1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for acquiring seismic data.
2. Discussion of the Background
During the past years, the interest in developing new oil and gas production fields has dramatically increased. However, the availability of land-based production fields is limited. Thus, the industry has now extended drilling to offshore locations, which appear to hold a vast amount of fossil fuel. Offshore drilling is an expensive process. Thus, those engaged in such a costly undertaking invest substantially in geophysical surveys in order to more accurately decide where to drill in order to avoid a dry well.
Marine seismic data acquisition and processing generate a profile (image) of the geophysical structure (subsurface) under the seafloor. While this profile does not provide an accurate location for the oil and gas, it suggests, to those trained in the field, the presence or absence of oil and/or gas. Thus, providing a high resolution image of the subsurface is an ongoing process for the exploration of natural resources, including, among others, oil and/or gas.
During a seismic gathering process, as shown in FIG. 1, a vessel 10 tows an array of acoustic detectors 12. The array of acoustic detectors 12 is disposed along a body 14. Body 14 together with its corresponding detectors 12 are sometimes referred to, by those skilled in the art, as a streamer 16. The vessel 10 may tow plural streamers 16 at the same time. The streamers may be disposed horizontally, i.e., lying at a constant depth z1 relative to a surface 18 of the ocean. Also, the plural streamers 16 may form a constant angle (i.e., the streamers may be slanted) with respect to the surface of the ocean as disclosed in U.S. Pat. No. 4,992,992, the entire content of which is incorporated herein by reference. FIG. 2 shows such a configuration in which all the detectors 12 are provided along a slanted straight body 14 making a constant angle α with a reference horizontal line 30.
With reference to FIG. 1, the vessel 10 may also tow a sound source 20 configured to generate an acoustic wave 22a. The acoustic wave 22a propagates downward and penetrates the seafloor 24, eventually being reflected by a reflecting structure 26 (reflector R). The reflected acoustic wave 22b propagates upwardly and may be detected by detector 12. For simplicity, FIG. 1 shows only two paths 22a corresponding to the acoustic wave. However, the acoustic wave emitted by the source 20 may be substantially a spherical wave, e.g., it propagates in all directions starting from the source 20. Parts of the reflected acoustic wave 22b (primary) are recorded by the various detectors 12 (the recorded signals are called traces) while parts of the reflected wave 22c pass the detectors 12 and arrive at the water surface 18. Since the interface between the water and air is well approximated as a quasi-perfect reflector (i.e., the water surface acts as a mirror for the acoustic waves), the reflected wave 22c travels back towards the detector 12 as shown by wave 22d in FIG. 1. Wave 22d is normally referred to as a ghost wave because this wave is due to a spurious reflection. The ghosts are also recorded by the detector 12, but with a reverse polarity and a time lag relative to the primary wave 22b. The degenerative effect that ghost waves have on bandwidth and resolution of seismic measurements are known. In essence, interference between primary and ghost arrivals causes, among other problems, notches, or gaps, in the frequency content of the data recorded by the detectors.
The traces may be used to determine the subsurface (i.e., earth structure below surface 24) and to determine the position and presence of reflectors 26. However, the ghosts disturb the accuracy of the final image of the subsurface and for at least this reason, various methods exist for removing the ghosts, i.e., deghosting, from the results of a seismic analysis.
The streamer configuration illustrated in FIG. 2 is considered to provide a more accurate data acquisition then the configuration illustrated in FIG. 1. One reason for this difference is the fact that for each reflector, a time gap between the detection of the primary and ghost reflections becomes greater, the further the detector 12 is from the source 20, due to the slanted disposition of the detectors, thus facilitating deghosting.
However, the slanted streamer shown in FIG. 2 has the following limitation, which makes it impracticable. Current streamers have a typical length on the order of 6 to 10 km. Using a slanted streamer as suggested in U.S. Pat. No. 4,992,992, e.g., with a slope of 2 degrees relative to the horizontal line 30, would lead to a depth of about 280 m for the last detector, while in reality current marine detectors are designed to operate in water depths up to about 50 m. Thus, for current streamers, the approach proposed in the '992 patent would require detectors to be located in water depths beyond their current capabilities, thus resulting in detectors failure or the impossibility to provide the detectors at those depths.
