Gravity
The latitudinal model [1] is used to calculate the gravitational field:
The accuracy of this model is 0.01 mg, which is an orders of magnitude higher than the accuracy of MWD tools.
Manual input of the gravity reference is also supported.
Magnetic
The Earth’s magnetic field is modeled using the spherical harmonics method[2]:
Unlike MWD STD Advanced modeling the global and crustal fields, MWD STD Basic uses only the geomagnetic model of the Earth’s magnetic global field from NOAA [3], which consists of 12 first harmonics:
The error model is updated periodically, now the magnetic declination error is described by the following formula [4]:
The accuracy of the geomagnetic model fully satisfies the standard model of the MWD survey error[5], which is described by the formula:
For the total B and dip angle, the errors are 145 nT and 0.21 deg, respectively. This is slightly higher than the standard model (130 nT and 0.2 deg), but this does not affect the azimuth accuracy on the straight line.
The geomagnetic model is updated periodically every 5 years, in addition, NOAA monitors the accuracy performance of the model and, if necessary, releases an unscheduled update.
In addition, MWD STD Basic supports manually input or uploading of the geomagnetic reference if you are using BGGM, HDGM, or IFR.
Grid Convergence
Grid convergence is calculated for WGS84 only. UTM zone is selected automatically. If you are using a datum other than WGS84, enter the grid convergence value manually.
1. Moritz, H., Geodetic Reference System 1980
2. Brett, L., Methods of Spherical Harmonic Analysis, Quarterly Journal of the Royal Astronomical Society, Vol. 29, NO.2/JUN, P.129, 1988
3. NCEI: Arnaud Chulliat, et al. & BGS: William Brown, et al.; The US/UK World Magnetic Model for 2020-2025 : Technical Report
4. NCEI: Arnaud Chulliat, et al. & BGS: William Brown, et al.; State of the Geomagnetic Field, 2021
5. Williamson H.S. 2000. Accuracy Prediction for Directional Measurement While Drilling, SPE 67616, SPE Drilling and Completion, 15(4)