WO2019224354A1

WO2019224354A1 - Method for establishing position and timing of seismic recording devices that are deployed on a seafloor

Info

Publication number: WO2019224354A1
Application number: PCT/EP2019/063466
Authority: WO
Inventors: Jeroen Hoogeveen
Original assignee: Individual
Current assignee: Individual
Priority date: 2018-05-24
Filing date: 2019-05-24
Publication date: 2019-11-28
Anticipated expiration: 2020-11-24

Abstract

A seismic method comprises steps of deploying multiple immersible seismic recording devices on a seafloor, each seismic recording device containing sensors for registering at least one of motion, acceleration, pressure, a digital clock, electronics to record signal data provided by the sensors, a memory, a data handling system, and a battery or other power source. In the method, one or more passes are performed across the deployed seismic recording devices with one or more seismic sources that fire regularly during a period of time, wherein a source signature is varied or modulated from shot to shot. Further in the method, seismic data are recorded by the seismic recording devices during the period of time, and the seismic recording devices are retrieved. Next, data recorded by the seismic recording devices are transferred to a computer system. In a subsequent analysis the recorded data are processed by the computer system. This processing includes preparing seismic data from an individual seismic recording device so as to obtain a sequence of first break arrival times, identifying individual shots of the seismic sources, enabled by the varied or modulated source signatures, and establishing a position and local timing of the seismic recording device by solving a system of equations with known source locations, known absolute source firing times and the obtained relative first break arrival times.

Description

METHOD FOR ESTABLISHING POSITION AND TIMING OF SEISMIC

RECORDING DEVICES THAT ARE DEPLOYED ON A SEAFLOOR

TECHNICAL FIELD

The invention relates to a seismic method, and in particular to a method for establishing position and timing of seismic recording devices that are deployed on a seafloor and exposed to seismic sources that are fired during a period of time.

BACKGROUND

Methods for acquiring geophysical data in water covered areas are well known in the art. Such geophysical data is used for academic and commercial investigations into the earth’s geological structures. The experimental method used is often referred to as a seismic method, and is widely used in the oil and gas exploration industry and also for academic studies.

A seismic method is a generic experimental procedure which aims to record the earth’s response to excitation by a seismic source. The earth’s response consists of a complicated series of acoustic waves and shear waves which contain information about the earth’s sub-surface. The seismic excitation may be a generated by a seismic source such as an airgun array at the sea surface, but it may alternatively be generated by a vibrator at the sea-floor, or a source in a borehole, or a natural source such as an earthquake or any other method. Generally, the seismic excitation will produce both acoustic and shear waves, which may collectively be termed seismic waves or seismic energy. Water has a shear modulus of zero and therefore seismic excitation in the water column will only produce acoustic waves. From the point of excitation acoustic and shear waves propagate in patterns dictated by physical properties of the materials that the waves travel through. As waves travel into the earth, they are reflected, refracted, transmitted, attenuated and mode- converted according to the laws of physics and the physical properties of the rock types encountered. Some of the acoustic and shear wave energy after an excitation event can be recorded at the seafloor by sensitive, seismic recording devices. Analysis of the recorded data provides information about the physical properties of the materials through which the seismic energy propagated. The acoustic waves may be recorded by pressure sensors, but may also be recorded by particle motion sensors or accelerometers. Shear waves may be recorded by particle motion sensors or accelerometers. As shear waves cannot propagate through water, it is important that the sensors are located at the sea-floor and that they are coupled such that shear wave energy will propagate into the sensor unit.

For seismic recordings made by the recording devices to be of use, the locations of the recording devices are required. Also, the timing of the recordings relative to an external reference time is required.

Hence, there is a general need for a method to establish the position and timing of seismic recording devices that are deployed on a seafloor and exposed to seismic sources that are fired during a period of time.

US 8,995.222 B2 discloses a system and method of seismic exploration that produces improved locations and timings for ocean bottom seismometers. The method utilizes linearized inversion in conjunction with a conventionally accurate clock to provide both time and positioning for each ocean bottom seismometer unit with high accuracy. Inversion is one mathematical tool that effectively performs the requisite triangulation. Furthermore, the clock drift can be accounted for in the inversion scheme. The inversion determines the ocean bottom seismometer's position and shot timing errors and estimates the accuracy of the position and timing determination. In this method and system, it is necessary to determine a length of time since a clock associated with a selected receiver was last

synchronized.

