TILTMETER FOR GEOPHYSICAL PROSPECTING BY MEASUREMENT OF EARTH TIDES

Abstract: To locate potentially hydrocarbon-bearing subsurface formations or other subsurface anomalies, tiltmeters (TI-TS) measure the time of arrival, apparent direction and magnitude of a lunar-induced earth tide at an array of points on the earth's surface. The measurements are combined to determine the shape of the tidal wave in a region of interest. The shape is indicative of subsurface anomalies in the region. The tiltmeter is mounted on adjustable support legs (188) and includes fluidcarrying cups (168, 170) in which the fluid level changes relative to the cup with the rise and fall of the earth's surface. An optical array including a monochromatic light source (62) causes interferometric bands to be generated as the distance between the fluid surface and a fixed mirror (65) varies. The fixed mirror (65) is closely spaced from the fluid surface to produce multiple reflection (67) therebetween which significantly increases measurement sensitivity and logic circuitry (72-90) is provided for determining the number of bands and their direction of movement in order to establish the time and magnitude of the peak movements of the crust. A thermostatically controlled heating system (226) maintains both the fluid within the cups and the adjustable support legs at a constant temperature to eliminate measurement error due to the effects of thermal expansion, drift, and high frequency seismic noise.

Description: TILTMETER FOR GEOPHYSICAL PROSPECTING BY MEASUREMENT OF EARTH TIDES TECHNICAL FIELD The present invention relates to geophysical prospecting and more particularly to a highly accurate tiltmeter which can be used in a system for geophysical prospecting through measurement of earth tides. BACKGROUND OF PRIOR ART S cientific and commercial interest in knowing what lays below the earth's surface is increasing for a number of reasons. One of these reasons is that the world has, until recently, enjoyed an abundant supply of relatively inexpensiv fossil energy in the form of oil and natural gas. However, the known sources or proven reserves of such fossil energy have not kept pace with the increasing demand, leading to critical short term shortages and less critical but still serious long term shortages. While alternative sources of energy, such as nuclea energy or solar energy, are being explored it is not widely believed that such alternative sources will be available soon enough to avoid a severe international energy crisis. The most feasible short term solution for the fossi shortage is to find additional reserves of the fossil fuels. Various seismic techniques are used in efforts to locate certain kinds of subsurface formations using surface meas¬ urements. The nuclear, acoustic and electrical properties of jsuch formations can all be studied through the use of meas¬ urements made at surface detectors. The reason for making such measurements is to deter mine the most advantageous location for drilling an oil or ga well. Since the costs of drilling a well can be quite high, the geologist or other individual responsible for deciding whether to begin drilling needs to have as much information a possible about the subsurface formations in a region of interest before drilling to minimize the chances of a dry or nonproducing well. Similarly, the demand for uranium and other metals is beginning to outpace the known supplies, making it imperative that new sources of the ores are located. Finally, as the population of the world increases and tends to become concentrated in urban centers with their inevitable high rise buildings, concern over the potentially calamitous effects of earthquakes has increased. To properly assess the risks posed, subsurface faults must be found and evaluated. BRIEF SUMMARY OF INVENTION Knowledge of subsurface formations is acquired by using a tiltmeter constructed in accordance with the present invention in order to measure the rise and fall of the earth's surface due to lunar-induced gravitational forces. The tiltmeter includes a level fluid assembly comprising two fluid-containing cups having a connecting tube which permits the fluid to flow from one cup to the other when one cup rises relative to the other. An improved, high sensitivity interferometric sensing unit measures the distance between the fluid surface and a member mounted a fixed distance from the cup. Changes in this distance are due to the rise and fall of the earth's surface at one cup relative to the surface at the other cup. BRIEF DESCRIPTION OF DRAWINGS While the specification concludes with claims particularly pointing out and distinctly claiming that which is regarded as the present invention, details of a preferred embodiment of the invention may be more readily ascertained from the following detailed description when read in con¬ junction with the accompanying drawings wherein: FIGURE 1 is a simplified view of the earth and the moon showing the retardation of the earth tide; FIGURE 2 illustrates a section of earth tide in a region of uniform viscosity; FIGURE 3 illustrates a section of earth tide in a region of abnormal viscosity; FIGURE 4 is a schematic representation of a biaxial tiltmeter used to determine a time of arrival and apparent direction of movement of an earth tide at a single point; FIGURE 5 illustrates an array of tiltmeters used to track the movement of an earth tide through a region of interest; FIGURE 6 is a diagram of trigonometric relationships used in analyzing an earth tide traversing an array of tiltmeters; FIGURE 7 is a vector diagram showing gravitational force vectors in a high viscosity region; FIGURE 8 is a vector diagram showing gravitational force vectors in a low viscosity region; FIGURE 9 is a side elevational view of part of a tiltmeter in a level position, parts being broken away in section for clarity; FIGURE 10 is a view of the same part when tilted by a traversing earth tide; FIGURE 11 is a schematic diagram of an interfero- metric tiltmeter suitable for measuring the magnitude of an earth tide at a particular point on the earth's surface; FIGURE 12 illustrates the relationship between a preferred photodetector array in an optical head for a tiltmeter and interferometric bands generated by the inter¬ ferometer of FIGURE 11; FIGURE 13 is a block diagram of one portion of a circuit for measuring the magnitude of the earth tide at a point on the surface; FIGURE 14 is a block diagram of the remaining part of the circuit for measuring the magnitude of the earth tide at a point on the surface; FIGURE 15 is a log of tidal wave shapes in a region of interest at spaced time and distance intervals; FIGURE 16 is one