Is the velocity required to best migrate the seismic data and is related to the true interval velocity, not the stacking velocity. For significant structures ray-based depth conversion should be used. V avg is often used for depth conversion but is only valid where the velocity varies only vertically. Is the depth divided by the two way time to any interface. The application of DMO mostly removes the effects of dip from V stack such that V stack approximates V rms and interval velocities computed from the DIX equation should be stable. For dipping layers a higher velocity is required since Vstack = Vrms/cos(dip). For horizontal layers and small offsets V stack should equal V rms. However, it turns out that an appropriate choice can cover up for a multitude of assumptions made in the CMP stacking process. The choice of V stack can be rather subjective. Is the velocity required to best stack the data using the best-fit hyperbola over the available offset range. For many targets this can become important at offsets greater than around 3km. At large offsets more accurate NMO corrections can be performed by retaining the next term of the equation - this is usually referred to by contractors as fourth order or higher order NMO correction. The root-mean-square (RMS) velocity is calculated from interval velocities as shown in the figure.
#Promax seismic kickass series
V rmsįor multiple flat layers and assuming the offset is small compared with the depth, a hyperbolic moveout equation can be derived as a truncated power series in which V rms is used as velocity. The difference between V nmo and V stack is subtle. Is the velocity required to best NMO correct the data using the hyperbolic NMO assumption. V int can be approximately calculated from V rms using the DIX equation (the inverse of that shown in the figure). Is the constant velocity of a single layer (which can be very thin). Referring to definitions shown in the adjacent figure: V int Nevertheless seismic velocity is often used for depth conversion and migration purposes and can be calibrated to well information or used where well information is particularly sparse. Generally is its correct to stack the data with seismic velocity but little else. Sheriff and Geldart show further diagrams illustrating the typical trends of velocity with various parameters. Carbonates in particular show a large range in velocities depending on porosity. The adjacent figure summarises typical velocities for differing lithologies and porosities. Fluids within pores tend to make the rocks less compressible and lead to higher interval velocities for P-waves. Sediment velocities generally increase with depth due to increased pressure of the overburden. Seismic measurements of velocity are averaged over the horizontal distance through which the seismic energy travels. A zero-offset VSP or checkshot survey will measure the vertical velocity in the vicinity of the well. Seismic measurements are typically 5-10% faster.Ī sonic log measures the velocity variation in depth as seen immediately adjacent to the borehole. Anisotropy is also used to refer to the difference between well velocities (vertical measurements) versus seismic velocities (horizontal measurements). The CMP method generally ignores mode conversions and anisotropy (variation of a property with direction of measurement). In horizontally layered media such as shales the transverse anisotropy may cause the velocity to vary as much as 20% depending on dip. It is generally considered that velocity picking is an art. Velocity analysis is often carried out several times during processing resulting in an iterative improvement of velocity estimation. It is also probably the most critical stage since the velocity analysis is an initial interpretation of the data and it is important that the seismic interpreter is involved in the analysis and quality control stages. Nevertheless, velocity analysis is still one of the most time consuming parts of seismic processing. Several velocity analysis methods have been used in the past but today most velocities are picked interactively using combination displays on processing workstations. In this section we define several types of velocity commonly used in seismic processing and concentrate on methods used to determine and quality control velocity analysis. For a single flat layer the shape of the moveout curve is defined by the hyperbolic relationship between zero-offset time and velocity. The CMP method, NMO and the concept of stacking velocity was introduced in a previous discussion. Interactive Velocity Analysis explanation of sample display preconditioning, analysis interval, moveout stretch. VELOCITY ANALYSIS in practice version 1.0 released 29/1/99ĭefinition of velocities e.g.