Note: Descriptions are shown in the official language in which they were submitted.
CA 02341846 2001-03-21
BAH-001
A METHOD FOR CONDUCTING SEISMIC SURVEYS
UTILIZING AN AIRCRAFT DEPLOYED SEISMIC SOURCE
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to geophysical exploration. More
particularly, the
invention relates to a novel method for generating a seismic signal.
2. Description of the Prior Art
Reflection seismology is a well-known technique for prospecting for sub-
surface oil
and gas reservoirs, both on land and in marine environments. A seismic source
is utilized to
generate acoustic waves, normally at or near the earth's surface, and these
waves travel
downwardly into the earth's subsurface. A portion of the seismic wave energy
is reflected or
refracted from the subsurface interfaces between earth strata having different
acoustic
velocities, and this reflected or refracted energy is then detected by sensors
which are
normally deployed at the earth's surface. The detected signals are normally
recorded for later
signal processing. The travel time of the seismic signal from the seismic
source location
down to various subsurface interfaces is determined and this travel time along
with a velocity
profile of the earth's subsurface are utilized to determine the subsurface
location of these
reflecting interfaces. The velocity profile may be determined from the seismic
data or it may
have been predetermined. Sub-surface acoustically reflecting interfaces often
correspond to
the location of an oil and gas reservoir.
A wide variety of sources have been used, including dynamite, mechanical
impact
sources and seismic vibrator sources. A wide variety of impact sources are
known, including
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by way of example, but not limited to, those described in U. S. Patents
4,124,090; 4,421,198;
and 4,011,924.
The prior art shows certain uses of aircraft in geophysical exploration. It is
known,
for example, to conduct electromagnetic and gravity surveys with overflying
aircraft carrying
electromagnetic and gravity sensing instruments. It has also been proposed, in
U. S. Patent
3,704,764 to transport seismic sensors to a survey site by means of an
aircraft and to drop the
sensors to the earth's surface as the aircraft overflies the survey site.
There has been a long felt need for a system for conducting seismic
exploration that
reduces the impact on environmentally sensitive areas such as the arctic
region and land-
marine transition zones. There are also regions, such as the marine-land
transition zones and
mountainous regions, that are difficult to traverse with a transport vehicle.
Accordingly, it is
an object of this invention to generate a seismic signal without requiring
surface
transportation of the means for creating the seismic signal to the location
where the signal is
to be generated.
SUMMARY OF THE INVENTION
In an embodiment of the invention, a seismic signal is generated at a survey
site by
dropping a mass from an overflying aircraft so that the mass will impact the
earth's surface at
the survey site. Seismic sensors are deployed within the survey site to detect
seismic signals
resulting from the impact of the mass on the earth's surface.
DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a system layout useful for practicing the invention.
FIG. 2 shows the shape of a mass unit which is useful in practicing the
invention.
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FIG. 3 shows the change in energy imparted to the earth as the height from
which
an object is dropped varies.
FIG. 4 shows the invention utilized for performing a vertical seismic
profiling
survey.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 shows a plan view of a portion of a survey site according to an
embodiment
of the invention. As shown in FIG. 1, multiple lines 12 of seismic receiver
stations 10 are
deployed in the survey area. Each receiver station 10 is of the conventional
type for
receiving the seismic energy of interest, and as such may consist of a single
receiver, but
may preferably include multiple receivers. Multiple receivers associated with
a receiver
station 10 may be located near the same point, but each receiver station may
also include
multiple receivers spread out in an array, but interconnected to generate one
composite
signal.
The types of receivers included within receiver stations 10 will be determined
according to the type of seismic energy to be detected. Examples of
conventional receivers
useful in this embodiment of the invention include conventional geophones,
which detect
energy in the form of velocity. Conventional geophones may be configured to
detect
compressional, horizontal shear or vertical shear energy. Further, a single
geophone
installation may include multiple component geophones for detecting energy in
each of the
three orthogonal directions.
Alternatively, for receivers deployed under water, receiver stations 10 may be
conventional hydrophones. As is well known, hydrophones detect seismic energy
transmitted
as pressure, without a directional indication. For underwater detectors,
combinations of both
hydrophones and geophones for each receiver station 10 may be desirable, as
the directional
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information detected by the geophones can be used in de-ghosting the pressure
information
detected by the hydrophanes.