embodiment of a clock reset circuit for the system shown in block diagram form in FIGURE 13; FIGURE 17 is one embodiment of a two stage parallel shift register for the system shown in block diagram form in FIGURE 13; FIGURE 18 is one embodiment of a rise/fall detector for use in the system shown in block diagram form in both of FIGURES 13 and 14; OMPI FIGURE 19 is a combined side elevational view of, and electrical schematic diagram for, the preferred form of a tiltmeter in accordance with the present invention; and FIGURE 20 is a fragmentary, side elevational view taken on a larger scale, of supporting means for the tilt¬ meter shown in FIGURE 19, including a schematic diagram of an electrical circuit for a system for controlling the tempera¬ ture of the supporting means. OMP DETAILED DESCRIPTION OF INVENTION Referring to FIGURE 1, the effects of the gravitational pull of the moon 10 on the oceans of earth 12 have been noted and studied for many centuries. As the earth 12 turns on its axis in the counterclockwise direction indicated by arrow 14, (resulting in relative movement of the moon 10 in the direction indicated by arrow 16) the gravita¬ tional attraction of the moon generally causes ocean tides at the particular point on the earth's surface which is on the center line 18 between the center of the earth's mass and the center of the moon's mass. It is not widely recognized that the gravitational pull of the moon also causes the earth's crustal surface to rise and fall in a single advancing wave, referred to here¬ after as an earth tide. Unlike high ocean tide, high earth tide coes not occur at the center line 18. Because of the resistance or drag of the earth's crustal rocks, the maximum earth rise lags behind the center line 18 from 1° to 7° with approximately 5° retardation being normal. The characteristics of the advancing earth tide are influenced by changes in the density and viscosity of the medium through which it propagates; namely the earth's crustal surface. For example, in regions of lower-than- normal visσoscity, the velocity will decrease. Referring to FIGURE 2, an earth tide 20 traversing a region of uniform viscosity in the direction indicated by arrow 22 will be seen to have a relatively straight leading edge and uniform magnitude. In contrast, FIGURE 3 depicts a wave 24 propagating in a direction indicated by arrow 26 in a region of lower-than-normal viscosity. The section 28 of the wave above the low viscosity region is bowed outwardly in the direction of propagation and has a greater-than- normal magnitude. Conversely, an earth tide traversing an area of higher-than-normal viscosity will tend to have a bow in a direction opposite the direction of propagation and will have a reduced magnitude. Subsurface formations containing water or hydrocarbons, such as oil and/or natural gas will exhibit lower viscosity than "dry" formations. By measuring the earth's tide at a number of points on the earth's surface, the shape of the earth tidal wave in a region of interest can be established. Forwardly bowed sections of the earth wave will indicate potentially hydrocarbon-bearing, low- viscosity subsurface formations. To establish the wave shape, the time of arrival and apparent direction of the wave at each of a plurality of points is measured using devices known as biaxial tiltmeters. A preferred embodiment, of such a device is described in detail later. Basically, a biaxial tiltmeter is a device for measuring the rise and fall of the earth's surface along two orthogonal axes as the earth tide traverses a region of interest. Referring to FIGURE 4, a biaxial tiltmeter would include a first arm Tl for measuring the rise and fall of the earth due to the wave component traveling in a first direc¬ tion, arbitrarily designated as east to west and a second arm for measuring the rise and fall of the earth's surface due to the wave component in a second direction, arbitrarily desig¬ nated as north to south. Each tiltmeter arm is responsive to the rise and fall of the earth only along a single axis. For example, if an earth tide is traveling due west, arm Tl will (on a scale of 0 to 10) have a reading of 0. Conversely, if an earth tide is moving due south, arm T2 will have a reading of 10 while arm Tl will have a reading of 0. An earth tide approaching the biaxial tiltmeter from some direction other than along one of the axes will cause both Tl and T2 to measure some rise in the earth's surface. If an earth rise is detected at both Tl and T2, the angle of the approaching earth tide relative to one of the axes, designated as due east, can be calculated as a tangent of T2/T1. For example, if Tl has a reading of 7 while T2 has a reading of 3, the angle _ will be the arctangent of 7/3 or approximately 24°. OM WIP While the apparent direction of movement and the time of arrival of an earth tide can be found from a single biaxial tiltmeter, the distortion or dynamic changes in the wave shape can be established only by taking readings at a plurality of points in a region of interest. FIGURE 5 illustrates a suitable array of tiltmeters Tl, T2, T3, T4 and T5. To simplify computations, tiltmeters Tl, T2, T4 and T5 are located at the corners of a square of known size while tiltmeter T3 is located at the center of that square. While this arrangement is preferred for computational purposes, any placement of tiltmeters could be used in which the distance and relative bearings between tiltmeters is shown. FIGURE 6 illustrates the type of calculations which can be performed to establish the velocity of the wave front and to determine whether the wave front is straight or distorted. In that figure, it is assumed that the wave front has arrived at tiltmeter Tl.and relative readings along orthogonal axes have established that an angle -9 exists between due east and a normal 32 to the wave front 30. Basic geometry indicates that the angles θ 2 between the wave front and due south and Θ5 between due east and a normal 34 to the wave front are equal to θ. The normal 34 drawn from the wave front 30 to the tiltmeter T5 forms the third side of a right angle triangle having interior angles of θr and 75 where 75 equals 90°- 5°. Since the distance d W 5 between tiltmeter Tl and tiltmeter T5 is known, the distance d w5 between the wave front 30 and the tiltmeter T5 can be computed as d W 5 = d-__^ sin γ,-. The velocity of the wave front along the normal 34 is equal to the distance d W 5 divided by the time interval between the arrival of the wave at tiltmeter Tl and the arrival of the wave at tiltmeter T5. Similar trigonometric relationships are employed in determining that the distance d W 2 = dj_2 sin θ . If the wave is not distorting, its velocity along a normal 36 between the wave front and tiltmeter T2 will be equal to the velocity of the wave front along the normal 34 at tiltmeter T5. If the y - , _, ι.