WO2015082010A1

WO2015082010A1 - Geophysical data acquisition systems

Info

Publication number: WO2015082010A1
Application number: PCT/EP2013/075716
Authority: WO
Inventors: Paul Meldahl
Original assignee: Statoil Petroleum As
Priority date: 2013-12-05
Filing date: 2013-12-05
Publication date: 2015-06-11

Abstract

A geophysical data acquisition system comprises: at least one submarine vehicle carrying or towing a seismic source; at least one autonomous underwater vehicle, AUV; and at least one camera, carried or towed by said AUV, for taking a plurality of images of the ocean bottom to measure the response of the earth to said seismic source.

Description

Geophysical data acquisition systems

FIELD OF THE INVENTION The invention relates to a system and method for acquiring geophysical data. BACKGROUND OF THE INVENTION

It is a requirement for seismic 4C acquisition and some EM acquisition systems that the sources, receivers and navigation-system need to interact with the ocean surface and bottom during acquisition of data. This interaction affects the operation downtime and also prevents 4C and EM data acquisition from being integrated and synchronised.

The following links describe some of the existing technology.

PGS towed EM

http://www.pgs.com/en/Geophysical-Services/Towed-Streamer-EM/ Implosive Source

http://www.mpl.ucsd.edu/people/ldorman/abst/GPY000Q19.pdf Reflection Seismic

http://en.wikipedia.org/wiki/Reflection_seismology EM geophysics

http://en.wikipedia.org/wiki/Electromagnetism

The existing technology is subject to a number of drawbacks, which are discussed below.

4C and EM data:

Generally 4C and EM data should be combined since the rock parameter estimation from multi component seismic data will increase quality of EM interpretations and visa versa. The seismic waves consist of two compressional waves, known as P-wave, which move fastest and can move in water; and b) shear waves, known as S-waves, which move slower, they move in the rock skeleton and are independent of water content, and do not move in water.

4C seismic data are normally not acquired with proper spatial sampling density for analysis of S-waves and without the intention to use both P and S waves 4C data are rarely acquired due to the high cost related to the complex and expensive 4C sensor logistics.

Existing 4C and EM acquisition cannot be very well integrated and synchronized because of a mismatch in preferred tow depths of EM sources (0-100m above ocean bottom) and Seismic sources (5-10m below sea surface.)

4C sensors are normally coupled to the ocean bottom while the contactless EM- sensors can move while recording.

Navigation:

The navigation network for geophysical source and receiver systems are normally linked to the source and receiver vessel and further to a GPS system. These navigation systems have limited working range under ice. In this document AUV (Autonomous Underwater Vehicle) and submarine related navigation systems are introduced.

Seismic sources:

The most efficient geophysical sources are towed by a surface vessel. The EM sources have to be deep towed while the conventional seismic sources have to be towed shallow. As a consequence EM and seismic sources cannot easily be transported by the same vessel.

Since the towing speed of deep tow source systems are restricted, a deep towed EM source and a shallow towed Seismic sources cannot readily be synchronized in an integrated operation. Further surface towing is not possible in the case of surface ice. The commonly used seismic sources (air gun arrays) cannot be towed as deep as the EM sources because they lack efficiency in a high pressure environment.

On the other hand the efficiency on implosive seismic sources increases with increasing pressure. See the following web link:

http://www.mpl.ucsd.edu/people/ldorman/abst/GPY000Q19.pdf 4C sensors:

Conventional 4C sensors which measure S-waves are commonly placed on the ocean bottom while recording. As a consequence the sensors become part of the sub-surface and therefore influence the seismic wave field they are supposed to record. Further the logistics of handling sensors on the ocean bottom become quite complicated and expensive even for robotized systems.

Seismic vessels: Seismic vessels are specialized to store, operate and handle huge seismic systems and to transport sensors and receivers around the survey area. Vessel cost is quite large. In addition the cost of transportation of these vessels from site to site around the world is quite high. In the case of containerised and robotic geophysical acquisition systems, the vessel requirements could allow the use of a vessel of opportunity for a "mother vessel" (=supply vessel for the geophysical operation). A vessel of opportunity is a vessel which is already available in a particular location, for example a vessel which is used for transporting general purpose metal containers for transporting goods. Instead of using a specialised seismic vessel of the type normally used for a seismic operation, a vessel which is already available close to the seismic survey site could be used as a supply vessel to the "contactless geophysical operation. The supply vessel could be used to transport containers with geophysical equipment including AUVs and submarines and tools, and for releasing equipment into the sea and taking equipment out of the sea. SUMMARY OF THE INVENTION

The invention provides a geophysical data acquisition system and method as set out in the accompanying claims.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 shows a system which includes a surface vessel, two submarine vehicles and a number of autonomous underwater vehicles, AUVs.

