Downhole referencing techniques in borehole surveying

A method for determining rotational offset between first and second gravity measurement devices, the first and second gravity measurement devices disposed at corresponding first and second positions on a downhole tool deployed in a borehole, the method comprising:

• (a) positioning the tool in a previously surveyed section of borehole, the previously surveyed section providing a historical survey including at least three previously surveyed azimuthal reference points within the previously surveyed section of the borehole;
• (b) measuring first and second gravity vector sets at each of five or more sites;
• (c) utilizing a projected rotational offset to determine a set of corrected gravity vectors at each of the five or more sites;
• (d) replacing one of the gravity vector sets at each of the five or more sites with the corrected gravity vectors at the corresponding sites;
• (e) utilizing the corrected gravity vectors to determine the local azimuths at each of the five or more sites;
• (f) comparing the local azimuths determined in (e) with the historical survey;
• (g) determining a rotational offset between the first and second measurement devices that gives a best fit in (f) between local azimuths determined in (e) and the historical survey.

A method for determining rotational offset between first and second gravity measurement devices, the first and second gravity measurement devices disposed at corresponding first and second positions on a downhole tool deployed in a borehole, the method comprising:

• (a) positioning the tool in a previously surveyed section of borehole, the previously surveyed section providing a historical survey including at least three previously surveyed azimuthal reference points within the previously surveyed section of borehole;
• (b) measuring first and second gravity vector sets using the first and second gravity measurement devices;
• (c) determining local azimuths using the gravity vectors measured in (b);
• (d) repeating (b) and (c) at two or more additional sites in the previously surveyed section of the borehole;
• (e) comparing the local azimuths determined in (c) and (d) with the historical survey;
• and
• (f) determining a rotational offset between the first and second measurement devices that gives a best fit in (e) between local azimuths determined in (c) and (d) and the historical survey.

A system for determining rotational offset between first and second gravity measurement devices deployed in a borehole, the system comprising:

• a down hole tool including first and second gravity measurement devices deployed thereon, the tool operable to be positioned in a previously surveyed section of borehole, the previously surveyed section providing a historical survey including at least three previously surveyed azimuthal reference points within the previously surveyed section of borehole;
• and
• a processor configured to determine:
• (A) local azimuths at three or more sites in the previously surveyed section of the borehole from readings taken from the first and second gravity measurement devices;
• (B) a comparison of local azimuths determined in (A) with the historical survey;
• and
• (C) a rotational offset between the first and second measurement devices that gives a best fit in (B) between local azimuths determined in (A) and the historical survey.

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 deployed at corresponding first and second positions in a borehole, the first and second positions located within a previously surveyed section of borehole;
• the processor further having access to a historical survey of the previously surveyed section of borehole, the historical survey including at least three previously surveyed azimuthal reference points within the previously surveyed section of borehole;
• the computer-readable logic further configured to instruct the processor to execute a method for determining rotational offset between the first and second gravity measurement devices, the method comprising:
• (a) determining local azimuths at three or more sites in the previously surveyed section of borehole using input from the first and second gravity measurement devices;
• (b) comparing local azimuths determined in (a) with the historical survey;
• and
• (c) determining a rotational offset between the first and second measurement devices that gives a best fit in (b) between local azimuths determined in (a) and the historical survey.