IL31835A

IL31835A - Process for measuring mechanical characteristics of rocks during drilling,a device which makes it possible to operate the said process and the use of this process and of this device for the automatization of drilling

Info

Publication number: IL31835A
Application number: IL31835A
Authority: IL
Original assignee: Aquitaine Petrole
Priority date: 1968-03-22
Filing date: 1969-03-18
Publication date: 1971-07-28
Also published as: AT316162B; IL31835A0; ES364970A1; GB1253717A; RO63741A; DE1911680C3; LU58207A1; SU528041A3; OA03313A; NL6904471A; BE730268A; DE1911680A1; NL166753C; FR1587350A; DE1911680B2; JPS4942562B1; NL166753B; CH510814A

Description

A PROCESS FOR MEASURING MECHA ICA L CHARACTERISTICS OP ROCKS DOSING DRILLING, DEVICE WHICH MAKES IT POSSIBLE TO OPERATE THE SA ID PBOCESS AND THE US OP THIS PROCESS AND OP THIS DEVICE FOR THE AUTOMATIZATION OP DRILLING .imp miis ·?α n^sT^oaiEns ntf? I JITQI -p^nm cnomm ητ τ ητι . .

The present invention relates to a method of continuously measuring the characteristics of geological formations which are being drilled, at the actual moment of drilling, in particular during the drilling of oil and gas wells and the like* The invention also relates to apparatus for carrying out this method and to the application of the method to the regulation of the parameters imposed on the drilling tool.

The Invention is particularly applicable to the drilling of rock formations but it is to be understood that although the terra "rock" is used hereinafter, the invention is applicable to the drilling of geological formations in general* Hitherto, the measurement of the characteristics of rock has been carried out subsequently to the drilling operations* Such measurement may be undertaken either on the basis of rock samples taken at the time of drilling, and possibly placed under conditions approximating to the conditions in which they were found at the bottom of the drill-hole, or i situ, after withdrawal of the drilling equipment, by means of diagraphs, for example acoustic diagraphs* Wheel-type tools work on the working face of a drill-hole by percussion, the rotation of the wheel from one tooth to the following tooth ensuring the destruction of the rock. The vibratory energy - - P.A. 31835/2 thus emitted by the drilling tool is distributed along two paths; the first path is formed by the r$ck, in which the transmitted energy serves for the destruction thereof; the second circuit is formed by the mechanical assembly comprising the drilling tool, the drill collars and drilling rods, which together will be referred to as the drilling assembly, and the suspension system for the said drilling assembly. A signal can thus be observed at the top of the drilling assembly, for example by means of pick-ups w&ich detect stress and acceleration The vibrations observed on the surface are the produce* of the chain of vibrations originating from the working face o the transfer function of the drilling assembly.

One of the objects of the present invention is to provide a method which will enable one to derive from this signal, data on the mechanical characteristics of the rock which is attacked by the tool.

Another object of the present invention is to provide a method which will enable one to utilise the data which is collected, for placing the drilling parameters under the control of certain mechanical characteristics of the rock attacked by the tool.

The method according to the present invention comprises collecting at the upper part of the drilling assembly at least one signal which is P.A. 3I835/2 representative of the vibrations of the upper part of the drilling assembly, selecting from the said one signal * fraction thereof in a definite pass band, generating a second signal of like nature to the said one signal and varying with the speed of rotation of the drilling assembly, comparing the said second signal with the said one signal while adjusting the second signal until/the signal representing the difference is cancelled out, and measuring at least one characteristic of the said second signal, which characteristic is connected with the mechanical properties of the rock being drilled and which serves for the control of the drilling pa amete s.

According to one feature of the present invention, the said second signal is generated by forming a sinusoidal value of multiple frequency from the speed of rotation of the drilling assembly and controlling, by means of the said value, a generator delivering a value of which the amplitude is proportional to the frequency, amplifying the amplitude value which is proportional to the frequency by means of an adjustable gain amplifier, regulating the phase of the amplified value by means of an adjustable phase-shift member, applying the resulting value to the input of a network formed of modules arranged in series and each comprising elements constituting a resonant circuit, and multiplying the signal entering the said network by the transfer P.A. 31 35 2 function of the said network.