- L OMPI wave traverses the normals at different velocities, the wave front is being distorted by subsurface formations of abnormally high or abnormally low viscosity. Naturally the operations of the various tiltmeters have to be syn¬ chronized on a time basis to permit velocity comparisons. Direct electrical connections between the tiltmeters with a single master clock might be used to establish the synchroni¬ zation. Using ground loop currents to carry a timing signal is another possibility. By taking readings at a number of spaced points in a region of interest, the shape of the wave front as it traverses the region can be established. By analyzing the wave shapes and the changes in those shapes, subsurface forma tions of abnormally high or abnormally low viscosity can be located in the region of interest. The wave shapes in the region of interest are not static as they traverse a region of abnormal viscosity but rather channel as they approach and leave the region due to the restoring forces of the moon's gravitational pull. Elementary, vector analysis indicates that the gravitational pull of the moon tends to maintain an earth tide having a relatively straight wave front. Referring to FIGURE 7, the gravitational vectors in a region of abnormally high viscosity are shown. The total of resultant vector Rg is a .function of the magnitudes of a vertical vector Vg and a horizontal vector Hg. While verti¬ cal vector Vg is relatively high in a high viscosity region due to the large included angle between the vector Rg and the vector Vg. . He horizontal vector Hg will tend to increase the velocity of the retarded portion of the wave until a normal wave shape is re-established. Figure 8 shows the same gravitational vectors in a region of abnormally low viscosity. The horizontal vector H g is relatively small in such a region causing any advanced sections of the wave to be gradually retarded relative to the rest of the wave as the earth tide passes beyond the region of abnormally low viscosity. To measure the rise and fall of. the crustal surface, a tiltmeter constructed in accordance with the present inven¬ tion is employed. FIGURE 9 is a greatly simplified, partially cut away view of such a device. The tiltmeter includes first and second open-topped cups 38 and 40 which communicate with each other through a connecting tube 42. This assembly is mounted on a base 44 which in turn may be placed on the earth's crustal surface 46. The assembly of cups 30 and 40 and the connecting tube 42 are filled with a liquid 52 such as mercury. A first sensing unit 48 is located above cup 38 while second similar sensing unit 50 may be located above cup 40. The function of each sensing unit, a preferred embodiment of which will be described in more detail later, is to measure the distance from the sensing unit to the surface of the liquid 52. If the unit is initially leveled on the earth's surface, the distance di between the sensing unit 48 and a surface of the liquid in cup 38 will be equal to the distance d 2 between the sensing unit 50 and the surface of the liquid in cup 40. When the earth tide passes the location of the unit, the entire unit will be tilted as the crustal surface of the earth rises and then falls. FIGURE 10 shows, in greatly exaggerated fashion, how the unit would be tilted relative to a normal level line 54 when the crustal surface of the earth has risen beneath cup 40 to a greater extent than beneath cup 38. While the surfaces of the liquid in the cups 38 and 40 remain at the same level relative to the normal level line 54, the level of liquid in cup 40 falls relative to the upper surface of the cup and the sensing unit 50. Similarly, the level of .liquid 52 in cup 58 rises relative to the sensing unit 48. Sensing unit 50 will detect a liquid- to-unit spacing of D 2 + Δr while sensing unit 48 will detect a liquid-to-unit separation of d]_ - Δr. Since the change in separation at one unit must be offset by an opposite, equal change at the other unit, only one sensing unit is needed to detect the rise at a given point on the earth's crustal surface. Two units would provide redundant data but may still be preferred to assure that data is still available in the event of failure of one of the sensing units. Also, since the data from the two units should correspond, any discrepancy between the outputs of the two units could be used as an indication that at least one of the units is malfunctioning or needs to be recalibrated. FIGURE 11 depicts an interferometric sensing unit for detecting changes in the level of a liquid 56 in a cup 58 relative to a mirror 60 positioned above the cup. The sensin unit includes a light source, preferably a monochromatic one such as laser 62, a beam splitter 64, a beam combining elemen 66, and an optical head 68 including a collimator tube 70 and a photodetector array 72. The light beam generated by laser 62 is split at a beam splitter 64 with a part being directed toward the surfac of the mirror 60. That part of the beam directed obliquely toward the surface of the fluid 56 is reflected between the mirror 65 and a plurality of points over such fluid surface, as indicated by the multiple arrows 67, before emerging towar the beam recombining element 66. The light beam reflected from the surface of the liquid 56 and from the surface of the mirror 60 are recombined in the beam combining element 66 and applied as a single beam through the collimator tube 70 to the photodector array 72. As the surface of the liquid 56 rises and falls, the changes .in the path length from beam splitter 64 to liquid 56 to beam combining elements 66 will cause the recombined beam to display an interferometric pattern consisting of a series of alternating bright and dark bands that appear to move in one direction when the liquid surface is rising and in the opposite direction if the liquid surface is falling. The employment of reflecting mirror 65 is signifi¬ cant in the present sensing unit because even minute changes in the level of fluid 56 within the cup 58 result in rela¬ tively large increases in the length of the optical path of that part of the light beam directed toward the fluid surface; by this feature, the sensitivity of the sensing unit is magnified and the accuracy of the resulting measurements is considerably increased. Referring to FIGURE.12, photodetectors PDl, PD2 and PD3 in array 72 are.used to count the number of inter¬ ferometric bands, such as dark bands 74 and alternating light bands 76, which traverse the