Borehole surveying
The invention relates to a method and an apparatus for use in surveying boreholes. The method of the invention comprises the following steps: providing an instrument package in a leading end of a drillstring, the instrument package comprising first and second single-axis sensors mounted for rotation with the drillstring about the rotational axis of the drillstring, the first sensor being an accelerometer and the second sensor being a magnetic fluxgate or a rate gyro; rotating the drillstring; deriving from the first sensor the inclination angle of the drillstring at the instrument package; and deriving from the second sensor the azimuth angle of the drillstring at the instrument package.
1. A method of surveying boreholes, comprising:
providing an instrument package in a leading end of a drillstring, said instrument package comprising first and second single-axis sensors mounted for rotation with the drillstring about the rotational axis of the drillstring, the first sensor being an accelerometer and the second sensor being a magnetic fluxgate or a rate gyro;
rotating the drillstring;
deriving solely from the first sensor the inclination angle of the drillstring at the instrument package; and
deriving solely from the first sensor and the second sensor the azimuth angle of the drillstring at the instrument package.
2. The method of claim 1 , wherein the sensors are radially spaced from the borehole axis and have their sensing axes in a plane containing the borehole axis and an axis perpendicular thereto.
3. The method of claim 1 , wherein the sensors are radially spaced from the borehole axis and have their sensing axes in a plane parallel with the borehole axis.
4. The method of claim 1 , wherein the drilling control rotation angle is obtained from the outputs of the sensors.
5. The method of claim 1 , wherein the outputs of the sensors are integrated over each of the four quadrants of rotation and the desired output angle is derived from the integrated output.
6. The method of claim 1 , wherein the instrument package suitably includes rotation angle reference means for use in the integration.
7. The method of claim 1 , wherein additional information is derived comprising the local gravitational and magnetic field vectors.
8. An apparatus for use in surveying boreholes, the apparatus comprising:
an instrument package adapted to be included in the leading end of drillstring, the instrument package comprising first and second single-axis sensors mounted for rotation with the drillstring about the rotational axis of the drillstring, the first sensor being an accelerometer and the second sensor being a magnetic fluxgate or rate-gyro; and
computing means for deriving solely from the first sensor while the drillstring is rotating the inclination angle of the drillstring at the instrument package, and or deriving solely from the first sensor and the second sensor while the drillstring is rotating the azimuth angle of the drillstring at the instrument package.
9. The apparatus of claim 8 , wherein the sensors are radially spaced from the borehole axis and have their sensing axes in a plane containing the borehole axis and an axis perpendicular thereto.
10. The apparatus of claim 8 , wherein the sensors are radially spaced from the borehole axis and have their sensing axes in a plane parallel with the borehole axis.
11. The apparatus of claim 8 , wherein the computing means operates to integrate the outputs of the sensors over each of the four quadrants of rotation and to derive the desired output angle from the integrated outputs.
12. The apparatus of claim 8 , further comprising rotation means for use in the integration.