The present invention relates to improvements in seismic source devices, and in particular to but not restricted to magnetostrictive sources for use during in borehole investigations of the sub-surface in oil and gas exploration.

Seismic sources are used to propagate an acoustic signal into the earth, which is to be subsequently detected at appropriately designed receivers that record the signal. The purpose of seismic source/receiver systems is to determine properties of the earth in the region between the source and receivers. Typically, the characteristic velocity of a rock formation, the acoustic impedance, or other reflection characteristics are investigated. Such investigations reveal information regarding the structure of the earth local to the source and receiver.

Seismic investigations can be conducted on a number of scales. In many cases it is desirable to deploy seismic sources and receiver arrays together in a single well to

investigate the formation properties near to the wellbore. Upon drilling of a well, data may be interpreted to assist and guide drilling of the well. Specifically, a recorded signal that has been reflected back from a particular target can provide early warning of a problematic structure that should not be drilled.

A seismic source is driven in a manner that enables a signal with a particular form to be generated and transmitted. In general, it is desirable to have accurate control of the source signal. An accurate and reproducible source signal facilitates processing of the receiver signal into an interpretable form. Signals may be constructed to have a particular frequency content or amplitude and phase attributes selected for a given task. In the case of sub-surface imaging, the ability to resolve sub surface structures at various ranges from the source is a major consideration for designing an appropriate source signal.

In downhole applications, a number of different sources have been used, but they suffer from a number of deficiencies and drawbacks. A common type of seismic source is an electro-mechanical source, which produces a seismic signal from a mechanical impact generated in response to an electrical input signal. These sources rely on the impact between components which, in general, is difficult to accurately repeat. For example, each time there is an impact, differing amounts of energy are dissipated as friction. Such unclean impacts make it difficult to generate a well-defined pulse. These sources operate over a limited frequency band, typically

up to about 400 Hz. This compromises resolution and the ability to process the data effectively.

Piezo-electric sources have been used in sonar applications for looking ahead of the bit whilst drilling. However, in general, such systems do not transfer energy efficiently to the source and into the earth.

A further type of seismic source is a magnetostrictive source, which contains an electric coil/magnet transducer for turning electric input pulses into mechanical pulses. These devices have been developed to generate signals over a wide frequency range. The best known magnetostrictive device is a reaction-mass source developed by Sandia National Laboratories, which was designed for environmental applications. In this source, the transducer displaces a large mass in order to create the seismic signal. As a result, a significant current must be generated in the transducer coil in order to move the mass. This requirement causes difficulties when generating a signal because as the current is switched, a significant back EMF results, which can burn out the transducer coils.

Another disadvantage of this system is that strong resonances are generated in the borehole which interfere with transmission of the primary signal. Furthermore, this source is designed to be deployed alone in a borehole separate without other downhole instruments deployed during drilling. Thus, in order that this source can be used for seismic imaging during drilling, the existing downhole tool string must be removed from

the borehole, prior to deploying the source. This generates significant expense and poses practical problems particularly where there is a jointed pipe system already in place.

Other problems are manifest in downhole source arrangements where the sources are attached to and suspended from individual wire lines, or, are attached to the tool strings, which are typically deployed for investigations in connection with the drilling process. Such source arrangements inhibit effective coupling into the source signal into the earth, and, as a result, a significant portion of the transmitter signal may become trapped in the well, for example, as a standing wave providing unwanted secondary coherent noise sources. Much of the transmitted signal power is also lost in these situations.

It is one aim of the invention to provide a downhole seismic source that mitigates or at least obviates limitations of prior art systems.

It is another aim of the invention to provide a downhole seismic source capable of transmitting a broadband acoustic signal during deployment of tool strings in the borehole.

It is another aim of the invention to provide a magnetostrictive downhole seismic source that couples effectively to the walls of the well.

Other aims and objects of the invention will become apparent upon reading the following description.

