Introduction

The invention is within the technical field of seismic prospecting, and more specifically relates to a method and system for seismic surveying which combines VSP, in particular HybridVSP with towed seismic survey.

Background

There are several different sensor systems for seismic applications. Examples of such are bore hole seismic systems using cables with seismic sensors integrated in the cable or cables where the cable themselves are continuous sensors. The last mentioned can be provided by fiber optic cables providing distributed acoustic sensing (DAS) based on Rayleigh scattering for providing distributed strain sensing.

There is also a combination of the two different mentioned sensor systems described in WO 2014/199300 A2 (Kjos). The lower part of a seismic cable comprising seismic sensors and an upper part comprising a fiber optic cable which itself is a sensor and which also transfer signals from the sensors in the lower part. The distance between the sensors and the number of sensors comprised in these cables are fixed and cannot be altered when such cables are provided for a specific seismic survey.

HybridVSP is an operation described in patent application WO 2016/202373. Dense seismic with short cables is illuminates reservoirs in difficult fields as described as a method to obtain accurate seismic. Due to the short cables it is easy to maneuver, and such an operation is well suited for circle or elliptical surveying. HybridVSP is particularly well suited to illuminate reservoirs below salt formations where the salt formation obstructs imaging due to density and poor signals.

HybridVSP uses a combination of DAS and 3C signals. The DAS signals are collected in the fibre optic cable and can be collected with signal distance as low as 25 cm. This will result in many signals, which provides good and accurate imaging of the reservoir. DAS signals are generally weak signal, but it has been shown that it is possible to process the signals, including noise extraction. A weak complete signal provides the exact same information as a strong signal. Processing is therefore an important issue to make the DAS marine system, near bottom system or subsea system cable work.

The distance between two 1C acquisitions in traditional cables is about 3m for dense acquisitions. With similar dense shooting, this results in good and accurate acquisition. Combined with HybridVSP acquisition, this provides very good data that can provide accurate reservoir data in places where the surface seismic methods cannot be used.

The acquisition path may comprise a circle or ellipse shaped shot line. The advantage of a circle or ellipse is that you avoid lost time for turn and avoids problems related to getting back into the line. Such paths require that the cables are short or that the shooting takes place without cables.

The applicant has found that by using short seismic cables being towed by the firing vessel, the shoot pattern can be a circle or ellipse with a distance down to 100 m between each line of the circle/ellipse. This firing pattern is then the same as on a HybridVSP and both of the surveys can be combined in the same survey. This provides great cost savings. The number of signals increases accordingly and the accuracy and quality of the seismic increases accordingly.

In this description hybridVSP is used as example, but it should be noted that also other kinds of VSP operations may be used as alternative or in combination with hybridVSP.

The object of the invention is to provide a method and a system for seismic surveying that increases effectivity, provides improved resolution and decreases surveying costs.

The object of the inventio is achieved by means of the features of the patent claims.

In one embodiment, a method for seismic surveying comprises performing a towed seismic survey by towing seismic cables and seismic sources from a vessel, performing a Vertical Seismic Profiling (VSP) simultaneously to the towed seismic survey and using the same seismic sources.

The towed seismic survey can be a surface survey, a near bottom seismic survey or a seabed seismic survey.

In one embodiment, the seismic cables are DAS cables. The DAS cables may comprise at least two fibre optic cables arranged in parallel and connected at their ends. In a fibre optic DAS cable, the can be recorded in the cable with a distance of 25 cm.

In one embodiment, the DAS cable comprises a main cable and at least one sub cable, where the sub cables are arranged substantially perpendicular to the main cable. The angle between the sub cable and the main cable may be substantially perpendicular to the each other. In one embodiment is the angle between the sub cable and the main cable such that it is ensured that the angle between the incident seismic signal and the sub cable is less than 90 degrees, and in some embodiments less than 80 degrees. In order to achieve the desired angle, the sub cable may comprise weights and/or control fins.

There is also provided a system for seismic surveying.

The invention will now be described by means of examples and by reference to the accompanying figures.

Figure 1 illustrates parts of prior art seismic acquisition operations

Figure 2 illustrates a traditional cable being towed.

