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For a single flat layer the shape of steamboat velocity analysis moveout curve is defined by the hyperbolic relationship between zero-offset time and velocity.

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. Several velocity analysis methods have been used in the past but today steamboat velocity analysis velocities are picked interactively using combination displays on processing workstations.

Nevertheless, velocity analysis steamboat velocity analysis still one of the most time consuming parts of seismic processing. It is also probably the most critical stage since the velocity analysis is an initial interpretation of the Steamboat Springs Vertical Drop Jack data and it is important that the seismic interpreter is involved in the analysis and quality control stages.

Velocity analysis is often carried out several times during processing resulting in an iterative improvement of velocity estimation. It is generally considered that velocity picking is an art. The CMP method generally ignores mode conversions and anisotropy variation of a property with direction of measurement.

Anisotropy is also used to refer to the difference between steamboat velocity analysis velocities vertical measurements steamboat velocity analysis seismic velocities horizontal measurements. A sonic log measures the velocity variation in depth as steamboat velocity analysis immediately adjacent to the borehole. A zero-offset VSP or checkshot survey will measure the vertical velocity in the vicinity of the. Seismic measurements of velocity are averaged over the horizontal distance through which the seismic energy travels.

Sediment velocities generally increase with steamboat velocity analysis due to increased pressure of the steamboat velocity analysis. Fluids within pores tend to make the rocks less compressible and lead to higher interval velocities for P-waves. The adjacent figure summarises typical velocities for differing lithologies and porosities. Carbonates in particular show a large range in velocities depending on porosity.

Sheriff and Geldart show further diagrams illustrating the typical trends of velocity with various parameters. Generally is its correct to stack the data with seismic velocity but little.

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. Is the constant velocity of a single layer which can be very. V int can be approximately calculated from V rms using the Steamboat velocity analysis equation the inverse of that shown in steamboat velocity analysis figure.

The difference between V nmo and V stack is subtle. For 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 root-mean-square RMS velocity is calculated from interval velocities as shown steamboat velocity analysis the figure.

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. Steamboat velocity analysis many targets this can become important at offsets greater than around 3km. Is the velocity required to best stack the data using the best-fit hyperbola over the available offset range.

The choice of V stack can be rather subjective. However, it turns out that an appropriate choice can cover up for a multitude of assumptions made in the CMP stacking process. For horizontal layers and small offsets V stack should equal V rms. Note this assumes no 3D effects. The application of DMO mostly removes the effects of dip from V stack such that V stack steamboat velocity analysis V rms and interval velocities computed from the DIX equation should steamboat velocity analysis stable.

Is the depth divided by the two way time to any interface. V avg is often used for depth conversion but is only valid where the velocity varies only vertically. For significant structures ray-based depth conversion should be used. Is the velocity required to best migrate the seismic data and steamboat velocity analysis related to the true interval velocity, not the stacking velocity.

The adjacent figure a shows steamboat velocity analysis synthetic with four events. As can be seen from the figure a and also the NMO equation the magnitude of moveout decreases at increasing time and with increasing velocity. This reduces resolution of velocity in the steamboat velocity analysis section. The moveout of the deeper event is increased by recording further offsets. However at offsets longer than 3km in a layered earth the hyperbolic assumption is less valid so the actual velocities picked may be in steamboat velocity analysis error by using the far-offset information.

Figure b shows the synthetic after NMO correction using steamboat velocity analysis correct velocity. A loss of temporal resolution and frequency content is seen on the far offsets and shallow times - this is called moveout stretch and occurs because the far steamboat velocity analysis are shifted more then the near offsets.

An interpolator is used in practise to attempt to minimise stretch and maintain resolution and recently contractors have been refining the accuracy with which this is performed.

However at some stage the stretched data should be muted. Figure c shows the results of muting using an automatic stretch algorithm available within Promax. The far offsets show marginal loss of resolution but this is unlikely to affect the quality of the stack trace.

The requirement for muting is steamboat velocity analysis usually too much of a problem since the far offsets and shallow times contain direct arrival, refracted and mode-converted energy which should also be muted.

This is illustrated for real data on the adjacent figure click here for an enlarged version. Figure a shows the CDP before moveout correction, figure b shows the data following NMO and with the appropriate picked mute shown. Note that in some cases the automatic mute may remove real data and it may be better to pick a mute by hand to avoid removing too many far offsets.

For data steamboat velocity analysis high signal-to-noise ratio a harsh mute may be tolerated but for poor SNR fold may be more important. Any AVO effects may also be present on far-offset traces and should be preserved.

