ORIENTATION INDEPENDENT GRAVITY SENSOR

• US 20090126486A1
• Filed: 11/20/2007
• Published: 05/21/2009
• Est. Priority Date: 11/20/2007
• Status: Abandoned Application

• Current Assignee: Baker Hughes Incorporated (Baker Hughes Company)
• Original Assignee: Baker Hughes Incorporated (Baker Hughes Company)
• Inventors: DiFoggio, Rocco, Fang, Sheng, Georgi, Daniel T., Edwards, Carl M., Estes, Robert
• Application Number:
  US11/943,200
• Publication Number:
  US 20090126486A1
• Time in Patent Office: Days
• US Class Current: 73/382.R
• CPC Class Codes: G01V 7/16 specially adapted for use on moving platforms, e.g. ship, aircraft

Abstract

An instrument for measuring gravitational acceleration, the instrument including: a plurality of accelerometers disposed about a three-dimensional structure, the plurality of accelerometers providing output used for measuring the gravitational acceleration; wherein each accelerometer in the plurality is implemented by at least one of a micro-electromechanical system (MEMS) and a nano-electromechanical system (NEMS).

Claims

1. A sensor for measuring gravitational acceleration, the sensor comprising:

a plurality of accelerometers disposed about a three-dimensional structure, the plurality of accelerometers providing output used for measuring the gravitational acceleration;

wherein each accelerometer in the plurality is implemented by at least one of a micro-electromechanical system (MEMS) and a nano-electromechanical system (NEMS).

2. The sensor as in claim 1, wherein the sensor is disposed in a logging instrument.

3. The sensor as in claim 1, wherein the plurality of accelerometers measures acceleration in each dimension of the three-dimensional structure.

4. The sensor as in claim 1, wherein the at least one of the MEMS and the NEMS comprises an interferometric displacement sensor coupled to a proof mass for measuring the gravitational acceleration.

5. The sensor as in claim 4, further comprising at least one spring coupled to the proof mass and to a support substrate, the spring providing a counterforce to a force of gravity acting upon the proof mass.

6. The sensor as in claim 1, wherein the structure comprises three surfaces, each surface about orthogonal to the other surfaces.

7. The sensor as in claim 1, wherein the structure comprises at least a curved surface.

8. The sensor as in claim 1, wherein the plurality comprises a density of over one hundred accelerometers per square inch.

9. The sensor as in claim 1, wherein a portion of the plurality of accelerometers are disposed about the three-dimensional structure in relation to a direction for each of the three dimensions.

10. A method for determining gravitational acceleration, the method comprising:

performing a measurement of gravitational acceleration with each accelerometer in a plurality of accelerometers, the plurality disposed about a three-dimensional structure; and

determining a net value of the gravitational acceleration from the measurements;

wherein each accelerometer in the plurality is implemented by at least one of a micro-electromechanical system (MEMS) and a nano-electromechanical system (NEMS).

11. The method as in claim 10, wherein determining comprises correcting each individual measurement to account for measuring a fraction of gravitational acceleration in line with a direction of measurement.

12. The method as in claim 10, wherein determining comprises solving

g_z=√

{square root over (A²+B²+C²)}

where g_z represents the gravitational acceleration and A, B, and C are determined by solving

