Kurang Mehta

Improving the virtual source method by wavefield separation

The virtual source method has recently been proposed to image and monitor below complex and time-varying overburden. The method requires surface shooting recorded at downhole receivers placed below the distorting or changing part of the overburden. Redatuming with the measured Green’s function allows the reconstruction of a complete downhole survey as if the sources were also buried at the receiver locations. There are still some challenges that need to be addressed in the virtual source method, such as limited acquisition aperture and energy coming from the overburden. We demonstrate that up-down wavefield separation can substantially improve the quality of virtual source data. First, it allows us to eliminate artifacts associated with the limited acquisition aperture typically used in practice. Second, it allows us to reconstruct a new optimized response in the absence of downgoing reflections and multiples from the overburden. These improvements are illustrated on a synthetic data set of a complex laye...

Controlled-source interferometric redatuming by crosscorrelation and multidimensional deconvolution in elastic media

Various researchers have shown that accurate redatuming of controlled seismic sources to downhole receiver locations can be achieved without requiring a velocity model. By placing receivers in a horizontal or deviated well and turning them into virtual sources, accurate images can be obtained even below a complex near-subsurface. Examples include controlled-source interferometry and the virtual-source method, both based on crosscorrelated signals at two downhole receiver locations, stacked over source locations at the surface. Because the required redatuming operators are taken directly from the data, even multiple scattered waveforms can be focused at the virtual-source location, and accurate redatuming can be achieved. To reach such precision in a solid earth, representations for elastic wave propagation that require multicomponent sources and receivers must be implemented. Wavefield decomposition prior to crosscorrelation allows us to enforce virtual sources to radiate only downward or only upward. Virtual-source focusing and undesired multiples from the overburden can be diagnosed with the interferometric point-spread function (PSF), which can be obtained directly from the data if an array of subsurface receivers is deployed. The quality of retrieved responses can be improved by filtering with the inverse of the PSF, a methodology referred to as multidimensional deconvolution.

Virtual source applications to imaging and reservoir monitoring

The virtual source method is a breakthrough that allows us to image and monitor the subsurface in cases where surface seismic or VSP fail to deliver. In settings where the overburden is complex or changing, traditional time-lapse signals are weak or nonrepeatable, leading to ineffective seismic input to reservoir modeling and little of value generated by repeated seismic surveys. On the other hand, the virtual source method allows us to image under complex overburden, yields repeatable data for reservoir monitoring, enables shear seismic, and may help us “look ahead” as we drill.

Acquisition geometry requirements for generating virtual-source data

Using model and field data, this article reviews the virtual-source method and its acquisition geometry requirements. Before we go into the details of the acquisition geometry requirements, let us briefly review the basic concept and the advantages of the virtual-source method. A typical surface seismic experiment has sources on the surface to excite waves that propagate through the subsurface. Surface receivers record the reflected waves. In order to image the subsurface, we migrate the reflected wavefield recorded by the receivers, using an estimate of the subsurface velocity model. However, the near surface is usually complex, and the velocity is difficult to estimate. These near-surface inhomogeneities, if not represented in the migration velocity model, defocus the deeper image. In order to avoid the estimation of the near-surface velocity model, Bakulin and Calvert (2006) proposed the virtual-source method, a technique that uses cross-correlation of the wavefield recorded by a given pair of receiver...

Strengthening the virtual-source method for time-lapse monitoring

Time-lapse monitoring is a powerful tool for tracking subsurface changes resulting from fluid migration. Conventional time-lapse monitoring can be done by observing differences between two seismic surveys over the surveillance period. Along with the changes in the subsurface, differences in the two seismic surveys are also caused by variations in the near-surface overburden and acquisition discrepancies. The virtual-source method monitors below the time-varying near-surface by redatuming the data down to the subsurface receiver locations. It crosscorrelates the signal that results from surface shooting recorded by subsurface receivers placed below the near-surface. For the Mars field data, redatuming the recorded response down to the permanently placed ocean-bottom cable (OBC) receivers using the virtual-source method allows one to reconstruct a survey as if virtualsources were buried at the OBC receiver locations and the medium above them were a homogeneous half-space. Separating the recorded wavefields ...

