Volker Dirks

A 3D subsalt tomography based on wave-equation migration-perturbation scans

We have developed a simple but practical methodology for updating subsalt velocities using wave-equation, migration-perturbation scans. For the sake of economy and scalability (with respect to full source-receiver migration) and accuracy (with respect to common-azimuth migration), we use shot-profile, wave-equation migration. As input for subsalt-velocity analysis, we provide wave-equation migration scans with velocity scanning limited to the subsalt sediments. Throughout the migration-scan sections, we look for the best focusing or structural positioning of characteristic seismic events. The picking on the migration stacks selects the value of the best perturbation attribute (alpha-scaling factor) along with the corresponding position and local dip for the chosen seismic events. The associated, locally coherent events are then demigrated to the base of the salt horizon. Our key observation is that this process is theoretically equivalent to performing a datuming to a base of salt followed by subsalt migr...

Fast Beam Migration - A Step Toward Interactive Imaging

This paper presents an effort toward interactive migration. A prestack depth-domain fast beam migration algorithm is implemented. To achieve efficiency, only picked events are migrated. Picked events in the time domain form a new dataset which is a small fraction of the original dataset in terms of volume. Furthermore, events are migrated wavelet by wavelet instead of sample by sample. The wavelet is mapped into depth domain in the format of a patch. Coherent noise and migration artifacts in migration image are reduced by multi-path migration and intrinsically used dipping information The idea of multi-path algorithm is borrowed from the Gaussian beam migration technique, but only picked events with certain p values (event slowness) are migrated. The resulting Sigsbee image using fast beam migration is superior to that of conventional Kirchhoff migration with a speed-up of at least one order of magnitude.

A Fast And Low Cost Alternative to Subsalt Wave Equation Migration Perturbation Scans

Subsalt velocity analysis using prestack wave equation migration scans through perturbed velocity models is an accurate but expensive approach. To reduce turnaround time and computation cost, we investigated an alternative approach for subsalt velocity analysis. In this approach, we first use the current best velocity model to perform one prestack migration to produce a subsalt image. We demigrate the subsalt image to the base of salt to produce demigrated zero-offset seismic data. Using this demigrated data as input at the base of salt datum, we perform a scan of post stack wave equation migrations through perturbed subsalt velocity models. We demonstrate the necessity of demigrating only to the base of salt in order to avoid significant image degradation and show the feasibility of this methodology using the 2D Sigsbee data set.

3D Finite Angle Tomography Based On Focusing Analysis

We have developed a new 3D finite angle tomography based on the analysis of focusing errors. The seismic input to this new velocity analysis is a set of migration panels that are stacked images formed at various imaging conditions, zerotime or non zero-time. By comparing these different common focusing error panels the best focused events and their corresponding focusing errors are picked. The picked focusing errors and associated local reflector attributes (position and dip) are then fed into the existing 3D finite angle tomography to update the velocity model. This new technique complements the input to our existing 3D tomography based on either residual curvature analysis or migration scan, and is a cost effective alternative to the latter. It is particularly appropriate in subsalt areas where migrated seismic data have poor signal to noise ratio.

Subsalt velocity analysis by combining wave equation based redatuming and Kirchhoff based migration velocity analysis

Due to the geometrical complexity of the typical Gulf of Mexico (GOM) velocity models, with embedded salt bodies of any shapes, wave equation migration is used preferentially over Kirchhoff methods for subsalt velocity model building. This preference is based on the ability of wave-equation based migrations to overcome the need for tracing complex ray paths through the salt bodies and for a better handling of multi-path arrivals via wavefield reconstruction. Subsalt velocity analysis uses prestack wave equation migration scans that are created from perturbed velocity models: this is an accurate albeit expensive approach that requires multiple runs of prestack wave equation migration. To reduce the computation cost, we present in this paper a low-cost alternative to perform subsalt velocity analysis. For those cases where sediment velocity structure is relatively simple, we perform a single one-time redatuming to the base of salt (BOS), using existing prestack wave equation tools. By redatuming, we remove the complexity of the wavefield caused by the salt bodies. Once having obtained a simplified wavefield by stripping off the effects of the complex overburden, we can employ less expensive Kirchhoff imaging algorithms for performing subsalt velocity model building.

3D Finite-offset Depth Tomography Model Building: Green Canyon, Gulf of Mexico

A more detailed velocity analysis is required for successful pre-stack depth migration model building and tomographic methods offer a potential solution. However, migration velocity analysis is often a underdetermined problem. We present a new finite-offset depth tomography scheme that overcomes the problems of non-linearity and allows us to perform automatic dense velocity analysis in structurally complex areas were classical linear methods fail. We demonstrate the advantage of the new tomography scheme on a deep offshore Gulf of Mexico dataset from the Green Canyon area.

Automatic, dense and volumetric picking for high‐resolution 3D tomographic model building

Summary As survey sizes are steadily increasing, 3D tomogra phy as the key component of today’s velocity model buildin g workflows has to be performed on large data volumes within acceptable turnaround times. To provide high resolution velocity models, dense volumetric pickin g is required. This leads to even larger data volumes. I t is therefore paramount to automate as many processing steps as possible to free the time of the geophysicist fo r the necessary QC. In this paper we present a modified workflow for high-resolution 3D tomography addressing the need for process automation. We are putting the emphasis of this paper on data preparation and the quality control of the inversion phase. The presented workf low allows performance of high-resolution tomography work on large data volumes over a significantly reduced cyc le time.

3D Beam Based Prestack Redatuming For Efficient Subsalt Velocity Analysis

Accurate velocity model building is a key in achieving high quality subsalt images. To reach this goal, we propose a target oriented and efficient velocity analysis methodology. Our methodology comprises of two steps. In the first step the surface data is downward extrapolated to a new datum plane using a beam based redatuming algorithm. In the second step we switch to a Kirchhoff or fast beam migration algorithm to perform the actual velocity analysis. The redatuming process has the advantage that it not only regularizes the subsurface data, but also reduces the data to a shorter recording times and a narrower offset range. Most importantly the resulting wave field at the new datum plane is generally much simpler in nature as supra salt sediment and top salt reflected as well as refracted arrivals have been removed – hence revealing the weaker subsalt events. A fast migration method can therefore be used to scan the subsalt velocity to determine the best image quality. As the beam based redatuming converts the irregular surface data into regular data at the datum level, it can also be used as a regularization tool.

Prestack wave-equation migration as a routine production tool

In recent years the search for hydrocarbon reservoirs has moved into deeper and more complex environments. In the Gulf of Mexico this has meant exploring below salt sheets, which distort the seismic raypaths and thus effectively mask the underlying geology. In northwest Europe subsalt plays face similar challenges and in the North Atlantic/Faroes imaging below basalt is a major issue.

High-resolution Images From 4-C Horizontal-well VSP Data

Drilling technology has advanced to the stage where horizontal-well completions are now common. The Rosa-3 horizontal well was designed to appraise an oligocene channel system within a graben structure in the deep Angolan offshore. This channel has been affected by a turtle-back anticline structure linked to a regional salt gravity tectonics. The mechanism has created an important conjugate faulting, which divides the structure into different fault blocks. Stratigraphically, the channel system consists of several channelised turbidite complexes. The objectives of the Rosa-3 well were to calibrate these turbidite complexes and check the feasibility of a subhorizontal well to overcome the compartmentalization of the reservoir. More precisely, the aim of the VSP on the horizontal drain was to complete the calibration with the 3D surface seismic and to provide an accurate channel reservoir image below the drain trajectory for better compartment positioning with the associated faults.