DESCRIPTION OF PREFERRED EMBODIMENTS

We have described, for example in US Patent Nos 7,660,188; 7,583,387; 7,933,003; 8,498,176; and 8,400,871 ; and US Patent Application No 12/920273, a system for analysing the response to a seismic source by using one or more cameras to record the movement of particles on the sea bottom in response to returning P- and S- waves. We refer to this system as "Temprei" in this document. In the Temprei system, each camera includes at least one interferometer, and may also include one or more vibration sensors. We refer to such a camera as a Temprei camera, or Temprei sensor. During operation of the Temprei system vibrations of the ocean bottom are induced as a result of seismic source, and the interferometer measures vibration of the ocean bottom relative to the camera. The vibration sensor measures vibrations of the Temprei camera itself, so that these vibrations can be eliminated from the measurements made by the interferometer.

Figure 1 shows a surface vessel 2 on the surface 4 of the ocean, together with two underwater (ie submarine) vehicles 6, and a plurality of autonomous underwater vehicles, AUVs, 8. Each underwater vehicle 6 may carry a seismic source 9, and optionally also an electromagnetic (EM) source 1 1 . Each AUV 8 carries a Temprei camera 12 and may optionally also carry an EM receiver 14. The Temprei camera 12 and/or the EM receiver 14 may also be towed by their respective AUV 8.

The surface vessel 2, underwater vehicles 6 and AUVs 8 may all navigate using any suitable means. The surface vessel (mother vessel) 2 may, for example, be provided with a satellite navigation system, and may have a navigation link to the underwater vehicles 6. The underwater vehicles may navigate, for example, by knowing the relative positions of the surface vessel 2 and/or the AUVs 8.

Each of the underwater vehicles 6 may be provided with any or all of the following features: a) a seismic source 9;

b) an electromagnetic (EM) source 1 1 ;

c) a navigation link to the surface vessel 2; and

d) a navigation link to the AUVs and submarine(s)8

Each of the AUVs 8 may be provided with any or all of the following: a) one or more Temprei cameras 12, which may be used for seismic multicomponent measurements;

b) one or more EM receivers; and

c) a navigation link to one or more of the underwater vehicles 6. The navigation links may for example be acoustic links.

The AUVs 8, and hence also the Temprei cameras 12, may move at a velocity of, for example, 1 to 3 m/s relative to the ocean bottom 10. The AUVs 8 and Temprei cameras 12 also operate close to the ocean bottom 10, for example within a range of 1.5 to 5 metres from the ocean bottom 10. Preferably the AUVs 8 and Temprei cameras 12 operate no more than 30 metres from the ocean bottom 10.

As noted above, conventional 4C sensors must be used on the ocean bottom 10, which is costly and also influences the seismic wave field at the ocean bottom 10. In contrast, the Temprei multicomponent seismic sensors 12 are contactless and can therefore be transported say 1 -5 m above the ocean bottom 10.

By using a Temprei camera as a 4C seismic sensor we provide a contactless geophysical system with integrated measurement of 4C (seismic) and EM (electromagnetic) data. The system of Figure 1 uses integrated and synchronized seismic and EM sources transported by a common submarine 6, and receivers transported by a swarm of AUVs 8. The system can be used without contact with the ocean surface 4 or bottom 10, which increases operation and acquisition efficiency and also allows seismic imaging under ice or during bad weather when the ocean is rough. The system described herein is therefore "contactless", in that it can operate without contact with either the ocean surface 4 or the ocean bottom 10. In particular, there is no need for either the underwater vehicles 6 or the AUVs 8 to be in contact with the surface vessel 2 at all times.

The combination of 4C and EM systems allows the detection of earth anomalies which could not even be seen by a far more expensive 3D seismic grid.

The system may use an acoustic link for data transfer between the AUVs 8 and the or each submarine 6. Such data could for example be used for quality control data/instructions and/or navigation purposes. Loss of full integration due to different EM and 4C source offset requirements can be compensated by adding extra seismic sources to the system.

It is also possible to position one or more AUVs 8 between the ocean bottom 10 and the submarine(s) 6 in order to support an extended communication link, which may for example use light and/or acoustics (which may be high frequency, HF) to transmit data.

Seismic and EM sources can be towed by a surface vessel, or may be operated from one or more of the submarines 6 which sail at some distance from the ocean bottom 10. Preferably an implosive seismic source is used. The distance influences EM signal penetration and the strength of any seismic implosive source which may be used.

Each submarine 6 has to recharge power for the operation of the sources and for its own propulsion after a certain operation time. The sources, both seismic 9 and electromagnetic 1 1 , may be either carried by a submarine 6 directly, or towed on a streamer behind a submarine 6. Therefore all references in this specification to a submarine "carrying" a source should be construed to mean either directly carrying the source or towing the source.

