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From the field, tends of thousands of raw second wave reflections
recorded digital computer tapes are sent to a geophysical processing
center. Specialists at the center turn the raw date into several
different 3 D seismic data "volumes". Signal enhanced 3D seismic
volumes are primarily used to map the structural geology of subsurface
rock layers. In some rock formations and areas. Such
as the Cretaceous Forbes in the eh Northern California Sacramento
Basin, Preserved Amplitude 3D data volumes are used to identify
unusually high amplitude reflection, called "bright spots", which
are commonly associated with natural gas saturated reservoirs.
Seismic wave data in the eh "gather" from is used to perform Amplitude
vs. Offset distance (AVO) analysis on the bright spot. The
ultimate goal of all the analysis is to select promising drilling
locations.

In general, an individual seismic trace can be thought of as a single
reflection of the surface shockwave directed downward throughout
the subsurface rock layers. The seismic wave travels down
throughout the many rock layers, both changing shape as it encounters
different rock horizons and reflecting a bit of its energy back
to the surface depending upon the velocity and density differences
of the stratigraphic boundaries it penetrates. Upon close
inspection, a single seismic wave appears to look like a profile
of surface waves turned sideways, similar to a bow wave created
by a boat on the surface of a still lake. Seismic waves,
like all waves have peaks and throughout of varying size.
The peaks and troughs represent approximate stratigraphic boundaries
of rock formation, although it is important to note that not all
rock layers necessarily cause reflections. Natural gas
in certain rock formations and situations dramatically lowers the
velocity of the fluid-filled reservoir rock and often helps create
a very intense reflection signal as the seismic wave encounters
it.
For the North Arbuckle Prospect are, Vector Seismic, Inc., In
Houston, Texas created a very large 3D volume covering more than
30 square miles and 90 cubic miles. 2, 575 individual shots
were made (one at a time) with up to 1,200 live receiver-geophone
stations arrayed on the surface to record the reflections of the
surface shockwave directed to rock layers deep in the earth. Reflections
were recorded down to more than 4 seconds of two way time (down
from and back to the surface from the deep rocks at about 20,000
feet.) This design insured that potential gas reservoirs in the
eh Forbes formation, believed to be present from 1.1 to 1.7 seconds
(about 4,300 to 6,800 feet deep were adequately sampled.

The individual seismic wave traces totaled 2,617,193.
In 3D processing traces that sample the same point in the subsurface
and are "gathered" together into a common reflection
bins. Binning is essentially a three dimensional grinding and sorting
process. The seismic grid area at Arbuckle was 232 bins wide
and 410 bins long and contained a total of 95,120 bins which are
110 feet apart at bin center.
Traces collected in a bin are called the gather. Although
traces gathered together in a bin share a common reflection point
in the subsurface, they have varying distances between their respective
source and receiver stations on the surface, normally from 150 feet
to over 8000 feet. Looking at these traces in a single bin
before there are combined is the essence of Amplitude vs. Offset
distance analysis. In 1984 Chevron Oil Company in San Francisco
California revealed publicly that, based on seismic data in the
Sacramento Basin, the traces with greater offsets distance tended
to have more amplitude than the near traces when reflected off the
same gas filled strata. This was called Amplitude vs. Offset
analysis. Although several companies had been using variations
of the technique in other parts of the world AVO was officially
born.
Once gathered, traces assigned to a bin in are mathematically
combined or "stacked" to reduce ambient noise and bring out the
true rock reflection peaks and troughs. A single composite trace
is generated for each of the 95,125 bins. This trace approximates
what a single seismic wave would appear to be if one could direct
a single shot straight down and straight back to a geophone in the
center of the bin on the surface.

The 95,120 traces are then stacked and filtered into a variety
of 3D seismic volumes. such as the Preserved Amplitude version to
search for bright spots. The volumes are interpreted on Royale's
3D workstation in their San Diego office. The geologist creates
geophysical cross section views of the earth called seismic sections
to identify bright spots, faults and other subsurface geological
phenomenon. It is also used to make geophysical and geological
maps, predict the depth and a real extent of the objective reservoir
and select the final drilling locations.

Seismic data is expensive and requires a great deal of planning
and exacting execution to insure a successful 3D seismic program.
The program is considered a success if the data quality and coverage
is sufficient to allow the selection of drill sites that yields
commercial gas wells. Royale Energy believes that the North Arbuckle
3D seismic volume, the first of its kind in the area, will yield
a significant of new gas pools. Judging by the initial analysis
several promising drill sites have been selected. By
the end of 1998 the results of the initial drilling should herald
the beginning of several years of successful drilling in the area.
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