As illustrated in FIG. I, receiver stations 10 are deployed in multiple lines
12 which
may be substantially parallel to one another within the survey area. In this
example, each line
12 includes a plurality of receiver stations 10, together with suitable
conventional telemetry
equipment for communication of electrical signals corresponding to the
detected seismic
energry. The spacing, n, between receiver Lines 12, typically, is between 50
meters and 2000
meters. By way of example, in the system described herein, it will be assumed
that the
separation between the fines of receivers is one thousand meters. Each line 12
includes a
number of receiver stations 10, and the spacing between receiver stations
within a survey line
might typically be 50 meters, although this spacing could vary substantially,
depending on the
particular survey needs. A typical length, m, of each tine of detectors may be
b000 meters,
but this is also subject to substantial variation, depending on geographical
constraints, the
survey objectives and other logistical considerations. The number of receiver
lines 12
deployed for a particular survey will vary, but eight receiver fines is a
typical number. The
seismic sensors included in each receiver line 12 are typically included in a
cable which is
rolled out to configure the receiver line. However, the receiver stations may
each be
individually positioned at the receiver locations.
The signals detected by the seismic receivers are normally recorded for later
processing. In a particular embodiment of the invention, the detected signals
are transmitted
along communication channels 1 S extending along the length of receiver lines
12 and
conveyed to a central control and recording system I4. Transmission media
useful for
transmitting the detected signals include, without limitation, electrical
conductors, fiber optic
cables and radio waves. Signals corresponding to energy detected by receiver
stations 10 are
communicated to central control and recording system 14 for conventional
storage, and for
conventional analysis such as move-out correction, common midpoint (CMP) trace
gather
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formation, static corrections, migrations and the like. It is also been
proposed in the prior art
to include a recording system at each receiver station.
The invention described herein is especially useful in environmentally
sensitive areas,
such as the arctic region and land-marine transition zones, where vehicular
traffic, especially
from heavy vehicles required to transport vibrator sources, is potentially
damaging to the
environment.
In practicing the present invention, an aircraft 16, which in a preferred
embodiment
could be a fixed-wing aircraft such as a Skyvan, which was manufactured by
Shorts Aircraft,
or a C-130, manufactured by Lockheed Martin, is utilized for transporting a
mass unit over
the survey site, and as the aircraft flies over the survey site the mass unit
is dropped from the
aircraft. An aircraft which has been previously adapted for carrying sky
divers may be
especially useful in practicing the invention. Clther aircraft, including but
not limited to other
fixed wing aircraft, heliccspters or dirigibles could also be used and all
such delivery means are
within the scope of this invention. Normally, a plurality of such mass units
will be loaded
onto the aircraft, and these mass units will be dropped from the aircraft from
aerial positions
such that the mass units will impact the earth's surface substantially at
selected locations at
the survey site. The impact of the mass units an the earth's surface will
generate seismic
shock waves which will travel through the earth as substantially spherical
wave fronts. The
magnitude of the seismic energy that is generated as the mass units strike the
ground
increases as a function of impact velocity, and for that reason it is
desirable to configure the
mass units 22 into a streamlined cylindrical shape, such as the shape shown in
Fig. 2, in order
to maximize the impact velocity. The mass units 22 may also include fins 23
attached at one
end of the mass units to increase the stability of the mass units 22 as they
fall toward the
earth's surface. Handles may also be added to the mass unit so that they will
be easier to
handle.
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In a preferred embodiment, the flight path of the aircraft as it crosses the
survey site
may be parallel to the direction of the receiver lines. Other flight paths,
such as flight paths
which are transverse to, or diagonal to the receiver lines are also within the
scope of this
invention. Representative flight paths, designated by numerals 18 and 19 are
shown in Fig. 1.
Although the specific aircraft selected for use in practicing the invention
will affect the speed
at which the aircraft is flown in traversing the survey site, a fixed wing
aircraft might typically
be flown at ground speeds within the range of i 50 to 500 kilometersJhour. The
spacing
between the locations at which it is desired to generate a seismic signal will
vary, depending
on survey needs, but if mass units are dropped from an overflying aircraft
flying at 300
kilometers per hour at six second intervals, the spacing between the seismic
source locations
at which the mass units strike the earth's surface will be about 500 meters.
The source
locations designated by numeral 20 shown in Fig. 1 are intended to be
representative, and are
shown by way of example. The source locations in relation to the receiver
lines and receiver
stations may be selected to meet the needs of a particular survey according to
criteria known
15 to those of ordinary skill in the art. The invention also permits sources
to be placed in
efficient patterns selected for a specific survey to reduce acquisition
footprint artifacts.
After a seismic signal is generated it is necessary to wait for a "listening
time" for the
signal to travel down into the subsurface where the signal is reflected from
subsurface
interfaces and then travels back to the earth's surface where it is detected
by a receiver. The
20 required listening time may vary depending on the acoustic velocity of the
subsurface and
specific survey needs, but a listening time of five seconds or more would be
typical.
In a particular embodiment of the invernion, the mass units which are dropped
from
the aircraft 16 to generate the seismic signal may be concrete slugs or ice
slugs which may be
formed into a cylindrical, streamlined shape such as shown in Fig. 2,
designated by reference
numeral 22. Other materials that may be used to form the mass units include,
but are not
limited to, water-filled balloons. Such balloons would normally be formed from
an
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elastomeric material, and such water filled balloons may be especially useful
for practicing the
invention because of their tendency to form themselves into a streamlined
shape as they fall
through the atmosphere, which results in maximizing the impact velocity and
the magnitude
of the resulting seismic signal. The mass units may also comprise explosive
devices, which
may be plastic explosives of the type typically used in the geophysical
exploration industry.