According to a first aspect of the present invention, there is provided a system for use in a borehole in an earth formation comprising: (a) a downhole assembly conveyed in the borehole; (b) a magnetostrictive transducer on the downhole assembly which produces a mechanical displacement of a coupling member when activated by an electrical signal from a source thereof; (c) at least one contact member in contact with a wall of the borehole and hydraulically coupled to the transducer, the at least one contact member producing a seismic signal in the formation in response to the mechanical displacement of the coupling member.

Preferably, the magnetostrictive transducer comprises an alloy of terbium, dysprosium, and iron.

The electrical signal may be selected from the group consisting of (i) a pseudo-random binary sequence, and (ii) a swept frequency signal.

The system may further comprise a plurality of extension devices which maintain the transducer substantially in a fixed position in the borehole.

The hydraulic coupling may be provided by: (i) a master cylinder having an axis substantially parallel to an axis of the borehole, and (ϋ) at least one slave cylinder coupled through an extension device to the at least one contact

member, the at least one slave cylinder oriented substantially orthogonal to the master cylinder.

The signal may further comprise a pseudo-random binary sequence, and the source of the electrical signal may further comprise a shift register provided with a negative feedback loop.

The at least one contact member may further comprise a pair of opposed contact members.

Conveniently, the system further comprises a conveyance device which conveys the downhole assembly into the borehole, the conveyance device selected from (i) a wireline, (ii) a slickline, and (iii) a drilling tubular. The system may further comprise a receiver which produces an output responsive to the propagation of the seismic signal through the earth formation. Conveniently, the system further comprises a processor which determines from the output of the receiver and the electrical signal a travel time of the seismic signal from the at least one contact member to the receiver.

According to a second aspect of the present invention, there is provided a method of evaluating an earth formation comprising: (a) conveying a downhole assembly into a borehole in the earth formation; (b) providing an electrical signal to a magnetostrictive transducer in the downhole assembly and producing a mechanical

displacement of a coupling member coupled to the transducer; and (c) hydraulically coupling at least one contact member in contact with a wall of the borehole to the coupling member and producing a seismic signal in the formation in response to the mechanical displacement.

The step of providing the electrical signal may further comprise using at least one of (i) a pseudo-random binary sequence, and (ii) a swept frequency signal.

The method may further comprise using a plurality of extension devices for maintaining the transducer substantially in a fixed position in the borehole.

Conveniently, the step of hydraulically coupling the at least one contact member in contact with the wall of the borehole further comprises using a master cylinder and at least one slave cylinder coupled through an extension device to the at least one contact member.

The signal may comprise a pseudo-random binary sequence, and the step of providing the electrical signal may further comprise using a shift register provided with a negative feedback loop.

Conveniently, the at least one contact member may further comprise a pair of opposed contact members.

The method may further comprise conveying the magnetostrictive transducer into the borehole on one of

(i) a wireline, (ii) a slickline, and (iii) a drilling tubular.

Conveniently, the method further comprises the step of detecting the propagation of the seismic signal through the earth formation.

The method may also comprise determining from the detected signal a travel time of the seismic signal through the formation.

According to a third aspect of the present invention, there is provided a computer readable medium for use with a system for use in a borehole in an earth formation, the system comprising: (a) a downhole assembly conveyed in the borehole; (b) a magnetostrictive transducer on the downhole assembly hydraulically coupled to at lest one contact member in contact with a wall of the borehole; the medium comprising instructions which enable a processor to provide a signal to the magnetostrictive transducer which causes the at least one contact member to produce a seismic signal in the formation.

Preferably, the medium further comprises at least one of (i) a ROM, (ii) an EPROM, (iii) an EAROM, (iv) a flash memory, and (v) an optical disk.

According to a fourth aspect of the invention there is provided a seismic source comprising a magnetostrictive transducer, and coupling means for coupling the

transducer to a formation, wherein the coupling means includes a hydraulic coupling arrangement.

Preferably, the seismic source is adapted to be run in and coupled to a borehole.

