GB2055467A - Geophysical exploration utilizing telemetered seismic data and airborne receivers - Google Patents

Abstract

A method of seismic data gathering employing a plurality of data acquisition units, each having a detector and a radio transmitter; a plurality of sources; and a control station equipped with a plurality of receivers, one for receiving on a respective different frequency a transmission from each of the data unit transmitters operated in conjunction with the activation of the first source and then in conjunction with the activation of the second source and so on in roll-along fashion. The control station is preferably airborne and the data acquisition units each include a receiver for receiving command control signals and, in one embodiment, a recorder for retransmission purposes upon receipt of a proper command request signal.

Description

SPECIFICATION Geophysical exploration utilizing telemetered seismic data and airborne receivers This application pertains to seismic communications systems and more particularly to a method of seismic data gathering utilizing radio communications among the individual data acquisition units, seismic source units and the control station, where command signals are produced and data is recorded.

The traditional technique for running a seismic survey utilizes a plurality of seismic detectors (e.g., geophone strings and hydrophones), a plurality of seismic sources, and a recording station. Each of these basic items of the system may be complex apparatus. For example, the recording station typically includes seismic amplifiers, analog-todigital converters, time and frequency demultiplexers and the like. Nevertheless, the basic system still comprises those items listed above.

In preparing for the taking of a survey, "jug crews" place the seismic detectors in position along the proposed path, the detectors being evenly spaced, such as at 200-foot intervals. The detectors within a group are conventionally located within 10 to 12 feet of each other A cable is connected to the detectors for providing communications of the seismic reflections in the form of data from the detectors to a recording station. The initial seismic source is also positioned in line with the path formed by the detectors at a location somewhat removed from the first of the detectors. Typically, this source-tofirst-detector spacing is in the range between 300 and 1000 feet. The source in some prior art systems is also cable-connected to the recording station. Many sources are mechanical and not hazardous to an operator positioned at the source.

Other sources include explosives, such as dynamite, and are hence activated remotely by a trigger signal communicated along a cable or by radio transmission to the source.

To activate the prior art system just described, the source is activated, thereby also producing a time base signal down the cable to the detectors.

As the seismic reflections are received at the respective detectors or data acquisition units, the information is converted to electrical impulses and transmitted by cable to a recorder. The use of separate wires or time multiplexing permits the sorting or identification of the information signals of one unit from another.

Once recordings are made, then the source is advanced, or if the source has been expended, another source is located at an advance position along the path. The detectors are also advanced, usually taking the ones that are on the rear end of the line and placing them on the front end.

Another reading from each of the detectors is recorded in conjunction with activation of the second source in the same manner as previously described. Successive operations of sources and detectors provide a complete roll-along survey.

The signal from the source and the signals from each of the detectors occur in time sequence, but do overlap one another. Delays may be inserted to insure against overlap. Alternatively, each unit may include frequency multiplexing equipment.

Field units in more sophisticated systems even include individual recorders, which recorders can be activated for simultaneous play back in response to a command signal. Each unit transmitter may operate in conjunction with a different frequency or in conjunction with a different signal wire so as to maintain isolation between channels. In any event, communications is via the cable or cables or wires that connect the various system items or components together.

The cable-type system just described works well in relatively clear terrain where deploying of the detectors and sources in the manner set forth above is not too difficult. The system does not always work well in adverse or rough environmental conditions. For example, with snow or ice covering the ground, moving along the path is slow and difficult. When the path is mountainous or swampy or heavily wooded, deploying the various system components may become extremely difficult, is often extremely time-consuming, and may be so difficult or hazardous as to make it totally impractical to accomplish a complete survey. For example, if the path is to proceed up the side of a relatively steep incline, the decision is often made just to leave a "gap" in the data information being gathered, rather than to take the time, or the risk necessary for providing complete geographic coverage.

In addition, difficult terrain may even require the building of bridges, the cutting of swaths through trees and underbrush and the like. Not only does this make an impact on the ecology, it often requires additional construction equipment and improved communications between personnei. Typically, communications are provided by CB radio, radio-telephone, or other government-licensed radio communications.

In addition to the above, an additional disadvantage of a cable system is that in case of cable separation from the break outward all communications in the system are lost. Cable wear or impairment also can cause intermittent communication operation and/or channel interference. The adverse conditions discussed above, such as operation in frozen or swampy conditions, increase the probability of these cableinduced communication difficulties.