photodetectors. Signals pro- viced by the photodetectors PDl, PD2 and PD3 also provide the information required to determine whether the bands are moving in one direction or the other. For purposes of explaining the invention, it is arbitrarily assumed that movement of the bands from left to right indicates a falling liquid level (which corresponds to a rise in the earth's crustal surfaces) while movement of the. bands from right to left indicates a rising liquid surface or a fall in the earth's crustal surface. The photodetectors PDl, PD2 and PD3 are spaced from one another such that no more than one of the photodetectors is in a dark bank 74 at any given time. If an output from a photodetector in a light band is assumed to be a binary 1, at least two out of the three photodetectors and periodically all three, will produce binary 1 signals at any given time. FIGURE 13 is a block diagram of a portion of the logic circuits required to establish the magnitude and direction of changes in the elevation of the earth's crustal surface. Each of the three outputs from the photodetector array 72 is inverted by inverter amplifier 78. After inver¬ sion, no more than one of the signals will be at a binary 1 level at any given time. .The inverted signals are applied in parallel to a clock reset circuit 80, to an OR gate 82 and to a two stage parallel shift register 84. Both the clock reset circuit 80 and the two stage parallel shift register 34 will be described in more detail later. . The output of OR gate 82 is applied to set input terminal of a flip flop 86. The output of the clock reset circuit is applied to the reset input of this flip flop. OR gate 82 provides a setting input OMPI to the flip flop 86 whenever one of the photodetectors in array 72 is in a dark bank region. Clock reset circuit 80 on the other hand provides a reset pulse only when all three photodetectors are located in one of the light bands. When flip flop 86. is set, clock circuit 88 provides a clock pulse to the shift register 84 which shifts the three binary signals stored in the first stage to the second stage while permitting the signals at the output of the inverter amplifiers78 to be entered and stored in the first stage. Signals from both stages of the shift register 84 are applied to a rise/fall detector circuit 90 which decodes changes in the signal patterns to determine whether the light bands are moving .to the right or to the left; that is, whether the earth's crustal surface is rising or falling. The rise/ fall detector 90, which is shown again in block diagram form in FIGURE 14, will be described in more detail later. A rise or R output from detector 90 is applied to a set input termi- nal of a flip flop 92 and to one input of a"NAND gage 94. A fall or F output from detector 90 is applied to the reset input terminal of flip flop 92 and to the second input of the NAND gate 94. The flip flop 92 provides a count up/count down signal to a binary counter 96 which counts pulses generated by a monostable multivibrator 98 at the output of NAND gate 94. A second monostable multivibrator 100 provides a "hand shake" signal which triggers a digital recorder 102. Digital recorder 102 records the contents of a binary counter 96. A plotter 194 is shown connected to the output of the digital recorder 102. Plotter 104 can accept data recorders in similar, sensing circuits to plot the shape of the earth tidal wave in a region of interest. Changes in the shape of the wave indicate changes in subsurface viscosity which, ' in turn, indicate potentially hydrocarbon-bearing formations. FIGURE 15 depicts different shapes which an earth tide wave front assumes as it traverses a region of abnor¬ mally low viscosity from left to right. The successive wave OM / , ~ WI shapes are displaced from one another in both time and distance. As a.matter of convenience, the successive wave shapes are discussed below with reference only to the distance displacement. Wave front 106 is basically a straight wave front such, as occurs in a region of uniform viscosity. Wave form 108, which represents the earth tide at some distance Δd to the left of the location at which wave front 106 is detected, is slightly distorted to the left indicating that the earth tide is approaching a region of abnormally low viscosity. Wave form 110 is closer to the center of region of abnormally low viscosity and correspondingly is distorted more than wave form 108. Wave form 112 is -highly distorted which indicates that the earth tide is .-directly above a region of abnormally -low viscosity. As the tide begins to leave the low viscosity region, the amount of distortion lessens as indicated by wave shape 114 and later occurring wave shape 116. When the earth tide has passed beyond the region of abnormally low viscosity•and into a region of uniform viscosity, the wave shape again will be a basically straight line, as indicated by the last wave shape 118 in FIGURE 15. The dynamic changes in the shapes of the wave fronts locate the region of abnormal viscosity with the most dis¬ torted wave occurring above or near the leading edge of the low viscosity region. The absolute magnitude of the distor¬ tion is not critical in determining the location of the low viscosity region. Only the relative magnitude of the dis¬ tortion from one wave to the next is critical. The magnitude of the distortion does, however, provide at least a qualitative indication of the subsurface depth of the low viscosity formation. A low viscosity formation which is relatively near the earth's surface will cause highly distorted wave shapes while a deep, low vis¬ cosity formation will result in wave shapes having rela¬ tively lesser distortion. The components of a system suitable for generating the information ' required to plot the shape of earth tide wave from over a region of interest has already been described in block diagram form. FIGURE 16 is a detailed logic diagram of the clock reset circuit 80 used to reset the previously described clock-controlling flip flop 86. The clock reset circuit includes a trio and AND gates 120, 122 and 124, each having an input from the inverter amplifier connected to each of the photodetector outputs. Each of the AND gates, 120, 122 and 124 recognizes a unique combination of photodetector outputs going high only when that combination exists. For example, the PD2 and PD3 inputs to AND gate 120 are inverted as is conventionally indicated by the small ' circles interpose between the gate 120 and the leads labeled PD2 and PD3. Therefore, AND gate will go high only when PDl = 0, PD2 = 0 and PD3 = 0. Table 1 below indicates which of the gates is high for a particular combination of PDl, PD2 and PD3 signals. TABLE