Patent Application
20120191352
A method and an apparatus for the acquisition, processing and inversion of marine CSEM data are described. According to the invention, the system provides data acquisition and processing of the responses measured simultaneously by multiple receivers placed in the near zone and partly in the intermediate zone at different distances around the transmitter.
A common method of CSEM (Control Source Electro Magnetic) exploration is carried out by locating multiple electromagnetic n recorders along a straight line on the sea floor. A powerful electric current source (transmitter) is located on a vessel, and pushing current pulses into a cable embedded in sea water produces an electromagnetic field which induces an electromagnetic field in a subsea structure. The resulting electromagnetic field is recorded for later analysis. CSEM is described in, i.a., US 2003/0052685 A (Ellingsrud et al., 2003), U.S. Pat. No. 6,628,119 B1 (Eidesmo et al., 2003), WO 02/14906 A1 (Ellingsrud et al., 2002), WO 03/034096 A1 (Sinha et al., 2003), WO 03/048812 A1 (MacGregor et al., 2003) and WO 2007/053025 (Barsukov et al., 2007).
A known problem in CSEM exploration is that of distinguishing the desired electromagnetic signals, which are induced in the subsea structure, from electromagnetic signals originating from geomagnetic pulsations, tides, streams, wind and swell, and other internal signals produced by instrumentation (ADC (analog-digital converter), drift of electrodes etc.) and by moving of transmitter and receiver cables. All these signals are recorded and are commonly referred to as “noise”. A known technique for suppressing noise is commonly referred to as “stacking”.
Such stacking technique involves the use of long and repeated measurements that decrease the productivity of CSEM surveying. At the same time, this technique can minimize the noise only in the cases when the noise is a random function of time. Quite often, the noise originates from sources which do not depend on time, for example from local anomalies of the sea floor relief, the inclination of recording lines, streams, etc. In such cases, stacking in time is useless.
However, in such cases another way of suppressing noise can n be applied, namely stacking in the space domain. Such an approach, named common depth point (CDP) or, in some cases, common mid-point (CMP), is used widely in seismics. The basic idea of the CDP method is stacking (accumulation and averaging) of reflections of waves from common pieces of layers at different locations of sources and receivers. Besides, it is assumed that the boundaries of the layers are inclined slightly (less than ˜3 degrees). Such an approach was suggested for the processing of radar data by Shafers (US 4430643) in 1984. An example of successful application of CDP processing for high-frequency electromagnetic (EM) data by ground-penetration radar (GPR) was demonstrated by Belov et al.
However, there is a principle difference between seismics and high-frequency EM sounding on the one hand and CSEM sounding on the other. The radar works at a very high frequency range (from 10 MHz to 5 GHz), and the EM field corresponds to the same wave formulas as the seismic field. In this case, the CDP seismic technology can be used directly on EM data. But at such a high frequency range, the EM field attenuates very quickly in conductive sea water and in underlying structures and cannot be used for hydrocarbon prospecting.
Strack (US 0071709 A1, 2008) has suggested a method of accumulating EM data called “common mid-point” (CMP) with subsequent “normal moveout correction” as a seismic method. According to this method, all the EM measurements are recalculated into apparent resistivity and then averaged. For the calculation of apparent resistivity, late-time asymptotics and a long distance (offset) are used, in reality a direct-current regime by large offsets, and transients are not utilized. Such sounding thereby has a low spatial resolution as far as a hydrocarbon target is concerned.
Thomsen et al. (U.S. Pat. No. 7,502,690 B2, 2009) have proposed the processing of t-CSEM data (t-CSEM technology) mostly as seismic data. It is mentioned that, in principle, the EM field used in the t-CSEM technology is different from a seismic field, but is still suggested that remnants of EM fields (which do not exist in diffusion EM fields) be analysed, for data then to be stacked with some empirical weighting. With such limitations, the results received and their resolution cannot be checked.
The invention has for its object to remedy or reduce at least one of the drawbacks of the prior art.
The object is achieved through features which are specified in the description below and in the claims that follow.
It is an object of the present invention to provide new and improved method of CSEM data acquisition for obtaining a greater amount of high-quality data concerning a subsea hydrocarbon structure.
It is another object of the invention to provide a new and improved CSEM processing method to enable suppression of the noise caused by local geoelectric inhomogeneities of section, by instrumentation, stream etc.
It is a further object of the invention to provide a new and improved CSEM method which gives the possibility of controlling the process of data acquisition simultaneously with the processing of the data.
The invention relates to a new method and apparatus for electromagnetic (EM) data acquisition, processing and inversion, providing effective accumulation (stacking) of EM responses containing information on an underground structure (layers) and electrical properties (resistivity). Together with seismic, well logging and other geological and geophysical data, this information gives the possibility of determining whether there are/is hydrocarbons or water in the reservoir.
