Imaging Ocean-Bottom Seismic Data with Acoustic Kirchhoff Pre-Stack Depth Migration: A Numerical Investigation of Migration Responses and Crosstalk Artifacts

A12 EVOLUTION OF A MULTICOMPONENT AUTONOMOUS OBS: THE ROLE OF GEOPHYSICS Abstract 1 Multi-component ocean bottom seismic (OBS) data for oil and gas exploration have traditionally been acquired with systems in which many seismometers are physically linked with a cable. An alternative approach is to record data utilizing a set of distributed nodes each operating autonomously. The high degree of flexibility in the receiver geometry permitted by a node based system is a motivating factor for employing nodes for some geophysical objectives such as wide azimuth imaging. Our development of such a system through several iterations has provided insight into

Summary The ocean contains low-frequency passive acoustic waves that are readily detected on seismic recording devices. One source of this ambient noise is from wind and wave action at the sea surface that creates passive sound waves that propagate in all directions. Passive energy recorded during an ocean bottom seismic (OBS) survey in 1000 m of water is analyzed using seismic interferometry and a lowfrequency dispersive arrival is observed that is interpreted using the normal mode theory of water waves. The interferometry arrivals allow the detection of clock drifts on individual sensors by analyzing the time asymmetry between the causal and acausal arrivals. The clock drifts are found to increase with time in a manner consistent with other observations. A second method for clock drift detection is developed that takes advantage of the long wavelengths in the passive energy that are typically many times the separation distance between OBS sensors. A synthetic co-located node is created at the location of an existing sensor by interpolation of the data from neighboring sensors. Comparing the arrival times on the synthetic and real data allows for the timing drift error identification. This technique produces results consistent with the interferometry observations. Both of these techniques are useful for improving the timing fidelity of OBS data. In existing OBS node designs, up to 50% of the battery energy is used to power the ovenheated clocks that maintain a stable known temperature. The new methods open up the possibility of significantly extending node life by using less energy-intensive clocks.

The application of a wide range of geophysical technologies (ranging from Narrow-Azimuth (NAZ), Wide-Azimuth (WAZ) streamer and Ocean-Bottom-Seismic (OBS) node acquisition, to anisotropic seismic imaging) has resulted in considerable business impact on Shell's GOM field development. In this paper, results will be shown for several GOM deepwater fields. Introduction Whereas seismic processing for large Gulf of Mexico (GOM) exploration projects typically focuses on imaging the unseen (often imaging below complex salt structures), seismic processing for development projects often has to focus on adding further fine detail to the seismic data. Typical business drivers demanding such details for development are:–Well-Positioning requiring enhanced lateral and vertical positioning–Reservoir Modeling requiring enhanced seismic resolution to enhance stratigraphic details–Reservoir monitoring requiring time-lapse signal To address these challenges, respectively, a variety of seismic processing techniques like anisotropic velocity model building + PreStackDepthMigration (PreSDM), High Definition Seismic imaging, and 4D TimeLapse processing are commonly applied in our development projects. Hereby, various seismic acquisition methods are used (ranging from Narrow-Azimuth (NAZ), and Wide-Azimuth (WAZ) streamer, to WAZ Ocean-Bottom-Seismic (OBS) node acquisition). The application of all these geophysical technologies has resulted in considerable business impact on Shell's GOM field development. In this paper, results will be shown for several GOM deepwater fields. Geophysical Technologies Anisotropy and Pre-Stack Migration Anisotropy impacts the positioning and focusing of subsurface events. Taking anisotropy properly into account in PreSDM can significantly reduce subsurface uncertainties, improve the seismic to well match, and impact well positioning. In the GOM with its many salt structures embedded in sediments, we especially recognize the effect of differential anisotropy: isotropic salt bodies next to anisotropic sediments. This can easily lead to false structures next to and below salt. The typical velocity model building procedure used here has been described in detail by Stopin et al. (2008) and consists of several migration and update cycles where the velocity and the anisotropy parameters (d and (or ?)) are inverted for. Final acceptance criteria are PreSDM results with flat image gathers and small seismic to well misties (typically not more than 100 ft). It should be noted that this anisotropic velocity model updating procedure is a highly integrated iterative effort involving processing, interpretation, and well-positioning staff.