I The table indicates that at least one of the gates will have a high output for any combination of PDl, PD2 and PD3 signals except where all of those signals are low or 0. Under the latter conditions, all of the AND gate outputs are low. The outputs from the three AND.gates 120, 122 and 124 are supplied to. a NOR .gate 126 which goes high only when the AND gates outputs are all. low; that is, only when PDl, PD2 and PD3 are all equal to 0. The output of the NOR gate 126 is applied to the reset input terminal of--the flip flop 86, causing that flip flop to be reset, preventing further generation of clock pulses by clock 88, when the inverted outputs from all of the photodetectors remain at a low level. The two stage parallel shift register 84 is described in detail with reference to FIGURE 17. The first stage of the shift register includes three J- flip flops 128, 130 and 132, each having its J and K input terminals tied to the output.of one of the inverter amplifiers 78. With both the J and the K inputs connected in common to the input, such a flip flop will transfer its input signal to its Q output terminal when a clock pulse is applied to the C or clock input terminal. The second stage of the shift register is the second group of three flip flops 124, 136 and 138. The J input terminal to each of the flip flops is connected direc¬ tly to the Q output terminal of an associated flip flop in the first stage while the input to the second stage flip flop is connected to the Q output of the associated first stage flip flop, with the first and second stages connected in this manner, the application of a clock pulse to a flip flop in the second stage will cause the binary signal appearing in the J input terminal of the second stage flip flop to be transferred to the Q output terminal of that flip flop. For example, a binary 1 on the J input terminal to flip flop 134 will be transferred to the Al output terminal when that flip flop is clocked. The Q output terminals for the second stage flip flops 124, 136 and 138 carry a first set of signals A* j _, B*-_, C± , respectively, representing the inverted outputs of the photodetectors at a particular point in time. The Q output terminals of the first stage flip flops 120, 130, 132 carry a set of output signals A 2 , B , C , respectively, representing the inverted photodetector outputs at a later point in time. As the interferometric bands sweep across the photodetector array, the photodetector outputs will change in a predetermined pattern which differs depending on whether the bands are moving to the left or to the right. Assuming the bands are moving to the right, the sequence of inverted photodetector outputs is shown in Table 2. TABLE 2 If the interferometric bands are moving from right to left, the sequence of signal patterns is reversed with the binary 1 propagating from right to left in a cycle having four possible signals states. The pattern sequence is shown below in Table 3. TABLE 3 An inspection of Tables 2 and 3 indicates that the patterns change in a predetermined sequence. By inspecting successive patterns, it is possible to determine whether the interferometric bands are moving to the right or to the left; that is, whether the crustal surface of the earth is rising or falling. A suitable rise/fall detector is illustrated in FIGURE 18. The rise/fall detector includes a first set of AND gates 140, 142 and 144 having inputs from the output terminal of the flip flop 128, 130 and 132 in the first stage of the two stage shift register. Each of the AND gates 140, 142 and 144 will go high for one and only one combina¬ tion of ones and zeros in the first stage of the two stage shift register. Each of the AND gates in the second set also recognizes a unique combination of ones and zeros in the second stage shift register. The output of AND gate 140 provides an input to AND gates 152 and 154. AND gate 142 is connected to both AND gate 156 and AND gate 158 while AND gate 144 is connected to AND gate 160 and 162. The OMPI second input to each of these gates is provided by the AND gates 146, 148 and 150. For example, AND gate 146 is connected to AND gates 160 and 158. Because of the manner in which the clock reset circuit operates, the two stage shift register does not receive any signals when all of the photodetector outputs are low. The rise/fall detector circuit 90 operates as if that particular signal pattern did not exist. Since the two inputs to each of the AND gates 152, 156, 160, 154, 158 and 162 represents inverted photodetector outputs at different periods of time, those AND gates can be used to determine the changes in the photodetector signal patterns. For example, AND gate" 152 will go high only when the second stage of the shift register has a 010 signal pattern while the first stage of the shift register has a 100 signal pattern. Table 3 indicates that this particular sequence of signal patterns occurs when interferometric bands are moving from right to left. Simple analysis of the input signals for.each of the other AND gates will indicate when the output of the gate will go high. Basically, AND gates 152, 156 and 160 go high when the interferometric bands are moving from right to left whereas AND gates 154, 158 and 162 go high when the band are moving from left to right. The AND gates 152, 156 and 160 provide inputs to an OR gate 164 which is connected to the set input terminal of the flip flop 92 (FIGURE 14). AND gates.154, 158 and 162 provide inputs to a second OR gate ' 166, the output of which is connected to a reset input terminal of the flip flop 92. Attention is now directed to FIGURES 19 and 20 wherein the preferred form of a tiltmeter is displayed. The tiltmeter includes a pair of spaced apart, fluid-containing cups 168 and 170, respectively, fluid coupled by tube means 172 to a fluid reservoir 174. Fluid reservoir 174 is adapted to contain a predetermined volume of a selected fluid such as mercury or gallium, and is provided with a fluid filler tube 176 to allow filling of the reservoir 174 with the appropriat OMPI fluid. Each of the cups 168 and 170 is provided with a corresponding sensing unit 178 and 180 adjacent thereto which are essentially identical to those sensing units previously described. The above-mentioned components are mounted by any suitable means, to the upper surface of a base plate 182. Base plate 182 is preferably constructed from rigid materials in a manner to minimize- longitudinal flexion or bending of the base plate 182. Support assemblies 184 and 186 are shiftably mounted on the opposite extremities of base plate 182. Each of the support assemblies 184 and 186 comprises an elongate, leg member 188 having intermediate