OPTICAL SEISMIC DETECTOR

Abstract: The invention relates to a servo-accelerometer which is particularly suited for use as an optical seismic detector. The servo-accelerometer comprises at least one permanent magnet (17), at least one coil (16) for producing a magnetic field which can interact with the magnet, coil supporting means (15) movable with respect to the magnet, a light source (18), light receiving means (20) for producing a signal indicating the position of impingement of the light beam emitted by the source (18), an optical system (14) mounted on the supporting means for causing the light beam to be focussed on the light receiving means (20), and a servo-amplifier (22) for applying a driving current to the coil (16) in response to the signal from said light receiving means (20).

Description: The present invention relates generally to a detector for detecting the acceleration of a point as defined in claim 1. More specifically, the present invention relates to a servo accelerometer which is particularly suited for use as an optical seismic detector for detecting and mapping an underground stratum structure. Background Of The Invention Seismic detectors called geophones are well known in the art. The geophone is basically a seismic-to-electrical transducer, and, when used, it is typically planted in the surface of the earth. When a disturbance is generated at a location away from the place where the geophone is planted, it propagates into the ground and the reflected disturbance is collected by the geophone for use in the analysis of the underground structure. In this case, the geophone basically detects the motion of the ground surface, i.e., the acceleration, the velocity or the displacement of a point of the ground surface. Although there are several variations, the geophone is mainly comprised of a moving mass, a spring for suspending the mass and a case for housing therein the moving mass suspended by the spring. With this structure, when the case is set in motion, for example, due to reception of the reflected disturbance, there is produced a relative motion between the moving mass and the case through the spring and such a relative motion is utilized to produce a variable, such as a voltage, for the measurement of the acceleration, the velocity or the displacement of a point of interest. On the other hand, a servo accelerometer has recently come to draw much attention. The servo accelerometer is basically an accelerometer with a feedback circuit and thus it defines a closed-loop detector system. The accelerometer has been used for measuring the vibration of machine tools and buildings, and it is mainly comprised of a moving mass for detecting the acceleration and a spring for supporting the moving mass. In such an accelerometer, the force applied to the moving mass due to the acceleration is electrically detected for measurement of the acceleration. Such an accelerometer usually defines an open-loop detector system. Thus, the acceleration-responsive geophone may be considered as a kind of accelerometer. The servo accelerometer is an improvement over the above-described normal accelerometer and it is so structured to measure the acceleration from an electric current which is required to produce a force or torque to bring the moving mass back to the original position. Thus, the servo accelerometer is significantly enhanced in accuracy as well as stability as compared with the normal open-loop type accelerometer. One of the remarkable features of the servo accelerometer is the static acceleration sensitivity, which allows the servo accelerometer to be used for the measurement of the angle or orientation, indicating the possibility for the application in the attitude control. US Patent -A-2 695 165 (Hansen) describes a servo accelerometer including the features described in the preamble of claim 1. As described above, the servo accelerometer is far superior to the open-loop type accelerometer in many respects, but still it needs to be further improved so as to allow wider and more extensive applications. Summary Of The Invention The present invention has been made in view of the above-described circumstances, and, it has its primary object to provide an improved detector capable of detecting the acceleration of a point of interest with high resolution and increased sensitivity. A further object of the present invention is to provide a detector for detecting the acceleration, the velocity or the displacement of a point of interest, which is reliable in operation and compact in size. A servo accelerometer according to the invention is defined in claim 1. Hereinafter, the present invention will be described in detail by way of embodiments with reference to the accompanying drawings. Brief Description of the Drawings Fig. 1 is a schematic illustration showing a system of offshore vertical seismic measurement to which the optical seismic detector embodying the present invention may be advantageously applied; Fig. 2 is a schematic illustration showing the overall structure of the optical seismic detector constructed on the principle of a servo accelerometer; Fig. 3 is a plan view showing the structure of a dual photodetector suitable for use in the present invention; Fig. 4 is a circuit diagram showing the detailed structure of the servo amplifier 22 shown in Fig. 2; and Figs. 5 and 6 are plan views showing alternative structures of the dual photodetector which are suitable for use in the detector shown in Fig. 2. Detailed Description Of The Preferred Embodiments Referring now to Fig. 1, there is schematically shown a system of offshore vertical seismic measurement, in which an optical