Multi-component ocean bottom seismic (OBS) data for oil and gas exploration have traditionally been acquired with systems in which many seismometers are physically linked with a cable. An alternative approach is to record data utilizing a set of distributed nodes, each operating autonomously. In such a system each node marks time and records the sensor outputs continuously for the duration of its deployment on the seafloor, which may be days or weeks. The high degree of flexibility in the receiver geometry permitted by a node based system is a motivating factor for employing nodes for some geophysical objectives, such as wide azimuth imaging. The data quality issues for node based OBS recordings are much the same as those for cable based acquisition: vector fidelity, coupling, bandwidth, signal-to-noise ratio, and repeatability. We acquired data in the deep water Gulf of Mexico with six test nodes and analyzed the data with respect to these key issues. Physical Description of a Node Cylindrical in shape (diameter 22.5 inches, height 10 inches) and machined from a single piece of aluminum stock, the unit presents a rugged exterior (Figure 1). Geophones and electronics are contained entirely within the case, while a hydrophone contacts the water from inside a recess in the case wall. Rechargeable batteries occupy the bulk of the interior and contribute significantly to the unit’s weight (200 lbs in air). Seismic data, stored in flash memory, are downloaded through a USB interface on recovery of the unit. Compass and tilt measurements—the geophones are fixed, not gimbaled—are also recorded. The unit is designed for 28 days of continuous 4-component recording at 2 ms sample interval. It has been successfully pressure tested to a water depth of 10000 ft.

We describe an experimental four-component (4C) nodal ocean-bottom seismic (OBS) survey that was conducted in 2001 at Thunder Horse Field in the deepwater Gulf of Mexico—in fact, the water depth of 6000 ft makes this, to our knowledge, the deepest-water OBS survey shot by industry to date. The goal of this survey was to investigate the technical feasibility of deepwater OBS surveys, which hold the promise of improved imaging due to the wide-azimuth nature of the data. We present the results from prestack depth migration of these OBS data and compare them to an image computed from towed-streamer data. The results lead us to conclude that deepwater OBS is feasible and may be a powerful technology for future deepwater imaging projects.

We describe an experimental four‐component (4C) nodal ocean‐bottom seismic (OBS) survey that was conducted in 2001 at Thunder Horse field in the deep water of the Gulf of Mexico. At a water depth of 6000 ft, this is to our knowledge the deepest‐water OBS survey shot by industry to date. The goal of this survey was to investigate the technical feasibility of deep‐water OBS surveys, which hold the promise of improved imaging due to the wide‐azimuth nature of the data. We present the results from prestack depth migration of these OBS data and compare them to an image computed from towed‐streamer data. The results lead us to conclude that deep water OBS is feasible and may be a powerful technology for future deep‐water imaging projects.

In many parts of the world, pre-stack time migration (PSTM) still represents the majority of seismic imaging activity in the industry. The reason for this is the simple efficiency and robustness of time imaging and its ability to focus seismic reflectors for many geological settings. Limitations of PSTM appear in the case of strong lateral velocity variations, where the more rigorous imaging and more accurate velocity models offered by Pre-Stack Depth Migration (PSDM) are required. In areas of moderate complexity, where PSTM begins to struggle we introduce a new, accurate method, ‘Beyond Dix’, to help bridge the gap between PSTM and PSDM. Beyond Dix provides an accurate fast-track PSDM from PSTM outputs. It takes full advantage of the efficiency, good focusing, and high signal-to-noise ratio available from time imaging to jump-start the PSDM process. The name comes from the fact that we are going beyond the limitations of the 1D Dix inversion commonly used to derive depth interval velocities from the PSTM velocities. It uses the full kinematic information available in the time migrated domain to directly build an accurate depth velocity model and bypass the 1D Dix inversion altogether. Not only does this approach extract the maximum value from time imaging, it also adds great flexibility to imaging projects by allowing a seamless and fast transition from PSTM to PSDM and thus avoiding the need to choose at an early stage between a time or depth approach. It is clear that in addition to providing a fast-track PSDM, Beyond Dix has a range of possible applications, including building more accurate initial models for full depth imaging projects. In this paper we explain how the detailed information freely available within the time migrated domain can be used directly to build an accurate depth velocity model. We also illustrate the application of Beyond Dix with two examples from different geological settings. In particular, we demonstrate that the resulting PSDM images converted back to time exhibit significantly improved focusing and structural delineation compared to equivalent PSTM images. Depth velocity model building The estimation of an accurate 3D depth velocity model certainly remains one of the greatest challenges in seismic imaging. Depth velocity model building typically involves: N Initial PSDM N Residual moveout (RMO) picking on common image point (CIP) gathers N Structural dip picking in the migrated domain N Update of the depth velocity model by ray-based tomographic inversion Starting from an initial velocity model, an initial PSDM is performed to build CIP gathers for RMO picking. The objective of the velocity model building/update is to minimize the observed RMO (related to the velocity error) in the migrated domain, flattening the events on the CIP gathers. From the knowledge of the RMO and the structural dip one can implement a linearized solution to tomographically update the velocity model with the objective of reducing RMO in the next PSDM run (Al-Yahya, 1989; Liu and Bleistein, 1995). The tomography is usually repeated through several iterations to solve for complex non-linear effects. To avoid the extra delay involved in running a full PSDM and re-picking RMO and dip after each tomographic update iteration, Guillaume et al. (2001) proposed an efficient kinematic approach. This same kinematic approach is used by Beyond Dix.