The invention provides a new method for electromagnetic data acquisition in CSEM surveying. This method, which is further named Control Source Electro Magnetic Common Depth Point (CSEM CDP), is based on an idea of joint stacking of the response electromagnetic signal in both the time domain and the space domain with the aim of minimizing any noise and maximizing the signal-to-noise ratio.
Preferably, two conditions are taken as the basis for CSEM CDP:
a) the applied method of sounding must work in time domain, and
b) the relief and other underlying layers change smoothly in the vicinity of the sounding area.
Preferably, the electromagnetic surveying is carried out in series of groups along profiles or within the area previously identified as potentially containing a subsea hydrocarbon reservoir. Each CDP group consists of multiple receivers installed and working on the sea floor in the near zone around the transmitter.
The transmitter impresses, on a cable embedded in sea water, the current pulses with sharp fronts, and the receivers measure the EM responses.
Preferably, all the receivers are located at different distances (offsets) from the transmitter; that is to say, far enough to avoid an influence of IP (induced polarization) effect and close enough to have a signal-to-noise ratio acceptable for measurements.
Preferably, all the raw electromagnetic data recorded by receivers during surveying are stored, and the apparatus performs processing and inversion of stored marine electromagnetic data essentially in real time in accordance with the commands of an operator.
Preferably, all the data measured at a distance r between the transmitter and the receivers located around it, satisfying the condition of a near zone—that is to say, large enough for sufficient attenuation of the IP effect and small enough to provide consistent registration of EM response—are inverted all together and the result of the inversion is concerned with the circle centre of the radius r, the centre of the circle being the CDP.
The process may be controlled continuously by measurements and accumulation of data and, if necessary, the operator installs additional receivers to provide acceptable quality of the result.
After receiving acceptable results in one common depth point (CDP), the transmitter and the plurality of receivers can be moved to the next point along the profile or area.
In a first aspect, the invention relates more specifically to a method for the acquisition, processing and inversion of marine electromagnetic data recorded by a system consisting of a plurality of synchronously working devices arranged to register an electromagnetic field and installed on or near a sea floor while an electromagnetic field is excited by pulses of to electric current pumped in sea water by a pulse generator installed on board a vessel, said marine electromagnetic data being common depth point (CDP) marine electromagnetic data, characterized by said CDP marine electromagnetic data being the data selected from a plurality of raw records of the is electromagnetic field measured in time domain at a distance (offset) between a transmitter and a receiver, the distance satisfying the following conditions:
a) CDP marine electromagnetic data consist of only the galvanic mode of the electromagnetic field; and
b) all the receivers are located at a distance (offset) satisfying the condition r1<r<r2, in which r1 is the distance from the transmitter at which the effect of induced polarization is insignificant as compared with the measured response signal, whereas r2 is the distance from the transmitter at which the measured response signal is still considerable as compared with the noise and, besides, the receiver is still inside the near zone of the electromagnetic field determined by the condition r≦√{square root over (107ρt/2)}.
Said processing may involve inversion of said CDP electromagnetic data with respect to the resistivity of layers existing within the earth, and the vertical extent of said layers.
Said common depth point (CDP) electromagnetic data may be acquired by carrying out marine electromagnetic surveying operations comprising the steps of:
In a second aspect, the invention relates more specifically to an apparatus for the acquisition, processing and inversion of marine electromagnetic data, characterized by the fact that said marine electromagnetic data may be common depth point (CDP) marine electromagnetic data selected from a plurality of electromagnetic field records measured in the time domain at a distance (offset) between a transmitter and a receiver satisfying the conditions
a) CDP marine electromagnetic data consist of only the galvanic mode of an electromagnetic field; and
b) the receivers are located at a distance (offset) satisfying the condition r1<r<r2, in which r1 is the distance from the transmitter at which the effect of induced polarization is insignificant as compared with the measured response signal, whereas r2 is the distance from the transmitter at which the measured response signal is still considerable as compared with the noise, and the receiver is still inside the near zone of the electromagnetic field which is being generated by the transmitter.
The apparatus may include:
For a better understanding of the present invention, together with other and further objects and features thereof, advantages of the proposed method as well as disadvantages of existing methods applied for marine electromagnetic surveying of hydrocarbons, reference is made to the description of the invention that now follows, referring to the appended drawings.