seismic detector constructed in accordance with one embodiment of the present invention can be advantageously used. As shown, there is shown an offshore rig l to support thereon a measurement and control unit 2. A downhole cable 4 extends from the measurement and control unit 2 through a borehole 3 drilled in the earth and is provided with a downhole tool 5 at its bottom end. The downhole tool 5 houses therein an optical seismic detector of the present invention, which is operatively coupled to the measurement and control unit 2 via the cable 4. As will be described in detail later, the optical seismic detector provided inside of the downhole tool 5 is fundamentally constructed from a servo accelerometer and thus it detects the acceleration at the site of the downhole tool 5. A boat 6 floating at the surface of the water at a location away from the offshore rig l has a seismic source 7, which may be comprised of an air gun. When the seismic source 7 generates a seismic wave or disturbance, the seismic wave propagates into the earth which includes several strata 8a, 8b, 8c and 8d in the illustrated example. Here, the stratum 8c may be assumed as an oil reservoir. The seismic wave 7a propagates through the strata 8a, 8b and 8c and is reflected at the interface between the strata 8b and 8d to reach the optical seismic detector inside of the downhole tool 5. On the other hand, another seismic wave 7c directly reaches the optical seismic detector inside of the downhole tool 5 after travelling through the strata 8a, 8b and 8c. In this manner, various seismic waves reach the optical seismic detector inside of the downhole tool 5 to apply different accelerations, so that the analysis of the underground structure can be carried out from the seismic data thus collected. It is to be noted that the tool 5 will normally make measurements at a plurality of different depths in the borehole 3. Fig. 2 schematically shows in perspective an optical seismic detector l0 constructed in accordance with the present invention, and, as will be appreciated, the illustrated optical seismic detector has a basic structure of a servo accelerometer. Although not shown for the purpose of clarity, in the practical application, the entire structure shown in Fig. 2 is completely enclosed in a sealed case, which, in turn, is fixedly mounted inside of the downhole tool 5. As shown in Fig. 2, the optical seismic detector l0 includes a base plate ll, which may be fixedly attached to the above-mentioned case (not shown). On the base plate l0 is fixedly mounted a flexible hinge or a deflectable connector plate l3 as standing upright. The connector plate l3 is generally rectangular in shape and provided with a horizontal groove l3a on opposite sides to make the plate l3 locally extremely thin to remove the springy characteristic as much as possible. The connector plate l3 is also provided with a center rectangular hole to provide an even enhanced deflectability. It will be appreciated that the connector plate l3 deflects only in one direction, i.e., in the direction perpendicular to the plane of the connector plate l3 in the undeflected condition. A vertical supporting rod l2 is fixedly attached to the top end surface of the connector plate l3 as extending vertically, and a horizontal supporting rod l5 is fixedly mounted on the vertical supporting rod l2. As shown, the vertical supporting rod l2 extends through the horizontal supporting rod l5 at the center thereof perpendicularly, thereby defining a cross-shaped supporting structure. The horizontal supporting rod l5 effectively defines a moving mass in the present accelerometer structure. Above the horizontal supporting rod l5 is provided a l/4 pitch gradient index rod lens l4 (also referred to as selfoc lens) as extending through the top portion of the vertical supporting rod l2 as fixedly attached thereto. It is to be noted that the present invention is not limited to the gradient index rod lens having the l/4 pitch and use may be made of a gradient index rod lens of any pitch; however, the use of such a rod lens is preferred because of its high structural integrity and manufacturing simplicity. That is, it is only necessary to provide a hole in the vertical supporting rod l2 and to have the rod lens l4 inserted into the hole thus formed with an appropriate adhesive agent. In the preferred embodiment, the rod lens has a diameter ranging from l - 2 mm and the longitudinal length ranging from 2 to 5 mm. A pair of torque coils l6a and l6b are provided as wound around the horizontal supporting rod l5, one at each side of the vertical supporting rod l2. And a pair of permanent magnets l7a and l7b are disposed on both sides of the horizontal supporting rod l5 such that each of the magnets l7a and l7b has a predetermined magnetic pole, e.g., N or S pole, located facing to the corresponding end surface of the supporting rod l5 as spaced apart over a predetermined distance. Thus, by controlling the direction of the current passing through the torque coils l6a and l6b, the horizontal supporting rod l5 may be moved closer to the selected one of the permanent magnets l7a and l7b as deflected as the connector plate l3. These permanent magnets l7a and l7b are provided fixed in position and they are preferably provided as fixedly mounted on the before-mentioned case. Also provided is a light source l8, which is preferably comprised of a laser diode or a light-emitting diode, and the light source l8 is connected to a power supply l9 which supplies a constant driving current to the light source l8. The light source l8 is also preferably fixedly mounted on the before mentioned case. At the side