The 3D ocean-bottom seismic (OBS) surveys acquired by BP over the Azeri-Chirag-Gunashli (ACG) structure in the southern Caspian Sea prompted the development of new techniques for velocity model building, tomography, anisotropy estimation and event matching, to generate prestack depth-migrated datasets of both pressure-wave (PZ) and mode-converted shear-wave (PS). The OBS data are better able to resolve previously poorly imaged mud volcano features, reducing structural uncertainty particularly over the crest of the anticline, and provide a greater degree of confidence in well positioning. Subsequent drilling in the area has verified the viability of the techniques developed.

Multicomponent data increased the information of seismic data. Recent developments of ocean bottom seismic (OBS) technology make more multicomponent data available. From these data, both P-P and P-S sections can be obtained. Since these waves carry different properties of the target reflectors, combining them together can provide us with petrophysical parameters that cannot be obtained with P-wave data alone. However, extracting the new information requires more physically rigorous techniques. Many attempts have been made to carry out elastic wave migrations. Chang and McMechan (1987, 1994) conducted 2D and 3D elastic reverse-time migrations using a full wave finite-difference method. Zhe and Greenhalgh (1997) used potentials, instead of displacements, to propagate P and S waves. Hokstad (2000) proposed a multicomponent Kirchhoff migration method and a specially designed imaging condition for multicomponent elastic waves. On the other hand, others chose to treat elastic waves as scalar waves with different speeds and used scalar wave propagator to do the migration. Using scalar wave propagators to simulate elastic wave propagation, although straightforward, sometimes over simplifies the real physical processes. On the other hand, the phase screen method for scalar wave migration has been used for many years (Stoffa et al., 1990). Several modifications of this method have been made to improve its accuracy under wide-angle and large velocity contrasts (e.g., Ristow and Ruhl, 1994; Jin, et al., 1998; Xie and Wu, 1998; Huang, et al., 1999; Huang and Fehler, 2000; Xie et al., 2000). Compared with other methods, this one-way wave equation based method provided an efficient, high quality propagator for scalar wave migration. The scalar wave screen propagator has also been extended into elastic wave case (Wu, 1994, 1996; Xie and Wu, 1996, 2000; Wild and Hudson ,1998). Wu and Xie (1994) did some primary work to test the elastic screen method as a backpropagator for multi-component elastic migration. Based on the above-mentioned progresses, it is natural to extend the scalar wave screen migration method to the multicomponent elastic wave case. In the following sections, we propose an elastic screen propagator and vector imaging conditions for prestack depth migration and imaging of multicomponent seismic data.

Ocean Bottom Seismic (OBS) survey acquired over the Johan Sverdrup field provided a superior image to the streamer data from P-wave imaging. When processing PS data we observed that PS image ties reasonably well with PP image in depth. However, the PS image below chalk is poorer than the PP image. Despite a generally simple overburden without big shallow gas anomalies, the presence of the high velocity contrast injectite intrusions and a high contrast chalk layer, makes PS imaging difficult in the Johan Sverdrup case. The most likely reason for poor PS image below Shetland could be the mode conversion occurring at the chalk interface. Reverse Time Migration (RTM) of PP data shows improvement when compared to Kirchhoff. Similar improvement is expected from RTM migration on PS data, due better handling of multi pathing and mode converted events.

Noise suppression and signal enhancement prior to prestack depth migration (PreSDM) may significantly increase the resolution of the depth image, and the effectiveness of the PreSDM workflow. The Common-Reflection-Surface (CRS) technique was previously used for this enhancement of seismic prestack data, providing so-called CRS gathers with regularized CMP and offset coverage, and with a strong noise suppression. These CRS gathers considerably improved the depth image in Kirchhoff PreSDM but were not suited for shot-based PreSDM algorithms. This case study now presents a straightforward way to produce geometry-preserving CRS gathers that similarly increase the signal-to-noise ratio. In a first implementation, CRS prestack data interpolation is performed at the existing trace locations providing a straightforward and automatic preservation of the original shot geometry. Application to 3D seismic land data demonstrates the improved signal-to-noise ratio and resolution both in the geometry-preserving CRS shot gathers, and in the corresponding QC stack. As in the Kirchhoff migration of regularized CRS gathers, such enhancements are expected for Reverse Time Migration of CRS shot gathers as well.