In accordance with one mode of carrying out the method according to the presentinvention, the signal received at the upper part of the drilling assembly is an electric signal* According to a preferred mode of carrying out the method of the present invention, there a e received at the upper part of the drilling assembly, simultaneously but nevertheless independently of one another, a signa which represents the stresses set up in the upper part of the drilling assembly by the vibrationsof the latter and a signal representing the accelerations resulting from vibrations of the upper part of the drilling assembly, in each of these two signals there is selected a fraction of the said signals having the same pass band, the the means frequency of this pass band being a multiple of the frequency associated with the speed of rotation of the drilling assembly, the partial signal representing the accelerations is Integrated, thus obtaining a partial signal representing the speed of displacement, this partial signal is amplifie and the amplified partial signal is added to the partial signal derived: from the signal representing the stress, which will be called the force signal, the resulting signal is amplified in an amplifier having a given gain and is shifted in phase, the amplitude of the resultant signal is compared in an amplitude comparator with the amplitude of the signal generated as indicated above, yielding a difference signal, the difference signal is applied to an integrator, the integrated difference signal is used for controlling the adjustable gain amplifier, the resultant signal phase is compared in a phase comparator with the phase of the eignal generated as indicated above, the difference signal is used for regulating the adjustable phase-shift member and the gain ofthe adjustable gain amplifie and the shift in phase of the phase-shift member are measured independently.

In a modification of this procedure, pick-ups or strain gauges collecting the signal representing the stresses or strains are positioned at the upper part of the drilling assembly atrotoe the rotatable driving table or platform and comprise two gauges having a vertical axis and two gauges havin a horizontal axis, the mounting being effected in the form of a complete bridge for measuring longitudinal vibrations only, whereas the $ick-ups receiving the signal representing the accelerations are placed in the vicinit of the first pick-ups, the mounting in this case being effected in a differential manner and the axis o the said pickups being parallel to the axis of the drilling assembly.

In another modifications of this procedure, the pick-ups receiving the signal representing the stresses are placed at the upper part of the drillin assembly below the rotary driving table or « · platform and are formed by four gauges inclined at 45° to the axis of the drilling assembly, the mounting being effected as a complete bridge, while the pick«ups receiving the signal representing the accelerations are positioned in the vicinity of the first pick-ups, the mounting b£ing effected In a differential manne and the axis of the said pickups being perpendicular to the axis of the drilling assembly.

In a more highly developed form of this procedure, the measurement of the amplifie gai measurement of the shift in phase are combined, so that the resultant measurement is a measure of the impedance of the rock* The invention is also concerned with apparatus for aarrying the above method into effect, such apparatus comprising an alternating voltage generator controlled by pulses and delivering an alternating voltage with a frequency which is a multiple o the number of pulses received per second, the frequency of the said generator controlling two filters havin the same pass band, a current generato controlled by the alternating voltage generator and deliverin a current proportional to the frequency, a variable gain amplifier amplifying the said current, an adjustable phase-shift device for altering the phase of the said amplified current, a delay line multiplying the said amplified and depahsed current by a transfer function which is adjustable as a function of the P.A. 31835/2 geometrical parameters of the drilling assembly, comparison members receiving, on the one hand , the voltage leavin the delay line and, on the other hand, a voltage established from the voltages received by the pick-ups at the upper part of the drilling assembly, the said voltage being established by simultaneous filtering of the two voltages which are collected by the filters under the control of the alternating voltage generator, one or other of the collected and filtered voltages being applied to an Integrator and to an amplifier, and an adding unit receiving the second collected voltage and filtered voltage originating from the second pick-up, the said adding unit being followed by an amplifier and a phase-shift device* Xn oae preferred embodiment of the arrangement according to the invention, the generated voltage is compared with a voltage established from the vofctages collected by the strain gaugues and the acceleration pick-ups .

In a modification of this arrangement, the strain gauges and the acceleration pick-ups are positioned at the upper part of the drilling assembl above a rotary table, the said pick-ups being arranged in such a manner that the longitudinal accelerations and the stresses or strains are obtained.

In another modification of this embodiment, the stress pick-ups and the acceleration pick-ups art Ρ.Δ. 31835/2 placed at the upper pa t o ; the drilling assembly below the rotary table, the said pick-ups being positioned in such a manner that the stressed and torsional, accelerations are detected.