surface areas thereof threaded. Leg members 188 are threadably received within thread bores through the base plate 182 and extend downwardly past the lower surface of the latter. Preferably, each of the leg members 188 will be manufactured from a material having a low co-efficient of expansion, such as a plastic material impregnated with Invar or titanium. Support assemblies 184 and 186 each further include a foot member 190 which presents a surface 192 for supporting the tiltmeter on the earth's surface, and is pivotally connec¬ ted to the lower extremity of the corresponding leg member 188. Desirably, base plate 182 will be sufficiently elongate to maximize the spacing between cups 168 and 170 in order to increase corrulated measurement reliability, with respect to the measurements taken by sensing units 172 and 180. Support assemblies 184 permit the tiltmeter to be *< leveled on the earth's surface prior to the taking of earth tide measurements. This is accomplished by screwing certain ones of the leg members 188 in the appropriate direction within base plate 182, which in turn alters the spacing 'between the surface 192 (and therefore the earth's surface) and the surface of the fluid within the corresponding cup 168 or 170. It may be appreciated since earth tide measurements OMPI ,. WIPO « are taken atop the natural, terrain of the earth's surface, significant differences in elevation may exist in the terrain between the extremities of .the tiltmeter so that the foot members 190 must be differently spaced from the base plate 18 in order to compensate for these differences in terrain. Thus, under field conditions, significant variations may exist in the length of that portion of the leg members 188 which extend between the foot members 190 and base plate 182. Since the tiltmeters are employed to take measurements over a (24) twenty-four hour period, significant variations in ambient temperature are encountered which adversely affect the accuracy of the earth tide measurements that are taken. More specifically, even a temperature variation of a single degree can produce expansion or contraction of the leg mem¬ bers 188 which raise or lower one extremity of the tiltmeter, albeit minutely, thereby producing erroneous tilt measurements. This problem has been overcome in the present invention by providing a thermostatically controlled heating system which compensates for the effects of variations in ambient temperature and maintains the leg members 188 at a constant temperature whereby to eliminate thermal expansion or contraction. This compensation system includes a heat activated switchable circuit element, such as a thermistor 194 • attached to each of the leg members 188 which functions to sense or detect the temperature of associated leg member 188 and closes a switch therein when a temperature of a pre¬ scribed magnitude is detected. The switchable contacts of thermistor 194 are respectively electrically coupled by lines 196 and 198 to one side of an electrical power source 200 and to one side of a relay coil 202. Line 204 connects the opposite sides of relay coil 202 and the power source 200, which power source 200 may comprise a battery as shown in FIGURE 20. Each of the leg members 188 has a suitable electrically energizable, heating element (not shown) connec¬ ted thereto, and preferably formed integral therewith. The OM contacts of the heating element are respectively coupled through lines 204 and 206 to one side of a power source 208 and a relay switch 210 which is controlled by the coil 202. The opposite sides of..relay 210 and power source 208 are coupled together by line 212. In operation, when thermistor 194 senses that the temperature of leg member 188 falls below a preselected value, thermistor 194 switches closed thereby coupling lines 196 and 198 together, in turn connecting the power source 200 to the relay coil 202, whereby to energize the latter. Upon energization of relay coil 202, the normally open relay switch 210 is closed, thereby connecting power source 208 via lines 204 and 212 to the heating element connected to, or formed integral with, the leg member 188. Power flowing through lines 204 and 212 to the heating element of leg member 188 results in heating of the latter to a desired temperature. Thermistor 194 may be of a double switching type which switches open when a preselected maximum tempera¬ ture is reached whereupon the power source 200 is discon¬ nected from the coil 202 thereby resulting in the opening of relay switch 210 and the disconnection of the power source 208 from the heating element. In this manner, the tempera¬ ture of the leg member 188 is thermostatically controlled within a relatively narrow range, regardless of substantial variations in the ambient temperature. Changes in ambient temperature may also adversely effect the earth tide measurements which are taken because of thermal expansion and contraction of the fluid within the cups 168 and 170. Thus, the present invention provides for a thermal compensation system including heating means and a control system for maintaining the fluid in cups 168 and 170 at an essentially constant temperature, regardless of changes in the ambient temperature. Thermistors 214 and 216, or similar conventional heat sensitive switching elements, are mounted in the cups 168 and 170, respectively, in communication with the fluid therein. Thermistors 214 and 216 are coupled.in series relationship with each other by line 218. Line 218 couples a suitable source of elec¬ trical power 220, a manually operable switch 222 and a relay coil 224 in series relationship with each other, and between the thermistors 214 and 216. A hearing element 226 surrounding the fluid reservoir 174 has the electrical contacts thereof coupled in series relationship with a suitable power source 228, such as a battery, and a normally open relay switch 230, by line 232. The manually operable switch 222 is provided to disable the thermal compensation system during transport of the tiltmeter- Issuπminα. Row-a-tre**--, tka-t ewtt^ ill ia --n. i-te- closed position, when the temperature of the fluid in cups 168 and 170 falls below a preselected magnitude, the corres¬ ponding thermistors 214 and 216 are actuated thereby closing the circuit between power source 220 and relay coil 224, thereby energizing the latter. Upon energization of relay coil 224, relay switch .230 switches to its closed position thereby applying power source 228 to the heating element 226. As heating element 226 is energized, heat is transferred by conductance to the fluid within reservoir 174, which heat is transferred via the fluid within tube means 172 to that portion of the fluid within cups 168 