It is contemplated that personnel onboard the aircraft will drop the mass
units from
the aircraft at periodic time intervals which are selected so that the mass
units will impact the
earth's surface to generate seismic shock waves substantially at preselected
locations. Such
personnel would normally wear a safety harness secured to the interior of the
aircraft for
safety reasons. It is also contemplated that a large container of water could
be carried
onboard the aircraft and that equipment of the type typically used in the
bottling industry
could be utilized for filling elastomeric balloons on board the aircraft, as
required, and that
apparatus typically used by the defense industry for mechanically dropping
devices from
aircraft could be utilized for dropping the water filled balloons or other
mass units from the
overflying aircraft.
Objects dropped from an aircraft rapidly increase in velocity as they fall
toward the
earth. Since the impact energy of a falling object as it strikes the earth is
proportional to
velocity squared, the energy of a dropped object also increases rapidly as it
falls toward the
earth's surface. Fig. 3 shows the impact energy as a function of drop height
of a thirty
kilogram streamlined cylindrical mass unit having a drag coe~cient of 0.7,
with a diameter of
0.21 meters at standard atmospheric conditions. As shown in Fig. 3, very
little increase in
impact energy is realized by dropping a mass from flight altitudes greater
than about 4000
meters above the earth's surface because air friction will limit any increase
in velocity of the
falling object. At a drop height of about 2000 meters (referenced to the
earth's surface at sea
level}, a thirty kilogram streamlined cylindrical mass will generate about 262
kiloJoules of
energy upon impact. This energy is about twelve times the energy that is
generated by
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dropping a 2000 pound (909 kilogram) weight from a height of eight feet
(2.4384 meters),
which is a practical configuration for weight-drop seismic saurces deployed on
the earth's
surface.
Processing of the recorded seismic data detected by seismic sensors 10 to
determine
the location of subsurface reflecting interfaces and other subsurface
attributes, requires
knowledge of the locations of the seismic sensors 10 and the source locations
20. The
seismic sensors will normally be placed manually on the ground and their
locations may be
precisely determined. Because of the difficulty in flying an aircraft to a
precise drop point,
and varying air currents which may cause the dropping mass to drift, it may be
difficult to
control with great precision the location at which the dropped mass unit will
impact the
earth's surface.
Seismic signals will be transmitted from the impact locations 20 along the
earth's
surface as well as into the subsurface, and the time of reception of the
"first break" signal
detected by at least three of the sensors stations 10 on the earth's surface
may be utilized, by
triangulation methods, to determine the time and location of the impact of a
mass unit on the
earth's surface after the mass unit is dropped from an aircraft. Triangulation
methods are
commonly used in marine and transition zone seismic operations for determining
seismic
sensor locations, and such methods are well known to those of ordinary skill
in the art. Other
methods may also be utilized to determine the precise location of the impact,
including but
not limited to, visual sightings and detection of electromagnetic signals
generated by the
impact of the mass on the earth's surface.
The foregoing embodiment of the invention has been described in terms of a
surface
seismic survey in which the seismic sensors are deployed substantially at the
earth's surface.
However, the invention may also be employed to perform vertical seismic
profiling surveys,
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in which seismic sensors are deployed within a wellbore drilled into the
earth's surface.
Figure 4 shows receiver stations l0A deployed by means of a wireline 30 within
a wellbore
32 which has been drilled into the earth's subsurface. Aircraft 16 is flown
over the survey
site and drops mass units 22 onto the earth's surface 34. The resulting
seismic signal travels
from the impact location 20A along raypaths, such as the direct arrival
raypath 38, and
raypath 3b which is reflected from a subsurface reflecting interface 40, and
the signals reach
the location of seismic receiver stations 10A in the wellbore. Signals which
are detected by
senors IOA are transmitted up the wireline 30 to the earth's surface, where
they are recorded
by central recording and control system I4A. Sensors 28 may also be deployed
on the
I O earth's surface, and the "first break" signal detected by these surface
sensors may be utilized
for precisely determining, by triangulation methods known to those of ordinary
skill in the
art, the time and locations of the impact of the mass unit 22 on the earth's
surface.
Global Positioning System receivers may also be utilized to navigate the
aircraft to
drop positions that have been corrected for atmospheric conditions so that the
impact
locations of the mass units on the earth's surface may be more precisely
controlled.
It will be appreciated that various modifications and variations may be made
to the
invention without departing from the scope of the invention as defined in the
appended
claims. It is the intent to cover within the scope of the appended claims all
such modifications
and variations.
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