Local crustal contributions
Global references such as BGGM include the core field and some regional crustal contributions. Magnetic rocks can add shorter-wavelength contributions that the selected global reference leaves unresolved. IFR1 uses local magnetic observations and modeling to estimate those contributions. See the BGS description of global and local references.
At a fixed location, crustal contributions are approximately constant over drilling timescales. They vary with position and depth. Their importance depends on the local geology, magnetic data coverage, and the survey method.
A defined declination-error example
Consider a straight horizontal 3,000 m section with a constant 1° azimuth offset caused by a declination error. Assume the same starting position and otherwise correct survey inputs. The cross-track endpoint displacement is approximately:
3,000 m × sin(1°) = 52.4 m
This is the displacement from a specified angular error. An uncertainty ellipse requires an error model and a selected scale or confidence level. The example alone cannot establish collision clearance or a shut-in decision. Those decisions depend on spacing, both wells' uncertainties, reference correlations, and the applicable rules. See Collision Avoidance.
How IFR1 estimates the reference
Aeromagnetic and marine surveys usually measure total field strength. Mathematical transformations estimate directional components from those scalar measurements. Local vector observations, including suitable ground surveys, can also support IFR1. Both approaches retain uncertainty. See the BGS IFR method description and scalar-to-vector validation.
A global model already returns values for the requested coordinates and date. IFR1 adds local information to produce estimates of declination, magnetic dip, and total field within a defined area and depth range. The reference record must identify coverage, spatial resolution, coordinates, height or depth datum, survey date, and model version.
TALLENTECH's described workflow combines BGGM with local crustal information. Other IFR1 products can use different global references: Monarch's IFR1 uses HDGM. The input package must identify whether it contains complete reference values or residual crustal corrections, and which global reference it uses. Combine the values according to the provider's definitions to avoid adding the global field twice.
References are evaluated at the survey-station coordinates and dates. Application depends on valid geographic and depth coverage, suitable input data, and QC. The Main Field Model page describes the reference records and checks.
Uncertainty and a modeled comparison
IFR1 changes the reference error budget. In Rev5, resolving crustal contributions can remove omission terms and introduce commission terms for the estimated field. Remaining uncertainty includes unresolved contributions and temporal variation. The supplied estimates and survey QC must support the selected tool model.
The position effect depends on the reference data, survey method, geometry, and other errors. A modeled reduction does not establish measured field accuracy or a smaller ellipse for every well. See the ISCWSA definition and our error-model article.
The figure compares generic LRGM, HRGM, and IFR1 calculations on the 8,000 m ISCWSA #1 test well. It opens with the lateral rotated toward magnetic north. Each axis is scaled to 2σ.
The incremental HRGM-to-IFR1 reduction in modeled lateral uncertainty at TD is approximately 29% in that orientation and 8% for the published orientation. The overall reductions, approximately 46% and 17%, include both reference changes. The vertical value is unchanged in this example. These are results for the selected geometry and generic models, rather than performance estimates for a particular IFR dataset.
| Case | Engine code | 2σ LAT | 2σ VERT | Step change |
|---|---|---|---|---|
| Generic low-resolution reference | MWD+LRGM | 135.8 m | 45.3 m | Baseline |
| MWD+HRGM | 103.6 m | 45.3 m | −24% LAT | |
| MWD+IFR1 | 73.9 m | 45.3 m | −29% LAT | |
| Overall, first to last | −46% LAT |
Select a case to draw its step: the case above in amber, this case in blue.
NoteIn this modeled north–south case, azimuth sensitivity to axial magnetic interference is small, so reference uncertainty has a larger influence on lateral uncertainty.
Evaluating where IFR1 helps
Compare the actual reference error budget and available data with the survey requirements:
- Local magnetic contributions: assess the contributions unresolved by the chosen global reference. Inconsistent offset-well declination also requires checks for other survey errors; it does not identify a crustal cause by itself.
- Weak horizontal field: a given transverse magnetic error has a larger azimuth effect where the horizontal field is weak, often at high magnetic latitudes. A fixed declination error in degrees remains an angular offset.
- Collision clearance: evaluate both wells' uncertainty and reference correlations under the selected separation rule.
- Geological targets: assess the relevant direction of position uncertainty. Thin targets can depend on TVD, inclination, and depth uncertainty as well as azimuth.
See NOAA's horizontal-field uncertainty explanation, the ISCWSA correlation supplement, and Geological Target Analysis.
IFR1 does not correct rapid temporal variation. IFR2 estimates that variation at the survey time by transferring suitable observatory data to the wellsite. It can support real-time or retrospective processing when timestamps and data coverage are suitable. It retains uncertainty. See the BGS IFR services brochure.
Using an existing IFR1 cube or map
TALLENTECH can assess supplied third-party data for use in survey processing. The assessment checks the format, units, reference baseline, coordinates, height or depth datum, geographic and depth coverage, version, and supplied uncertainty. Complete reference values and residual corrections require different handling.
Suitable data can be applied to eligible survey stations, with the reference, selected error-model parameters, and QC results recorded. Reuse depends on compatibility and coverage; it does not establish an automatic accuracy gain or a fixed acquisition cost.
References
- BGS: IFR method and location-specific uncertainty, BGGM specifications, and IFR services brochure, 2025.
- Macmillan (2008): Validation and communication of geomagnetic reference uncertainty.
- ISCWSA: Reference categories, error-model definition, Rev5.13, and geomagnetic correlation supplement.
- NOAA NCEI: WMM limitations and horizontal-field-dependent uncertainty.
- Monarch Magnetics: IFR1 product and global reference.