More preferably, the seismic source is adapted to be run on a drill string.

Preferably, the magnetostrictive transducer comprises an alloy of terbium, dysprosium, and iron.

More preferably, the magnetostrictive transducer is a Terfenol-D ™ transducer.

Terfenol-D is a trade mark of Etrema Products, Inc of Indiana, USA.

Preferably, the coupling arrangement comprises a hydraulic master cylinder coupled to the transducer.

More preferably, the hydraulic master cylinder is coupled to the transducer on a substantially longitudinal axis of the borehole.

Preferably, the coupling arrangement comprises a plurality of slave cylinders coupled to the master cylinder and the borehole.

More preferably, the coupling arrangement is adapted to transmit a substantially non-directional signal in the formation.

More preferably, the coupling arrangement comprises pressure pads adapted to transmit an output signal from the slave cylinders to the formation.

Preferably, the slave cylinders are oriented substantially radially in the borehole.

More preferably, two slave cylinders are provided oriented substantially in the same axis.

Optionally, the coupling means comprises a pair of fixing cylinders adapted to fix the transducer radially within the wellbore.

Preferably, activating means is provided for activating the coupling arrangement by moving it from a first position in which it is free from the borehole to a second position in which it is coupled to the borehole.

More preferably, the fixing means increases the hydraulic pressure in the slave cylinders and the fixing cylinders.

The seismic source may be adapted to be driven by a digital binary electronic signal.

The digital binary electronic signal may be a pseudo- random binary sequence (PRBS) .

Preferably, the PRBS is provided at low voltages.

Preferably, the PRBS is a maximal length (ML) sequence, providing broadband frequency signal characteristics.

Preferably, the ML PRBS can be generated using a 10-stage negative feedback shift register, having two feedback taps combined into an XOR logic gate and modulo-2 summed.

According to a fifth aspect of the invention there is provided a seismic source comprising a magnetostrictive transducer, and coupling means for coupling the transducer to a formation, wherein the seismic source is adapted to be driven by a pseudo-random binary sequence (PRBS) .

Preferably, the PRBS is provided at low voltages.

Preferably, the PRBS is a maximal length (ML) sequence, providing broadband frequency signal characteristics.

Preferably, the ML PRBS is generated using a 10-stage negative feedback shift register, having two feedback taps combined into an XOR logic gate and modulo-2 summed.

According to a sixth aspect of the invention there is provided a seismic source comprising a transducer and coupling means for coupling the transducer to a formation, wherein the coupling means includes a hydraulic coupling arrangement.

Preferably, the seismic source is adapted to be run in and coupled to a borehole.

More preferably, the seismic source is adapted to be run on a drill string.

According to a seventh aspect of the invention there is provided a seismic signal transmission system comprising; - a seismic source coupled to a formation, - means for driving the seismic source using a pseudo-random binary sequence (PRBS) to cause a signal to propagate in a formation, - means for receiving a signal from the formation.

Preferably, the PRBS is provided at low voltages.

Preferably, the PRBS is a maximal length (ML) sequence, providing broadband frequency signal characteristics.

Preferably, the ML PRBS is generated using an nth stage negative feedback shift register, having two feedback taps combined into an XOR logic gate and modulo-2 summed.

Preferably, the seismic source is adapted to be run in and coupled to a borehole.

More preferably, the seismic source is adapted to be run on a drill string.

According to an eighth aspect of the invention there is provided a method of transmitting an acoustic signal in a formation the method comprising the steps of; a. providing a seismic source, b. coupling the seismic source to the formation by a hydraulic coupling arrangement, c. driving the seismic source to cause an acoustic signal to be transmitted in the formation.

Preferably, step a. involves running the seismic source on a drill string.

Preferably, the step c. is carried out using a pseudo- random binary sequence (PRBS) .

Preferably, the seismic source is the seismic source of any of the fourth, fifth or sixth aspects of the invention.

