Magnetic variation at the survey time
A static reference can differ from the magnetic field during a survey. IFR2 uses suitable observatory data to estimate temporal contributions at the wellsite. Both regular daily variation and irregular geomagnetic activity can matter to magnetic MWD processing.
The regular solar quiet (Sq) variation comes from electric currents driven by winds in the ionosphere. Solar radiation supplies the ionisation that makes this region conductive. Solar-wind disturbances drive additional current systems in the magnetosphere and ionosphere. Ground observations also include currents induced within the Earth. See our diurnal variation article and the Sq physics review.
The effect depends on geomagnetic latitude, local time, season, activity, and the survey method. Quiet-day variation can exceed the required reference accuracy. Compare its contribution with the actual error budget rather than assigning it a fixed rank among survey errors.
Two observed days at Meanook
The charts show declination at Meanook Magnetic Observatory, Alberta, on 11 May 2024 during the Gannon storm, and a lower-variation comparison day, 3 April 2024. They use GSC definitive one-minute data, published on 1 April 2026 and retrieved through INTERMAGNET.
The storm-day range is approximately 8.33°, over the available observations. The comparison-day range is approximately 0.20°. Neither range is a reference uncertainty or an azimuth error for a well. The second curve shows the day's observed variation; it does not isolate Sq from every other contribution.
A wellsite's field can differ from the observatory's field. A static reference can produce substantial azimuth error at some times during a storm, but the error depends on the site, timestamp, reference, and survey method. Field-strength and dip checks can identify some problems; they do not directly validate declination. The North Sea drilling study describes these reference dependencies.
How the reference is estimated
IFR2 adds an estimate of temporal variation to IFR1. In TALLENTECH's BGGM-based processing, calibrated observatory values are compared with the selected global reference at the observatory. The observatory and wellsite baselines use the same model and reference epoch.
The current workflow interpolates between observation times at the selected observatory. It transfers the resulting field differences to the wellsite, assuming that those differences are representative there. This temporal interpolation is distinct from spatial interpolation across an observatory network. Data quality and spatial representativeness must support the transfer.
The reference baseline must be defined. Separating a quiet-day pattern from irregular activity can help describe the data, but the applicable temporal contribution includes both Sq and irregular variation. Subtracting a quiet-day pattern and applying only the remaining storm signal would leave the regular variation unaccounted for.
TALLENTECH's described reference sequence is:
| Reference | Estimates |
|---|---|
| BGGM | Core and resolved regional crustal contributions, with secular variation |
| IFR1 | The global reference plus estimated local crustal contributions |
| IFR2 | The IFR1 reference plus estimated temporal contributions at the survey time |
These are reference estimates with remaining uncertainty. The ISCWSA IFR2 category requires IFR1 and suitable local observed temporal-field data. Estimating a temporal contribution alone does not establish eligibility for that tool-model category.
Timing, coverage, and missing observations
Use the survey acquisition time, with the time zone and any supported clock adjustment recorded. The time when a survey arrives for processing is not its measurement time. The selected observations must cover the measurement interval at an appropriate cadence.
Observatory selection requires more than a distance check. Geomagnetic latitude, field gradients, activity, calibration, and data quality affect whether observations represent the wellsite. ISCWSA specifies observations within approximately 100 km, depending on geomagnetic latitude. BGS's separate service guidance uses one or more observatories within 500 km. These are different requirements; neither distance alone validates a transfer.
IFR2 can support real-time or retrospective processing when suitable observations and timestamps are available. Archived data can reveal conditions that were not reviewed during drilling. See the BGS services brochure.
Missing or unusable observations can prevent an observed-data correction. Where a modeled fallback is used, identify it separately and review its limitations. A quiet-time diurnal model cannot reproduce an observed storm-time disturbance. A fallback must not be treated as measured storm data or assigned reduced uncertainty without supporting evidence.
Monitoring and survey review
TALLENTECH's platform ingests observatory data around the clock. It supports calibration records, field comparisons, gap reports, outlier scans, and review of flagged samples. These functions support assessment; they do not guarantee that every bad value is identified before use. Rapid changes can also be real geomagnetic activity rather than an instrument fault.
Monitoring and reference records should distinguish the observation source, measurement time, baseline, calibration, and processing status. They should identify missing data and modeled substitutes. A QC flag alone does not establish that the reference meets the survey requirements.
Uncertainty and deliverables
An IFR2 estimate retains measurement, calibration, and transfer uncertainty. Other survey errors remain. The selected error-model terms and magnitudes must be supported by the supplied reference, remaining temporal uncertainty, and QC. See the ISCWSA definition and our error-model article.
The reference package should identify the timestamps, observatory, baseline and model versions, calibration, data status, applied values, and uncertainty with its scale or confidence definition. Its processing record should identify any missing data, substitute estimates, and review results.
IFR2 is worth evaluating where temporal reference uncertainty matters to the target or collision-clearance requirements and suitable data are available. Its benefit depends on the well geometry, survey method, other errors, and the quality of the transfer. An audit requirement alone does not establish that IFR2 is necessary.
References
- BGS: IFR method, IFR2 service guidance, and services brochure, 2025.
- ISCWSA: Geomagnetic reference categories and error-model definition, Rev5.13.
- Yamazaki and Maute: Sq and the ionospheric wind dynamo.
- Reay et al. (2005): Space weather effects on drilling accuracy in the North Sea.
- GSC/INTERMAGNET: Definitive Meanook data, 11 May 2024 and 3 April 2024.