and 170. In this manner, rising and falling of the fluid within cups 168 and 170 due to thermal expansion produced by variations in the ambient temperature are eliminated. It is to be incidentally noted here, that the provision of a reservoir 174 functions as a filter for removing high frequency seismic noise from the system. It is to be also noted that the thermistors 214 and 216 may be adapted by those skilled in the art to switch to an open position whereby to disconnect the power source 220 from the relay coil 224, thereby de-energizing heating element 226 when a prescribed maximum temperature of the fluid within cups 168 and 170 has been attained. OM /.. WIP It has been found that the use of gallium as a fluid within the reservoir 174 and cups 168 and 170 is particularly desirable since it is particularly effective in wetting surfaces, and thereby provides an especially flat surface in the cups 168 and 170 from which measurements are taken. Moreover, since- gallium freezes at 29.8 degrees centigrade, the fluid assumes a solid physical state when the heating element 226. is de-energized, as during the transport of the tiltmeter from site to site. Thus, the thermal compensation system described above not only substan¬ tially reduces measurement error in the system, but also makes possible the use of gallium in the system which con¬ tributes considerably to the portability of the tiltmeter. Moreover, portability. is further increased by the fact that gallium is considerably lighter than suitable alternative fluids such as mercury. While there has been described what is considered to be a preferred embodiment of the present invention, variations and modifications therein will occur to those skilled in the art once they become acquainted with the basic concepts of the invention. Therefore, it is intended that the appended claims shall be construed to include all such variations and modifications as fall within the true spirit and scope of the invention. _ OMPI _

Claims: CLAIMS 1. For use in evaluating subsurface formations, a tiltmeter for measuring the rise and fall of the earth's surface, said tiltmeter comprising: a level fluid assembly including first and second fluid-containing cups and tube means connecting said first and second cups to permit fluid to flow between said cups when one of said cups is elevated relative to the other; and an interferometric sensing unit including - a source of optical energy for producing an optical energy beam, means for splitting said energy beam into a first portion directed onto the surface of the fluid in one of said cups at an acute angle with respect to said fluid surface, and into a second portion, means spaced from said beam splitting means for recombining said first and second beam portions, a first member spaced from said' fluid surface and disposed in the optical path of said second beam portion, a second member mounted a fixed distance above said fluid surface and including a beam reflecting surface spaced from said fluid surface at a distance to produce a plurality of reflections of said first beam portion between said fluid surface and said reflecting surface whereby to increase the optical path length of said first beam portion, and means for measuring the separation between said fluid surface in said one cup and said first member. 2. The tiltmeter of Claim 1, wherein said reflecting surface of said second member extends essentially parallel to said fluid surface and said second member is interposed between said first member and said fluid surface. 3. The tiltmeter of Claim 1, wherein: said first member includes a reflecting element for reflecting said second portion of said beam toward said beam recombining means, and IPO • said measuring means includes means responsive to the interferometric pattern of the recombined beam to determine the relative movement between said first member and said fluid surface, said interferometric pattern responsive means including an optical head having, a plurality of photodetec¬ tors located in the path of the recombined beam, said photo¬ detectors being spaced from one another in the direction of apparent movement of the interferometric bands, said interferometric pattern responsive means further including logic means connected to said plurality of photodetectors and .responsive to electrical signals pro¬ duced thereby to determine the direction of apparent movement of interferometric bands and to count the number of bands. 4. The tiltmeter of Claim 3 wherein said logic means comprises: a two stage parallel shift register having parallel inputs from said plurality of photodetectors; clock means for shifting photodetector signals through said shift register in parallel one stage at a time whenever at least one of said photodetectors produces a predetermined signal; . means responsive to the signals stored in both stages of said shift register at a particular time for generating a signal indicating the apparent direction of movement of the interferometric bands; and up/down binary counter means connected to said last named means for counting the number of interferometric bands traversing said photodetectors. 5. The tiltmeter of Claim 4, wherein said signal responsive means comprises: a first plurality of AND gates having parallel inputs from the first stage of said shift register, each of said AND gates having one or more signal inverters at its inputs to cause the gate to be enabled only upon the occur¬ rence of a unique signal pattern in said first stage; OMPI WIPO a second plurality of AND gates having parallel unputs from the second stage of said shift register, each of said AND gates having one or more signal inverters at its inputs to cause the gate to be enabled only upon the occur¬ rence of a unique signal, pattern in said second stage; a third plurality of AND gates, each having a first input from one of the AND gates in said first plurality and a second input from one of the AND gates in said second plurality; and a plurality of OR gates for logically combining outputs from selected ones of the AND gates in said third plurality. 6. A tiltmeter as recited in Claim 1, wherein said second member comprises a mirror, said optical energy source comprises a monochromatic light source generating- a beam directed along a normal to a line between said mirror and the fluid surface, said beam splitting means comprises an optical element located in the beam bath at a point intermediate the light source and the fluid-to-mirror line, and said beam recombining means comprises a second optical element aligned with but on the opposite side of the fluid- to-mirror line from the generated beam. 7. The tiltmeter of Claim 1 wherein is provided a pair of said interferometric sensing units respectively associated with said first and second fluid containing cups, each of said cups including open portions in upper regions thereof placing the fluid surface therein in optical com¬ munication with the respectively associated sensing unit. 8. The tiltmeter of Claim 2, wherein said beam splitting means and said beam recombining means are respec¬ tively spaced on opposite sides of a reference axis extending perpendicularly from said fluid surface toward said first member. 