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Collaborating Authors
Amundsen, Lasse
ABSTRACT Marine seismic data are distorted by ghosts as waves propagating upwards reflect downwards from the sea surface. Ghosts appear both on the source-side as well as on the receiver-side. However, whereas the receiver-side ghost problem has been studied in detail and many different solutions have been proposed and implemented commercially, the source-side ghost problem has remained largely "unsolved" with few satisfactory commercial solutions available. In this paper we propose a radically new and simple method to remove sea-surface ghosts that relies on using sources at different depths but not at the same lateral positions. The new method promises to be particularly suitable for 3D applications on sparse or incomplete acquisition geometries. Presentation Date: Tuesday, October 18, 2016 Start Time: 4:10:00 PM Location: 163/165 Presentation Type: ORAL
On seismic deghosting using Green’s theorem
Amundsen, Lasse (Statoil Research Centre, The Norwegian University of Science and Technology) | Reitan, Arne (Arne Reitan Consulting) | Weglein, Arthur B. (University of Houston) | Ursin, Bjørn (Universidade Federal da Bahia, The Norwegian University of Science and Technology)
ABSTRACT We have examined theoretically how receiver-side deghosting of pressure measurements can be derived from the Green’s theorem method. We split the Green’s function that obeys Dirichlet boundary conditions on the sea surface and at the receiver plane into two contributions: the first emitting energy downward only from its source location and the other emitting energy only upward. Using the normal derivative of the source-side downgoing Green’s function in the Green’s theorem evaluation over the receiver plane, the upgoing part of the pressure field is predicted. This is the receiver-side deghosted field. By inserting the source-side upgoing normal derivative Green’s function in Green’s theorem, its evaluation over the receiver plane predicts the downgoing part of the pressure field. For a plane horizontal receiver surface, the required Green’s function can be derived using the image series expansion method. To display the fundamental frequencies of this Green’s function, we have applied a Fourier series expansion of the Green’s function. Our theory gives a new understanding of and generalizes and simplifies previously published theories on Green’s theorem-based receiver-side deghosting of pressure wavefields.
ABSTRACT Ghost cavitation, which is a term describing that cavitation bubbles are generated acoustically, has been hypothesized to occur when the ghost reflected signals from many individual air guns beneath the sea surface produce a pressure that is close to zero in the water above the source array. Ghost cavitation is typically observed some milliseconds after the ghost reflection, and it may last for 5–15 ms, depending on the configuration of the source array. The cavitation process subsequently generates a weak high-frequency signal. To investigate this potential signal model and mechanism, we have performed a dedicated source experiment. We found that the distance between the source strings in a source array is a major factor that influences the amount and strength of the high-frequency signal. By increasing the separation distance from 6.5 to 8 m, we have observed a significant decrease in the high-frequency signal. Further, the amount of ghost cavitation can be reduced by increasing the distance between the guns. Also single sub-arrays may create ghost cavitation sound, of course weaker in signal strength compared with full arrays, in agreement with the model. Conventional air-gun modeling can be used to predict where ghost cavitation can occur. Therefore, in principle, a workflow could be developed to quantify grossly if and how much high-frequency signals could be generated by this mechanism, given the source array configuration, and further change the configuration to reduce to a very minimum the high-frequency signals, if deemed necessary. For an air-gun array consisting of two subarrays separated by 6 m and fired at 9 m depth, we found that the high-frequency signals emitted between 1 and 10 kHz were of similar strength to the noise from conventional cargo ships, depending on their size and the vessels’ speed.
