Questions
10–12 MCQs per paper
Difficulty
Medium-Hard
Importance
Core — never skip
Overview
Seismic methods utilize artificial shock waves to map subsurface structures by measuring the time-distance relationship of reflected or refracted waves. It is a cornerstone topic for ONGC and other PSU geophysics exams, focusing on wave propagation principles and the mathematical processing of seismic data. Mastering the transition from wave theory to signal processing is essential for solving high-frequency numerical problems.
Seismic Wave Theory
Seismic waves are elastic disturbances that travel through the earth, categorized primarily as body waves and surface waves. In the context of PSU exams, understanding the velocity relationships between compressional (P-waves) and shear (S-waves) is critical.
- Vp = sqrt((K + 4/3μ)/ρ)
- Vs = sqrt(μ/ρ)
- P-waves are longitudinal, S-waves are transverse
- Huygens' Principle describes wave front propagation
- Snell's Law: sin(θ1)/V1 = sin(θ2)/V2
Refraction & Reflection Methods
Reflection methods are used for deep subsurface imaging, while refraction methods are typically employed for shallow layer depth determination. You must be able to derive layer depths from intercept times and critical distances.
- Critical Angle (ic) = arcsin(V1/V2)
- Intercept time (ti) = 2z*sqrt(V2^2 - V1^2)/(V1*V2)
- Refraction is limited to layers with increasing velocities with depth
- Reflection travel time: t^2 = t0^2 + x^2/Vrms^2
- Crossover distance is the point where refracted waves overtake direct waves
Seismic Processing (NMO & Migration)
Processing transforms raw field data into interpretable images by correcting for geometry and velocity variations. Normal Moveout (NMO) correction flattens reflection hyperbolas, while Migration shifts dipping reflectors to their true spatial position.
- NMO equation: Δtnmo = sqrt(x^2 + (Vrms*t0)^2)/Vrms - t0
- Migration removes the distortion of dipping reflectors
- Stolt migration is a common frequency-wavenumber method
- Vrms is calculated using Dix Equation for interval velocity
- DMO (Dip Moveout) handles events with non-zero dip
Formula Sheet
Vp = sqrt((K + 4/3μ)/ρ)
Vs = sqrt(μ/ρ)
sin(θ1)/V1 = sin(θ2)/V2
t = x/V (Direct wave)
t = x/V2 + ti (Refracted wave)
t^2 = t0^2 + x^2/Vrms^2 (Reflection hyperbola)
V_int = sqrt((V2^2*t2 - V1^2*t1)/(t2-t1)) (Dix Equation)
Exam Tip
Always verify units before plugging numbers into the NMO or depth calculation formulas, as PSU exams often trap students with millisecond vs. second conversions.
Common Mistakes
- Confusing Vrms (Root Mean Square velocity) with Average velocity and Interval velocity.
- Ignoring the condition V2 > V1 for head wave generation in refraction methods.
- Applying the normal moveout formula without accounting for the offset distance correctly.
More Revision Notes
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