New oil and gas production from geographies not having adverse terrain conditions is decreasing. New production generally comes from discoveries made in remote areas, which are also environmentally difficult. Therefore, the elimination of cables from seismic communication systems is highly desirable. Radio systems that have been employed in seismic operations in the prior art to avoid the cable difficulties overcome some of the problems discussed above encountered with difficult terrain, but certainly not all. In addition, radio communication operation introduces new problems.

For example, operations on a number of different channels is sometimes difficult because it requires obtaining government licenses which may not be readily available. The easier to acquire licenses are for the UHF FM band. Operation in this range, however, is spotty and uncertain in mountainous terrain. Line-of-sight is required for non-fading reliable communications.

The individual data acquisition units in such a radio system also must have power available from a self-contained source since cable-connected power is no longer available. Continuously operating batteries have limited life, thereby requiring rapid surveying (not practical for the popular roll-along survey described above) or battery recharging when battery-powered data acquisition units are used. Battery recharging obviously is time consuming.

The command or control station for recording the system data also must be located fairly near the field units since transmitters for such units have a limited range. Moreover, radio communications between all of the field units and the control station must be line-of-sight when employing the VHF FM band. Mountainous terrain again is a problem with the reliability of such systems.

U.S. Patent No. 2,700,753 discloses a method and apparatus for seismic prospecting employing an AM transmitter operating on a different frequency for each field detector unit. Heterodyne operations of four receivers at each control station permit the receipt of four frequencies for each receiver, or a total of sixteen simultaneous recording channels. The particular circuits in the receivers minimize detrimental cross-channel modulation products. Nothing in this patent suggests operation in a roll-along fashion or separate receiver operation for each channel. In today's surveys, it is not uncommon for 48, 96 or even more channels to be employed for single comparison recordings, a concept totally unfathomable with this prior art apparatus.

U.S. Patent 3,062,315 discloses a radio-link system of seismic exploration in which the control station is located aboard an aircraft. Transmission from each field unit in this second patent is on the same frequency channel. Time delays are employed at each of the field units to separate the individual data channels, thereby achieving time multiplexing. There is no frequency multiplexing.

In fact, the single frequency operation is promoted as a distinct advantage in simplifying the communication equipment that would be required for multifrequency operation.

The present invention aims to overcome the problems noted above and is directed to a method of seismic data gathering, which comprises the steps of locating a plurality of self-contained data acquisition units in an array to be seismically surveyed, each of said units including at least one seismic detector, a radio transmitter connected for transmitting seismic data detected by said detector, each of said transmitters being set for transmission on a preselected channel so that a preselected number thereof in the array are each operating on a different frequency channel from the other of said transmitters, locating a first seismic source at a first shotpoint, activating said first source, detecting seismic reflections at a preselected number of said units and transmitting simultaneously said detected seismic reflection data, each of said transmissions being on a separate channel, receiving and recording at a master control station each of said transmissions, locating a second seismic source at a second shotpoint, activating said second source, detecting seismic reflections at the same preselected number of said units involved with the first shot and transmitting simultaneously said detected seismic reflection data, each of said transmissions being on a separate frequency channel, and receiving and recording at the master control station each of said transmissions.

One of the features of a preferred aspect of this invention is to provide an airborne controlled radio telemetry communications system for seismic operation using multiple channels of UHF FM signals for greatly expediting a roll-along seismic survey.

It is another feature of the present invention to provide a radio telemetry seismic survey communications system using, for example, 72 different frequency channels and providing line-ofsight communications regardless of how mountainous the terrain.

It is still another feature of the present invention to provide an improved seismic telemetry system that has the flexibility of being readily expansible both with respect to the length of a survey path and with respect to geographic area coverage.

It is yet another feature of the present invention to provide a radio telemetry seismic communications system employing field data acquisition units that are self-contained, such units being operable in conjunction with a recording interrogation signal from a remotely located control station, the transmissions from the respective units not necessarily being at real time.

It is still another feature of the present invention to provide an improved seismic exploration system wherein the movement of apparatus in adverse environmental conditions increases the safety of handling the hazardous components and preserves the reliability of handling the more sensitive components, when compared with prior art systems.

It is yet another feature of the present invention to provide an improved seismic exploration system whose communications iink maintains high data quality in spite of adverse environmental conditions encountered in the survey site.