SUMMARY

A general object of the present invention is to provide an improved method for establishing position and timing of seismic recording devices that are deployed on a seafloor and exposed to seismic sources that are fired during a period of time.

A particular object of the invention is to provide an improved method for establishing position and timing of seismic recording devices that are deployed on a seafloor and exposed to seismic sources that are fired during a period of time which overcomes disadvantages of the related art. For instance, a possible object of the invention is to provide such a method that does not rely on the determining of a length of time since a clock associated with a selected recording device or receiver was last synchronized.

The invention has been defined by the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS Features and advantages of the invention will be explained in closer detail in the detailed description below, with reference to the non-limiting examples illustrated in the figures. Figure 1 is a schematic flow chart illustrating a method according to the invention,

Figure 2 is a graph illustrating an example of observed first break arrival times at a recorder, Figure 3 is a graph illustrating an example of locus plot (2D view) given by plotting unadjusted radii derived from first break data, and

Figure 4 is a graph illustrating an example of locus plot (2D view) result after solving equations.

DETAILED DESCRIPTION

The invention will be described in the following as non-limiting examples, which illustrate principles of the invention as claimed. Figure 1 is a schematic flow chart illustrating a seismic method according to the invention.

The illustrated method is a method for establishing position and timing of seismic recording devices that are deployed on a seafloor and exposed to seismic sources that are fired during a period of time.

The illustrated method is initiated at the initiating step 100.

First, deploying step 110 is performed. The deploying step 1 10 includes deploying multiple immersible seismic recording devices on a seafloor.

This may be done by deploying individual recording devices on a rope attached to a buoy for later retrieval, possibly with the aid of an anchor or weight. Alternatively, the recording devices may be attached to short spur lines from a main deployment line and deployed in series. Still alternatively, the recording devices may be deployed by divers or with the aid of submersible vehicle(s).

Each seismic recording device contains sensors for registering at least one of motion, acceleration and pressure.

Hence, in an example, the seismic recording device may include a sensor for registering motion, in another example the seismic recording device may include a sensor for registering acceleration, and in still another example, the seismic recording device may include a sensor for registering pressure. Alternatively, the seismic recording device may include sensors for registering both motion and acceleration, or motion and pressure, or acceleration and pressure, or motion, acceleration and pressure.

By example, the seismic recording device may include a hydrophone sensitive in the bandwidth of 0-500Hz and geophones mounted in a gimballed fashion to record motion in the same frequency range.

Each seismic recording device further contains a digital clock and electronics to record signal data provided by the sensors. Each seismic recording device further includes a memory, a data handling system, and a battery or other power source.

The data handling system included in the seismic recording device may include a processing device such as a microprocessor or a microcontroller. The processing device, the memory and the electronics to record signal data provided by the sensors may be communicatively interconnected by a digital bus.

The battery or other power source included in the seismic recording device may be interconnected with all power-demanding elements of the seismic recording device.

In an advantageous aspect, the battery may be chargeable, and each seismic recording device may advantageously include a wireless charging device. The wireless charging device may be arranged to charge the battery in the seismic recording device inductively.

In an advantageous aspect, each seismic recording device used in the method may contain additional sensors, which may include at least one of a motion sensor, an acceleration sensor, an inclinometer and a compass.

In another advantageous aspect, each seismic recording device may include a wireless communication device.

The wireless communication device may be used to transfer data from the seismic recording device to the computer system that is arranged to process/ analyse the recorded data.

In an advantageous aspect, each seismic recording device is fluidly sealed to ensure that no water penetrates the seismic recording device. This may be obtained by means of a watertight housing which encloses the seismic recording device.

In an advantageous aspect, each seismic recording device is a standalone recording device. In this case, the multiple seismic recording devices are non-interconnected, i.e., they may operate independently without any communication between them. Alternatively, the seismic recording devices may be provided with means for communication between them. In an aspect, a plurality of seismic recording devices may be tethered to a string before they are deployed on the seafloor.

Returning now to the method as illustrated in figure 1 , the method further includes the performance of the firing step 120. In the firing step 120, one or more passes are performed across the deployed seismic recording devices with one or more seismic sources that fire during a period of time. In the firing step 120, a source signature is varied or modulated from shot to shot, i.e., from one shot to another.

The seismic source may comprise a single airgun with known position and output or an array of airguns with known geometry and output. The seismic source could be any device which generates a measurable controlled seismic disturbance. The timing of each source event may be registered for later use.