opposite to the light source l8 with respect to the rod lens l4 is disposed a light-receiving element 20 which is a one-chip dual-photodiode element in the illustrated embodiment. In other words, the element 20 is comprised of a single semiconductor chip in a monolithic structure and it is provided with a pair of photodiodes arranged side-by-side with a predetermined small gap therebetween. As will be described more in detail later, it is to be noted that the gap between the pair of built-in photodiodes may be made extremely small, e.g. in the range between a few microns and l0 microns, and, thus, together with the focusing function provided by the rod lens l4, which allows to provide a light beam having a similarly small spot diameter, the sensitivity or resolution of the present system can be significantly enhanced. The illustrated photodiode element 20 is also provided with a third photodiode 20c, which is generally U-shaped and thus substantially surrounds the paired photodiodes 20a and 20b. This third photodiode 20c is used for the purpose of alignment. That is, in the present system, since the spot of a light beam used in the present system is very small, it is sometimes difficult to find where the beam spot is located. And, thus, it is desirous to provide such an alignment region surrounding the pair of detector photodiodes 20a and 20b. The photodiode element 20 is also preferably fixedly mounted on the before-mentioned case and it is electrically connected to a servo amplifier 22 through three separate lines 2la, 2lb and 2lc which are connected to the built-in photodiodes 20a, 20b and 20c, respectively. The servo amplifier 22, the detailed structure of which will be described later, is connected to a terminal 25 for a.c. output and also to one end of the paired torque coils l6a, l6b through a line l7. The other end of the paired torque coils l6a, l6b is connected to another terminal 24 for d.c. output and to one end of an output resistor 23, the other end of which is connected to ground, via a line 26. In operation, under the equilibrium condition, the horizontal supporting rod l5 is located at the center between the pair of permanent magnets l7a and l7b, and, thus, the rod lens l4 is positioned in alignment with the optical path defined between the light source l8 and the photodetector 20. When an acceleration is applied to the horizontal supporting rod l5, the rod l5 swings from the center position due to the inertial force resulting from the applied acceleration. Such a swinging motion is detected by the photodetector 20, which thus produces an electrical signal having the information as to the amount of such a swinging motion. Then, the electrical signal thus produced is applied to the servo amplifier 22 which then supplies a feedback current to the pair of torque coils l6a and l6b such that the torque coils l6a and l6b produce electromagnetic fields for bringing the horizontal supporting rod l5 back into the original center position. That is, the torque coils l6a and l6b are so located to reside in the respective magnetic fields generated by the permanent magnets l7a and l7b, respectively, so that when a feedback current is passed through the torque coils l6a and l6b, there are produced electromagnetic fields which can interact with the magnetic fields emanating from the permanent magnets l7a and l7b. Since the permanent magnets l7a and l7b are fixed in position and the rod l5 is swingable, the rod l5 is caused to move as a result of the interaction between these two magnetic fields. The feedback current supplied to the torque coils l6a and l6b from the servo amplifier 22 is proportional to the acceleration applied to the rod l5 (here, it is assumed that the moving mass is substantially defined by the mass of the horizontally supporting rod l5, but, in reality, the moving mass includes other elements, such as coils l6a and l6b, vertical supporting rod l2 and rod lens l4), so that the acceleration applied to the rod l5 may be measured by measuring the feedback current. In this manner, the servo accelerometer differs significantly in structure from the open-loop type accelerometer, in which the force applied to the moving mass is directly measured in an open-loop circuit. Thus, the servo accelerometer is not easily susceptible to external disturbance and is excellent in frequency characteristic, which thus allows to obtain high accuracy and stability. As will be described in detail later, the dual photodetector 20 includes a pair of detector photodiodes 20a and 20b as arranged side-by-side with a small gap therebetween. In the preferred embodiment, the beam spot is substantially equal in size to the gap between the pair of detector photodiodes 20a and 20b and in the order of a few microns to l0 microns. When the rod l5 is located at the equilibrium or center position, the light beam emitted from the light source l8 is focused onto the center position between the pair of detector photodiodes 20a and 20b through the rod lens l4. However, when the rod l5 swings in either direction according to the direction of acceleration applied thereto, the rod lens l4 also moves thereby causing the light beam passing therethrough to shift in position on the surface of the photodetector 20. In other words, the beam spot gradually moves into the corresponding one of the pair of detector photodiodes 20a and 20b, thereby causing the corresponding photodiode 20a or 20b to produce an electrical signal having the information as to the degree of deviation of the beam spot from the center position. Fig. 3 shows in plan view the detailed structure of the dual photodetector 20 provided in the optical