In the figures, Tr, Tr1 . . . , Tr4 indicate transmitters arranged to induce an electromagnetic field in sea water and an underlying structure including one or more layers Lb of different resistivities ρ1, ρ2, ρ3, ρ4. The transmitters Tr, Tr1 . . . , Tr4 are installed in sea water Sw above a sea floor Sb and are in signal communication with a signal generator Sg (see
A number of receivers Rz, Rz1, . . . , Rz9; Rz1-1, . . . , Rz1-9; Rz2-1, . . . , Rz2-9 are arranged to record the field strength and communicate the signal values to data storage means (not shown). Rp, Rp 1, Rp 2 indicate receiver profiles.
A mainframe computer (not shown) includes means arranged to accept operator commands and means arranged to receive data from said data storage means.
An array processor unit (not shown) is arranged to receive said commands and said data from said mainframe computer unit and process and invert said data in accordance with said operator commands and visualize the results essentially in real time.
In
In
It is well known that for increasing a signal-to-noise ratio, two ways of accumulating signals from measurements of an electric field result, namely accumulation in time and accumulation in space. Accumulation in time is convenient for all existing CSEM methods.
However, not all existing CSEM methods used for hydrocarbon marine electromagnetic exploration can be improved by a spatial accumulation. This is explained by low spatial resolution of the methods working in the frequency domain, and is based on the principle of geometric sounding, for example, the SBL method (Ellingsrud et al., 2003; Eidesmo et al., 2002, 2003; Greer et al., 2004; MacGregor at al., 2000, 2003, 2004; Tompkins et al., 2004; Wicklund et al., 2004; Sinha et al., 2003 etc.), which is a marine modification of a wellknown direct-current sounding method proposed by C. Schlumberger in the 1920s.
Accumulation in space is possible only when applied to transient electromagnetic methods operating with the galvanic mode of the EM field in the near zone; for example, the TEMP-VEL and TEMP-OEL methods used only the vertical component of the electric field.
Simultaneous measurements and averaging of the vertical component of the electric field in N receivers Rz1, . . . , RzN located within a small territory around a source (transmitter) Tr1, . . . , Tr4 gives the possibility of increasing a signal-to-noise ratio proportional to factor √{square root over (N)}, because the noise in these receivers Rz1, . . . , RzN is uncorrelated since the vertical component of the electric field from the noise is caused mainly by local inhomogeneities and cannot be the same for all receivers located in different places.
However, there are two limitations which give no possibility of using direct averaging of the signal.
First, the induced signal containing useful information about the cross section depends on distance between the transmitter and the receiver and, therefore, has different amplitudes at different distances.
Second, the useful signal is complicated by the effect of induced polarization (IP) which masks the signal and makes the inversion and interpretation of the acquired data hard.
The problem of these limitations can be solved by an appropriate choice of distance (offset). The optimal CDP area (circular ring) must satisfy the condition r1<r<r2, in which r1 is the internal radius (offset) of the ring and r2 is the external one. Both radii can be found from the next consideration.
The maximal distance (r2 boundary) is determined by the end of the far zone of the EM field. In the time domain, the boundary of the far zone takes place at a time t at which the EM signal changes its sign. The time t depends mainly on the resistivity ρ of sedimentary rocks (till ˜500) and on the distance (offset): r˜√{square root over (107ρt/2)}. The maximal distance r2 is Selected to be such that it is possible to provide the effective inversion of sounding data in the time range not less than tmax/t>30, in which tmax is the maximal time of signal registration (this time is determined by the noise level). For example, if ρ=2 Ωm, tmax=10 s and t ˜0.3 s, then r<1.7 km.
IP effect forces the use of the distances r>rIP; here, rIP is the distance at which the IP effect is sufficiently small. For example, at IP ˜0.3% (the background value) and p=2 μm, the distance rIP>1 km provides conditions sufficient for suppressing the IP distortions to 30% at the time tmax=10 s.
Therefore, in our example, the CDP area lies within the circular ring of 1 km<r<1.7 km.
The spatial accumulation in the CDP method is carried out by means of an 1D inversion scheme using simultaneously multiple transient processes; that is to say, a search of parameters of a layered section for which the experimental responses Ez (r1), Ez(r2), Ez(rN) corresponding to receivers located in points r1, r2, rN do minimize the functional
in which Rz1, Rz2, . . . , RzN are the calculated responses corresponding to the model found. The final section gives a common result for the Tr-Rz1-Rz2, . . . group.
The simplest profile scheme of transmitter and receivers location shown in
to
The efficiency of surveying can be increased by the application of an apparatus producing processing and inversion synchronously with the measurement row data acquisition and, if necessary, a decision to add receivers and repeat the measurements.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20092699 | Jul 2009 | NO | national |
| 20100945 | Jun 2010 | NO | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/NO2010/000281 | 7/12/2010 | WO | 00 | 3/12/2012 |
