Inclination and high side angle of a borehole is measured by a tool that may be positioned inside a drill string and lowered into the borehole. The tool includes three gravity transducers positioned in a novel array. The transducers are positioned in a case whose axis coincides with that of the borehole and are arranged with their axes angularly displaced from each other around the case axis by approximately 120 degrees and from the case axis by approximately 45 degrees. An array of fluxgate magnetic transducers may be added to the apparatus to provide combination surveying and steering capability.
When surveying a borehole using an instrument responsive to the earth's magnetic field, a length of non-magnetic drill collar is necessary to house means for measuring the magnetic field in the borehole perpendicular to the direction of the borehole axis. The instrument determines the inclination angle and the highside angle from the gravitation measurements, with these measurements and the magnetic measurements, the azimuth angle is determined. Using the method of this invention a minimum length of non-magnetic material necessary for an accurate measurement may be calculated and used.
Apparatus for monitoring vibration of a bottom hole assembly used to drill a well includes at least one accelerometer mounted in the bottom hole assembly to generate data in the form of electrical signals corresponding to the acceleration experienced by the assembly. The computer in the assembly is programmed to collect data from the accelerometer and compute magnitude of the assembly acceleration. Means are provided for selecting from the collected data a value which exceeds a preset level. In one form, when the collected data has a value above a preset level, an alarm signal is sent to the surface so corrective measures can be taken to prevent damage to downhole equipment.
A method is disclosed for determining wellbore azimuth accurately, notwithstanding the corrupting influence of axial and transverse magnetic field biasing error components. A reliable value of the axial magnetic field component (B.sub.x) is determined by minimizing the vector distance between B (as measured) and its expected value B. Preferably, this is accomplished utilizing a method of radicals to solve a quadratic equation. Given a reliable B.sub.x, the transverse magnetic field components (B.sub.x and B.sub.x) can be determined from taking a number of surveys as the drillstring is rotated about. B.sub.x and B.sub.x can be determined by utilizing the assumption that the transverse magnetic field components should remain invariant as the drillstring is rotated.
A method is provided for deriving wellbore azimuth in a manner which reduces the impact of axial magnetic field errors, which utilizes a plurality of estimates of the axial component of the magnetic field which are combined in a manner which emphasizes the better estimates and deemphasizes the poorer estimates.
