Rongrong Lu

A Novel Application of Time Reversed Acoustics: Salt Dome Flank Imaging Using Walkaway VSP surveys

In this paper we present initial results of applying Time-Reversed Acoustics (TRA) technology to saltdome flank, seismic imaging. We created a set of synthetic traces representing a multilevel, walkaway VSP for a model composed of a simplified Gulf of Mexico vertical-velocity gradient and an embedded salt dome. We first applied the concepts of TRA to the synthetic traces to create a set of redatummed traces without having to perform velocity analysis, moveout corrections, or complicated processing. Each redatummed trace approximates the output of a zero-offset, downhole source and receiver pair. To produce the final salt-dome flank image, we then applied conventional, poststack, depth migration to the zero-offset section. Our results show a very good image of the salt when compared to an image derived using data from a downhole, zero-offset source and receiver pairs. The simplicity of our TRA implementation provides a virtually automated method to estimate a zero-offset, seismic section as if it had been collected from the reference frame of the borehole containing the VSP survey.

Redatuming through a salt canopy and target-oriented salt-flank imaging

We describe a new shortcut strategy for imaging the sediments and salt edge around a salt flank through an overburden salt canopy. We tested its performance and capabilities on 2D synthetic acoustic seismic data from a Gulf of Mexico style model. We first redatumed surface shots, using seismic interferometry, from a walkaway vertical seismic profile survey as if the source and receiver pairs had been located in the borehole at the positions of the receivers. This process creates effective downhole shot gathers by completely moving surface shots through the salt canopy, without any knowledge of overburden velocity structure. After redatuming, we can apply multiple passes of prestack migration from the reference datum of the bore-hole. In our example, first-pass migration, using only a simple vertical velocity gradient model, reveals the outline of the salt edge. A second pass of reverse-time, prestack depth migration using full two-way wave equation was performed with an updated velocity model that consisted of the velocity gradient and salt dome. The second-pass migration brings out dipping sediments abutting the salt flank because these reflectors were illuminated by energy that bounced off the salt flank, forming prismatic reflections. In this target-oriented strategy, the computationally fast redatuming process eliminates the need for the traditional complex process of velocity estimation, model building, and iterative depth migration to remove effects of the salt canopy and surrounding overburden. This might allow this strategy to be used in the field in near real time.

Fracture quality from integrating time-lapse VSP and microseismic data

Tight gas reservoirs are problematic to produce, often requiring multiple stages of hydraulic fracturing in order to create connected pathways through which hydrocarbons may flow. In this paper, we propose a new methodology to characterize the quality of hydraulic fractures. Using synthetic VSP and microseismic data, we test the concept that the rock volume containing open, gas-filled fractures will scatter seismic energy more profusely than a volume containing closed, nonproductive fractures. By measuring the amount of scattered energy in a time-lapse 3D VSP study taken before and after the hydraulic fracturing episode, we hope to compare the productive flow quality of different regions of the hydraulically fractured rock. The microseismic recordings allow us to locate areas which have been hydraulically fractured and create imaging operators to extract the scattered signals from the time-lapse VSP data.

Imaging dipping sediments at a salt dome flank - VSP seismic interferometry and reverse-time migration

Summary We present results of applying seismic interferometry to image dipping sediments abutting a salt dome. We create a set of synthetic traces representing a multi-level, walk away Vertical Seismic Profile (VSP) for a model composed of a simplified Gulf of Mexico vertical-velocity gradient and an embedded overhanging salt dome. The sediment reflectors in the model dip up towards the salt dome flank. To process these data, we create a set of redatummed traces using seismic interferometry. This is done without having to perform any velocity analysis or moveout corrections. Each of these redatummed traces mimics the output of a downhole source and down-hole receiver pair. The linear v(z) gradient enables the redattumed data set to illuminate and capture reflections from both the salt-dome flank and the upward turning sediments. We then apply pre-stack depth migration to these traces to produce the final image of the beds and the salt dome flank. The final migrated results demonstrate that the reflected turning ray energy from both the salt flank and sediments are adequate to create structurally correct images using the combination of seismic interferometry and prestack depth migration.

