Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
SIMULTANEOUS CONVENTIONAL AND PHASE ENCODED
SEISMIC ACQUISITION
FIELD OF THE INVENTION
[0003] This invention relates to seismic prospecting for hydrocarbon
resources and
especially to acquisition of seismic data using sweep-type seismic sources.
BACKGROUND OF THE INVENTION
[0004] In the process of acquiring seismic data, a crew is typically
deployed across
several square miles of a survey area positioning cables and seismic receivers
while
seismic sources move from predetermined point to predetermined point to
deliver
vibrational seismic energy into the earth. The receivers capture the reflected
signals that
are recorded and subsequently processed to develop images of geologic
structures under
the surface.
[0005] Occasionally, two crews will end up working survey areas that are
sufficiently
close that recordings in one survey area will include seismic energy delivered
in the other
survey area. As both crews try to acquire useful seismic data, both typically
end up with
obscured data that is difficult to process and unhelpful for resolving the
subsurface
geology. Typically, when these situations arise, the crews end up time sharing
such that
only one crew is permitted to deliver seismic energy at any moment in time.
While the
data is much more useful and valuable, the time for acquiring the data is
prolonged and
much more costly.
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[0006] Phase encoded seismic sweeps by sweep vibrators on separate
source points,
sometimes described as Zenseise seismic prospecting, increases seismic survey
productivity over conventional seismic by acquiring data at several source
points at the
same time. If two phase encoded surveys are being conducted at the same time
in close
proximity, as long as the start times for each sweep are reasonably separated
and the
phase encoding of each crew is optimally tuned, each may generally proceed
without
time sharing. However, a conventional seismic survey crew receives the phase
encoded
sweeps as significant noise and, heretofore, have not been able to proceed
when another
crew is in the area.
BRIEF SUMMARY OF THE DISCLOSURE
[0007] The invention more particularly includes a process for
conducting two
adjacent seismic surveys in close proximity and concurrently where each survey
utilizes
multiple sweep type seismic vibrators and where one survey is conducted with
the
seismic vibrators using sweeps of about twice the length of time or longer as
the sweeps
of the seismic vibrators of the other survey.
[0008] In one embodiment, one survey is shaking one shot point at a
time while the
other shakes multiple shot points concurrently. The sweeps may begin at first
frequency
and end at a second frequency and progress from the first to the second
wherein the first
frequency of one survey is at least five percent different than the first
frequency of the
other survey. The second frequency of one survey may be at least five percent
different
than the second frequency of the other survey. Additionally, one survey may
utilize an
upsweep while the other utilizes a downsweep.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] A more complete understanding of the present invention and
benefits thereof
may be acquired by referring to the follow description taken in conjunction
with the
accompanying drawings in which:
[0010] Figure 1 is perspective view of a first seismic crew acquiring
data in a
conventional manner;
[0011] Figure 2 is a perspective view of a second seismic crew
acquiring data in a
phase encoded manner, and
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[0012] Figure 3 is a diagram illustrating hypothetical sweeps of the
two survey teams
shown in Figures 1 and 2 to point out how the sweeps may be distinguished and
thus,
where the two crews may continue to gather seismic data without interfering
with one
another.
DETAILED DESCRIPTION
[0013] Turning now to the detailed description of the preferred
arrangement or
arrangements of the present invention, it should be understood that the
inventive features
and concepts may be manifested in other arrangements and that the scope of the
invention
is not limited to the embodiments described or illustrated. The scope of the
invention is
intended only to be limited by the scope of the claims that follow.
[0014] An example of a conventional seismic survey crew is shown in
Figure 1,
where a team 10 of four sweep vibes 11, 12, 13, and 14 are arrayed around a
shot point A
and a plurality of receivers 18 are set to record the signals arriving back to
the surface
after reflecting off of subsurface structures. In the data set, all the energy
put into the
ground by the team 10 are seen as from the single point and may be essentially
summed
to be equivalent to one very big vibe. The team 10 will move together from
shot point to
shot point until every shot point in the survey has been shaken. The source
points may be
arranged in a variety of arrays depending on the paradigms of the designing
geophysicist.
[0015] In Figure 2, a phase encoded seismic survey team 20 is shown
with four vibes
21, 22, 23 and 24 each setting on shot points J, K L and M while a plurality
of receivers
are set to record the signals arriving back to the surface. The data recorded
at each
receiver 28 includes the energy from each of the shot points J. K, L, and M.
Thus, four
shot points are surveyed at the same time and the phase encoding of each
source provides
for subsequent separation of the data so that interpretations of the
subsurface geology
may be made based on data from each shot point to each receiver point. The
goal of
phase encoded seismic surveying is to create the same data set as acquired by
a
conventional seismic survey team, but in significantly less time Thus, it
should be noted
that with four vibes on one shot point will essentially put four times the
energy into the
ground in a set period of time as one vibe. Thus, in the phase encoded survey,
the vibes
spend more time delivering energy into the ground at each shot point as the
vibe.