The invention will be more fully understood from the following description, given by way of example only,' of. one constructional form of apparatus for carrying the method of the invention into effect, together with different applications of the apparatus. The description is illustrated by the accompanying drawings, in which: Fig 1 represents dia rammatically one embodiment of apparatus according to the invention; Fig. 2 shows curves representing variations in the amplitude of the vibrations received at the top of the drilling assembly, as a function of the frequency of the speed of rotation, for rocks having different mechanica characteristics, and Fig. 3 represents diagrammatically, a delay line which forms part of the apparatus of Fig. 1, in which there has been incorporated a tool of resilient behavbur.

Referring to Fig. 1, 1 represents the point of arrival of pulses originating from a proximity detector placed in the vicinity of the driving shaft of the drilling assembly. This proximity detector can be replaced by any other forms of tachometric measuring device.

P.A. 31835/2 A given cumber of pulses reaches 1 each time that the driving shaft makes one rotation* 2 is a piloted alternating voltage generator receiving the pulses arriving at 1 and delivering at its output, which is connected to the line 3, an alternating voltage of which the frequency is a multiple of the number of pulses received at 1 per second. 'nils alternating voltage acts on a generator 4 which delivers an alternating voltage, the strength of which is proportional to the frequency. On the other hand, by way of the line 5» there is received a signal picked up at the upper part of the drilling assembly. This signal is picked up by means of strain gauges, which measure the vibratory stresses occurring in the rod on account of vibrations emitted by the drilling tool. At the same time, by way of the line 6, there is received a signal representing the accelerations picked up in the upper part of the drilling assembly. The alternating voltage generator 2, through Its output 34, controls two frequency selection filters 7 and 8, which allow the passage of a partial signal in the pass band defined by the two filters 7 and 8, the average frequency of the partial signal being the frequency imposed by the voltage generator 2· After filtering by the filter 7» the partial signal originating from the line 5 is applied, by way of the line 9» to an adding unit 12. The partial signal originating from the line 6 is applied, after filtering by the filter 8 and by way of the line 10, to an amplifier 11 followed, by an integrator 11a · At this moment* there is thus available at the outlet of the integrator 11a, a signal which represents the product of the displacement speed at the upper part of the drilling assembly and the characteristic impedance of the rods, if the adjustment of the amplifie is correct.

This signal is applied to the adding unit 12» as indicated above. In this adding unit 12» the modified partial signal derived from the input 5 and the modified partial signal derived from the input 6 are added together. The resultant thereof is a combined sing e signal which is of great interest, as will hereinafter be explained. The combined single signal is delivered by way of the line 13 to an amplifier I and then by way of the line 15 to a phase-shift device l6. The signal as thus treated is applied, on the one hand, to a rectifier 17 and then to a differential amplifier 18 and, on the otherhand, through the line 26, to a phase-shift comparator 28· Another signal, which is established in a manner which will hereina er be defined, is applied to the second input of the differential amplifier 18.

As already mentioned, the generator k delivers a voltage, the strength of which is pBuportional to the frequency* and this voltage is applied by way of the line 2¾ , to the input of the variable gain amplifier 23» which amplifies with a given gain, as will hereinafter be explained, the voltage originating from the sinusoidal generator 4. » The amplified voltage from the amplifier 23 is applied through the line 25 to a phase-shift device 0 , the phase-shift of which is adjustable.

The resulting signal is applied, by way of the line 31 t *o delay line 32 which is formed by an assembly comprising modular units disposed in series and each composed of self-ind&ctances and capacitors arranged to form so-called wT-filterse.

The voltage resulting from the transformation of -fche voltage applied (by way of the line 31 ) to the delay line 32 is applied by way of a line 33 to a fixed earthed resistance or potentionmeter 3¾ · The voltage thus obtained is called the generated signal and is supplied, on the one hand, to the second input of the differential amplifier 18 by way of the rectifier 21 and on the other hand to the second input of the phase comparator 29· The voltage difference at the output of the differential amplifier 18 controls the variable gain amplifier 23 by way of line 19· In a similar manner, an electrical value proportional to the resultant, at the outlet from the comparator 28, of the two phases supplied through the lines 26 and 27» is supplied by way of line 29 and serves for regulating the phase-shift device 30.

The amplitude and the phase corresponding to the moment when the collected signal and the generated signa are equal, are received on the line 22 by means of the line 35 and on the line 29 by means of the line 36, respectively* The amplitude and the phase are collected by the device 37 and combined, and the direct reading of the rock impedance is obtained at 38· The recording of this value constitutes a geological diagraph which is obtained ins antaneously a s the drilling advances* The functioning of this apparatus can be described as follows: In actual fact the signal received at the output of the adding unit 12 is of the form Fjj+Zevu» in which is the force and is the integral of the acceleration* received in the uppar part of the drilling assembly. VJJ represents the speed of displacement at the upper part of the drilling assembly and ZQ is the impedance which is characteristic of the drilling rods.

