Roger Turpening

Seismic Sources In Cased And Cemented Boreholes

Far field, stationary phase approximations are often used to study the radiation of P and S-waves from seismic sources located in boreholes, usually with an assumption of low frequency and in application to uncased boreholes. We extend these solutions to apply to cased and cemented boreholes by solving boundary condition equations numerically with propagator matrices, as is generally done in the calculation of synthetic full waveform acoustic logs. In this way, no low frequency assumptions are made, and a generalized stationary phase solution for sources in concentrically layered borehole models is easily obtained. We use this solution to demonstrate the reduction of source amplitudes caused by the introduction of casing and cement into a borehole. The degree to which the source amplitude is reduced is a function of both the type of source (volume injection, radial stress, or axial stress) and of the formation velocity. Cross-hole data from the Earth Resources Laboratory test site, which includes several receiver gathers collected with and without casing in the source hole, confirms the validity of the theoretical results. The theoretical casing compensation factor calculated using the generalized stationary phase solution with a seismic velocity model of the test site is very close to the change in amplitude observed in the data set.

Imaging with reverse vertical seismic profiles using a downhole,hydraulic, axial vibrator.

Summary We present the analysis of a reverse vertical seismic profile (RVSP) acquired over a pinnacle reef in the northern Michigan reef trend. The survey exhibited two features of note: 1) a new, strong, downhole vertical vibrator, and 2) a random distribution of surface receiver locations. A short sequence of processing steps followed by diffraction summation migration provides a high resolution image of a portion of the target reef at 4600 feet depth. The high resolution of the image was largely due to the downhole source, which generated a high powered signal at frequencies up to several hundred Hz. The source signal was repeatable, allowing our processing scheme to recover these high frequencies. Due to adverse conditions, a large portion of the surface spread had to be abandoned. The reduced spatial coverage limits the extent of the migrated image, and therefore precludes an evaluation of the effectiveness of the random receiver spread. However, the partial image agrees with our previous interpretation of the reef. The high resolution offers new insight into the structure of the reef, although a detailed geological interpretation is not possible due to the limited extent of the image.

P- And S-wave Tomographic Images of An Oil Reservoir At MIT's Michigan Test Site

Massachusetts Institute of Technology. Earth Resources Laboratory. Reservoir Delineation Consortium

Elastic Migration/inversion of Three-component Seismic Data From the Springdale Reef

Explorationists have always known that S waves were wntributing to the noise in reflection images. Nevertheless, acoustic techniques were (and are) used in the name of computational speed or inadequate data (i.e., only vertical component data). Here we describe an elastic inversion technique and demonstrate its use on a reef target. Offset VSP is an excellent demonstration of the need for elastic inversion techniques because of the large number of converted waves in the data. Elastic inversion also puts some urgency on the need for good S wave velocities. The results are worth the small additional effort. In the example presented, we begin to see some details in the reef, whereas the surface reflection data and conventionally processed VSPs show only the nature of the B Salt/AZ Carbonate interface. This interface is not the reservoir at all and is only crudely related to the reservoir.