Geophysics Formulas for CSIR NET Earth Science: Gravity, Seismic, Magnetic and Electrical Methods
Geophysics is a high-scoring but high-fear section of CSIR NET Earth Science. The problem is not the number of formulas; the problem is knowing which formula belongs to which physical situation. This guide organizes the most useful geophysics formulas and the logic needed to apply them in Part B and Part C.
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1. Seismology Formulas
| Concept | Formula / Relation | Exam Use |
|---|---|---|
| Travel time | t = distance / velocity | Direct time, residual and layered path problems |
| P-wave velocity | Vp = sqrt((K + 4mu/3) / rho) | Elastic property interpretation |
| S-wave velocity | Vs = sqrt(mu / rho) | S waves vanish when rigidity mu = 0 |
| Poisson ratio | Related to Vp/Vs | Fluid/incompressible and rock-property questions |
| Time residual | Residual = observed time – reference time | Velocity perturbation problems |
2. Gravity Method Formulas
- Bouguer correction: delta gB = 2 pi G rho h.
- Free-air correction: approximately 0.3086 mGal per metre.
- Horizontal cylinder anomaly ratio: Delta g(x)/Delta gmax = z^2 / (x^2 + z^2).
- Sphere anomaly has a different distance-depth dependence; do not mix it with cylinder equations.
Exam tip: If a question says the anomaly falls to half or one-third of its maximum, convert the ratio into an equation and solve for x/z.
3. Magnetic Method Formulas and Concepts
- Total magnetic field: F = sqrt(H^2 + V^2).
- Inclination: tan I = V / H.
- At the magnetic equator, inclination is small and H dominates V.
- Magnetic declination records horizontal angular deviation and is useful for rotation about a vertical axis.
- Natural remanent magnetization can preserve paleomagnetic direction if not reset.
4. Electrical and EM Methods
| Concept | Meaning | CSIR Trap |
|---|---|---|
| Resistivity | Resistance normalized by geometry | Do not confuse resistance with resistivity |
| Reflection coefficient | Contrast response at interface | Range may be negative to positive |
| Transmission coefficient | Passed signal fraction in normalized form | Depends on definition used in question |
| SP log | Electrochemical potential response | Linked with permeability and salinity contrast |
5. Well Logging High-Yield Relations
- Neutron porosity decreases in gas-bearing zones.
- Sonic velocity decreases in overpressured zones.
- Wet shaly sandstone can reduce formation factor.
- SP response is affected by permeability and fluid salinity contrast.
How to Prepare Geophysics for CSIR NET
- Build a formula sheet grouped by method: seismic, gravity, magnetic, electrical and logging.
- For each formula, write the physical meaning of every symbol.
- Solve one direct question, one graph question and one Part C analytical question from each topic.
- Revise common traps: units, signs, ratios and square-root relations.
- Practice mixed PYQs because CSIR rarely announces the method in the question title.
Common Mistakes
- Using P-wave relations for S-wave problems.
- Forgetting that fluids cannot transmit S waves because shear modulus is zero.
- Mixing gravity anomaly formulas for spheres and cylinders.
- Ignoring whether a magnetic question asks inclination or declination.
- Not checking units before selecting the final option.
FAQ
Is geophysics compulsory for CSIR NET Earth Science?
Yes. Geophysics questions appear regularly and are especially important for Part C numerical and analytical scoring.
Can I score in geophysics without a geophysics background?
Yes, if you prepare formula logic, units and repeated PYQ patterns. You do not need to derive every equation from first principles for every question.
Which geophysics topic should I revise first?
Start with seismology and gravity because they produce direct numerical questions. Then revise magnetic, electrical and well logging concepts.
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