The signal which leaves the adding unit 12, a te ampli ication by the device Ik and phase-shift by the device l6, represents the vibratory state at the point of connection of the rods and of the casings therefo , and this makes it possible, in particular, to be free from the impedance of the suspension system of the drilling assembly* · The method under consideration consists in measuring by a comparison procedure the vibratory state at the point of connection of the drill rods .A. 31 35 2 The two components, amplitude and phase, are compared separately in two comparison devices, namely the differential amplifier 18 and the phase comparator 28· The signal coming from the driving shaft of the drilling assembly controls the alternating voltage generator 2 and causes a voltage to be supplied which represents the movement of the drilling tool.

The rotation imposed on the drilling assembly causes the movement of the teeth of the tool, and this sets up a train of longitudinal vibrations and torsion, the frequency of which is a multiple of the frequency corresponding to the speed of rotation of the drilling assembly.

The servo-amplifier or generator k transforms the voltage delivered by the generator 2, in such a way that the simulation of the operation of the tool is effected with constant displacement, that is to say, the displacement of the tool during each travel from one tooth to the next is constant. The signal leaving the generator and representing the work carried out during this constant displacement is amplified by the amplifier 23 and shifted in phase by the phase-shift device 30, until the signal supplied by way of the delay line 32, and after voltage division by means of the potentiometer jk, is equal to the signal supplied by the phase-shift device 16.

The delay line 3 simulates the drill collars, which have the effect of deforming, delaying and attenuating the signal by factors bearing a fundamental relationship to the geometrical dimensions of the said P.A. 3I835 2 The two adjustable elements 23 and 30 represent the behaviour of the rock. Thus results are obtained as a consequence of the electrical-mechanical analogy.

The drilling parameters which have to be applied to the tool, for the drilling operation to be carried out under the best possible conditions, are determined by the mechanical characteristics of the rock.. In particular, a hard rock has to be drilled with a low speed of rotation and a considerable pressure on the tool, whereas a soft rock has to be drilled at a high speed of rotation and with a low pressure on the tool. By determining a mechanical characteristic of the rock, which is connected with its hardness, it is possible to use the result of the measurement in order to control the drillingparameters.

One application of the present invention to the measurement of the characteristics of rocks, in order to allow automation of the drilling, consists in utilising the signal obtained by measuring the mechanical characteristics of the rock for controlling the speed of rotation of the drilling assembly and the control of the pressure applied to the tool.

In one method of carrying out this application of the invention, the signal representing the mechanical characteristics of the rock, which is recorded at 38, is used and the said signal is compared with a reference value determining the average positions of the control member of the motor which rotates the drilling assembly and of the brake which controls the unwinding of the cable so as to permit the pressure In another application of the present invention to the automation of drilling, the signal leaving the phase-shi member l6 is collected , its amplitude is compared with a predetermined fixed value and the difference between these two values is utilised for operating the control members of the motor which rotates the drilling assembly and the brake which controls the unwinding of the cable* 'nils application of the invention will now be described with reference to Fig. 2· Two curves are shown i Fig. 2, namely the curve 4.1 and the curve 42. The curve 4l expresses the amplitude of the modified vibrations received at the head of the drilling assembly, as a function of the rotational frequency of the said assembly. The amplitudes vary considerably, passing through a series of maxima with intervening minima · The curve 4l represents the amplitude for a certain rock characteristic. The curve 42 represents the amplitude of the vibrations received at the head of the drilling assembly for another rock characteristic.

I is thus seen that with a given speed of rotation and with a given rock characteristic, a given amplitude is received. It is thus possible to cause an electric voltage to correspond to this amplitude and to use this asa reference value. The variations relatively to this reference value will be representative of the variations of the mechanical characteristics of the rock within a certain sufficiently wide range for the P.A. 31835 2 of a given lithologicalformation. The amplitude variations will thus make it possible to co rect within a definite rante, the influence kf the mechanical-characteristics of the rock and in particular to adjust the pressure applied to the tool* if a fixed rotational speed is maintained.