Further features and advantages of the invention will now be described in connection with a preferred embodiment of the invention which is described below with reference to the accompanying drawings wherein: Fig. 1 is a pictorial illustration of an array of field data acquisition units and a source employed in one embodiment of the present invention; Fig. 2 is a profile illustration of field units of the seismic survey illustrated in Fig. 1 geographically positioned along mountainous terrain; and Fig. 3 is a schematic representation of the seismic radiotelemetry system employed in accordance with the present invention.

As noted above, geophysical prospecting in remote, mountainous locations is incompatible and unsuited for being conducted using cable systems, when compared with a radio telemetry system. Also,. demand for more complete information than previously requires increasing the numbers of detectors for each source activation, both in terms of single path 9overage as well as area coverage. A typical array of data acquisition field units in accordance with the present invention is illustrated in Fig. 1.

These field units are typically each a selfcontained seismic detector (e.g., geophone string or hydrophone), a fixed-frequency transmitter, and a receiver. One embodiment also includes a recorder in each of the data acquisition units.

"Self-contained" as used herein does not necessarily imply that all of the electrical and mechanical parts are necessarily packaged in the same physical package, only that there is no need for an external connection, such as to a remote source of power. The field unit is powered by a long-life, rechargeable battery also included as an integral part.

Field units in the odd series of numerals 1 through 31 are illustrated as being positioned along a straight primary path for which the survey is to be conducted. Typically, 96 units are charted along the path. Each unit is charted to be located horizontally 200 feet apart from the units on either side. Also charted are cross paths for additional units. For example, field units in the odd series of numerals 33 through 51 are respectively located horizontally 220 feet apart along a first cross path.

At a distance therefrom, such as a distance of six units (e.g., 1320 feet), field units in the odd series of numerals 53 through 65 are respectively located horizontally 220 feet apart along a second cross path parallel with the first cross path. For convenience, these cross paths are at right angles to the primary path.

A source unit 69 is also located along the primary path. This source unit typically includes an explosive device, such as dynamite, a receiver and transmitter. The first source unit is typically positioned or offset from the first field detector unit by approximately 3000 feet. Other source units (not shown) are deployed as hereinafter described in performing a roll-along survey. These additional source units are charted along the primary path at regular intervals. The distance between source unit positions typically is six spacings of field units, or, for the example, 1320 feet. Not all of the field detector units are illustrated in Fig. 1, only a few for illustrating a typical array.

In operation, one or more field crews, typically transported by helicopter, deploy the first source 69 and a plurality of the field data acquisition units along the path to be surveyed. The use of helicopter permits placement of units even on very steep grades or other difficult terrain to ensure against having data gaps. A greater number of field units can be set out than is used in conjunction with the first shot activation. The control plane is then positioned overhead in the vicinity of the survey, the first source 69 and the predetermined number of field units operated in concert therewith are activated, and the recordings are made in the control plane. In case a transmission from a particular unit is garbled, for the embodiment including a recorder, that unit can be reactivated to produce a retransmission.In fact, if desired, for the system embodiment employing recorders in the field data acquisition units, all of the field recording units can be reactivated, with appropriate time delays being inserted in the reactivation sequence to suitably align or stack the retransmissions to compensate for the distances between the respective units and to relate them to an index time related to the source occurrence.

After the data resulting from the first source activation is satisfactorily recorded at the control station, a second source (not shown), at an advance position along the path, is positioned and/or activated to produce data from the predetermined number of field units operated in conjunction therewith.

There is usuaily a large overlap in the number of field units operated in conjunction with the first and second source activations. Typically, for example, 48 channels of data from 48 separate field units are simultaneously produced. The second source (not shown) is advanced along the path by six field units. Hence, six units are dropped from the first source data collections and six units are added. It should be further noted that the six added units can be operated on the same respective frequency channels as the six dropped since data collection for a first source survey and a second source survey does not occur simultaneously, although the same respective frequency channels do not have to be used. The only requirement is that during a first source survey or a second source survey, there are no two field units operating on the same frequency channel at the same time.

Additional source surveys are conducted in like manner until the path is completely covered. This system of surveying an elongate path, that may be many miles in length, is referred to as a "rollalong" survey. Because of the large overlap of data collected, the integrity of the data is very good.