A‘shot’ or source event may refer to the firing of the single gun or the collective firing of the airguns within an array within a set period.

To enable individual shots to be recognized and identified in post processing, the firing time between shots may be varied so as to create an identifiable pattern. The variation in time may be random, pseudo random or pre-determined, as long as it is registered for later use. Alternatively, within an array, the sequence of firing within one shot may be similarly varied to create an identifiable output. Such variation within an array will modify the pulse shape which can make it identifiable.

Alternatively, the above variations may be in firing pressure instead of firing time, or a combination of firing pressure and firing time.

Further in the method, recording step 130 is performed. In the recording step 130, seismic data are reveiced and recorded by the seismic recording devices during the above-mentioned period of time, i.e., the period of time in which the seismic sources are fired. Hence, the firing step 120 and the recording step 130 may be performed concurrently, or simultaneously, or at least with an overlapping period of time.

The recording step may include, in each recording device, reading or inputting data from the recording device's sensors for registering motion, acceleration and/or pressure, into the recording device's memory. This may be accomplished by means of the electronic circuitry and the data handling system which is provided in each recording device.

Further, the method includes a retrieving step 140, which includes retrieving the seismic recording devices from the seafloor.

Retrieval or collection of the recording devices from the seafloor may be done by locating the buoy attached to the single recording device or the string of recording devices. The buoy may be recovered by the survey vessel, and the rope deployment rope transferred to a reel or capstan for further recovery. Upon completion of the retrieving step 140, i.e. when the retrieval of the seismic recording devices has been accomplished, the data transfer step 150 is performed. In the data transfer step 150 data recorded by the seismic recording devices are transferred to a computer system.

The transfer step 150 may advantageously make use of the wireless communication device included in each seismic recording devices, which may interoperate with a wireless communication device included in the computer system. Such wireless communication between the seismic recording devices and the computer system may be obtained by the use of any suitable wireless communication protocol, advantageously for local or limited range communication, e.g. WLAN, Bluetooth, Zigbee, etc. In alternative, possible aspects, the transfer step 150 may be performed by the use of wired or optical data communication.

After completion of the data transfer step 150, a data analysis process 160 is performed. In the analysis process, the recorded data obtained from the seismic recording devices are processed by the computer system. This processing includes the following processing steps:

First in the analysis process 160, the obtaining step 170 is performed. The obtaining step 170 includes preparing seismic data originating from an individual seismic recording device so as to obtain a sequence of first break arrival times.

Within the art of seismic processing the process step of first break picking, i.e., of obtaining a sequence of first break arrival times, is well known per se. This process step may include establishing a background signal level by calculating signal statistics and identifying the onset time of new signal train using signal processing techniques.

Next in the analysis process 160, the identifying step 180 is performed. In the identifying step 180, individual shots of the seismic sources are identified, enabled by the varied or modulated source signatures.

In an alternative aspect, the individual shots may be identified by using a proximity criterion, which will be explained separately.

In an aspect of the disclosed method, in particular in the firing step 120, the varying or modulation of the source signature includes modulation of the firing time of the seismic sources. Individual shots may be identified by correlating the identified first break arrival times and/or associated pulse amplitudes with the shot firing pattern as logged. Alternatively, individual shots may be identified by matching the pulse shape.

In an aspect of the disclosed method, in particular in the firing step 120, the seismic sources used in the method include a primary or main seismic source and at least one secondary seismic source. In particular, a secondary seismic source may be fired in the method, and the source signature of this secondary source may be varied or modulated from shot to shot.

In an alternative application of the method, in particular in the firing step 120, more than one seismic source may be used, whereby one or both sources are modulated and/or varied. Specifically, a smaller source, possibly a single airgun or pulse source, may be used in addition to the normal seismic source(s) for the specific purpose of establishing positioning and timing of the receivers. Recording of multiple (semi) simultaneous shots from different sources may make identifying first break arrival times of individual shots difficult, but the art of de-blending shot records is well established in the field of seismic processing.

In the identifying step 180, in order to identify individual shots on the receiver data, every individual shot should be made identifiable on the receiver data. This may be done by varying the timing between shots, as this would be observed as a very similar variation in first break arrival times in the receiver data, allowing correlation between source fire times and observed first break arrival times to be establish a direct relationship between individual shot and specific first break arrival time. Variations in pulse shape and amplitude may similarly be used.