- Europe (0.68)
- North America > United States (0.46)
A model-independent finite-difference method for removal of free-surface generated multiples
Vasmel, Marlies (ETH Zurich, Institute of Geophysics) | Robertsson, Johan O. A. (ETH Zurich, Institute of Geophysics) | Amundsen, Lasse (University of Science and Technology (NTNU) and STATOIL Research Centre)
ABSTRACT We have developed a new data-driven method to solve the free-surface-related multiple problem using forward space-time-domain modeling. We build a finite-difference (FD) modeling engine in which the interaction of any desired wavefield with an unknown subsurface (but without the reflecting sea surface on top) can be modeled. This can either be used to predict the desired primary reflections of the subsurface or to (simultaneously) model all orders of free-surface-related multiples. In this method, we use conventional forward-modeling techniques in combination with a novel type of boundary condition. The wave propagation in the known part of the medium (i.e., the homogeneous water layer) is modeled on an FD grid, whereas the wave propagation in the unknown part of the medium (i.e., the subsurface) is predicted using the recorded multicomponent data. The methodology dynamically links these two distinct domains. We demonstrate the exact method and an adapted version that works under realistic conditions on a synthetic data set for different acquisition geometries. A small-scale test on ocean-bottom cable field data shows that the proposed method readily predicted the expected multiple arrivals in the recorded data.
- Europe > Norway (0.28)
- Europe > Switzerland (0.28)
- Geophysics > Seismic Surveying > Seismic Processing (1.00)
- Geophysics > Seismic Surveying > Multicomponent Seismic Surveying (0.88)
ABSTRACT Methods for wavefield injection are used in, for instance, reverse time extrapolation of shot gathers in reverse time migration. For correct injection of recorded data without any ambiguity of the propagation direction, the wavefield-injection methodology requires pressure and particle velocity data such as multicomponent towed marine or seabed seismic recordings. We discovered that by carefully considering the models (medium parameters and boundary conditions) for injection, wavefield injection of multicomponent data can also be used to solve several long-standing challenges in marine seismic data processing by means of conventional time-space-domain finite-difference propagators. We outlined and demonstrated several of these important applications including up-down separation of wavefields (deghosting), direct-wave removal, source-signature estimation, multiple removal, and imaging using primaries and multiples. Only acoustic models are considered, but the concepts are straightforward to generalize to elastodynamic and electromagnetic models.
- Europe > Norway (0.46)
- Europe > Switzerland (0.28)
- North America > United States > Illinois > Madison County (0.25)
- Geophysics > Seismic Surveying > Surface Seismic Acquisition > Marine Seismic Acquisition (1.00)
- Geophysics > Seismic Surveying > Seismic Processing > Seismic Migration (1.00)
- Geophysics > Seismic Surveying > Seismic Modeling (1.00)
- Geophysics > Seismic Surveying > Multicomponent Seismic Surveying (1.00)
- Europe > Norway > North Sea > Northern North Sea > East Shetland Basin > PL 050 > Block 34/10 > Gullfaks Field > Statfjord Group (0.99)
- Europe > Norway > North Sea > Northern North Sea > East Shetland Basin > PL 050 > Block 34/10 > Gullfaks Field > Lunde Formation (0.99)
- Europe > Norway > North Sea > Northern North Sea > East Shetland Basin > PL 050 > Block 34/10 > Gullfaks Field > Lista Formation (0.99)
- (2 more...)
ABSTRACT We have developed a new and simple method for deghosting of conventional hydrophone streamer data towed at arbitrary variable depths. The method uses a time-space domain finite-difference (FD) solution to the wave equation with pressure field boundary conditions to predict and remove ghosts. Because it operates in the time domain, our method is unaffected by any number of notches in the frequency spectrum of the data and therefore will deghost “through notches.” Apart from the acquired hydrophone data, the method relies on the depth profile of the streamer recording the data beneath a sea surface with a known reflection coefficient as well as the propagation velocity in the water above the streamer. The method was applied to simple and more complex synthetic data, which illustrated its ability to deal with complex data and any acquisition geometry.