There will now be described, by way of example only, embodiments of the invention with reference to the following drawings, of which:

Figure 1 is a side view line drawing of a downhole magnetostrictive seismic source in accordance with an embodiment of the invention;

Figure 2A is a state transition diagram for a Finite State Machine (FSM) in accordance with an embodiment of the invention;

Figure 2B is a schematic representation of a downhole magnetostrictive seismic signal generation and interpretation system in accordance with an embodiment of the invention;

Figure 3 is a line graph of a processed output signal in accordance with an embodiment of the present invention; and

Figure. 4 illustrates the arrangement of source and sensors using the source of the present invention;

With reference firstly to Figure 1, there is depicted a downhole magnetostrictive seismic source at 1. The source 1 is located within the wellbore 3. The source 1 comprises a master cylinder 5 at an operational end, and, a magnetostrictive transducer 7 is located at an input end. The transducer 7 and the master cylinder 5 are connected along a longitudinal axis by a push rod 9, which displaces relative to the master cylinder 5 in accordance with mechanical displacement generated by the transducer 7. The transducer 7 converts electrical pulses into mechanical motion along the longitudinal axis of the transducer 7. The push rod 9 is attached to the transducer such that an electrical signal in the coil is transferred to a mechanical movement of the push rod 9. The push rod may be referred to as a coupling member. For the purposes of the present invention, the geometry of the coupling member is not important.

In this example, the source is a Terfenol-D ™ transducer, marketed by Etrema Products, Inc of Indiana, USA.

In this embodiment, a set of slave cylinders 11 are attached to the master cylinder 5 and are provided with large-area pressure pads 15 that are located against the walls 4 of the well bore 3. A set of fixing cylinders 17 attached to the transducer 7 have a set of pressure pads 19 located against the well bore wall 4. The individual cylinders of the slave cylinders 11 and the fixing cylinders 17 substantially oppose each other and are

positioned perpendicularly to the longitudinal axis of the source.

The above arrangement provides means for securing and locating the source in the borehole. The slave pistons 11 and fixing pistons 17 displace radially away from the main cylinder 5 and transducer 7 according to forces imposed on them due to the pressure exerted on the walls of the cylinder 5. In turn, this force is transferred to the pressure pads 15 and 19 and is applied against the borehole walls 4. In this case, the master cylinder 5 is a hydraulic cylinder containing hydraulic fluid. In a test environment (which will be described in more detail below) , initial pressurisation is provided manually by a hand operated pump. It will be appreciated, however, that pressure may be supplied in the downhole environment via control line or the like, or utilising wellbore pressure, through a control valve assembly. Pressurising the master cylinder 5 simultaneously attaches the slave cylinders and fixing cylinders via pressure pads 15 and 19. Once the source is attached, the hydraulic link between the fixing cylinder set 17 and the master cylinder 5 is terminated by sealing a valve between them. This removes the possibility of pressure variations occurring in the fixing cylinder set 17 as a result of mechanical displacement of the push rod 9 into the master cylinder 5.

Upon increasing the pressure within cylinder 5 by insertion of the push rod 9 into the master cylinder 5, the slave pistons 11 exert a force through pressure pads 15 to the wellbore walls 3. Through this mechanism there

is transmitted an acoustic signal to the rock formation 6.

Mechanical displacement of the rod 9, caused by the transducer 7, into the cylinder 5, exerts a fluid pressure force on the hydraulic fluid within the cylinder 5 and in turn causes an increase in pressure within the cylinder 5. The pressurised fluid exerts a force against the walls of the master cylinder 5, which is transferred to the slave cylinders 11 of the master cylinder 5, to the pressure pads 15 acting against the well formation 6. In accordance with this mechanism, a signal generated by mechanical displacement of the transducer 7 and push rod 9, leads to increases and decreases in force applied to the walls of the master cylinder 5, and, in turn, leads to pressure pulses applied to the well bore through the pressure pads 15. As a result a seismic signal is transmitted through the pads 15 into the well formation 6.

