Supplemental referencing techniques in borehole surveying

A method for surveying a borehole, the method comprising:

• (a) providing a downhole tool including first and second gravity measurement devices disposed at corresponding first and second positions in the borehole, the first and second gravity measurement devices being constrained from rotating with respect to one another about a substantially cylindrical borehole axis, and a supplemental reference measurement device disposed at the first position, the second position being located deeper in the borehole than the first position, the second position being selected to be within sensory range of magnetic flux from a drill bit assembly;
• (b) causing the first and second gravity measurement devices to measure corresponding first and second gravity vector sets;
• (c) processing the first and second gravity vector sets to determine a change in borehole azimuth between the first and second positions;
• (d) causing the supplemental reference measurement device to measure a magnetic field vector set at the first position;
• (e) processing the first gravity vector set and the magnetic field vector set to determine a reference borehole azimuth at the first position;
• (f) determining borehole azimuth at the second position by applying the change in borehole azimuth determined in (e) to the reference borehole azimuth determined in (c) when a difference between a total magnetic field and a magnetic field of the Earth at the first position is less than a predetermined threshold value.

A system for surveying a borehole, the system comprising:

• a downhole tool including first and second gravity measurement devices and a supplemental reference measurement device deployed thereon, the first and second gravity measurement devices disposed at corresponding first and second positions along a substantially cylindrical axis, the gravity measurement devices constrained from rotational movement with respect to one another about the cylindrical axis, the supplemental reference measurement device disposed at the first position, the tool operable to be positioned in a borehole, the second position being located deeper in the borehole than the first position upon deployment of the tool in the borehole, the second position being selected to be within sensory range of magnetic flux from a drill bit assembly;
• and
• a processor configured to determine:
• (A) a change in borehole azimuth between the first and second positions using outputs from the gravity measurement devices;
• (B) a reference borehole azimuth at the first position using an output from the supplemental reference measurement device;
• and
• (C) a borehole azimuth at the second position by applying the change in borehole azimuth determined in (A) to the reference borehole azimuth determined in (B).

A computer system comprising:

• at least one processor;
• and
• a storage device having computer-readable logic stored therein, the computer-readable logic accessible by and intelligible to the processor;
• the processor further disposed to receive input from first and second gravity measurement devices when said first and second measurement devices are (1) deployed at corresponding first and second positions in a borehole, and (2) also substantially constrained from rotating with respect to one another about a substantially cylindrical axis along the borehole;
• the processor further disposed to receive input from a supplemental reference measurement device when said supplemental reference measurement device is deployed at the first position in the borehole;
• the computer-readable logic further configured to instruct the processor to execute a method for determining borehole azimuth, the method comprising:
• (a) determining a reference borehole azimuth at the first position using input from the supplemental reference measurement device;
• (b) determining a change in borehole azimuth between the first and second positions using input from the first and second sets of gravity measurement devices;
• and
• (c) determining borehole azimuth at the second position by applying the change in borehole azimuth determined in (b) to the reference borehole azimuth measured in (a);
• and
• (d) repeating (a) through (d) to determine borehole azimuth at multiple selected locations in the borehole, wherein at least one contiguous pair of said multiple selected locations are selected to be a predetermined borehole distance apart, said predetermined borehole distance exceeding twice the borehole distance between the first position and the second position.

A method for surveying a borehole, the method comprising:

• (a) providing a downhole tool including first and second gravity measurement devices disposed at corresponding first and second positions in the borehole, the first and second gravity measurement devices being constrained from rotating with respect to one another about a substantially cylindrical borehole axis, the tool further including a supplemental reference measurement device disposed at the first position, the second position being located deeper in the borehole than the first position, the second position being selected to be within sensory range of magnetic flux from a drill bit assembly;
• (b) determining a reference borehole azimuth at the first position using the supplemental reference measurement device;
• (c) determining a change in borehole azimuth between the first and second positions using the first and second gravity measurement devices;
• (d) determining borehole azimuth at the second position by applying the change in borehole azimuth determined in (c) to the reference borehole azimuth determined in (b).