Ocean Bottom Seismic (OBS) has seabed planted multicomponent geophones that are used in combination with a conventional pressure source near the sea surface.

The exploiting of High Resolution (HR) Pre Stack Depth Migration (PSDM) 3D seismic volumes, normally used for Oil & Gas exploration, has been pushed forward in geomorphological and geohazard risk evaluation. The novel approach proposed here allows to carry out such activities very early in respect of the standard work flow. Early awareness of critical areas turns out to be crucial in fast-tracking projects and allows a design to cost optimization. The 3D HR PSDM outputs are processed in order to generate a detailed imaging of the shallower portion of the seismic volumes. The volumes are processed at a 2 meters depth interval and converted in time (DTT). Finally, a dedicated post migration time processing sequence, followed by time-to-depth conversion, is applied to generate a Higher Resolution Volume (HRV) in depth domain. The resulting 3D volume is then analyzed to study the seabed and the sub-bottom from a geomorphological standpoint. The analyses focus on the identification and mapping of the distribution of the "areas of instability" eventually classified according to a specific KPI (Safety Factor Index in static conditions), providing a quantitative slope stability assessment of the area. The new approach has been validated comparing the DTM (Digital Topographic Model) derived from the 3D HR PSDM volume and the available MBES (Multi Beam Echo Sounder) bathymetry. The proposed approach leads to a dramatic improvement in the detection capability, highlighting the major critical structures such as: canyon flanks, buried slides, creeps and tension cracks on the shelf break, boulders and compacted sediments, sediment banks and sediment waves reshaped by bottom currents, pockmark areas and fluid escapes, turbidity mass movements and furrows due to tectonic activities. The approach matches perfectly the detection capability of a traditional MBES approach. The described workflow is potentially highly beneficial for early de-risking assets and operations, especially for facilities installation. The proposed innovative approach allows a detailed planning of dedicated data acquisition campaigns, restricted to the most critical areas, with a tangible reduction in the turnaround times and cost savings crucial for project economics.

Unocal Indonesia is using 3-D prestack depth migration to better image Indonesian prospects in difficult seismic areas. The goal is to delineate the deep fault geometry beneath image-distorting carbonates. Previous 2-D prestack depth migration has resulted in significantly better structural imaging, which is motivating the 3-D work. Lateral velocity variations of 2 to 3 km/s within distances of less than 1 km make conventional time migration methods inadequate to accurately image structures and faulting. Iterative 2-D depth migration has provided better structural imaging in depth due to more accurate interval velocities and structural geometry resolution. 3-D prestack depth migration provides better migration velocity analyses and superior structural imaging, especially in comparison to 2-D lines acquired in directions oblique to major faults. The most important step in depth migration is construction of a 3-D interval velocity model. All available data are used in constructing the model: Dip Moveout (DMO) stacking velocities, poststack time migration, well logs, check-shot surveys, gravity and magnetic data, seismic interpretations, and the 3-D prestack depth migration velocity analyses. The model is developed iteratively from the surface downward. Modeling packages such as GOCAD assist the data integration and 3-D visualization quality control. The 3-D migration algorithm is a Unocal proprietary Kirchhoff migration with ray - trace traveltimes and migration computed on a 3 Gigaflop, 10 CPU Silicon Graphics Power Challenge.

Migration is important issue for seismic imaging in complex structure. In this decade, depth imaging becomes important tools for producing accurate image in depth imaging instead of time domain imaging. The challenge of depth migration method, however, is in revealing the complex structure of subsurface. There are many methods of depth migration with their advantages and weaknesses. In this paper, we show our propose method of pre-stack depth migration based on time domain inverse scattering wave equation. Hopefully this method can be as solution for imaging complex structure in Indonesia, especially in rich thrusting fault zones. In this research, we develop a recent advance wave equation migration based on time domain inverse scattering wave which use more natural wave propagation using scattering wave. This wave equation pre-stack depth migration use time domain inverse scattering wave equation based on Helmholtz equation. To provide true amplitude recovery, an inverse of divergence procedure and recovering transmission loss are considered of pre-stack migration. Benchmarking the propose inverse scattering pre-stack depth migration with the other migration methods are also presented, i.e.: wave equation pre-stack depth migration, waveequation depth migration, and pre-stack time migration method. This inverse scattering pre-stack depth migration could image successfully the rich fault zone which consist extremely dip and resulting superior quality of seismic image. The image quality of inverse scattering migration is much better than the others migration methods.