Untitled record

GERMAN -iiZTMW 'PATENT OFFICE

Laid

German KL: 42 c - 42

Number: 1237 342

Reference number: C 23287IX b / 42 c

1 237 342 Date of filing: 31st January 1961

Auslegetag: March 23, 1967

The present invention relates to seismic research methods in which one seismic signal is successively transmitted from different transmission locations into the earth by means of a vibration generator, signals returning from at least one remote receiving station are recorded and the recorded signals are registered on a recording medium.

In a known method of seismic exploration of the Earth's interior, an elastic wave train is transmitted into the earth by means of a vibration generator resting on the earth's surface, which has essentially the form of a frequency-modulated oscillation. The sent in the earth elastic wave train is in the Earth's interior to discontinuities u. like. reflected to the earth's surface and there converted at a location remote from the transmitting location by seismic receivers, called geophones, into electrical signals, which are then registered for later evaluation. The duration of the wave train is a function of the distance between the reflective discontinuity and the earth's surface.

The recorded received signals are usually evaluated by a method known as "correlation." This usually involves multiplying an image of the transmitted signal at different mutual phase angles and integrating the products. It is known that the transmitted signal should not repeat within a period of time that is much longer than the maximum possible duration of a received signal to obtain in the correlation sharp and unambiguous evaluation results.

The reflections of the vibration train sent into the earth are in practice not the only components of the received signal. The emitted vibration energy can also reach the receiver as a surface wave or as a broken, diffracted or scattered wave. In contrast to such interference waves, however, the useful signals based on reflections arrive perpendicular or substantially perpendicular to the earth's surface at the receiving location. It is therefore usually used at the receiving location a larger number of receivers or receiver groups (nests), which are achieved by the practically perpendicular to the earth's surface returning waves almost simultaneously, while the substantially parallel to the earth's surface migrating noise arrive at the individual receivers at different times. If one adds one of several methods and equipment for seismic research

applicant:

Continental Oil Company, Ponca City, Okla. (V. St. A.)

representative:

Dr.-Ing. E. Sommerfeld and dr. D. v. Bezold, Patent Attorneys, Munich 23, Dunantstr. 6

Receivers recorded signals, so reinforce the reflection-based, in-phase useful signals, while the no phase relationship having defined interference, which are recorded by the various receivers, cancel as far as possible. It is also known to improve the ratio of useful signal to interference signal to make several broadcasts from different transmission locations. The oscillator is thereby shifted between the individual programs by a certain distance, which, however, is generally small compared to the distance between the transmitting location and the receiving location. It has already been proposed in this connection to successively record the received signals corresponding to the individual programs in adjacent magnetic tracks and to read a larger number of such successively recorded tracks for evaluation by a single, wide, all tracks detecting read head, which generates a composite signal, whose instantaneous amplitude is equal to the algebraic sum of the instantaneous amplitudes of the individual signals recorded in the individual tracks.

It is obvious that an accurate evaluation of the received signals is only possible if there is a defined and known temporal association between the transmitted signal and the received signal. Therefore, for example, the transmitted signal has been wirelessly transmitted to the receiving location and used there directly for correlation. In practice, however, such a method has several disadvantages: an evaluation in the field has the disadvantage that large and sensitive equipment must be carried, which in unfavorable

709 520/92

environmental conditions is a significant impediment. A combination of several, to various programs dating back records requires a very precise time synchronization of the individual transmissions or the received tracks.

By the present invention, methods and devices are to be specified in which these disadvantages are avoided.

A method for seismic research in which one seismic signal is transmitted from different transmission locations to earth in succession by means of a vibration generator, signals coming back from at least one remote receiving station are recorded and the recorded signals are recorded on a recording medium, characterized according to the invention by that the course of the seismic signal to be transmitted is recorded, that the recording of the signal to be transmitted is played at each transmission and controls the vibrator and that the received signals are recorded synchronously with this playback.

Preferably, the course of the seismic signal to be transmitted is recorded on the same recording medium on which the received signals are later recorded.

The recorded seismic signal to be transmitted preferably does not repeat in a manner known per se for a period of time substantially longer than the longest duration of a return signal reaching the receiving station of usable amplitude. In particular, the frequency of the recorded seismic signal changes continuously, as is also known per se.