Fig. 2 charts a typically mountainous profile along the primary path. As illustrated, 96 field units are located along the path. The horizontal spacing is uniform, as shown by the vertical lines, but the vertical spacing, of course, is not. Since there are no cable lengths to consider in the system, it is possible to precisely position the receivers, as shown. Furthermore, since there are no cables involved in the communication links, transmission by line-of-sight also eliminates any errors that would be introduced by various cable lengths. The profile shown in Fig. 2 also highlights at least two other advantages of the present system. First, any location for a control station which is not airborne would more than likely be out of line-of-sight communications with at least one of the field units.Land-based antennas would not provide the freedom to conveniently position the receiver for radio reception from all units. In some severe circumstances, it may not be possible to locate a plane at one position for simultaneous reception, but for the system embodiment employing field data acquisition units with recorders, with the recording and retransmission capability of such a system, even this possibility is effectively eliminated.

Another advantage of using a plane in conjunction with a layout of field and source units along the route is the increased reliability of voice communications for people on the ground (messages can be relayed via the plane) and the increased safety to the personnel. Before a source is activated, the visibility advantages afforded by the plane can be used to assure that no persons are exposed to possible explosion hazards.

Now referring to Fig. 3, there is schematically shown a sequence of dots, each dot representing a field data acquisition unit as described above, each unit being associated with at least one seismic detector such as in the form of a geophone, a geophone string, or hydrophone.

Conveniently, a plurality of racks are employed carrying the field units to their positions of use and in recovering the units. A rack holds seven of the units, although only six of the units are normally employed at any one time. The seventh unit provides a spare for back-up purposes.

Additionally, each rack includes a battery charger for recharging the units overnight. The large rectangular blocks with the numbers "6", "12", "1 8" and so on each represents a rack of such six operable field units.

In laying down the units, it is convenient to use two-man crews. To minimize expenses, two twoman crews may be used for this purpose, one associated with a helicopter to deploy the more difficult units and one crew associated with pack horses, where the terrain is not too difficult. The units for a particular shot sequence may number 96, as shown in Fig. 3, although a lesser or greater number may be used depending upon the selected mode of operation. For example, only 48 traces may be used for a particular shot sequence.

In any event, it is typical to space the field units consecutively apart by 220 horizontal feet along the charted path. The first source location is then positioned ahead of the first field detector unit by about 3000 feet. A crew of five men typically comprise a shooting crew.

Illustrated in Fig. 3 is a set-up for developing 48 traces from source 101, which is shown offset from unit "24" by 3000 feet and from the next 47 traces by consecutive spacings of 220 feet.

Hence, unit 72 is positioned to develop trace 48.

When the first source sequence of traces has been developed, the next source position is established at an advance position of approximately six field units (1320 feet). The field unit crews pick up the six units at the trailing end of the path and put down six units at the front. A sequence for the second source is then developed. This is conveniently done by placing the crew with one or more pack horses at the trailing edge and the helicopter crew at the front end.

Returning to the trace development, the source is detonated, or otherwise activated in the case of a non-explosive source, and the transmission from each the source and the units is received at the control station aircraft. The detonation of the source can be triggered either from the control station aircraft or from a ground position. The control station does transmit an activate signal or trigger to each of the field unit receivers in conjunction with the activation of the source.

Such a signal switches the receiver from a standby condition, where battery drain is minimal, so that the seismic detection, recording and transmit sections of each of the units is powered before the first reflections are felt by the detector(s) at the unit, but not unnecessarily in between. The control station recording of the traces may be in conjunction with transmissions occurring simultaneously with the detection. For the system embodiment employing field data acquisition units with recorders, the recording of the traces may be in conjunction with a request from the control station for a retransmission made from the recordings.Retransmissions of all data may be requested to reinforce receptions missed the first time for any reason (e.g., blocked transmissions, noisy transmissions, cross-channel coupling which should be at a minimum because of the use of separate receivers, and the like).

Once the data is satisfactorily recorded at the control station, then location and activation of the second source is in order. This is accomplished in a substantially identical manner to the first source activation and recording. A total of 112 field units are included on 1 6 helicopter racks, but normally only 96 units are deployed. Transmissions are made on 96 channels. If a back-up unit is employed, then the frequency of operation is selected so that it does not operate on one of the frequencies of another of the units.

Following each predetermined shot sequence, as described above, the pick-up crew will load the trailing helicopter each with the six field units and associated detectors (e.g., geophone strings).

Upon completion of loading, the helicopter will hook up to the rack, transport it to the leading edge of the roll-along operation and disconnect the rack for the layout crew to again deploy them in the predetermined pattern.

The above discussion assumes a single path of 96 units in a line. This configuration or pattern is cited by way of typical example. In practice it may be desirable to provide cross paths, as shown in Fig. 1, or to use a lesser or greater number of units than 96 in the in-line path. Another popular configuration is the development along two simultaneous parallel paths.