As ever the choice will lie with the processor and interpretation geophysicist. Note that NMO stretch limits available fold in the shallower section and particularly limits the resolution of the seabed reflection. For a seabed around ms deep the near-trace offset must be within m to ensure a seabed reflection is recorded uncontaminated by the refraction.

For 3D acquisition this can present a problem since near offsets may be in excess of m for particular source-receiver combinations. Data must be steamboat velocity analysis pre-processed prior to velocity analysis. The processing to be applied will steamboat velocity analysis on the purpose of the analysis since velocities will be picked at several stages during a processing sequence. For the first pass of velocity analysis the data will usually have been deconvolved, sorted to CMP gathers and muted.

Subsequent passes may require application of multiple suppression, DMO or steamboat velocity analysis migration prior to the velocity analysis. Depending on the processing system being used the entire data may be supplied to the velocity analysis routine or steamboat velocity analysis usually the data is edited to a subset which defines the velocity analysis locations.

The data may also be bandpass filtered and scaled to reduce noise prior to velocity analysis computations. Severe noise contamination may be removed by dip-filtering. In areas where velocity analysis is particularly difficult sometimes harsher procedures may be used to prepare data for velocity analysis than would be tolerated in production processing. Steamboat velocity analysis are usually picked steamboat velocity analysis discrete spatial intervals along a seismic section and the velocity steamboat velocity analysis linearly interpolated between analysis points.

The spatial and temporal steamboat velocity analysis interval will depend on the degree of lateral velocity variation and should be sufficient to define the structures under consideration. In theory extra analysis points can be used at any point to more accurately define a particularly complex geological structure.

In practise this option is rarely pursued although some contractors now use automatic picking routines to attempt to in-fill velocities from hand-picked seed points. The reliability of these methods depends on the constraints employed within the picking algorithm. For the first pass of velocity analysis a coarse interval of m or m will be chosen. For a steamboat velocity analysis 20km 2D section, For a typical km 2 3D a 1km interval will result in around analysis points.

For a final pass of post-DMO velocity analysis a typical interval would steamboat velocity analysis m. At each analysis location the interpreter would typically pick 10 time-velocity pairs or knees required to stack the main reflection events.

Modern velocity analysis is almost exclusively based on the hyperbolic assumption derived steamboat velocity analysis a flat multi-layered earth. There are several methods of stacking velocity analysis. The preferred method depends on the data under consideration and the preferences of the velocity picker.

Almost all velocity analysis today is performed interactively on a screen using a combination display configured according to user preference. Animated displays are common and show the results of applying the NMO and stacking the data with the velocities chosen.

In the past velocity analysis was considered to be a computer intensive process and some shortcuts were taken such as reducing the fold of gathers. The power of modern computer systems means that short cuts are no longer required.

Some systems will calculate the velocity analysis on the fly as requested by the user but most systems expect the pre-computation of the velocity analysis displays. The subsequent speed of analysis is limited only by the speed of the picker and the graphics hardware steamboat velocity analysis used. On some displays the interpreter can pick several key horizons which the velocity interpreter can use as main velocity boundaries.

Depending on the geological province this method is critical, for example if velocities are to be picked for depth migration purposes. When picking horizons care should be taken to ensure the velocity interpolation stage can handle pinchouts and other more complex geological structures. The mini-stack panels are displayed next to each other and velocities picked where key events show the highest amplitude or greatest continuity.

The method shows what the data will steamboat velocity analysis like if stacked with the chosen velocity but has a resolution limited to the velocity interval chosen. This may be the best method for data with very steamboat velocity analysis SNR. Some attention should also be paid to the mutes applied for CVS analysis, particularly if multiples are present.

The following displays are available by default on the PROMAX interactive velocity analysis system see adjacent figure or click here for enlarged figure. The individual panels show high resolution but the Steamboat Restaurant Scarborough quality of the panels depends on the accuracy of the initial function used. A combination display would usually show the central gather NMO of the panels corrected using the range of function velocities. The gather and stack displays are interactively updated as picks are.

Stack display a is the stack with the currently picked velocity function, stack b provides an animate display of the original FVS stacks as the picks are altered. The colour background display reflects interval velocity. The maximum amplitude of coherence is expected where the hyperbola best fits a given high amplitude seismic event. The measure of coherence most often used is called semblance which is robust to noise, Steamboat Springs Things To Do Summer 77 spatial aliasing and lateral variations in amplitude.

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