For practicing such a method, a device is preferably used which includes a magnetic head for recording the seismic signal to be transmitted by a respective recording head for each receiving station and a holding and transporting device for moving a magnetic recording medium past the magnetic heads.

In the method according to the invention, therefore, the oscillator is controlled by one and the same recorded seismic signal in each individual transmission process. The recording of the seismic signal to be sent is also referred to below as the "control track". Since the control track moves synchronously with the recording medium on which the successively received signals are recorded, a defined and known temporal association between the transmitted signal and received signals is ensured. The received signals can be evaluated without difficulty at a later time in a laboratory, since an image of the transmitted signal is available for evaluation, which has a known temporal assignment to the recorded received signals.

The invention will be explained in more detail below with reference to an embodiment in conjunction with the drawing. It shows

F i g. 1 is a schematic representation of a seismic research facility to which the invention may find application;

Fig. 2 is a simplified plan view of a suitable for practicing the method according to the invention recording device and

F i g. FIG. 3 shows a simplified plan view of an evaluation device for the recordings, which by means of the method shown in FIG. 2 illustrated device were produced.

The in F i g. 1 schematically shows a vibration generator or vibrator 6, which rests on the earth's surface and allows to generate a longer-lasting vibration signal of variable frequency, which propagates as an elastic wave in the earth. Of the above-mentioned types of propagation, only one path 8 of a reflection wave and one path 10 of a surface wave are shown in FIG.

The vibration generator is provided with an operating frequency controlling, known per se control unit 14, which in turn is controlled by a line 18 supplied signals generated by a recording device 20 according to the invention, which is based on F i g. 2 will be explained in more detail.

At a distance from the transmitting location and from each other, a number of receivers 22, 24, 26, 28, 30 are arranged on the earth's surface, which convert the vibrations of the ground, thus also the reflection waves 8 and surface waves 10, into electrical signals. These electrical signals are supplied via lines 32 to the recording device 20. It should be noted that in practice the receivers 22 to 30 may consist of a single geophone or a so-called geophone nest. When working with Geophonnestern the signals generated by all the geophones of a nest are generally supplied together via a common line 32 to the recording device 20.

In the operation of a plant of the type shown in FIG. 1, the vibration generator 6 is controlled by a signal supplied via the line 18 from the recording device 20 and provides a frequency-modulated vibration train, while at the in F i g. 1 pulled out location shown. The components of the vibration train arriving at the receivers 22 to 30 are then converted into corresponding electrical signals and supplied to the recording device 20.

Subsequently, the vibration generator 6 is brought to another location 34 and generates there, under the control of the control signal supplied via the line 18, a new vibration train whose course coincides exactly with that of the vibration train first generated, as will be explained. The received signals are recorded again in the recorder 20. The vibrator 6 is then again brought to another transmitting location 36, and the process described is repeated. How many consignments are made and how many receivers exist depends on the geological conditions.

The in F i g. 2 recording device 20 includes a frame 28 in which a cylindrical drum 40 is rotatably mounted on a shaft 42. The shaft 42 is mounted in bearings 44 and is driven by a motor 38 attached to the frame 46 via a gear 48. The drum 40 may itself serve as a magnetic record carrier, but is preferably on its shell

sheet-shaped magnetic recording medium 50 fastened.

In the shell of the drum 40, preferably in the vicinity of its one end, two fixed magnetic heads 52,54 are arranged on the frame 38. The head 52 is used to record a time track on the carrier 50. The head 54 is used to record the control signal on a control track 56 and also also with each revolution of the drum 40, the recorded on the control track 56 control signal again, which then via the line 18th (F i g 1) is supplied to the vibration generator 6. Each vibration train generated by the vibration generator is thus controlled by the same signal recorded in the control track 56, so that the vibration trains emitted by the different transmission locations have an identical course. The timeline track associated with the head 52 serves as a time scale for the other tracks recorded on the carrier 50.