Redatumming Through a Salt Canopy – Another Salt Flank Imaging Strategy

We describe a new short cut strategy to image the sediments and salt edge around a salt flank through an overburden salt canopy. We demonstrate its performance and capabilities on a synthetic acoustic seismic data from a Gulf of Mexico (GOM) style model. In this strategy, we first redatum the surface shots from a walk away Vertical Seismic Profile (WVSP) survey to be as if the source and receiver pairs had been located in the borehole at the positions of the receivers. This process creates effective downhole shot gathers by completely moving the surface shots through the salt canopy without any knowledge of the overburden velocity structure. After redatumming, we apply reverse time prestack depth migration to the effective downhole shot records using a simple linear v(z) gradient velocity model. This first pass of migration reveals the salt dome edge quite well. Once the salt dome edge is defined, a second pass of reverse time prestack depth migration is performed with an updated velocity model that now consists of the v(z) gradient and the salt dome. The second pass migration brings out the dipping sediments abutting the salt flank because these reflectors were illuminated by energy that bounced off the salt flank forming prismatic reflections. In this target-oriented strategy, the computationally fast redatumming process eliminates the need for the traditional complex process of velocity estimation, model building, and iterative depth migration to remove the effects of the salt canopy and surrounding overburden.

Imaging a salt dome flank by directional redatuming of a field 3‐D VSP survey

We processed a large 3-D vertical seismic profile (VSP) around a Gulf of Mexico (GOM) salt dome. The VSP was acquired to address uncertainty in subsalt structure and in locating the dipping edge of the salt base on the surface seismic images. To simplify the processing, remove potential uncertainty in the velocity of the overburden, and image the salt edge, we create and apply a strategy of seismic interferometric, directional redatuming followed by prestack depth migration. The surface VSP shots are grouped into 10° wedges based on the shot to geophone azimuth. Each wedge is redatumed and migrated separately to create separately steered images of the salt flank. The final merged result complements and is compatible with the interpretation of the surface seismic data. The redatuming methodology does not require knowledge of the velocity structure between the surface shots and the downhole receivers, and the salt-flank reflections are easily seen on the intermediate redatumed, virtual source gathers.

Fracture Quality From Integrating Time-Lapse VSP and Microseismic Data

We present a new methodology to characterize the quality of hydraulic fractures by extracting the amount of scattered seismic energy from the induced fractures. A 3D VSP is collected over the field before and after the hydraulic fracturing is performed. Microseismic recordings of the hydraulic fracturing treatment form the basis for imaging operators. These imaging operators are used on the time lapse (difference) VSP volumes to extract the amount and angular variation in amplitude of scattered energy. Model results show that compliant, open fractures produce larger amounts of scattering than stiff, closed fractures. It may also be possible to use the azimuthal variation in scattered energy as an additional indicator of fracture compliance.

Application of TRA to the Pre- and Post-Stack Imaging of a Salt Dome Flank

We present results of applying Time-Reversed Acoustics (TRA) to seismic imaging of a salt dome flank. We create a set of synthetic traces representing a multi-level, walk away VSP for a model composed of a simplified Gulf of Mexico vertical-velocity gradient and an embedded overhanging salt dome. To process these data, we first apply source-receiver reciprocity and the concepts of TRA to the synthetic traces. This creates a set of redatummed traces without having to perform any velocity analysis, moveout corrections or other processing. Each of these redatummed traces is equivalent to the output of a down-hole source and down-hole receiver pair. For quick turn around analyses, we can select from these redatummed traces only the zero-offset, or collocated, down-hole source/receiver pairs. For a more thorough analysis we utilize all the redatummed traces. We then apply conventional post- or pre-stack depth migration, respectively, to these traces to produce the final image of the salt dome flank. The migrated results show that the redatummed turning ray energy reflected from the salt dome flank is handled correctly by this method and produces effective images of the salt.