Downhole Receiver Function: a Case Study

Receiver function is defined as the spectral ratio of the radial component and the vertical component of the ground motion. It is used to characterize converted waves. We extend the use of the receiver function to downhole data using waves recorded in a borehole, excited by an earthquake of magnitude 4.0 near San Fran- cisco, California, on 26 June 1994. The focal depth of the event was 6.6 km and the epicenter was located at a distance of 12.6 km from the borehole array. Six three- component sensors were located at different depths in a borehole. To extract a co- herent response of the near-surface from the incoherent earthquake waves, we de- convolve the waves recorded by the sensors at different depths with the waves recorded by the sensor on the surface. Deconvolution applied to the waves in the S- time window recorded by the radial component result in an upgoing and adowngoing wave propagating with S-wave velocity. For the waves in the P-time window re- corded by the radial component, deconvolution also gives an upgoing and a down- going wave propagating with S-wave velocity. This interesting result suggests a P- to-S conversion at a depth below the deepest sensor. To diagnose this we compute the receiverfunction forthe borehole recording of the earthquakewaves.Thereceiver function shows an upgoing wave with an arrival close to time t ! 0 for the deepest sensor. The agreement of the upgoing wave in the receiver function with the travel- time curve for the P-to-S converted wave, calculated using the P- and the S-wave velocity profile, supports the hypothesis of a pronounced P-to-S conversion. We present a synthetic example to illustrate that the first arrival of the receiver function applied to borehole data gives the upward-propagating P-to-S converted wave. To corroborate the observation of the mode conversion, we apply receiver function to a different earthquake data recorded by the same borehole array in 1998. The focal depth of the event was 6.9 km and the epicenter was located at a distance of 13 km from the borehole array. The receiver function for these data also show an upgoing wave with a pulse close to time t ! 0 at the deepest sensor. The moveout of the upgoing wave agrees with the travel-time curve for the P-to-S converted wave, hence supporting our observation of the mode conversion.

Virtual source method applied to crosswell and horizontal well geometries

The virtual source method is a useful tool for redatuming a seismic survey below complicated overburden by creating virtual sources at downhole receiver locations, hence generating data independent of the overburden and the time-lapse changes therein. In this article we first apply this technique to crosswell geometries, whereby a virtual crosswell survey is simulated by shooting a line of surface shots passing through two boreholes instrumented with downhole sensors. Using this acquisition geometry, receivers in one of the two wells are turned into virtual sources by correlating the wavefield recorded by those receivers with the recording at receivers in the other well, and summing the correlated data over the surface shots.

Comparing Virtual Versus Real Crosswell Surveys

The virtual source method (VSM) is a useful tool for imaging below complex overburden and monitoring in the presence of time-varying overburden. This concept, when extended to crosswell geometry produces data comparable to real crosswell data. Using a field data example we demonstrate that virtual crosswell data is kinematically comparable to real crosswell data, but the virtual crosswell method possesses flexibilities, which are difficult to achieve in a real crosswell survey. Some of these flexibilities include the ability of the virtual source to radiate either horizontally or vertically and the possibility for the virtual source to radiate only Por only S-waves. It is also possible to create virtual crosswell data that contain only the direct arrivals or only the reflections. These features of the virtual crosswell method should make it useful for crosswell tomography, imaging and reservoir monitoring for moderate interwell distances.

Reconstructing head waves with virtual source method

The original applications of the Virtual Source Method (VSM) concentrate on imaging and monitoring through complex and changing overburden. This can be accomplished by correlating the wavefields recorded by downhole geophones. There are a number of reasons to expect even better results when this concept is extended to using head waves for reservoir imaging and monitoring purposes. The current practice to create virtual source data is to correlate the gated direct arrival at virtual source with total wavefield at receivers. Using a simple acoustic 2D model with two half-spaces having different velocities, we demonstrate the usefulness of correlating the head waves with different types of waves, theoretically and numerically.

Virtual source gathers and attenuation of free‐surface multiples using OBC data:implementation issues and a case study

Virtual source imaging is a technique based on extracting the Green’s function that characterizes wave propagation between two receivers by cross-correlating the wave-fields recorded by these receivers. We focus on implementation issues in generating a virtual source gather from a multi-component OBC data recorded at the Mars field. The implementation issues include choice of the receiver that acts as the virtual source and the number of sources over which the cross-correlated data is stacked. The pre-stack correlated data (correlation gather) is a useful diagnostic for quality control and for assessing the source locations that give a stationary phase contribution. By stacking over specific source locations, we restrict the direction of the incoming energy and generate virtual source gathers containing arrivals within a specified horizontal slowness interval. We compare the virtual source gather generated by using a small number of sources to the virtual source gather generated by using a larger source aperture for stacking. Artifacts due to the traces at the edges of the source aperture can be suppressed by applying a taper before stacking the correlation gather. Another artifact observed in virtual source gathers is due to side-lobes of the auto-correlation of the source-time function. We show the use of dualsensor summation to separate the upand the down-going energy in the raw data and using that to generate virtual source gathers containing only the upgoing energy, hence attenuating the free-surface multiples.