On one side of the frame 38, a slide bar 60 is arranged, which leads a plurality of parallel to the longitudinal axis of the drum 40 slidable holder 58. The holders 58 carrying the recording heads 68 are slidable by a threaded spindle 62 rotatably supported in bearings 64. At one end of the threaded spindle 62, a scale and a hand wheel 66 are mounted, which allow a precise adjustment of the holder 58 by hand. Upon rotation of the spindle 62, the holders 58 move by equal distances along the drum axis. The recording heads 68 attached to the holders 58 serve to record tracks 70, 72, 74, 76, 78 on the record carrier 50. The number of holders and speech heads is arbitrary within the limits set by the size of the record carrier 50. Each speech head 68 is connected via a line 32 (Fig. 1) to a receiver 20 to 30, e.g. B. a Geophonnest connected.

In operation, the holders 58 of the recording device 20 are brought into their one end position, in which they g in F i. Write 2 solid drawn tracks. The drum 40 is then rotated, wherein the control signal taken from the magnetic track 54 from the control track 56 via the line 18 to the vibrator 6 is fed to control its operating frequency. The control signal, as mentioned, preferably has a continuously changing frequency. The vibration signals received from the various receivers 22 to 30 are recorded by the recording heads 68 in tracks 70 to 78.

The drum 40 is stopped after one revolution, and the recording heads 68 are shifted by rotating the screw 62 a certain distance to the right (as viewed in Fig. 2). After also the vibrator 6 has been brought to a new transmission location, the drum 40 is in turn rotated to control the vibrator 6 by the control signal 56 removed from the control signal. The signals corresponding to this second transmission, which are received by the receivers 22 to 30, are recorded by the recording heads 68 on the carrier 50 in tracks 70 a ... 78 a, which are shown in dashed lines. These process steps are repeated for each transmitting location of the vibrator 6, so that on the

Record carrier 50 is a series of parallel tracks corresponding to the received at the individual receiving locations vibration signals. It should be noted that corresponding locations of the tracks 70, 70 a, 72, 72 a ... 78, 78 a along the circumference of the drum with respect to the control track 56 correspond to the time difference between transmission and reception of the vibration signals when the drum 40 is rotated at a constant speed.

After a desired number of programs has been performed, the recording medium 50 is removed from the drum 40 of the recording apparatus and mounted on a drum 80 of a reproducing apparatus 82 shown in FIG. The reproducing apparatus 82 includes a frame 84 in which a shaft 86 is rotatably supported by a drum 80. The drum 80 is driven via a gear transmission 90 by means of a motor 88. Each set of tracks recorded by a recording head 68 of the recorder 20 is associated with a fixed earphone 92 fixed to the frame 84. The listening heads 92 are in a proposed manner so each so wide that they are able to scan all the tracks of the associated sentence. More specifically, therefore, a listening head 92 is disposed opposite to tracks 70 to 70c recorded on the carrier 50 by the recording head 68 nearest to the fixed head 54 in the recorder 20. For the other heads 92 applies accordingly. The listening heads 92 are thus substantially wider than the recording head 68 or at least substantially wider than the individual tracks. In a preferred embodiment, standard size heads 92 are used while the recording heads 68 are considerably narrower and provide tracks about 0.3 mm wide. Between each record, the recording heads 68 are each shifted by a distance such that there is a gap of about 0.05 mm between the successive tracks of a row.

In operation of the playback device 82, the drum 80 is driven by the motor 88 via the gear 90, and all traces of the individual sets of tracks are scanned by the respective listening heads 92 at the same time. Each listening head 92 thus provides a signal composed of the individual signals recorded in the individual tracks of the sentence whose instantaneous amplitude is equal to the algebraic sum of the instantaneous amplitudes of the individual signals. Since all the received signals of a series are recorded synchronously with respect to the programs, the useful signals add up while the interfering signals largely compensate each other.

On the frame 84 of the playback device 82 is also still another head 94 fixedly mounted, which reproduces the recorded on the track 56 control signal simultaneously with the composition of the individual signals through the wide heads 92. The temporal relationships between the transmitted signal and the received signals are retained. The reproduced control signal and the composite signals can then be correlated in a known manner.

In order to reduce the required storage space as much as possible, it is desirable to