Sliding weight borehole gravimeter

Abstract

A borehole tool including an interferometer, a light source, a chamber containing a sliding weight having a first optical prism, a second optical prism located within the chamber, a tilt measuring device, and a timing device operatively associated with the interferometer. The light source, the interferometer, and the first and second optical prisms are configured to cause light emitted by the light source to form a first beam and a second beam that interfere with each other. The interferometer measures distances traveled by the sliding weight in the upward and downward direction by counting the fringes caused by the interference between the first beam and the second beam. The tilt measuring device measures the angle of the chamber relative to vertical. The influence of friction on the sliding weight's motion is eliminated by comparing the distances traveled by it in its upward and downward path over an equal time interval.

1 . A borehole tool comprising:
 an interferometer;   a light source;   a chamber containing a sliding weight including a first optical prism;   a second optical prism located within the chamber;   a tilt measuring device; and   a timing device operatively associated with the interferometer; wherein:   the sliding weight is movable within the chamber;   the light source, the interferometer, the first optical prism, and the second optical prism are configured to cause at least a portion of light emitted by the light source to form a first beam and a second beam that interfere with each other;   the interferometer measures the interference between the first beam and the second beam; and   the tilt measuring device measures an angle of the chamber relative to vertical.   
   
   
       2 . The borehole tool of  claim 1 , further comprising a propulsion device configured to propel the sliding weight upward in the chamber. 
   
   
       3 . The borehole tool of  claim 2 , wherein the sliding weight is magnetic and the propulsion device comprises an induction coil. 
   
   
       4 . The borehole tool of  claim 1 , wherein the timing device comprises an atomic clock. 
   
   
       5 . The borehole tool of  claim 1 , wherein the light source comprises a laser. 
   
   
       6 . The borehole tool of  claim 1 , wherein the tilt measuring device comprises a borehole tilt-meter. 
   
   
       7 . The borehole tool of  claim 1 , wherein a distance traveled by the sliding weight during a select period of time is determined using the interferometer and the timing device. 
   
   
       8 . The borehole tool of  claim 2 , wherein a gravitational acceleration is determined by propelling the sliding weight upward in the chamber, determining an upward and a downward distance traveled by the sliding weight during a select time period using the interferometer and the timing device, measuring an angle of tilt of the chamber during the select time period using the tilt measuring device, and using the determined upward and downward distances, a length of time of the select time period, and the angle of tilt to calculate the gravitational acceleration. 
   
   
       9 . The borehole tool of  claim 1 , further comprising:
 a second interferometer;   a second sliding weight contained within the chamber and including a third optical prism; and   a fourth optical prism located within the chamber; wherein:   the second sliding weight is movable within the chamber;   the light source, the second interferometer, the third optical prism, and the fourth optical prism are configured to cause at least a portion of light emitted by the light source to form a third beam and a fourth beam that interfere with each other; and   the second interferometer measures the interference between the third beam and the fourth beam.   
   
   
       10 . The borehole tool of  claim 1 , further comprising
 a second interferometer;   a second chamber containing a second sliding weight including a third optical prism;   a fourth optical prism located within the second chamber;   a second tilt measuring device; and   the timing device operatively associated with the second interferometer; wherein:   the second sliding weight is movable within the second chamber;   the light source, the second interferometer, the third optical prism, and the fourth optical prism are configured to cause at least a portion of light emitted by the light source to form a third beam and a fourth beam that interfere with each other;   the second interferometer measures the interference between the third beam and the fourth beam; and   the second tilt measuring device measures an angle of the second chamber relative to vertical.   
   
   
       11 . A method for determining a gravitational acceleration comprising:
 providing a borehole tool comprising a chamber with a sliding weight;   propelling the sliding weight upward in the chamber;   determining an upward distance and a downward distance traveled by the propelled sliding weight during a select time period;   determining an angle of the chamber relative to vertical during the select period of time; and   determining the gravitational acceleration using the determined upward and downward distances traveled by the sliding weight, a time length of the select period of time, and the determined angle.   
   