In order to accurately locate deep reflectors, high-frequency acoustic waves are not suitable on account of the high attenuation they undergo during their propagation. Thus, low-frequency acoustic waves are desired to be present in the spectrum recorded by the detectors. Therefore, an octave is desirable to be gained in the low-frequency range of the traditional methods, thereby increasing the conventional bandwidth of 5-40 Hz to, e.g., a bandwidth of 2.5-40 Hz. To gain the extra octave, it is possible to increase the depth of the streamer. However, it is not sufficient to give priority to low-frequencies since high-frequencies are needed to estimate precisely the velocity model of the surface layers. Also, the signal-to-noise ratio should be improved for low-frequency acoustic waves without deteriorating it for the high-frequency acoustic waves.
Therefore, although a slanted streamer may partially extend the above-discussed bandwidth due to the constant depth increase of the detectors relative to the surface of the water, additional limitations exist in the approach proposed by the '992 patent, as further illustrated below.
FIGS. 3 and 4 illustrate the simulated effect of ghosts on the frequency spectrum (herein referred to as “effective spectrum”) corresponding to a shallow reflector (disposed at a depth of about 800 m) for a slanted streamer after stacking (stacking is a process in which different traces corresponding to the same common point are added together to reduce noise and improve overall data quality). In other words, spectra 34 and 38 correspond to different depths of the detectors with no ghosts while spectra 36 and 40 correspond to the same different depths of the detectors but with ghosts. These spectrum simulations are for a streamer having the first detector placed at a depth relative to the surface of the water of about 7.5 m and about 15 m. It is noted that relative values of the amplitudes of the frequencies are plotted against the frequencies in FIGS. 3 and 4. In both of the ghost free simulations (34 and 38), the last detector on the slanted streamer is placed at a depth of about 37.5 m relative to the water surface. The ghost free simulated spectra (curves 34 and 38, respectively) neglect the ghost effect, i.e., represent an “idealized” situation in which the presence of ghosts has been artificially removed from the simulation, so as to show the deficiencies of the conventional data acquisition methods. The effective spectra 36 and 40 are calculated without artificially removing the effect of the ghosts. As clearly shown in FIGS. 3 and 4, the two spectra 34 and 36 of the first configuration have different shapes as the effective spectrum 36 includes less energy for the low-frequencies (lower than about 10 Hz) and high-frequencies (higher than about 60 Hz) as compared to the ghost free simulated spectrum 34. As the final image of the subsurface is sensitive to the low- and high-frequencies, two spectra that have these portions different from each other are considered to be different and thus, the data corresponding to the effective spectrum 36 does not produce an accurate final image of the subsurface.
An advantage of increasing the depth of the first detector is to minimize the effect of swell noise, e.g., noise produced by swells at the surface of the water. The swell noise is known to mainly affect detectors close to the surface of the water. A simulated spectrum 38 for this situation and a corresponding effective spectrum 40 are shown in FIG. 4. However, even for this case, the effective spectrum 40 shows a notch at about 45 to 50 Hz that corresponds to a detector having a depth of about 15 m.
For a reflector at a depth of about 15 m, the data recorded by the detectors relatively close to the seismic source may have an overriding influence in stacking as the distant receivers make a less important contribution. Therefore, for a shallow reflector, mostly the recordings of the detectors positioned in the head portion of the streamer (closest to the vessel) are used. This means that the depth dynamics of the detectors, which determine the diversity of the notches, are insufficient for good quality ghost elimination.
As can be seen from the above summarized illustrative discussion, a substantial disparity at low- and high-frequencies of the spectrum are still present when using streamers with a constant slant, which results in a poor final image of the subsurface. Accordingly, it would be desirable to provide systems and methods that avoid or significantly reduce the afore-described problems and drawbacks of the conventional systems.