. A method as claimed in claim 22, in which the noise is generated by cracking and/or colliding of ice bodies.

24. A method as claimed in claim 23, in which the ice bodies are floating ice bodies.

25. A method as claimed in any one of claims 22 to 24, comprising the further step of performing interferometry to obtain the seismic reflection data.

26. An apparatus for performing a method as claimed in any one of the preceding claims.

27. Use of the movement of an ice body, on which at least one sensor arranged to perform passive surveying for obtaining data relating to the structure of a region of the earth to be surveyed is deployed, to obtain data at a plurality of different locations with respect to the region.