Figure 3 illustrates a DAS cable being towed near surface

Figure 4 illustrates a DAS cable being towed near bottom

Figure 5 illustrates a DAS cable used at the seabed

Figure 1 illustrates examples of a vessel and DAS cable in a well of a typical HybridVSP acquisition. The acquisition path may comprise a circle or ellipse shaped shot line.

Figure la shows the device on a vessel 10. Sources 11 are towed by the vessel 10, and the distance between the sources are determined by the spreaders 13. Figure lb shows the arrangement of HybridVSP in a well 16, using a DAS cable 15.

In figure lb and the following figures, the arrows represent the path of the acoustic signals from the acoustic sources, being reflected and received by the various seismic sensors.

Figure 2 illustrates a traditional prior art cable 20 which can be used in the method and system according to the invention. This cable is being towed at a depth of 3 to 5 m below the sea surface 21. The cable 20 is for example 100 m long. An air gun/source 24 is arranged at or near the surface 21 by means of a float 23. The air gun/seismic source 24 emits an acoustic signal which are reflected from the sea bed and/or the ground below the seabed. At the cable 20 there are arranged seismic sensors 22, making the acquisitions when the acoustic signal is reflected towards the surface. The seismic acquisitions are recorded as seismic raw data for later processing.

According to the invention, the traditional cable 20 can be used during a HybridVSP survey, by towing cables together with the shooting for the HybridVSP. In this way, two different surveys are performed simultaneously, providing significantly increased data acquisition and better resolution in the seismic images resulting from the surveys, without the costs for one of the surveys increasing proportionally. The innovative idea is therefore to run two surveying simultaneously. Therefore, with HybridVSP and dense cables, quality, number of signals, reservoir illumination and accuracy can be increased. As statistical methods in seismic processing are important, the number of signals is an important parameter. By using the described system, the number of signals may increase by a factor 20 compared to the typical amount of data used in a standard VSP processing. The operational costs can thus be reduced to substantially, possibly be halved.

Figure 3 illustrates an example of a new type of DAS cable 30 that only comprises fibre optic cables used in surface seismic surveys. These are two continuous, fibre optic cables 37,38 connected at their ends to achieve the double length of cable for receiving independent signals. This leads to a substantially improved amount of data that can be recorded by the cable. Since DAS signals require that the incoming, reflected signals 39 have an incident angle different from perpendicular to the cable, the cables in the surface cable have sub-cables 35 that are arranged substantially vertically to the direction of travel. The sub cables can for example be 5m long, but can in other embodiments have lengths between 3-5m. The angle a must be less than 80 degrees. The signals come along the vertical cables and can be captured as DAS signals, for examples every 25 cm along the cable. For a vertical cable at 5m you will be able to capture 20 levels per vertical cable. Assuming that the 5 m cable is oriented with an angle, a cable length of 3m main cable will have 20 levels per section or 20 times the amount of data signal compared to traditional collection.

In one embodiment, the vertical cable is equipped with weights and/or control fins so that a stable geometric shape is established. The cable can also comprise positioning instruments so that accurate coordinates can be provided for each shot. The main cable is equipped with spreaders so that the sources have a required distance, for example 50 m, to each side and can be used for flip/flop shooting.

Figure 4 shows a similar cable 30 as in figure 3, used in a near bottom seismic survey. The cable 30 is in this embodiment towed at a distance of 3-20 meters above the seabed 46. This system may be operated as a close to the bottom system, i.e. it can be towed with 3 to 5 knots, capturing 3C directional signals as well as S signals. This provides significant cost savings compared to traditional subsea seismic.

The DAS cable described in figure 3 and 4 may also be used at the seabed as illustrated in figure 5. The main cable 30 is arranged in the sand/seabed 46. In this embodiment, the vessel may only tow the seismic source. However, there are still performed two simultaneous surveys, a VSP and the seabed seismic survey. The cable 30 may also comprise 3C satellites. The vertical part 35 comprise or is connected to buoyancy elements, so that the vertical part 35 can be extended to for example 100m. With DAS signals this can give 400 signals per vertical cable. If a vertical cable 35 is arranged at the main cable 30 with a distance of 20m, this can give 2000 signals per 100 m. These signals will also comprise directional information, and will be able to replace more than 500 3C satellites on the seabed or 3 3C satellites per m. The seabed cable, even if it is horizontal, can receive signals from shots far from the cable, ie. the angle a of the incident signal is less than 80 degrees.