The or each seismic source 9 may be operated less than 200 metres above the ocean bottom, or preferably less than 100 metres above the ocean bottom. The seismic sources are preferably operated at least 5 metres from the ocean bottom. The seismic and EM sources 9 and 1 1 may for example be operated for example between 20 and 200 metres above the ocean bottom 10. Preferably the seismic and EM sources 9 and 1 1 are operated a maximum of 100 metres above the ocean bottom 10. The or each Temprei camera 12 is preferably operated less than 30 metres from the ocean bottom. The or each Temprei camera 12 is also preferably operated at least 1 metre from the ocean bottom.

The Temprei 4C sensors (ie the Temprei cameras) may for example be transported say 1 -5 m above the ocean bottom by AUVs. To allow the use of EM antennas each AUV 8 may be provided with wings 16 which extend outwardly from the body 18 of the AUV 8, like the wings of an airplane, thus allowing the wings 16 to accommodate an EM antenna. The AUVs may also have two bodies connected together like a catamaran, thus allowing an EM antenna to be positioned between the two bodies.

There may be one or more of the surface vessels 2, each of which acts as a support vessel, mother vessel and/or supply vessel, which may carry containers for source systems, AUVs 8, and an on-board handling system and docking system for the submarine(s) 6 and the AUVs 8. Ideally the or each support vessel could be a vessel of opportunity. The surface vessel 2 may be adapted to allow recharging of the AUVs 8 and/or the submarines 6. A docking station may also be provided in the ocean to allow such recharging.

Embodiments of the invention provide a new contactless geophysical data acquisition system which can be used for geophysical characterization of the surface and/or sub- surface and added structures within fields such as geohazard, exploration and pipeline inspections. The geophysical data acquisition system may include both moving autonomous source(s) and receivers disconnected from the ocean bottom and ocean surface. As a consequence the system can operate in bad weather conditions and under ice. The system may contain contactless multicomponent sensors and a seismic source or sources moving relatively close to the ocean bottom while acquiring seismic data. EM sources and receivers can be added to the multicomponent system and the operation as well.

Multicomponent seismic ("4C") and EM (electromagnetic) data acquisition can be carried out in several ways. Multicomponent seismic acquisition records one to three components of the ocean bottom particle velocity introduced by seismic waves.

Claims

CLAIMS:

1. A geophysical data acquisition system comprising: at least one submarine vehicle carrying or towing a seismic source; at least one autonomous underwater vehicle, AUV; and at least one camera, carried or towed by said AUV, for taking a plurality of images of the ocean bottom to measure the response of the earth to said seismic source.

2. A system as claimed in claim 1 , wherein said seismic source is an implosive seismic source.

3. A system as claimed in claim 1 or 2, wherein said AUV also carries an electromagnetic receiver for receiving electromagnetic waves from an electromagnetic source.

4. A system as claimed in any preceding claim, wherein said submarine vehicle also carries an electromagnetic source.

5. A system as claimed in any preceding claim, wherein said camera includes at least one interferometer.

6. A system as claimed in claim 5, wherein said interferometer directs light to the ocean bottom and measures said light after reflection or diffraction from the ocean bottom.

7. A system as claimed in any preceding claim, wherein said camera includes at least one vibration sensor adapted to measure vibration of said camera, and comprises means for reducing the effects of said vibration on measurements made by said camera.

8. A system as claimed in any preceding claim, where said camera is a Temprei sensor.

9. A system as claimed in any preceding claim, which further comprises a surface vessel with a navigation link to said at least one submarine vehicle.

10. A system as claimed in any preceding claim, which further comprises a navigation link between said at least one submarine vehicle and said at least one AUV.

1 1. A system a claimed in any preceding claim, wherein said submarine is an autonomous submarine or autonomous underwater vehicle.

12. A method of using a system as claimed in any preceding claim, said method including:

operating said at least one submarine and said at least one AUV under the ocean;

activating said seismic source; and

using said camera to measure the response of the ocean bottom to said seismic source;

wherein, during activation of said seismic source, said at least one submarine is operated less than 200 metres from the ocean bottom.

13. A method as claimed in claim 12, wherein, during activation of said seismic source, said at least one submarine is operated less than 100 metres from the ocean bottom.

14. A method as claimed in claim 12 or 13, wherein, during activation of said seismic source, said at least one submarine is operated more than 5 metres from the ocean bottom.

15. A method as claimed in any one of claims 12 to 14, wherein said camera is operated less than 30 metres from the ocean bottom.

16. A method as claimed in any one of claims 12 to 15, wherein said camera is operated more than 1 metre from the ocean bottom.