Hydraulic attachment of the source to the borehole provides advantages. Because the pressure pads are in contact with the rock formation, the applied source signal has an effective coupling to the formation. This coupling minimizes any source energy trapped within the borehole, thus, reducing the occurrence of tube standing waves and resonances, which are otherwise susceptible to occur.

In addition, this source arrangement provides a method through which relatively small displacements of the push rod 9 into the cylinder 5 yield the production of significant pressure at the pressure pads 11.

Furthermore, this system is considerably lighter than existing magnetostrictive downhole sources/systems, making it easier to deploy. The clamp design and relatively low power requirements for driving the source allows attachment of the source to the tool string deployed in connection with drilling of a well. This prevents any need to remove the tool string prior to carrying out a seismic borehole investigation, for example, for sub-surface structural investigations for use in the direction of the drilling process.

Furthermore, the source couples effectively with the well formation and tends not to damage the interior of the borehole, which otherwise would occur for a single pulse electromechanical source.

In this operational configuration the source signal from the transducer 7 may generally be considered a varying signal around the DC pressure required for attaching the source to the borehole walls 6.

With reference to Figures 2A and 2B, there is depicted a signal generation and interpretation system depicted at 41 for use with the downhole magnetostrictive seismic source 1 that was described in the above embodiment. In this system, the input signal to the source and consequently the signal output into the well formation 49 is derived from a digital negative feedback shift register 43. The purpose of the shift register 43 is to provide the source 1 with a sequence of binary pulses- through electrical connection 44.

The shift register 43 is provided with a negative feedback loop 47, which, upon activation, provides a continuous stream of binary pulses until otherwise requested. The binary state of the shift register (i.e. the binary values at each flip-flop 46) , is set upon commencement of transmission. The transfer of binary digits into and out of the shift register is governed by clock pulses provided to the register 43 during transmission. In this example, the shift register 43 is configured to provide a maximal length, pseudo-random binary sequence (PRBS) of pulses. This is achieved by putting two binary feedback taps 45 through an XOR logic gate and modulo-2 adding as part of the negative feedback loop 47.

This configuration provides a maximal length PRBS of length 2 ³ -l. In this case, 7 states are cycled between during transmission. The zero state, where all flip- flops have binary value 0, is a locked state where transmissions continue to further 0 states. This is provided by a non-zero initial state upon signal generation of a programmed clock and reset device 47.

The provision of a PRBS to the well formation using the source 1 has a number of advantages. In this system, the signal transmitted by the source propagates through the rock formation to receiving instruments 50 that record the signal 49. The seismic signal detected at the receivers is correlated with the input signal, which is known to be a PRBS signal, which reveals high correlation where the input and output signals match.

The provision of a PRBS signal, allows significant processing gain to be achieved as a result of the inherent autocorrelation properties of such a signal. Specifically, upon autocorrelation, the signal-to-noise ratio of the output signal is substantially greater than the signal-to-noise ratio of the input signal.

This provides the possibility of a signal being detected and retrieved at the receiver despite being originally transmitted as a sequence of pulses below the ambient noise level. This feature of the PRBS signal, used in this embodiment, permits the use of a low-voltage current input to the source transducer. Thus, the switching of the electrical current in the transducer coil can take place safely without the risk of coil burnout due to back EMFs upon switching. This mechanism provides for maintaining prolonged use of the source components thus saving maintenance costs.

Use of a PRBS signal also provides for transmission of a broadband signal into the earth. This provides significant resolution benefits, with high-frequencies used for detection over short length scales.

In an alternate embodiment of the invention, instead of pulsing the source with a PRBS signal, a chirp signal is used. A chirp pulse that is a linear frequency modulated pulse with a start frequency fs and chirp rate γ is denoted as:

x _chirp (t) 0 < t < T _chirp ( 6 ) .

