Buried wellbore location from surface magnetic measurements

A method for locating a buried casing stub comprising:

• a) identifying a target region for the buried casing stub;
• b) providing a casing stub locator at each of a plurality of surface survey points in the target region, the casing stub locator including a vector magnetometer, the casing sub locator positioned at the surface of the earth;
• c) providing a total station in the target region, wherein the total station has a line of sight to the casing stub locator, determining the location of the total station, and using the total station to measure at least one of the position of the casing stub locator within the target region or the azimuth relative to true north of the casing stub locator;
• d) using the vector magnetometer to measure the magnitude and direction of the magnetic field at each of the plurality of surface survey points so as to generate a plurality of magnetic field measurements;
• e) using the plurality of magnetic field measurements to generate a model of the magnetic field of the target region;
• f) fitting the model generated in step e) to an expected model of a magnetic anomaly created by the casing stub so as to generate model fit information;
• and
• g) determining the position and the depth of the casing stub using the model fit information.