David Sixta

Tomostatics: Turning‐ray tomography + static corrections

Applications of tomographic velocities to static corrections have drawn much attention recently. In Computing field statics with the help of seismic tomography (Geophysical Prospecting 1987), W.N. de Amorin et al. showed that the waves refracted at the base of a low‐velocity layer can be used successfully for computing and refining field static corrections. In Tomographic inversion for velocity plus statics (SEG Expanded Abstracts 1991), Livia Squires et al. pointed out that velocity variations and static shifts caused by near‐surface effects and positional uncertainties can be solved simultaneously using constraint tomography.

Overthrust imaging with tomo‐datuming: A case study

Through the use of iterative turning‐ray tomography followed by wave‐equation datuming (or tomo‐datuming) and prestack depth migration, we generate accurate prestack images of seismic data in overthrust areas containing both highly variable near‐surface velocities and rough topography. In tomo‐datuming, we downward continue shot records from the topography to a horizontal datum using velocities estimated from tomography. Turning‐ray tomography often provides a more accurate near‐surface velocity model than that from refraction statics. The main advantage of tomo‐datuming over tomo‐statics (tomography plus static corrections) or refraction statics is that instead of applying a vertical time‐shift to the data, tomo‐datuming propagates the recorded wavefield to the new datum. We find that tomo‐datuming better reconstructs diffractions and reflections, subsequently providing better images after migration. In the datuming process, we use a recursive finite‐difference (FD) scheme to extrapolate wavefield withou...

Resolving the K-2 salt structure in the Gulf of Mexico An integrated approach using prestack depth imaging and full tensor gravity gradiometry

While potential field techniques have long been applied in regional studies and for mineral exploration, the recently-commercialized full tensor gradiometry (FTG) system possesses the resolution and sensitivity required in detailed mapping for oil and gas exploration, even for deep objectives. Used in combination with prestack depth imaging, this provides a potent mapping capability especially in areas of structural complexity. We illustrate this with a subsalt case study from the Gulf of Mexico where an integrated approach using wave-equation depth imaging and FTG in-version succeeds in resolving the base of salt in an area where Kirchhoff depth imaging alone fails.

Seismic imaging of hydraulic fractures in Carthage tight sands; a pilot study

An understanding of fracture geometry and growth is critically important in the design of hydraulic fracture treatment programs to optimize production from a petroleum reservoir. For example, treatment cost increases exponentially with fracture half‐length (Figure 1a) where fracture half‐length is defined as the lateral distance that an assumed vertical, symmetric planar fracture grows from the well bore as a result of hydraulic fracturing.

Are pore pressure related drilling problems predictable? The value of using seismic before and while drilling.

Over-pressured sedimentary formations are a common feature within the more prolific drilling areas within the US such as offshore Gulf of Mexico, South Louisiana and South Texas. While over-pressured zones could serve as prospect evaluation tool for the explorationist, they are a significant safety concern to the driller. Accurate knowledge of formation porepressure and fracture pressure is critical for safe and economical drilling in these areas. While it has been established that the pore-pressure can be estimated from surface seismic velocities, reliance on use of offset well casing and mud programs is still very common leading to costly drilling problems. Furthermore, the 3-D seismic velocities more commonly available today, are rarely used to generate 3-D pore-pressure cubes that could be an important input to prospect evaluation and well design. In this paper, we present a case study where we have utilized 3-D seismic velocities and well data to generate a 3-D pore-pressure cube over a 100 sq. mile region in South Louisiana. The technique is validated by comparing pore-pressure to mud weight programs in nearby wells in the 3-D survey. We also show how inversion of an intermediate VSP could be used to get higher resolution pore-pressure ahead of the bit before drilling through the critical zones indicated by the predrill pore-pressure prediction. Such integrated predrill and while-drilling analysis can greatly reduce uncertainty and risks involved in drilling in overpressured regions.

Comparison of Sign-Bit And Conventional Seismic Recording In Eastern Colorado

Close comparison of SV and SH, PISv and SV/P, etc., will eventually provide additional information, mainly about anisotropy. This might be very useful for some field studies, because fracture direction should be involved. As of now, no examples are known. Example 1. Study in carbonate environment. On an initial experimental line P, SH and P/STwaves were recorded, using an explosive source. The sections that were obtained are good enough for automatic evaluation of the yT coefficient, using correlation techniques. On this line the yT coefficient obtained from P and SH , and the yr coefficient obtained from P and P/S” modes can be compared. They are close, and can be used in the same way (Figures 4 and 5). &phones: For recording in P and P/SV modes only, acquisition can be carried out in the same run, using the same P-wave source, merely doubling the number of recording channels. Each geophone station corresponds to two field traces: one with conventional vertical geophones, the other with horizontal X oriented geophones. For such purposes, it is convenient to use biphones, which include both vertical and horizontal detectors. Example 2. A second experimental line was recorded using a P-wave source only, with Z and X oriented detectors. Results were satisfactory, and provided an estimate of the yT coefficient.