9. For use in evaluating subsurface formations, a tiltmeter for measuring the rise and fall of the earth's surface, said tiltmeter comprising: OMPI_ WIPO a level fluid assembly including first and second fluid-containing cups and tube means connecting said cups to permit fluid flow between said cups when one of said cups is elevated relative to the other of said cups; means for sensing the change in elevation of at least one of said cups in accordance with the rise and fall of the earth's surface; means for supporting said level fluid assembly on the earth's surface, said supporting means including a base having said fluid-containing, cups mounted thereon and a plurality of support members mounted on said base, at least certain of said support members being selectively shiftable toward and away from said earth and between a plurality of choices of positions to allow adjustment of the elevation of said cups; hearing means operably coupled with at least one of said support members and in heat transferring relation¬ ship to the latter, said heating means being adapted to be operably coupled with a source of energy and selectively energizable for heating said one support member; and control means operably coupled with said one support member and with said heating means for controlling the operation of the latter, said control means including first means for detecting the temperature of said one support member and means coupled with said first detecting means for energizing said heating means when the detected temperature of said one support member reaches a predetermined magnitude. said control means and said heating means coopera¬ ting to maintain the temperature of said one support means essentially constant whereby to eliminate error in the thermal expansion or contraction of said one support member. 10. The tiltmeter of Claim 9, wherein: each of said plurality of support members is shiftably mounted on said base for shifting toward and away OMPI from said earth and between a plurality of choices of positions to allow adjustment of the elevation of said cups, each of said plurality of said support members having one of said heating means and said control means operably coupled herewith, said first detecting means comprising a heat response switch, said control means further including switchable circuit means operably coupled with said heat responsive switch said heating means and said energy source and operative to couple with the latter with said hearing means when said predetermined temperature is detected. 11. The tiltmeter of Claim 9, wherein each of said plurality of support is elongate and comprises a plastic material impregnated with metal. 12. The tiltmeter of Claim 9, including: reservoir means coupled with said tube means and adapted to hold a quantity of said fluid therein: a selectively energizable heating system adapted to be selectively coupled with a source of electrical energy and coupled with said reservoir for heating the fluid within said reservoir; second means for detecting the temperature of the fluid in each of said cups; and switchable circuit means under control of said second detecting means and operably coupled with said heating system for selectively coupling the latter with said electrical energy source in accordance with the magnitude of the temperature sensed by said second detecting means. 13. The tiltmeter of Claim 12, wherein said fluid comprises gallium. 14. The tiltmeter of Claim 9, wherein said means for sensing the change in cup elevation comprises an inter¬ ferometric sensing unit, said sensing unit including: a source of optical energy for producing an optical energy beam. OMPI means for splitting said, energy beam into a first portion directed onto the surface of said fluid in one of said cups at an. acute angle with respect to said fluid surface, and into a second portion, means spaced from said beam splitting means for recombining said first and second beam portions, a member spaced a fixed distance above said one cup including a beam reflecting surface extending parallel to said fluid surface and being spaced from the latter a distance to produce a plurality of reflections of said first beam portion between said fluid surface and said reflecting surface, and means for measuring the separation between the surface in said cup and said member. 15. A tiltmeter for measuring the rise and fall of the earth's surface, comprising: a level fluid assembly including at least one fluid-containing cup adapted to be coupled with a source of fluid; an interferometric sensing unit for sensing the change of fluid level within said one cup, said change of said fluid level in said one cup. being correlatable with the magnitude of rise or fall in the earth's surface, said sensing unit including - (1) a source of optical energy for producing an energy beam, (2) means for splitting said energy beam into a plurality of portions and for directing one of said portions onto the surface of said fluid, within, said one cup at an acute angle with respect to said surface, (3) a member including a beam reflecting surface spaced above said fluid surface and extending essentially parallel to the latter to produce a plurality of reflections between said fluid surface and said reflecting surface, OMPI , IPO (4) means for recombining said beam portions, (5) means for. measuring the separation between said fluid surface and said reflecting surface; a base for supporting said level fluid assembly thereon; a plurality .of leg means on said base for supporting the latter on the earth and selectively shiftable on said base between a plurality of choices of positions to allow adjust¬ ment of the spacing of said one fluid cup from the earth; and means operably coupled with each of said leg means for controlling the temperature of the latter whereby to prevent thermal expansion of said leg means due to changes in ambient temperature. 16. The invention of Claim 15, including means operably coupled with said one cup for controlling the temperature of the fluid within said cup. O

Inventors: GEIGER A
Applicants: GEIGER A
Application year: 1980
Main classification: G01C9
IPC codes: G01C9