A particular advantage of using a chirp signal downhole is that if the frequency band is sufficiently large, the autocorrelation is a unit spike in the time domain. For band limited chirp signals, the autocorrelation is the sync function. Like the PRBS, a chirp signal has relatively low power requirements but is able to get a relatively large amount of energy into the formation.

For the case where a chirp signal is used, or even for the case where a PRBS is use, instead of the circuitry illustrated in Figures 2A, 2B, a processor may be used.

The principles of the present invention were demonstrated in a laboratory environment, in which a 10cm diameter, Im deep hole was cored in a 5 ton granite block. The transducer arrangement as described with reference to Figure 1 was used, and six accelerometers were placed on the surface of the block to monitor the performance of the transducer.

Figure 3 is a graph of the autocorrelation of an output signal with the input signal at 60. The graph reveals a substantial amplitude peak at 62 corresponding to the time at which the signal arrives at the receiver. At earlier times, in the region 64, the autocorrelated signal 60 remains within the ambient noise. In this case, the receiver is an accelerometer is located in a granite block away from the magnetostrictive source.

The results indicate that an improved signal-to-noise is achieved by autocorrelating a PRBS signal generated by the seismic source of the present invention. Furthermore, it provides confirmation that the source is

capable of successfully transmitting a PRBS seismic signal into a rock formation. This has significant application to single well imaging systems in particular, where tube waves generated by the source tend to obscure first arrivals and reflections.

Turning now to Figure 4, an example is shown of the source of the present invention conveyed in a wellbore in an earth formation. The arrangement is suitable for carrying out what is called a reverse Vertical Seismic Profile (VSP) . Shown is a drillbit 150 near the bottom of a borehole 126. The drillbit is part of a bottomhole assembly (BHA) conveyed in the borehole on a drilling tubular such as a drillstring or coiled tubing. A downhole source is denoted by 153 and a reference receiver at the surface is denoted by Rl. 155 shows an exemplary raypath for seismic waves originating at the source 153 and received by the receiver Rl. The source 153 is usually in a fixed relation to the drillbit in the bottom hole assembly. Also shown in Figure 4 is a raypath 155' when the BHA (and the source) are at a different position 153' near the bottom of the borehole. Raypaths 155 and 155' are shown as curved. This ray- bending commonly happens due to the fact that the velocity of propagation of seismic waves in the earth generally increases with depth. Also shown in Figure 4 is a reflected ray 161 corresponding to seismic waves that have been produced by the source at 153, reflected by an interface such as 163, and received by the receiver at Rl.

For the configuration shown in Figure 4, it is necessary to have the downhole clock on which the source operates and a surface clock where the data are received and/or recorded in synchronization. Without the synchronization, it is not possible to determine accurate propagation times for the seismic waves from the source to the receiver. Synchronization is also necessary to be able to properly perform the cross- correlation operations needed when a PRMS or a chirp signal is used. This synchronization is not necessary for looking ahead of the drillbit where the receiver(s) would be on or near the BHA and could be on the same bus.

The source may also be conveyed downhole on a wireline as part of a string of logging tools. For the purposes of this invention, the BHA and a one or more logging tools are referred to as a downhole assembly.

A processor is used to determine the time of propagation of the seismic waves from the source to the receiver from the received and transmitted signals. For looking ahead of the drillbit, a downhole processor is necessary. For the example shown in Figure 4, a downhole processor would be used for controlling the source and typically a surface processor is used for analyzing the signals.

The processing of the data may be accomplished by processor. Implicit in the control and processing of the data is the use of a computer program implemented on a suitable machine readable medium that enables the

processor to perform the control and processing. The machine readable medium may include ROMs, EPROMs, EAROMs, Flash Memories and Optical disks.

The invention has been described with reference to a logging tool conveyed on a drillstring. The method of the present invention is equally applicable for use with NMR logging tools conveyed on a wireline, slickline or coiled tubing.

It should be appreciated that other source arrangements and driving methods could be envisaged without departing from the scope of the invention defined by the first to fifth aspects above.