- Europe > Norway (0.46)
- Europe > Switzerland (0.28)
- Europe > Norway > North Sea > Northern North Sea > East Shetland Basin > PL 050 > Block 34/10 > Gullfaks Field > Statfjord Group (0.99)
- Europe > Norway > North Sea > Northern North Sea > East Shetland Basin > PL 050 > Block 34/10 > Gullfaks Field > Lunde Formation (0.99)
- Europe > Norway > North Sea > Northern North Sea > East Shetland Basin > PL 050 > Block 34/10 > Gullfaks Field > Lista Formation (0.99)
- (2 more...)
Summary We present a novel method for receiver- and source-side deghosting of conventional hydrophone streamer data towed at arbitrary variable depths. The method uses a time-space domain finite-difference solution to the wave equation with pressure field boundary conditions to predict and remove ghosts. Since it operates in the time domain, the method is unaffected of any number of notches in the frequency spectrum of the data and will deghost "through notches". Apart from the acquired hydrophone data, the method relies on the depth profile of the streamer.
- Europe > Norway > North Sea > Northern North Sea > East Shetland Basin > PL 050 > Block 34/10 > Gullfaks Field > Statfjord Group (0.99)
- Europe > Norway > North Sea > Northern North Sea > East Shetland Basin > PL 050 > Block 34/10 > Gullfaks Field > Lunde Formation (0.99)
- Europe > Norway > North Sea > Northern North Sea > East Shetland Basin > PL 050 > Block 34/10 > Gullfaks Field > Lista Formation (0.99)
- (2 more...)
Summary We show that by injecting the seismic wavefield measured on multicomponent sensors in streamers or on the sea bed into a time-space domain modeling scheme such as finite differences (FD), various wavefields can be predicted in a radically new way for use in processing, imaging and waveform inversion. Examples of wavefields that can be predicted per shot gather by using simple boundary conditions are the upgoing and downgoing waves (receiver-side ghosts), and the source radiation pattern (direct wave with source ghost). Applying more advanced boundary conditions at the edge of the injection model that use all the recorded data to account for the interaction with the unknown subsurface allows the prediction of the wavefield that would be recorded with the free-surface absent, or the prediction of the surface-related multiples of all orders. These novel modeling processes require no information of the subsurface. We also demonstrate how the upgoing and downgoing wavefields can be used in RTM with multiples (given a velocity model of the subsurface).
Summary Analysis of geophone recordings from an OBS survey offshore Norway shows that excellent long-offset signals may be measured for frequencies below 2 Hz, and as low as 0.25 Hz. This observation is in contrast to the hydrophone recordings that do not show such excellent long-offset low frequency signals. We think that there are at least two factors that cause this: First, the fact that the receiver ghost spectrum is non-zero for the geophone component while the hydrophone ghost spectrum approaches zero for low frequencies, respectively. Second, the hydrophone component is more affected by noise created by (long wave length) ocean water waves compared to the vertical geophone component. Furthermore, a critical factor is the intrinsic low frequency response of the geophone and the hydrophone, which is a critical factor that controls what frequencies we may record. In our case, we find that the low frequency geophone response is sufficient to measure signals that are above the background noise level. We think that this opens a new possibility for low frequency seismic acquisition: Use seabed geophones that are specifically designed to measure low frequency (0.1-5 Hz) signals. If successful, this might open new possibilities for imaging below complex salt structures, below basalt and for full waveform inversion.
ABSTRACT In marine seismic acquisition, it used to be commonly accepted that it is optimal to tow the source deep to enhance the low-frequency content in the seismic data. However, Mayne and Quay found in 1971 that the low-frequency response of air guns actually improves as the source depth decreases. We evaluated a simple ad hoc theory that demonstrates that two effects are counteracting each other: The free-surface effect favors deep-towed sources, whereas the bubble time period (increasing with decreasing source depth) favors shallower tow depths. From a fjord test, we found that combining several source depths in an air gun array might flatten and improve the low-frequency part of the source spectrum. The experiment confirms that various source depths result in local, characteristic maxima in the low-frequency spectrum, corresponding to the bubble time period of the air gun.