$\begin{array}{cc}\left(\begin{array}{c}\sum <br><br>{\cos }^2<br><br>{\mathrm{\theta <br><br>}}_i\\ \sum <br><br>\sin <br><br><br><br>{\mathrm{\theta <br><br>}}_i<br><br>\cos <br><br><br><br>{\mathrm{\theta <br><br>}}_i<br><br>\cos <br><br><br><br>{\mathrm{\phi <br><br>}}_i\\ \sum <br><br>\sin <br><br><br><br>{\mathrm{\theta <br><br>}}_i<br><br>\cos <br><br><br><br>{\mathrm{\theta <br><br>}}_i<br><br>\sin <br><br><br><br>{\mathrm{\phi <br><br>}}_i\end{array}<br><br>\begin{array}{cc}-\sum <br><br>\sin <br><br><br><br>{\mathrm{\theta <br><br>}}_i<br><br>\cos <br><br><br><br>{\mathrm{\theta <br><br>}}_i<br><br>\cos <br><br><br><br>{\mathrm{\phi <br><br>}}_i& -\sum <br><br>\sin <br><br><br><br>{\mathrm{\theta <br><br>}}_i<br><br>\cos <br><br><br><br>{\mathrm{\theta <br><br>}}_i<br><br>\sin <br><br><br><br>{\mathrm{\phi <br><br>}}_i\\ -\sum <br><br>{\sin }^2<br><br>{\mathrm{\theta <br><br>}}_i<br><br>{\cos }^2<br><br>{\mathrm{\phi <br><br>}}_i& -\sum <br><br>{\sin }^2<br><br>{\mathrm{\theta <br><br>}}_i<br><br>\sin <br><br><br><br>{\mathrm{\phi <br><br>}}_i<br><br>\cos <br><br><br><br>{\mathrm{\phi <br><br>}}_i\\ -\sum <br><br>{\sin }^2<br><br>{\mathrm{\theta <br><br>}}_i<br><br>\sin <br><br><br><br>{\mathrm{\phi <br><br>}}_i<br><br>\cos <br><br><br><br>{\mathrm{\phi <br><br>}}_i& -\sum <br><br>{\sin }^2<br><br>{\mathrm{\theta <br><br>}}_i<br><br>{\sin }^2<br><br>{\mathrm{\phi <br><br>}}_i\end{array}\right)<br><br>\left(\begin{array}{c}A\\ B\\ C\end{array}\right)=\left(\begin{array}{c}\sum <br><br>d_i<br><br>\cos <br><br><br><br>{\mathrm{\theta <br><br>}}_i\\ \sum <br><br>d_i<br><br>\sin <br><br><br><br>{\mathrm{\theta <br><br>}}_i<br><br>\cos <br><br><br><br>{\mathrm{\phi <br><br>}}_i\\ \sum <br><br>d_i<br><br>\sin <br><br><br><br>{\mathrm{\theta <br><br>}}_i<br><br>\sin <br><br><br><br>{\mathrm{\phi <br><br>}}_i\end{array}\right)& \left(11\right)\end{array}$

with respect to a spherical coordinate system used to locate each accelerometer of the plurality wherein the Z axis is the direction of the gravitational acceleration, θ

is an angle measured from the Z axis, φ

is an angle measured from an arbitrarily designated X axis, and d_i is the measurement of gravitational acceleration by the i-th of I accelerometers in the plurality.

13. The method as in claim 12, further comprising determining an angle of rotation, α

, with respect to the Z-axis and an angle of rotation, β

, with respect to the X-axis by calculating

$\begin{array}{c}\mathrm{\alpha <br><br>}={\tan }^{-1}<br><br>\frac{\sqrt{B^2+C^2}}{A}<br><br><br><br>\mathrm{and}\\ \mathrm{\beta <br><br>}={\tan }^{-1}<br><br>\frac{C}{B}.\end{array}$

14. The method as in claim 10, wherein determining comprises calculating a square root of the sum of the squares of each individual measurement.

15. An apparatus for measuring gravitational acceleration in a borehole, the apparatus comprising:

a logging instrument;

a plurality of accelerometers disposed about a three-dimensional structure, the plurality of accelerometers providing output used for measuring the gravitational acceleration; and

a data collector for providing measurement data to a user;

wherein each accelerometer in the plurality is implemented by at least one of a micro-electromechanical system (MEMS) and a nano-electromechanical system (NEMS).

16. The apparatus as in claim 15, further comprising a computer program product stored on machine-readable media for determining gravitational acceleration, the product comprising machine-executable instructions for:

performing a measurement of gravitational acceleration with each accelerometer in the plurality of accelerometers;

determining a net value of the gravitational acceleration from the measurements; and

collecting data from each accelerometer in the plurality.