Typically, a phase encoded seismic survey team with 4 vibes will deliver at
least four
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sweeps of distinctly longer length in time than a conventional survey team
will deliver.
Simply stated, if a conventional survey were to have four vibes provide a
common set of
four sweeps of 10 seconds each for each shot point, a phase encoded seismic
survey team
would have each vibe deliver at least four 20 second sweeps or 80 total
seconds of
energy.
[0016] Turning to the issue of conflicts between two crews in close
proximity to one
another, seismic receivers are not discriminatory in what signals are received
and
recorded. A conventional seismic survey crew has no practical way of
separating out
stray signals from other crews, especially if the other crews are close enough
that the
signals received are of comparable intensity to their own signals. 'The energy
from four
vibes in a nearby phase encoded seismic survey team is comparable to the
energy of four
vibes in a conventional survey. However, the data recorded at each receiver
point in the
conventional survey will be contaminated with information from the vibes in
the phase
encoded survey. Without inforniation about the phase encoding and the start
time, the
identification of the separate vibes is at least impractical and is more
accurately described
as practically impossible. At the same time, the conventional seismic crew
will be
contaminating the phase encoded survey, but the energy from the conventional
vibes is
typically easier to identify, especially once there are many sweeps. Moreover,
with
successive sweeps and the total energy of the sweeps delivered over a longer
time leads
mathematically to stacking out of the errant data and retention of sufficient
useful data.
[0017] The inventive process comprises cooperation from the adjacent
crews and
some tailoring of the sweeps to be used. For example, a sweep is characterized
by a
starting frequency and ending frequency, and a sweep duration. The moment a
sweep
starts is also helpful information to separate sweeps. Most sweeps are
upsweeps, but
using down sweeps for one crew while the other uses upsweeps with further
characterize
the sweeps for distinction in the data record for both crews. So, if one crew
starts its
sweeps at about 4 hertz while the other starts at 10 hertz, and one sweeps
quickly to an
upper frequency of. for example, sweeps to 75 hertz in 10 seconds while the
other sweeps
to 85 hertz in 24 seconds, the return echoes from the vibes of one crew will
appear
distinctive from the other. Thus, both crews can continue to gather seismic
data in what
previously had been too close for concurrent operations.
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[0018] Turning to Figure 3, a chart presents a first hypothetical set
of sweeps for the
conventional crew "1" and the phase encoded crew "2". The phase encoded crew
will
conduct four sweeps 2a. 2b, 2c and 2d of 16 seconds starting at 9 hertz to 65
hertz while
the conventional crew will conduct two sweeps la andlb of 8 seconds in length
starting
at 4 hertz and ending at 72 hertz. In this arrangement, the sweeps will
overlap in at most
two places, labeled "X" and "Y", and for only a brief moment in time. At the
overlap
points, the data will be suspect, but it is unlikely that both X and Y will be
at the same
frequency_ Thus, at the overlap frequency X, the data from the sweep lb will
provide the
gap filling data. The sweep la will provide the data for the overlap frequency
Y. It
should also be noted that each crew will require time to move the vibes from
point to
point. It is expected that a lot of seismic data for each crew will be
gathered while the
other is moving its vibes.
[0019] One aspect that should be noted is that with the phase encoded
system, the
vibes will spend more time on each shot point and will therefore not be able
to move to as
many shot points in a single day as a conventional survey. However, each vibe
in a phase
encoded survey provides the energy, by itself, for each shot point. Thus, when
the vibes
move in a phase encoded survey, four shot points are completed versus a single
shot point
in a conventional survey. Thus, it should be easily seen that a phase encoded
survey is a
much more productive and ultimately time efficient survey_
[0020] In closing, it should be noted that the discussion of any
reference is not an
admission that it is prior art to the present invention, especially any
reference that may
have a publication date after the priority date of this application. At the
same time, each
and every claim below is hereby incorporated into this detailed description or
specification as additional embodiments of the present invention.
[0021] Although the systems and processes described herein have been
described in
detail, it should be understood that various changes, substitutions, and
alterations can be
made without departing from the spirit and scope of the invention as defined
by the
following claims. Those skilled in the art may be able to study the preferred
embodiments and identify other ways to practice the invention that are not
exactly as
described herein. It is the intent of the inventors that variations and
equivalents of the
invention arc within the scope of the claims while the description, abstract
and drawings
are not to be used to limit the scope of the invention. The invention is
specifically
intended to be as broad as the claims below and their equivalents.
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