In the early stages of planning exploration and development in a new area, surface seismic surveys are used extensively to delineate prospective structural or stratigraphic traps.
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MSc. Degree or higher degrees
More than 15 years of experience for under-graduates to intermediate courses
More than 20 years of experience for intermediate to experts courses
When wells are drilled, opportunities exist to improve this situation through the use of well logs. After editing and calibrating against check shots, open hole sonic and density logs can be used to generate synthetic seismograms.
If open hole data are not available, in many instances a cased hole sonic log can be recorded for this purpose. These synthetics are extremely valuable in verifying reflection events in a seismic section and relating seismic features to geological structures. Velocity anomalies, which may cause exploration wells to be drilled off-structure, can be resolved.
A more recent geophysical application of wireline logging measurements involves the preparation of a vertical seismic profile (VSP). In this technique, an air gun Vibroseis, or other seismic source on the surface generates the input signal that is detected by a down-hole geophone.
As the sound energy travels only once through the weathered surface layers, the resultant profile has much better resolution than the surface seismic around the borehole, and, in favorable cases, can identify reflectors far below the total depth of the well.
Digital Check-Shoot Survey:
At each depth, the interval velocity of the formations between the source and the borehole geophone is measured. With an air gun source, the hydrophone monitors the signature and timing of the source signal, and the down-hole geophone records the direct and reflected arrivals.
Transit time is measured from the first break of the hydrophone (surface) recording to the first break of the geophone (down-hole) recording. Several shots are usually made at the same level and stacked in order to improve the signal-to-noise ratio.
If the hole is deviated or if there is a significant source offset, the transit times obtained must be converted to true vertical depth (TVD) transit times. Correction to the seismic reference datum (SRD) is also necessary if the source is above or below the seismic datum. The corrected, stacked magnetic tape data can then be converted to a standard SEGY tape format.
Time-To-Depth Conversion and Velocity Profile
Check-shot surveys are used to correct the velocities obtained by the integration of the sonic interval transit times. The adjusted sonic may then be used for the translation of surface seismic time into depth and in the calculation of formation acoustic impedance necessary for the generation of a Geogram* synthetic seismogram and for other applications.
Formation velocities obtained by the integration of sonic logs may differ from those obtained by surface and checkshot surveys for the following reasons:
Because of velocity dispersion with frequency, seismic velocities (measured at roughly 50 Hz) may be as much as 6% lower than sonic velocities (measured at 20,000 THz).
Borehole effects, such as those caused by formation alteration, may decrease the apparent sonic log velocities.
The sonic transit time measurement is fundamentally different from the surñce seismic measurement. The sonic log velocity is measured in a continuous manner alongside the borehole, while the seismic waves reaching the geophone(s) take the most direct acoustic (shortest) path.
The long-spaced or Array-Sonic tools are required for cased hole logs and provide better data than BHC Sonics in open holes. However, all recorded sonic logs should be edited to correct for borehole effects. To adjust a sonic log correctly, check shots ar
Vertical seismic profile (vsp):
Vertical seismic profiling is a technique of simultaneously recording the up-going and down-going wave trains
The total wave-field recorded at the detector in the borehole consists of signals arriving from above the tool (down-going) and the signals arriving from below the tool (up-going).
The down-going signals are the direct (first) arrivals and the down-going multiples. The up-going signals consist of the direct reflections and the up-going multiples.
Advantages of the vertical seismic profile technique include:
Recording a real seismic trace in the borehole rather than relying on a synthetically generated seismogram
Measuring the spectral content of the down
going seismic signal as a function of depth
Establishment of a precise link between the surface seismic results and well logs, since the VSP is a high
resolution measurement
The recording of signals with an improved high
frequency content, since they cross the highly absorptive low
velocity layers near the surface only once
Improved seismic resolution of subtle stratigraphic features around the well, such as faults or pinchouts
The recording of deep reflector signals that are not received at surface; this is particularly useful in structurally complex areas
An excellent record of the band
limited reflection coefficient series through deconvolution of the VSP
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by :: GeoGim
GeoGim Instructors Qualifications:
MSc. Degree or higher degrees
More than 15 years of experience for under-graduates to intermediate courses
More than 20 years of experience for intermediate to experts courses