Further in the identifying step 180, individual shots may be identified on the receiver recording by a proximity criteria which entails observing the pattern in first break arrival times and using the approximate receiver location as a-priori knowledge. The approximate receiver location may be derived by dead-reckoning on deployment. In this approach, the signal as recorded by the receiver in the recording step 130 may be split into a sequence of receiver records which are equal in length to the shot interval. Such splitting of the recording requires a choice of the time at which to start the sequence of receiver records. The time at which to start the splitting into receiver records is normally chosen such that the first break arrival times in the receiver records arrive at approximately the expected travel time when taking the approximate receiver location and the known source location into account. When first break arrival times from such‘split’ receiver records, as obtained in the obtaining step 170 and split as per the method described above, are plotted for successive receiver records split by the above described method, a pattern may be observed which is related to the travel time of the signal from the shot to the receiver. In simple terms, the shortest travel time must correspond to the source event closest to the receiver. This observation then allows one to associate this source event with a specific recorded first break on a receiver record. And hence earlier and later source events may be associated with earlier and later receiver records.

In the identifying step 180, a one-on-one relationship between source events and recorded first breaks was established. This allows one to split the recorded signal into individual receiver records, each associated with a source event as identified by step 180. The time at which to start the splitting into receiver records is normally chosen such that the first break arrival times in the receiver records arrive at approximately the expected travel time. As the shots intervals (the time between individual source events) are known and correlated to the receiver recording, one may split the receiver recording according to the associated shot interval. Each receiver record now starts at zero and has its first break at tij , whereby i stands for the receiver number and j for the identified associated source event number. As the choice of time to start the split sequence is approximate, the tij does not represent the true travel time from source event to receiver, but contains an error. This error is varying only according to local receiver clock drift and may be considered constant for source events within a time window, depending on the constraints of clock drift.

Next in the analysis process 160, the establishing step 190 is performed.

The establishing step 190 includes establishing a position and local timing of the seismic recording device, i.e., the position and local time of the seismic recording device during its deployment on the seafloor by solving a system of equations with known source locations, known absolute source firing times and first break arrival times tij that were obtained in the obtaining step 170.

In the disclosed method, the recordings made by a data recording device on the sea floor, in particular in the recording step 130, may be used to derive position and local timing of said recording device. Specifically, the first break arrival times, together with the known source positions may be used to derive an over-determined set of equations which may be solved in an iterative manner and/or with an error minimization criterion and/or holistically. It is also possible to use geophysical constraints to derive additional equations, notably by analyzing multiple recordings possibly also including data from proximal recorders.

In the following, further possible details are disclosed regarding the feature of solving the system of equations with known source locations, known absolute source firing times and the obtained first break arrival times, as included in the establishing step 190 of the analysis process 160.

Numerical methods may be used to solve the equations. Such methods are based on starting with a best guess of the receiver position, timing correction and velocity model and calculating an expected arrival time. This calculated arrival time is then compared to the observed arrival time - usually expressed as c-o (calculated minus observed). One or more of the input variables (position, time, and velocity model) are then varied and trends in c-o are identified and used to adjust the input variables.

Alternatively or additionally, the system of overdetermined equations may be solved by minimizing overall error, e.g. by holistically minimizing the overall error, in positioning and timing. It is in theory possible to derive an exact solution, however this is very complicated due to the high number of input variables.

In the method, a causal relationship is established between a source event and the corresponding registered first beak arrival at the recorder. As explained the establishment of a causal relationship may be done by various means including correlation of the shot sequences with observed first break events. This may also be achieved by recognition of deliberate or accidental perturbations and/or modulation of the source output, or by use of a proximity criterion.

The key set of equations may be based on the distance between source and recorder, for which the first break arrival time is a proxy:

Note there can be a plurality (n) equations for each recorder; n is the number of source firing locations taken into account, usually comprising a part or whole sail line. The sources fire at known location S_n(xsn,ysn,z_Sn) and known time t_s.

Furthermore, we have

with V[z] is water column velocity profile and C[t] = To + c[t] represents the unknown start time of the record (To) and the difference in timing between the reference time and the recorder time c[t].

Figure 2 is a graph illustrating an example of observed first break arrival times at a recording device. An example of a plot of arrival times tn through tin for source si through to s_n is shown in Figure 2. Figure 2 shows the parabolic nature of the observed arrival times. Note that the synthetic arrival times in this example have been corrupted with unknown start of record To and unknown timing errors.