seismic detector shown in Fig. 2. As shown, the dual photodetector 20 is constructed in the form of a single semiconductor chip and it is provided with a pair of first and second photodiodes 20a and 20b spaced apart from each other over a small distance d , which preferably ranges from a few microns to l0 microns, and with a third photodiode 20c, which is generally U-shaped and which substantially surrounds the pair of first and second photodiodes 20a and 20b. The pair of first and second photodiodes 20a and 20b are somewhat elongated in the vertical direction, and the first and second photodiodes 20a and 20b are connected to electrode pads 30a and 30b through leads 3la and 3lb, respectively, for connection to external circuity. Similarly, the third photodiode 20c is connected to the remaining electrode pad 30c via a lead 3lc. It is to be noted that since the dual photodetector 20 is manufactured in the form of a single semiconductor chip, the gap d between the pair of detector photodiodes 20a and 20b can be determined as small as practically possible, which, thus, allows to increase the resolution significantly. The importance of this feature is increased when combined with the rod lens l4, defining a focusing optical system, which causes the light beam from the light source l8 to be focused onto the surface of the dual photodetector 20 thereby allowing to obtain a beam spot of any desired size, which also contributes to enhance the resolution and thus the sensitivity of the entire detector system. Fig. 4 is a circuit diagram showing one embodiment of the servo amplifier 22 provided in the optical seismic detector shown in Fig. 2. As shown, the servo amplifier 22 includes a first input terminal 32 which is connected to the first detector photodiode 20a and a second input terminal 33 which is connected to the second detector photodiode 20b. The first input terminal 32 is connected to the inverting input of operational amplifier (hereinafter, simply referred to as "op amp") 34 whose output is connected through a resistor R5 to the inverting output of op amp 38 and to the inverting input of op amp 39 through a resistor R8. Similarly, the second input terminal 33 is connected to the inverting input of op amp 35 which has its output connected to the non-inverting input of op amp 38 through a resistor R7 and to the inverting input of op amp 39 through a resistor R9. Thus, the op amps 34 and 35 each convert the photoelectric current signals from the respective detector photodiodes 20a and 20b into voltage signals. Test terminals 36 and 37 are provided as connected to the output of op amps 34 and 35, respectively. The op amp 38, in effect, functions as a comparator and has its output connected to the inverting input of an op amp 48 through a resistor Rl5 and to the inverting input of an op amp 46 through a resistor Rl3. Thus, the op amp 38 supplies at its output a difference signal between the two detection signals from the detector photodiodes 20a and 20b, which is proportional to the displacement of the lens l4 with respect to the center position. Provided as connected to the output of op amp 38 is an a.c. output terminal 45 which is connected to the terminal 25 shown in Fig. 2. On the other hand, the op amp 46 has its non-inverting input connected to ground through a resistor Rl4 and has its output connected to a d.c. output terminal 47, which, in turn, is connected to the terminal 24 shown in Fig. 2. The op amp 46 is, in effect, a low pass filter for filtering out the high frequency component beyond a predetermined cut-off frequency thereby allowing to obtain the low frequency component. The op amp 48 has its non-inverting input connected to ground through a resistor Rl8 and has its output connected to a signal output terminal 49, which is connected to one end of the torque coils l6a and l6b shown in Fig. 2. Thus, the op amp 48 receives a sum of the displacement signal from the op amp 38 and the low frequency component of the displacement signal from the op amp 46 and then supplies a driving current to the coils l6a and l6b proportional to the summed signal. Accordingly, the input signals supplied at the first and second input terminals 32 and 33 are, after having been suitably amplified by the respective op amps 34 and 35, are compared at the op amp 38, serving as a comparator, whose output signal is supplied to the signal output terminal 49 after amplification by the op amp 48. It is to be noted that the present system functions as a servo accelerometer if the system operates in a frequency region which is lower than the cut-off frequency of op amp 46. In the present invention, the time constant of the servo mechanism in the structure shown in Fig. 2 is set to range from l msec to 3 sec. On the other hand, the remaining op amp 39 has its non-inverting input connected to a variable resistor 40, which is connected in series with a resistor Rl0 but in parallel with a Zener diode 4l serving as a voltage reference, and its output connected to a Darlington pair 42, which, in turn, is connected to a light source output terminal 44 and to ground through a diode 43. The Darlington pair 42 is also connected to a voltage supply, -l5V in the illustrated example. The light source output terminal 44 may be connected to the power supply l9 shown in Fig. 2. Thus, the op amp 39 receives a sum of the two signals from the respective detector photodiodes 20a and 20b and compares this sum signal with a comparison signal produced by a combination of voltage reference 4l and variable resistor 40 thereby supplying a difference signal as its output. This difference signal is amplified by the Darlington pair 42 to be used