   
       12 . The method of  claim 11 , wherein the borehole tool further comprises a tilt measuring device to measure the angle of the chamber relative to vertical. 
   
   
       13 . The method of  claim 11 , wherein:
 the borehole tool further comprises an interferometer, a timing device, a first optical prism, and a light source;   the sliding weight includes a second optical prism; and   the interferometer, the timing device, the first and second optical prisms, and the light source are operatively associated for determining the upward and downward distance traveled by the sliding weight during the select period of time.   
   
   
       14 . The method of  claim 11 , wherein the select period of time includes a time when the sliding weight reaches a maximum upward position. 
   
   
       15 . The method of  claim 11 , wherein the select period of time includes an upward time period and a downward time period. 
   
   
       16 . The method of  claim 15 , wherein the upward and the downward time period are substantially equal. 
   
   
       17 . The method of  claim 16 , wherein the gravitational acceleration is determined using the following equation:
     g   0 =( X   i   +X   j )/(cos (θ)· t   2 )   
     where g 0  is the gravitational acceleration;
 X i  is the downward distance traveled by the sliding weight during the upward time period; 
 X j  is the upward distance traveled by the sliding weight during downward time period; 
 θ is the angle of the first chamber relative to vertical; and 
 t is either the upward time period or the downward time period. 
 
   
   
       18 . A method for determining a density of an underground rock layer comprising:
 providing a borehole tool comprising an interferometer, a light source operatively associated with the interferometer, a chamber containing a sliding weight including a first optical prism operatively associated with the interferometer, a propulsion device operatively associated with the sliding weight, a second optical prism located within the chamber and operatively associated with the first optical prism and the interferometer, a tilt measuring device operatively associated with the chamber, and a timing device operatively associated with the interferometer;   positioning the borehole tool at a first select depth underground near the underground rock layer;   launching the sliding weight upward;   determining an upward distance and a downward distance traveled by the launched sliding weight during a select period of time;   determining an angle from vertical of the chamber during the select period of time; and   determining a gravitational acceleration using the determined upward and determined downward distances traveled by the sliding weight, a time length of the select time period, and the determined angle from vertical of the chamber.   
   
   
       19 . The method of  claim 18 , further comprising:
 positioning the borehole tool at a second select depth underground near the underground rock layer; and   repeating the steps of launching the sliding weight, determining an upward distance and a downward distance, determining an angle, and determining a gravitational acceleration.   
   
   
       20 . The method of  claim 19 , further comprising determining a density of the underground rock layer using the first and second determined gravitational accelerations, the first select depth, and the second select depth. 
   
   
       21 . The method of  claim 20 , wherein the density of the underground rock layer is determined using the following equation: 
     
       
         
           
             
               ρ 
               b 
             
             ÷ 
             
               
                 Δ 
                  
                 
                     
                 
                  
                 g 
               
               
                 Δ 
                  
                 
                     
                 
                  
                 z 
               
             
           
         
       
     
     where ρ b  is the density of the underground rock layer;
 Δg is the difference between the first and second determined gravitational accelerations; and 
 Δz is the difference between the first and second select depths. 
 
   
   
       22 . A method of  claim 18 , further comprising:
 the borehole tool further comprising a second interferometer, the light source operatively associated with the second interferometer, a second chamber containing a second sliding weight including a third optical prism operatively associated with the second interferometer, a second propulsion device operatively associated with the second sliding weight, a fourth optical prism located within the second chamber and operatively associated with the third optical prism and the second interferometer, a second tilt measuring device operatively associated with the second chamber, and the timing device operatively associated with the second interferometer;   launching the second sliding weight upward;   determining an upward distance and a downward distance traveled by the launched second sliding weight during a second select period of time;   determining an angle from vertical of the second chamber during the second select period of time; and   determining a second gravitational acceleration using the determined upward and determined downward distances traveled by the second sliding weight, a time length of the second select time period, and the determined second angle from vertical of the second chamber.   
   
