AU2813201A - A method for conducting seismic surveys utilizing an aircraft deployed seismic source - Google Patents

Description

S&FRef: 550234

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

ORIGINAL

Name and Address of Applicants: Phillip A. Bahorich 21191 Miramar Lane Huntington Beach California 92646 United States of America Michael S. Bahorich 11425 Calico Lane Houston Texas 77042 United States of America Actual Inventor(s): Address for Service: Invention Title: Phillip A. Bahorich and Michael S. Bahorich Spruson Ferguson St Martins Tower,Level 31 Market Street Sydney NSW 2000 A Method for Conducting Seismic Surveys Utilizing an Aircraft Deployed Seismic Source The following statement is a full description of this invention, including the best method of performing it known to me/us:- 5845c 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 10 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 S 15 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 BAI-0O 1 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 the impact on environmentally sensitive areas such as the arctic region and landmarine 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.

BAH-001 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 BAH-01 information detected by the geophones can be used in de-ghosting the pressure information detected by the hydrophones.

As illustrated in FIG. 1, receiver stations 10 are deployed in multiple lines 12 which may be substantially parallel to one another within the survey area. In this examuple, 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 energy. The spacing, n, between receiver lines 12, typically, is between 50 meters and 2000 meters. By way of example, in the systemn described herein, it will be assumed that the separation between the lines 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 line of detectors may be 6000 meters, but this is also subject to substantial variation, depending on geographical constraints, the survey objectives and other logistical considerations. The mnber of receiver lines 12 deployed for a particular survey will vary, but eight receiver lines 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 5: 20 processing. In a particular embodimet of the invention, the detected signals are transmitted along communication channels 15 extending along the length of receiver lines 12 and conveyed to a central control and recording sysem 14. Transisasion media usefu 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 systemn 14 for conventional storage, and for conventional analysis such as move-ut correction, common midpoint, (CMP) trace gather BAH-001 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 10 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. Other aircraft, including but not limited to other fixed wing aircraft, helicpters or dirigibles could also be used and al 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 on the earth's surface will generate seismic shock waves which will travel through the earth as substantially spherical wave fronts. The magnitude of the seismnic energy that is generated as the mass units strike the ground 20 increases as a ffunction 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 toicras the stability of the mass units 22 as they fall toward the earth's surfatce. Handles my also be added to the mass unit so that they will be easier to handle.

BAH-001 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 150 to 500 kilometers/hour. 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. I 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 suface where it is detected by a receiver. The 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 invention, 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 BAH-001 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 10 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 flling 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 15 overflying aircraft.

Objects dropped from an aircraft rapidly increase in velocity as they fall toward the V earth. Since the impact energy of a falling object as it strikes the earth is proportional to Svelocity 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 20 kilogram streamlined cylindrical mass unit having a drag coefficient 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 BAH-001 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 sources deployed on the earth's surface.

Processing of the recorded seismic data detected by seismic sensors 10 to determnine 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 i1 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 srace 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, BAH-001 in which seismic sensors are deployed within a weilbore drilled into the earth's surface.

Figure 4 shows receiver stations IlOA deployed by means of a wireline 30 within a weilbore 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 36 which is reflected from a subsurface reflecting interface 40, and the signals reach the location of seismnic receiver stations I OA 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 14A. Sensors 28 may also be deployed on the 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 wili 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 claimns. It is the intent to cover within the scope of the appended claims aUl such modifications and variations.

Claims (8)

1. A method for performing seismic exploration comprising: deploying seismic sensors within a seismic survey site; generating seismic signals at said survey site by dropping a mass onto sad survey site from an overflying aircraft, detecting seismic signals with said seismic sensors resulting from the impact of said mass on the earth's surface.

2. The method of claim 1 wherein said aircraft is a fixed-wing aircraft.

3. The method of claim I wherein said aircraft is a helicopter.

4. The method of claim I wherein said aircraft is a dirigible. The method of claim I wherein said mass is a volume of concrete fanned into a streamlined cylindrical shape.

6. The method of claim I wherein said mass is a volume of ice.

7. The method of claim 1 wherein said mass is an elastomeric balloon Niled with water.

8. The method of claim 1 wherein said mass unit is an explosive device. BAH-001 1 9. The method of claim I whrinsadsurvefs asr e ismif c srvy. 1 10. The method of claim I wherein said survey is a vertical seismic profiling survey. 1 11. A method for performing seismic exploration comprising dropping a mass f-rm an 2 overflying aircraft onto a seismic survey site at which seismic sensors have been 3 deployed in order to generate a seismic signal when said mass impacts the earth's 4 surface, 1 12. The method of claim 11I wherein said aircraft is a fixed-wing aircraft. 1 13. The method of claim I1I wherein said aircraft is a helicopter. 1 14. The method of claim IlIwherein said aircraft is adiigible. 1 15 h ehdo li hri.si asi oueo ocee 1 15. The method of claimn I I whlerein said mass is a volume of icoe. 1 16. The method of claim I1I wherein said mass utis an olumeoiedvie 1 19. Tie method of claim I1I wherein said survey is asurface seismiic survey. 1 20. The method of claim I1I wheren said survy is aveticalseismic profilig survey.

21. A method for performing seismic exploration, said method being substantially as hereinbefore described with reference to the accompanying drawings. Dated 20 March, 2001 Philip A. Bahorich Michael S. Bahorich Patent Attorneys for the Applicant/Nominated Person SPRUSON FERGUSON .0 0*0 0 6: [I:\DayLib\LIBLL] 1 0804.doc:oon