Diurnal Variation

Key Takeaways

• Geomagnetic diurnal variation correction in aeromagnetic surveys uses a stationary base station magnetometer (a ground-based fluxgate or proton precession magnetometer installed at a fixed location within the survey area) that continuously records the total field intensity at 1 to 10-second intervals throughout the survey flight; the base station record captures the diurnal variation as a time series that is subtracted from each aircraft survey measurement at the corresponding measurement time to remove the temporally varying background field, isolating the spatially varying anomaly field caused by subsurface geology; the correction accuracy depends on the spatial gradient of the diurnal variation across the survey area (which is small over areas less than 500 kilometers in extent but may require multiple base stations for large surveys) and the time synchronization between the aircraft and base station clocks (which must agree to within 1 second to avoid applying the wrong correction increment from the rapidly changing diurnal waveform); aeromagnetic surveys typically require that the diurnal variation at the time of measurement is less than 50 nT (quiet day) for surveys targeting anomalies with amplitudes of 10 to 100 nT, and surveys are suspended or re-flown during magnetically disturbed periods (magnetic storms associated with solar activity) when the diurnal variation may exceed hundreds to thousands of nT and the correction becomes unreliable.
• Magnetic storm interference is an extreme form of diurnal variation caused by the interaction of solar wind particles with the Earth's magnetosphere during periods of high solar activity (solar flares, coronal mass ejections, high-speed solar wind streams): magnetic storms can cause geomagnetic field variations of hundreds to thousands of nT within hours, completely overwhelming any geological anomaly signal in a magnetic survey and rendering any data collected during a storm uncorrectable by the standard diurnal correction procedure; the Kp index (a global geomagnetic activity index ranging from 0 to 9, computed from 13 mid-latitude magnetometer observatories) is used by aeromagnetic survey operators to assess the magnetic activity level before and during survey operations, with flights typically suspended when Kp exceeds 2 or 3 for high-resolution surveys and when Kp exceeds 4 for reconnaissance surveys; solar storm forecasting (from NOAA's Space Weather Prediction Center) provides 1 to 3 day advance warning of elevated geomagnetic activity, allowing survey scheduling to avoid the most disturbed periods and maximizing the proportion of survey data collected during quiet magnetic conditions.
• Borehole magnetic surveys (for directional well surveying using MWD magnetic tools) are affected by diurnal variation through its effect on the measured total field intensity and inclination of the Earth's field at the wellbore location: MWD magnetic direction measurements (azimuth, declination correction) depend on accurate knowledge of the Earth's magnetic field vector at the survey location and time; diurnal variations in field intensity of 20 to 100 nT represent 0.01 to 0.1 percent of the Earth's total field (~50,000 nT at mid-latitudes) but can cause azimuth errors of 0.1 to 0.5 degrees if not corrected; for most directional drilling applications, this level of azimuth error (0.1 to 0.5 degrees per 1,000 meters of horizontal reach) is acceptable, but for high-precision applications (magnetic ranging for SAGD well-pair placement, anti-collision in dense well clusters, or wellbore targeting tolerances of less than 5 meters at depth), real-time in-situ reference field measurement using an Independent Simultaneous Observation (ISO) tool (a separate magnetometer deployed at the surface or in an observation wellbore) can monitor and correct for the real-time geomagnetic field including diurnal effects, improving azimuth accuracy significantly beyond the static International Geomagnetic Reference Field (IGRF) model.
• Diurnal correction in gravity surveys removes the tidal and drift components of the Bouguer gravity anomaly: the Earth's gravity field varies diurnally by 0.1 to 0.2 milligal over a 24-hour cycle due to the gravitational attraction of the Moon and Sun (the tidal component) and the thermal expansion and contraction of the gravimeter spring as ambient temperature changes throughout the day (the instrumental drift component); gravity surveys require repeated measurements at a base station (where the measured gravity change between visits is attributed to tidal and instrument drift effects) and interpolation of the base station drift correction to each field measurement time to remove these components from the observed gravity values before computing the free-air and Bouguer anomaly; modern relative gravimeters (LaCoste and Romberg, Scintrex) have temperature compensation systems that reduce instrumental drift to less than 0.05 milligal per day, but the tidal component must still be modeled and removed for any survey with density anomaly amplitudes below 0.5 milligal.
• Temperature diurnal variation affects the velocity of near-surface seismic waves and consequently the static corrections applied in seismic data processing: the upper few meters of the Earth's surface experience significant diurnal temperature variation (10 to 20 degrees Celsius at mid-latitudes under clear sky conditions), which alters the seismic velocity of dry, partially saturated near-surface soils (thermal expansion of soil particles reduces contact stiffness and decreases P-wave velocity in dry granular media); in very high-resolution seismic surveys (near-surface characterization, engineering site investigation) acquired over multiple days and different times of day, the diurnal velocity variation in the near-surface layer can cause apparent static shifts between seismic lines acquired at different times, degrading the tie between overlapping lines; seismic refraction statics programs that estimate near-surface velocity from first arrival times must account for diurnal velocity changes if the survey spans multiple days with significant temperature variation, and re-surveying reference shot points at different times of day can quantify the diurnal velocity effect for correction in the statics model.