   
       23 . The method of  claim 22 , further comprising determining a density of the underground rock layer using the first and second determined gravitational accelerations. 
   
   
       24 . The method of  claim 23 , wherein the first interferometer and the second interferometer are a select distance apart. 
   
   
       25 . The method of  claim 24 , wherein the density of the underground rock layer is determined using the following equation
   η b ÷Δg/Δz   
     where ρ b  is the average density of the underground rock layer proximate the first select depth;
 Δg is the difference between the first and second determined gravitational accelerations; and 
 Δz is the select distance between the first and second interferometers. 
 
   
   
       26 . A borehole tool system comprising:
 a weight in a tube, wherein the weight is displaceable along the tube an upward distance and a downward distance during a displacement cycle, wherein the upward and downward distances are substantially similar;   an incline measuring device configured to measure a tube incline during the displacement cycle;   a timer configured to measure a duration of the upward and downward distances; and   a processor for calculating a gravitational acceleration from the upward and downward distances, the tube incline and the duration of the upward and downward distances.   
   
   
       27 . The system of  claim 26 , further comprising a second weight in a second tube. 
   
   
       28 . The system of  claim 26 , wherein the tube is a vacuum tube and the system does not include an ion vacuum pump. 
   
   
       29 . The system of  claim 26 , further comprising a light source, a stationary prism, and an interferometer that receives a light beam, that is generated by the light source and reflected from the stationary prism. 
   
   
       30 . The system of  claim 29 , wherein the weight includes a prism that reflects the light beam. 
   
   
       31 . A system of  claim 26 , further comprising a second weight in the tube. 
   
   
       32 . A method for determining a density of an underground rock layer comprising:
 providing a borehole tool including a first borehole gravimeter and a second borehole gravimeter positioned a select distance from the first borehole gravimeter;   positioning the borehole tool at a first select depth underground near the underground rock layer;   determining a first gravitational acceleration using the first borehole gravimeter and a second gravitational acceleration using the second borehole gravimeter; and   determining a density of the underground rock layer proximate the first select depth using the first gravitational acceleration and the second gravitational acceleration.   
   
   
       33 . The method of  claim 32 , wherein the density of the underground rock layer at the first select depth is determined using the following equation: 
     
       
         
           
             
               ρ 
               b 
             
             ÷ 
             
               
                 Δ 
                  
                 
                     
                 
                  
                 g 
               
               
                 Δ 
                  
                 
                     
                 
                  
                 z 
               
             
           
         
       
     
     where ρ b  is the average density of the underground rock layer proximate the first select depth;
 Δg is the difference between the first and second determined gravitational accelerations; and 
 Δz is the select distance between the first and second gravimeters. 
 
   
   
       34 . The method of  claim 32 , wherein the distance between the first and second gravimeters is calibrated on the surface of the earth using an absolute surface gravimeter. 
   
   
       35 . The method of  claim 32 , wherein the determined density represents an average density of the underground rock layer adjacent to the borehole tool between the first borehole gravimeter and the second borehole gravimeter. 
   
   
       36 . A borehole tool comprising:
 a first interferometer;   a light source;   a chamber containing a first sliding weight including a first optical prism and a second sliding weight including a second optical prism;   a third optical prism located within the chamber;   a tilt measuring device; and   a timing device operatively associated with the first interferometer; wherein:   the first sliding weight is movable within the chamber;   the second sliding weight is movable within the chamber;   the light source, the first interferometer, the first optical prism, and the third optical prism are configured to cause at least a portion of light emitted by the light source to form a first beam and a second beam that interfere with each other;   the first interferometer measures the interference between the first beam and the second beam; and   the tilt measuring device measures an angle of the chamber relative to vertical.   
   