Even allowing for variations in water column variations and by allowing a time varying difference between recorder time and reference time, the system of equations is overdetermined and can be solved to yield recorder positions

Rn(xrn,yrn,Zm), the unknown start time for the sequence of records To (To is constant for one sequence of recordings), and the difference in timing between the reference time and the recorder time as represented by c(t). As a by-product the water column velocity profile may also be estimated. Figure 3 is a graph illustrating an example of locus plot (2D view) given by plotting unadjusted radii derived from first break data.

The problem may be visualized by imagining the possible locus of each receiver as defined from a single arrival time to be a perturbed sphere as per Figure 3. The perturbations represent the errors from the water column velocity profile, and the recorder timing. The error in size of the sphere is a result of unknown To and imperfect velocity model. As long as the recorder stays in one location, it must lie on the point where the loci defined by the multiple source arrival times coincide. Mathematically the problem may be approached by taking sets of the three radii with associated source positions, which define three spheres in space. Each sphere represents the possible receiver position with the radius being defined by the estimates of the travel time and velocity model. The receiver position is calculated by trilateration, but will contain errors due to the errors in velocity model, To estimation and experimental errors. Many sets of three radii may be used to calculate many potential receiver positions. By adjusting the velocity model, and To, and by using the consistency of receiver position as a measure of accuracy, a best receiver position may be obtained.

With some physically realistic constraints on recorder timing variations (drift and random error) and limited variability in time of the water column velocity, the system of equations may be solved with an error that is well within geophysically required limit of lms timing and the equivalent spatial error. This is visualized in Figure 4. The disclosed method may be used to determine location, including 3D location, of a standalone seismic recording device or a plurality of seismic recording devices. These devices may or not be connected by any means and may or may not be in contact by whatever means with each other.

Use of a-priori information may significantly aid the accuracy of the result by placing hard constraints on the solution. The water depth may be obtained by independent means from echo-sounder data, multi-beam data or a prior survey. This allows one to fix the depth (z) coordinate of the receiver position, which removes one variable from the problem. Similarly, the water column velocity may be independently measured removing this variable from the problem.

In another advantageous aspect of the method, water column velocity variations are accounted for. Possible features of this aspect will be explained in the following.

Calculation of the position and timing relative to the source timing relies on calculation of the expected arrival time of the direct wave (or the refracted wave if offsets are larger than the critical offset). In the simplest case, the arrival time of the direct wave is calculated from the shot and receiver positions, giving a travel distance, and then using an average water velocity to get the estimated arrival time. However, it is known that the velocity of water in the sea varies in time and space. This is due to temperature and salinity variations, compounded by currents. The velocity of the sea water column and the shallow sediment layers, may be modelled to estimate the arrival times. As current variations are slow compared to the measurement cycle, the model may be considered static for the purposes of calculation. The velocity model adds more variables that need to be estimated, but the variables are constrained and only a limited number of variables are required to make a sufficiently accurate prediction of first break arrival times. As there are many more observations than model variables, the equations are sufficiently constrained to allow solving.

In an aspect of the disclosed method, the first break arrival times includes direct arrival times. In another aspect, the first break arrival times includes non-direct arrival times. In the latter case, the method may account for first arrivals through substrata. Expected refracted arrival times may be calculated from the thickness of the layers and the layer velocities. These velocities may be anisotropic. The layer thickness, layer velocity and anisotropy are extra variables that will need to be accounted for in the solution.

In an advantageous aspect of the disclosed method, data from multiple seismic recording devices are used to improve an accuracy of the timing and positioning solution. In this aspect, data from a single shot, as identified by means described above, may be recorded by more than one recording device. For each recording device a position, and local timing relative to the shot fire times may be derived as per step 190. With the absolute shot firing times known, the local first break arrival times known, and the relationship established between local recording device time and absolute shot firing time by means of step 190, it is possible to correct for the difference between absolute time and local time for each recording device. Thus - after such correction - recordings from different recording devices may be presented in the same time reference. After such timing correction, data from the same shot for two or more proximal recording devices may be presented in a so called‘shot gather’, whereby the offset from the source position is one axis and the time is on the second axis. The first break arrival times within such a shot gather are confined by the physics of wave propagation and may be calculated. In the simplest instance this would be the direct arrival through the water, but with increased offset refracted energy will arrive first. Any errors in the receiver position, the time correction or the velocities used to calculate the direct arrival or refracted arrival times, will show up as misalignments of the first break arrival times in the shot gather. These misalignments may then be used as input to further refine the receiver position, the time correction and the velocities.