for driving the light source l8. As a result, it is so controlled that the light source l8 always emits constant light proportional to the comparison signal. There are also provided other resistors R and capacitors C as connected as shown. It should be understood that the particular structure shown in Fig. 4 is suitable for use as the servo amplifier 22 in the optical seismic detector shown in Fig. 2, but the present invention should not be limited only to the servo amplifier illustrated and a servo amplifier of an other structure may also be used. Figs. 5 and 6 show alternative structures of the dual photodetector 20, which are suitable for use in the present displacement detector. In the structure shown in Fig. 5, a photodetector 50 in the form of a single semiconductor chip is provided with a pair of detector photodiodes 55a and 55b arranged side-by-side with a predetermined gap d therebetween. For example, if the gap d is set approximately at 10 microns using the beam spot having a diameter of approximately 10 microns as magnified approximately 5 times by the lens, the displacement of the lens in the order of 2 angstroms can be detected. The photodetector 55 is also provided with four alignment photodiodes 55c-55f, which are arranged to substantially surround the pair of detector photodiodes 55a and 55b. On the other hand, Fig. 6 shows a multiple photodetector 56 which includes a plurality (6 in the illustrated embodiment) of photodiodes 56a arranged in a line as spaced apart from one another over a predetermined distance d with their longitudinal axis directed perpendicular to the line of arrangement. Such a structure allows to expand the operational range of the present displacement detector. The photodetector 56 is also provided with alignment photodiodes 56b and 56c. As described in detail above, in accordance with the present invention, there is provided a detector for detecting the acceleration, the velocity, the displacement or the vibration of a point of interest at high accuracy and sensitivity. When use is made of a dual photodetector in the form of a single semiconductor chip, the resolution and sensitivity can be increased significantly and the mounting process can be greatly simplified since it does not require the alignment between the two photodiodes. Besides, since the two or more photodiodes are provided in a particular pattern on the same single substrate, the structural integrity is extremely high thereby allowing for use in the roughest condition without causing any problem. Besides, when use is made of a rod lens, e.g., selfoc lens, as an optical system for focusing the light beam from the light source onto the photodetector, the structural integrity of the detector can be further increased while facilitating the manufacturing process.

Claims: A servo accelerometer comprising: a fixed support (11) and a movable support (12,13,15); at least one permanent magnet (17a,17b) and at least one coil (16a,16b) positioned such as to be capable of producing a magnetic field which can magnetically interact with said permanent magnet, said coil and said magnet (16a,16b, 17a,17b) being respectively mounted on said supports (11 and 12,13,15); a light source (18) fixed in position for emitting a light beam; light-receiving means (20) fixed in position for receiving said light beam from said light source (18), said light receiving means (20) producing a signal indicative the position of impingment of said light beam; an optical system (14) fixedly mounted on said movable support (12,13,15) for directing said light beam from said light source towards said light-receiving means (20); and a servo amplifier (22) for applying a driving current to said coil (16a,16b) in response to said signal produced from said light-receiving means (20); characterized in that: said optical system comprises a gradient index rod lens attached to said movable support (12,13,15) for focusing said light beam onto said light-receiving means (20), and said light-receiving means includes, on a single semiconductor chip, at least two detector photodiodes (20a,20b) spaced apart from each other over a predetermined distance and at least one alignment photodiode (20c) surrounding said detector photodiodes (20a,20b) at least partly. The accelerometer of claim 1, characterized in that the distance between said two detector photodiodes (20a,20b) ranges from a few micrometers to approximately 10 micrometers. The accelerometer of claim 1 or 2, characterized in that said light source (18) is selected from the group consisting of light-emitting diode, laser diode, E.L. device and plasma discharge device. The accelerometer of any one of claims 1 to 3, characterized in that said movable support (12,13,15) includes: a vertical supporting rod (12) in which said rod lens (14) is fixedly inserted; and a deflectable supporting plate (13) having one end fixed in position and its opposite end fixedly connected to the bottom of said vertical supporting rod (12). The accelerometer of claim 4, characterized in that said deflectable connector plate (13) is provided with a horizontal groove (13a) on opposite surfaces thereof to provide a thinned region thereby allowing said connector plate (13) to deflect along said thinned region. The accelerometer of claim 5, characterized in that said deflectable connector plate (13) is provided with a center hole (13b) to provide an enhanced deflectability.

Inventors: KONDO, MASAMICHI, NISHIDA, KATSUHIKO, SHINDO, MASANARI, SUZUKI, MASAHARU
Applicants: SCHLUMBERGER LIMITED, SOCIETE DE PROSPECTION ELECTRIQUE  SCHLUMBERGER
Application year: 1986
Main classification: G01P15
IPC codes: G01V1, G01P15