: Another modification of this procedure consists in causing the speed of rotation to vary. It is possible at a given instant to carry out a "sweep" in rotational speeds that is to say, it is possible to cause the speed of rotation to vary within a definite tange in accordance with a give law. For a given rock characteristic, a portion of the curve l is obtained. If the rock characteristics are not those which are expected, the curve U becomes a curve such as 42, and the deviations observed make it possible to define the characteristics of the rock, and thus to vary the drilling parameters so as to impose the optimum values on these parameters.

Another way of making the drilling automatic, in accordance with the present invention, consists in measuring the transfer function of the drill collars, as observed at the terminals of the delay lino 32 in Fig. 1. In this way, the vibratory state of the collars is determined.

One particularly impo tant application consists in interposing into a series of casings a tool which has a resilient behaviour, for instance, a tool in the nature of a vibration damper, formed of two rigid P.A * 31835/2 parts connected by an element of a material having an elastic behaviour, such as rubber, in interposing at a suitable position in the delay line a capacitatibe element which is in shunt and in finding the maximum vibratory state* Fig. 3 represents such a capacitative element 3 interposed bet een two parts 32a and 32b of a delay line simulating the drill collars} the element 43 is connected on the other hand to earth. In order to be placed in the best drilling conditions and to obtain the effect of maximum percussions of the tool, a rotational speed ••sweep" is carried out and there is measured the force on entering the assembly formed by the ay lines 32a and. 32b and the capacitor 3. The maximum amplitude of the force is sought and then the drilling parameters are established under these conditions.

This application permits, in particular^ *he use of considerable percussion during rotary drilling, and of obtaining speeds of advance which are obtained in conventional rotary drilling.

Claims

P.A. 31835/2 WHAT IS CLAIMED ISi-

1. Amethod of measuring mechanical prope ties of rock during drilling operations carried out with a rotary drillin assembly having a lower part extending within a drill-hole and an upper part extending from the drill-hole and having at the said lower part, a tool for breaking down the rock, comprising a plurality of cutting elements rendered operative by rotation of the tool, which method is characterized by the following steps: (a) receiving,at the upper part of the drillin assembly, at least one signal which is representative of vibrations resulting from displacements and stresses created by the action of the tool on the rock and from the said displacements and stresses being transmitted through the drilling assembly; (b) selectin a part of the said received signal, by filtering it by means of a frequency filter haying a pass band centered on a frequency defined by the product of the speed of rotation of the drilling assembly and the number of the said cutting elements of the tool; (c) generating a signal having at least one chara teristic of the same nature as one characteristic of the received signal; (d) comparing the generated signal with the said composite signal and producing a difference signal; (e) adjusting the amplitude and phase characteristics of the generated signal until the difference P.A. 31835/2 (f) measuring at least one of the said character istics of the generated signal at the moment when the difference signal is zero; (g) correlating the value of the said characteristic or characteristics of the generated signalwith the de th of the said tool in the drill-hole.

2. . A method according to Claim 1, in which sa d signals are electrical signals.

3. A method accordin to Claim 1» in which the amplitude characteristic of the generated signal, corresponding to the difference signal being zero, is recorded as a function of the depth of the tool in the drill-hole.

4. A method according to Claim 1, in which aCalculator device is used to establish control values contan&ling parameters which are imposed, on the tool and in which the amplitude of the generated, signal » corresponding to the difference signal being zero, is used as an input signal for the said calculator device.

5. A method according to Claim 1, in which the amplitude and phase characteristics of a plurality of received signals are measured, the measured values being combined so as to obtain a value indicative of the mechanical properties of the rock.

6. A method according to Claim 1, in which a periodic frequency value is established, which is a multiple of the speed of rotation of the drilling assembly, the said value serving as the said generated signal, controlling by means of the said value a P.A. 31835/2 generator delivering a value of which the amplitude is a proportional to the frequency* amplifying this amplitude value which is propo tional to the frequency by means of an. adjustable gain amplifier, regulating the phase of the amplified value by means of an adjustable phase-shift member , applying the resulting value to the input of a network formed of modular units arranged in series and each comprising elements which constitute a resonant circuit and multiplying the signal entering the said network by the transfer function of the said network,