In an advantageous aspect, multiple recording devices may be tethered to a string and deployed on the seafloor, in the deploying step 1 10, and left there for an extended period of time, or extended periods of time, in order to record seismic data in the recording step 130 and the corresponding firing step 120. Multiple passes may be made across the deployed recording devices with one or more seismic sources which fire regularly, creating groupings of data which may be denoted sail line passes.

Upon retrieval of the recording devices, i.e., upon the retrieving step 140, the data recorded in the recording devices are transferred to a computer system, whereupon the data are analysed and processed, in the analysing process 160, to determine the position of each recording device while it was deployed and also to determine the relative time within each recording device compared to an external reference time. Advantageously, data from such a sail line pass are analysed to establish receiver position in three dimensions (x,y,z) and local timing t, though the solution may be more accurate if data from multiple sail line passes are analysed. Advantageously, position of the recording devices is determined every several hours or more often if necessary. The relative time within each recording device is computed as often as necessary, which may be from once every 30 seconds to longer periods.

The invention has been described in detail above as non-limiting examples. It should be understood that the invention can be modified to include various alterations and substitutions. Hence, the invention is not limited by the foregoing detailed description, but by the scope of the claims.

Claims

1. Seismic method, comprising

deploying multiple immersible seismic recording devices on a seafloor, each seismic recording device containing sensors for registering at least one of motion, acceleration and pressure, a digital clock, electronics to record signal data provided by the sensors, a memory, a data handling system, and a battery or other power source;

performing one or more passes across the deployed seismic recording devices with one or more seismic sources that fire regularly during a period of time, wherein a source signature is varied or modulated from shot to shot;

recording, by the seismic recording devices, seismic data during said period of time; retrieving the seismic recording devices;

transferring data recorded by the seismic recording devices to a computer system; in a subsequent analysis, by the computer system, processing the recorded data by - preparing seismic data from an individual seismic recording device so as to obtain a sequence of first break arrival times;

identifying individual shots of the seismic sources, enabled by the varied or modulated source signatures, and

establishing a position and local timing of the seismic recording device by solving a system of equations with known source locations, known absolute source firing times and the obtained relative first break arrival times.

2. Method according to claim, 1 wherein data from multiple seismic recording devices are used to improve an accuracy of the timing and positioning solution.

3. Method according to claim 1 or 2, wherein water column velocity variations are accounted for.

4. Method according to one of the claims 1-3, wherein the system of

overdetermined equations is solved in an iterative manner and/or by minimizing an overall error in positioning and timing.

5. Method according to one of the claims 1-4,

wherein each seismic recording device further contains additional sensors.

6. Method according to claim 5,

wherein the additional sensors include at least one of a motion sensor, an acceleration sensor, an inclinometer and a compass.

7. Method according to one of the claims 1-6,

wherein each seismic recording device includes a wireless communication device.

8. Method according to one of the claims 1-7,

wherein each seismic recording device includes a wireless charging device.

9. Method according to one of the claims 1-8,

wherein each seismic recording device is sealed to ensure that no water penetrates the seismic recording device.

10. Method according to one of the claims 1-9,

wherein each seismic recording device is a standalone recording device, the multiple seismic recording devices being non-interconnected.

11. Method according to claim 1 ,

wherein the varying or modulation of the source signature includes modulation of the firing time of the seismic sources.

12. Method according to claim 1,

wherein the seismic sources includes a primary seismic source and at least one secondary seismic source.

13. Method according to claim 1,

wherein the first break arrival times includes direct arrival times.

14. Method according to claim 1,

wherein the first break arrival times includes non-direct arrival times.

15. Method according to claim 1,

wherein the step of solving the system of equations includes imposing a constraint which includes a priori water depth information.

16. The method according to claim 1, whereby the relationship between source event and observed first break arrival time is established by means of a proximity criterion based on an estimated receiver position.

17. Method according to claim 1, further comprising associating each first break event with a specific source event by correlating source event timing and first break event timings.

18. Method according to claim 1, further comprising associating each first break event with a specific source event by correlating source event amplitudes and receiver event amplitudes.