While particular embodiments of the invention has been shown and described, it will be understood that the invention is not limited thereto, since modifications may be made and will become apparent to those skilled in the art. For example, a survey using a preselected grid array or even randomly arrayed units could be conducted using the method hereinabove described, if desired. Also, field camp locations or base camps may be established at intervals, as shown in Fig. 3, where the five men typically on the shooting crew and the two men on each of the jug crews can bed down overnight.

Claims (14)

1. A method of seismic data gathering, which comprises the steps of locating a plurality of selfcontained data acquisition units in an array to be seismically surveyed, each of said units including at least one seismic detector, a radio transmitter connected for transmitting seismic data detected by said detector, each of said transmitters being set for transmission on a preselected channel so that a preselected number thereof in the array are each operating on a different frequency channel from the other of said transmitters, locating a first seismic source at a first shotpoint, activating said first source, detecting seismic reflections at a preselected number of said units and transmitting simultaneously said detected seismic reflection data, each of said transmissions being on a separate channel, receiving and recording at a master control station each of said transmissions, locating a second.seismic source at a second shotpoint, activating said second source, detecting seismic reflections at the same preselected number of said units involved with the first shot and transmitting simultaneously said detected seismic reflection data, each of said transmissions being on a separate frequency channel, and receiving and recording at the master control station each of said transmissions.

2. The method of claim 1, including the step of producing a recall signal from the master control station for causing retransmission of data recorded at each of said units.

3. The method of claim 1 or 2, wherein transmissions from said master control station and from said units are frequency modulation radio transmissions.

4. The method of claim 1,2 or 3, wherein seismic reflections are detected at a preselected number of 48 units.

5. The method of claim 1,2 or 3, wherein seismic reflections are detected at a preselected number of 96 units.

6. The method of any of claims 1 to 5, wherein said first source and said second source are located at a location for a data acquisition unit.

7. The method of any of claims 1 to 6, wherein said master control station is aircraft mounted.

8. The method of any of claims 1 to 7, wherein said units in the array are placed along a path, said first shotpoint is located at the head of the path at a predetermined distance from the first of said units involved in receiving reflections from said first source, and said second shotpoint is located along the path at a predetermined distance from the first of said units involved in receiving reflections from said second source, said first unit involved with said second shot being separated from said first unit involved with said first shot by a predetermined number of said units, thereby producing a roll-along survey.

9. The method of claim 8, wherein said second seismic source is advanced from said first seismic source location by a predetermined number of 6 units.

10. The method of any of claims 1 to 9, including the additional steps of detecting seismic reflections resulting from activation of said first source from a preselected number of units along a second path in the array not in line with said firstnamed path and transmitting simultaneously said detected seismic reflection data, each of said transmissions being on a separate channel, and receiving and recording at the master control station each of said transmissions.

11. The method of claim 10, wherein the simultaneous transmission of said detected seismic reflections from said second path units is also simultaneous with the transmissions of said detected seismic reflections from said first path units resulting from activation of said first source.

12. The method of any of claims 1 to 11, wherein each of said units includes a radio receiver for receiving instructional transmissions, and a recorder for recording the seismic data detected by the unit, said method including the additional step preceding activation of said first source of simultaneously turning on the recording at each of said units involved in receiving reflections from said first source and conditioning the detector at each of such units for detection, and the additional step preceding activation of said second source of simultaneously turning on the recorder at each of said units involved in receiving reflections from said second source and conditioning the detector at each of said units for detection.

13. The method of claim 12, wherein activation of said first source and said second source is by radio transmission from the master control station.

14. The method of claim 12 or 13, wherein each of said recorders is turned on and each of said detectors is conditioned by radio transmission from the master control station.

1 5. The method of any of claims 1 to 14, wherein at least one of the transmissions of data resulting from detecting seismic reflections caused by activation of said second source is on the same frequency channel as one of the transmissions of data resulting from detecting seismic reflections caused by activation of said first source, thereby achieving time demultiplexing of said transmissions.

1 6. The method of claim 15, including the step of time demultiplexing at the master control station to separate the received data from transmissions of data resulting from detecting seismic reflections caused by activation of said first source from the data received from transmissions of data resulting from detecting seismic reflections caused by activation of said second seismic source.

1 7. The method of seismic data gathering substantially as herein described with reference to the accompanying disclosure.