Questions
8 questions in major PSU papers
Difficulty
Medium-Hard
Importance
Core - never skip for ONGC/OIL exams
Overview
Gravity and Magnetic methods are potential field exploration techniques used to map subsurface density and susceptibility variations. Mastering these is critical for PSU exams like ONGC and OIL, as they form the backbone of hydrocarbon and mineral resource exploration.
Gravity Survey Corrections
Gravity surveys require specific corrections to isolate the target anomaly from regional and instrument-induced effects. Understanding the sign and purpose of these corrections is a frequent testing point.
- Latitude Correction: Accounts for Earth's oblateness and rotation.
- Free-Air Correction (FAC): Corrects for elevation above the datum.
- Bouguer Correction (BC): Corrects for the mass of the rock between the station and the datum.
- Terrain Correction: Always positive, accounts for surrounding topography.
- FAC = 0.3086 h (mGal), where h is in meters.
- BC = 0.04193 rho h (mGal), where rho is density.
Gravity Anomaly Interpretation
Interpretation focuses on the Bouguer anomaly map, which reflects subsurface density contrasts. Candidates must distinguish between regional trends and localized anomalies.
- Bouguer Anomaly = g_obs - g_theoretical + FAC - BC + TC.
- Regional anomaly: Large-scale features from deep-seated density variations.
- Residual anomaly: Small-scale, localized anomalies of interest.
- Ambiguity Principle: Gravity data can be explained by multiple density distributions.
- Half-width rule: Used to estimate the maximum depth of an anomalous body.
Magnetic Properties of Rocks
Magnetic surveying measures variations in the Earth's magnetic field due to the induced or remanent magnetization of rocks. Induced magnetization is directly proportional to the Earth's field and rock susceptibility.
- Induced magnetization (I) = kH, where k is magnetic susceptibility.
- Diamagnetic materials have negative susceptibility.
- Paramagnetic and Ferromagnetic materials have positive susceptibility.
- Curie Temperature: Temperature above which ferromagnetic materials lose their magnetism.
- Remanent Magnetization (NRM): Permanent magnetization independent of the present field.
Aeromagnetic Surveys
Aeromagnetic surveys provide rapid, large-scale coverage for mapping basement topography and tectonic lineaments. They are the standard for regional structural mapping in PSU exploration projects.
- Fluxgate magnetometers were traditionally used.
- Proton precession magnetometers are highly accurate for total field measurement.
- Diurnal Correction: Removes time-varying solar field fluctuations.
- IGRF (International Geomagnetic Reference Field) is used for regional field removal.
- Higher altitude increases spatial resolution (smoothing effect).
Formula Sheet
FAC = 0.3086 * h
BC = 2 * pi * G * rho * h
Anomaly = g_observed - g_predicted
I = k * H
g_lat = 978032.7 * (1 + 0.0053024*sin^2(phi) - 0.0000058*sin^2(2*phi))
Exam Tip
Always verify units—ensure density is in g/cc and elevation is in meters when calculating Bouguer corrections to avoid order-of-magnitude errors.
Common Mistakes
- Confusing the sign of the Bouguer correction with the Terrain correction (both are added).
- Assuming gravity anomalies are unique solutions, failing to account for the non-uniqueness/ambiguity principle.
- Neglecting the impact of latitude correction in global gravity modeling.
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