7. · A method according to Claim 1, in which there are received at the upper part of the drilling assembly, simultaneously but indpenjsdently of one another, a signal representing staains set up in the upper part of the drilling assembly by vibration of the latter and a signal representing accelerations resulting from vibration of the upper part of the drilling assembly, a part of each of these two signals in the same pass band is selected, the mean frequency of this pass, band being equal to the product of the frequency associated, with the speed of rotation of the drilling assembly and the number of cutting elements of the tool, the partial signal representin the accelerations is integrated, yielding a partial signal representing the displacement speeds, the said partial signal is amplified, this amplified partial signal and a part of the signal representing the stress are added, the resultant signa is amplified in an ad ustable gain amplifier as a function of the length of the drilling assembly* this signal is shifted in phase by an amount related to the length of the drilling assembly, the amplitude of the resulting signal is compared in an amplitude comparator with the amplitude of the said generated signal, yielding a difference signal which is applied to an integator* the integrated difference signal is used for controlling the gain of the variable gain amplifier* the phase of the resulting signal is compared in a phase comparator with the phase of the said generated signal* the difference^ signal* is used for regulating the phase-shift of the adjustable phase-shift device* and the gain of the variable gain amplifier and the phase shift of the phase-shift device are measured independen ly*

8. , ffin drilling apparatus for drilling rock* and comprising a rotary drilling assembly having a lower part adapted to extend within a drill-hole and an upper part adapted to extend from the drill-hole and having at the aaid lower pa a tool for breaking down the rock* meaas for measuring the mechanical properties of the rock during drilling* characterized by pick-up devices at the said upper part of the drilling assembly for establishing a voltage representative of vibrations of the said upper part* an alternating voltage generator adapted to be controlled by pulses and to deliver an alternating voltage with a frequency which is a multiple of the number of pulses received per second* two filters having the same pass band and adapted to have applied thereto* P.A. 31835/2 the frequency of the said generator, a current generator connected to be controlled by the alternating voltage generator for delivering a current proportional to the frequency, a variable gain amplifier for amplifying the said current, an adjustable phase-shift device for altering the phase of the said amplified current, a delay line for multiplying the said amplified and dephased current by a transfer function which is adjustable as a function of the geometrical parameters of the drilling assembly, comparison devices for receiving, on the one hand, the voltage from the delay line and on the other hand, a voltage established from voltages from the said pick-ups by simultaneous filtering by the said filters under the control of the .a.ternating ^eolta e generator, an integrator and an amplifier adapted to have one of the filtered voltages applied thereto and an adding unit adapted to receive the other voltage originating from a second one of the pickups and an amplifier and a phase-shift device following the said adding unit.

9. * An automatic drilling method using a rotary drilling assembly having a lower part extending within a drillhole and an upper part extending from the drill-hole and having at the said lower part, a tool for breaking down rock in the drill-hole, comprising a plurality of cutting elements rendered operative by rotation of the cutting elements rendered operative by rotation of the tool which P.A. 3I835/2 method comprises meanering mechanical properties o the rock by means of the following steps: (a) receiving, at the upper part of the drilling assembly, a plurality of signals which are representative of vibrations resulting from displacements and stresses created by the action of the tool on the rock and (b) selecting a part of each of the said received signals, by filtering them by means of a frequency filter having a pass band centered on a frequency defined by the product of the speed of rotation of the drilling assembly and the number of the said cutting elements of the tool; (c) combining together the selected parts of the said signals to form a composite signal; (Λ) generating a signal having the same nature as the received signals; (e) comparing the generated signal with the said composite signal and producing a difference signal; (f) adjusting the amplitude and phase characteristics o the generated signal until the difference signal becomes zero; (g) measuring at least one of the said characteristics of the generated signal at the moment whe the difference signal is zero; (h) correlating the value of the said characteristic or characteristics of the generated signal with the depth of the said tool in the drill-hole; and then P.A. 31835/2 the moment of equality of the latter and of the received signal* and using the difference to control devices vhich determine the parameters of the drilling operation*

10. A drilling method according to Claim 9, consisting in detecting the amplitude of a generated signal at the input to a delay line, determining the maximum value of the signal as a function of the speed of rotation of the drilling equipment and maintaining the speed of rotation at the value giving the maximum amplitude.

11. » A drilling method according to Claim 10, in which a tool of elastic behaviour is used, which is interposed i the assembly of drilling collars, the method consisting in determining the maximum amplitude of a generated signal generated at the entry to the delay line and in maintaining the drilling parameters at srarlues such that this amplitude remains at a maximum.

12. A drilling method according to Claim 11, in which the mechanical impedance of the drilled rock is continuously recorded, thus establishing an instantaneous log of the progress of the drilling, and this log is used for geological interpretation. COHEN ZEDEK & SPISBACH