Main Field Model

Magnetic reference values

Standard six-axis magnetic MWD uses three accelerometers and three magnetometers to calculate inclination and magnetic azimuth. Declination converts magnetic azimuth to true azimuth. Reference magnetic dip and total field also support survey QC and selected correction methods, including axial magnetic correction.

A common declination error adds a common azimuth offset. Dip and total-field errors can affect corrected azimuth differently as tool attitude changes. Sensor calibration does not generally remove reference errors, although QC can reveal some mismatches. See the MWD methods described by Reay et al. (2005).

Reference uncertainty contributes to the position error budget. Its importance depends on the survey method, well geometry, and other error sources. Select a reference using its validity, data coverage, applicable uncertainty, and the survey requirements.

What the reference represents

The natural magnetic reference includes three main contributions:

• Core field: generated by fluid motion in the Earth's outer core. Its gradual change is called secular variation.
• Crustal field: contributions from magnetised rocks. At a fixed location these are approximately constant over drilling timescales, but they vary spatially. Global products resolve some crustal wavelengths; local contributions can remain unresolved.
• Temporal variation: contributions from ionospheric and magnetospheric currents, with induced responses in the Earth. Solar-quiet daily variation mainly comes from ionospheric wind dynamo currents in a region ionised by solar radiation. Solar-wind disturbances drive storms and substorms.

The core main field is one contribution. Global reference products such as BGGM and HDGM also include crustal content. A downhole measurement can additionally contain BHA, casing, or other artificial magnetic interference; those effects require separate evaluation.

IFR1 uses local observations or transformed aeromagnetic or marine data to estimate unresolved crustal contributions. IFR2 adds an estimate of temporal variation at the survey time. Both retain uncertainty. See the BGS reference description and review of solar-quiet variation.

Selecting a geomagnetic model

The table identifies model versions available in October 2026. Spherical harmonic degree describes spatial detail, separately from the release year.

Provider specifications: IGRF, WMM, WMMHR, BGGM, and HDGM. ISCWSA includes IGRF and standard WMM in its low-resolution reference category.

Five-year models use secular variation to calculate values for the requested date. Forecast uncertainty can increase with the prediction interval. Annual releases can incorporate new observations and shorten that interval. Check the product's validity period: WMM2025 covers January 2025 through December 2029; HDGM2026's main field and secular variation are valid through December 2026.

WMMHR is freely available and includes crustal content. BGGM's theoretical full-wavelength resolution is approximately 28 km; HDGM2026's is approximately 38 km at the equator, also reported as 19 km half wavelength. Resolution alone does not establish local accuracy. Near-surface magnetic data coverage and remaining uncertainty matter.

HDGM provider update: NOAA's final HDGM and HDGM-RT releases are the 2026 models. Existing users remain supported through the end of 2026. Monarch Magnetics will provide subsequent models under a University of Colorado license and identifies HDGM2027 as its first annual release. See the NOAA transition notice and Monarch's information.

For the detailed trade-offs, see Choosing a magnetic reference model for MWD surveys.

Reference uncertainty in the survey model

The following values are rounded root-sum-square totals of the generic ISCWSA Rev5 declination-family sub-terms.

These totals omit the horizontal-field-dependent reference terms and the dip and total-field contributions used with axial correction. They are generic defaults, rather than a complete azimuth or position uncertainty budget. See the ISCWSA definition, section 6.2.3.

The horizontal-field-dependent terms have their own magnitudes and weighting. Their azimuth effect increases where the horizontal field is weak, often at high magnetic latitudes. A fixed declination error in degrees remains an angular offset. Latitude alone does not rank the benefit of an enhancement.

BGGM provides location- and date-specific 1σ estimates. HDGM, WMM, and WMMHR also provide uncertainty estimates with different definitions. BGS's public global/random outputs do not directly replace every Rev5 component. DECG and DECR already include the horizontal-field-dependent contributions. Use the supplied mapping to avoid counting those contributions twice. See the BGS parameter definitions.

A modeled test-well comparison

The figure compares generic LRGM and HRGM uncertainty calculations on the 8,000 m ISCWSA #1 extended-reach test well. It opens with the trajectory rotated so its lateral faces magnetic north. Each axis is scaled to 2σ.

The modeled lateral reduction at TD is approximately 24% in that orientation and 10% for the published orientation. These percentages depend on the selected geometry and error budget. The figure uses generic reference classes; it does not evaluate a particular site's BGGM values.

Location-specific estimates and correlation

In Beggan et al. (2021), Table 3, location-specific estimates reduced modeled 1σ lateral semi-major axes at TD by approximately 7.8–26% across the three ISCWSA test wells and reported methods. The broader 5.2–26% range includes a fourth well offshore Brazil. These are modeled uncertainty comparisons.

Some Brazil orientations increased lateral uncertainty by up to 24% with axial correction near horizontal magnetic east–west drilling. Location-specific estimates can exceed generic values where field conditions warrant it.

Rev5 represents reference errors with global, well-by-well, and random components. Shared errors fully cancel in relative position only when the position responses match. A common reference alone does not establish cancellation. The calculation uses cross-covariance and applicable correlation assumptions. See the ISCWSA correlation supplement and our error-model article.

Reference values along the trajectory

Check whether one surface reference adequately represents the surveyed section. Field variation, the trajectory, height or depth datum, and required uncertainty affect that decision. A model's ability to return a value below surface does not establish its downhole accuracy. Check the selected product's limits and applicable uncertainty, including WMM's height and depth limits.

The BGS IFR brochure, Table 1, reports IFR1-derived declination values of 0.196° at depth zero and 0.031° at 1,860 m for a North Sea well: a difference of 0.165°. The table labels the coordinate as depth without specifying measured depth. These are reference declinations, rather than a separately reported crustal correction. BGS describes 0.1° as a desired declination accuracy; the survey requirements must also specify the applicable confidence level.

Planned station coordinates support planned references. Final processing uses the survey-station positions and survey dates, with the required datum conversions and checks.

Local and temporal corrections

Higher resolution can remove omission components and introduce commission components for the field being modeled. Corrections retain uncertainty. Evaluate the resulting position error budget for the actual reference, survey method, geometry, and target or clearance requirements. The ISCWSA reference categories describe these different reference types.

Reference records and checks

TALLENTECH uses BGGM with BGS location-specific estimates and records the selected model version per well. Reference checks use available observatory and local magnetic data where suitable for the site, date, and intended comparison.

The well record includes declination, dip, total field, the model version, survey date, coordinates, height or depth datum, uncertainty mapping, and QC results. Recording the inputs, corrections, and checks supports review and reprocessing of the definitive survey.

References

• NOAA NCEI: IGRF, WMM, WMMHR, and HDGM.
• BGS: BGGM specifications and error estimates and IFR services brochure, 2025.
• ISCWSA: Error model definition, Rev5.13, reference categories, and geomagnetic correlation supplement.
• Beggan et al. (2021): Location-specific geomagnetic uncertainties.
• Reay et al. (2005): Magnetic MWD methods and reference values.
• Yamazaki and Maute: Solar-quiet daily magnetic variation.
• NOAA: HDGM provider transition. Monarch Magnetics: Provider information.