   
       37 . The borehole tool of  claim 36 , further comprising:
 a second interferometer;   a fourth optical prism located within the chamber;   the light source, the second interferometer, the second optical prism, and the fourth optical prism are configured to cause at least a portion of light emitted by the light source to form a third beam and a fourth beam that interfere with each other; and   the second interferometer measures the interference between the third beam and the fourth beam.   
   
   
       38 . The borehole tool of  claim 37 , wherein a product of a gravitational acceleration by a coefficient of friction is determined by dropping the first and second sliding weights in the chamber, determining a downward distance traveled by the first sliding weight during a select time period using the first interferometer and the timing device, determining a downward distance traveled by the second sliding weight during the select time period using the second interferometer and the timing device, measuring an angle of tilt of the chamber during the select time period using the tilt measuring device, and using the determined downward distances of the first and second sliding weights, a length of time of the select time period, and the angle of tilt to calculate the product of the gravitational acceleration by the coefficient of friction. 
   
   
       39 . A borehole tool comprising:
 a means for containing a weight, the weight displaceable within the weight containing means for an upward and a downward distance during a displacement cycle and the upward and downward distances are substantially similar;   a means for measuring an incline of the weight containing means during the displacement cycle;   a means for measuring a duration of the upward and downward distances; and   a means for calculating a gravitational acceleration from the upward and downward distances, the incline of the weight containing means, and the durations of the upward and downward distances.   
   
   
       40 . The borehole tool of  claim 39 , further comprising a second weight in a second means for containing a weight. 
   
   
       41 . The borehole tool of  claim 39 , further comprising:
 a means for producing a light beam;   a means for at least partially redirecting the light beam; and   a means for determining interferences between a first and a second portion of the light beam.   
   
   
       42 . The borehole tool of  claim 41 , wherein the weight includes a second means for at least partially redirecting the light beam. 
   
   
       43 . A borehole tool comprising:
 a means for determining interferences between at two light beams;   a means for producing a light beam;   a means for containing a sliding weight including a first means for at least partially reflecting the light beam;   a second means for at least partially reflecting the light beam, the second means located within the sliding weight containing means;   a means for measuring an angle; and   a means for measuring time operatively associated with the interference determining means; wherein:   the sliding weight is movable within the sliding weight containing means;   the light producing means, the interference determining means, the first reflective means and the second reflective means are configured to cause at least a portion of light emitted by the light producing means to form a first beam and a second beam that interfere with each other;   the interference determining means measures the interference between the first beam and the second beam; and   the angle measuring means measures an angle of the sliding weight containing means relative to vertical.   
   
   
       44 . The borehole tool of  claim 43 , further comprising a means for propelling the sliding weight upward in the sliding weight containing means. 
   
   
       45 . The borehole tool of  claim 43 , further comprising:
 a second means for determining interferences between at least two light beams;   a second sliding weight contained within the sliding weight containing means, the second weight including a third means for at least partially reflecting the light beam;   a fourth means for at least partially reflecting the light beam located within the sliding weight containing means; wherein:   the second sliding weight is movable within the sliding weight containing means;   the light producing means, the second interference determining means, the third and fourth reflective means are configured to cause at least a portion of light emitted by the light producing means to form a third beam and a fourth beam that interfere with each other; and   the second interference determining means measures the interference between the third beam and the fourth beam.   
   
   
       46 . The borehole tool of  claim 43 , further comprising
 a second means for determining interferences between at least two light beams;   a second means for containing a second sliding weight including a third means for at least partially reflecting the light beam;   a fourth means for at least partially reflecting the light beam, the fourth reflective means located within the second sliding weight containing means;   a second means for measuring an angle; and   the timing measuring means operatively associated with the second interference determining means; wherein:   the second sliding weight is movable within the second sliding weight containing means;   the light producing means, the second interference determining means, the third and fourth reflective means are configured to cause at least a portion of light emitted by the light producing means to form a third beam and a fourth beam that interfere with each other;   the second interference determining means measures the interference between the third beam and the fourth beam; and   the second angle measuring means measures an angle of the second sliding weight containing means relative to vertical.