Geophysics

Applied Geophysics: Gravity & Seismics

Gravity corrections, seismic refraction and resistivity — the field methods that image the subsurface without drilling.

PART 1

Topic Breakdown & Traps

The Engineering Principle

Geophysics infers subsurface structure from physical fields. In gravity surveying, raw readings are reduced with the free-air (

0.3086 h

mGal) and Bouguer (

0.04193 ρ h

mGal) corrections; the residual Bouguer anomaly maps density contrasts (positive over dense bodies). In seismic refraction, energy critically refracts when

sin θ_{c} = V_{1} / V_{2}

; layer depth comes from the intercept time or crossover distance. Electrical resistivity (Wenner array) gives apparent resistivity

ρ_{a} = 2 π a R

The Core Formula Matrix

Free-air correction:

0.3086 h

mGal (

h

in m).

Bouguer correction:

0.04193 ρ h

mGal (

ρ

in g/cm³).

Critical angle:

sin θ_{c} = V_{1} / V_{2}

.

Refractor depth:

h = \frac{t _{i}}{2} \frac{V _{1} V _{2}}{V _{2}^{2} - V _{1}^{2}}

.

Wenner resistivity:

ρ_{a} = 2 π a (V / I)

The ‘IIT Traps’

⚠
Free-air vs Bouguer. Free-air corrects for elevation only; Bouguer also removes the rock slab's mass.
⚠
**Critical angle needs $V_{2} > V_{1}$ .** Refraction surveys require velocity increasing with depth.
⚠
P faster than S. P-waves arrive first; $V_{p} > V_{s}$ always.

📚 Standard references

An Introduction to Geophysical Exploration — Kearey, Brooks & Hill
Applied Geophysics — Telford, Geldart & Sheriff

PART 2

Progressive 3-Tier Question Suite

Q1BASIC1 Mark · MCQ

The seismic body wave that is fastest and arrives first is the

Q2MEDIUM2 Marks · NAT

A gravity station sits

500

m above the datum. The free-air correction is _____ mGal.

mGal

Q3HARD2 Marks · NAT

A refraction survey has

V_{1} = 2000

and

V_{2} = 4000

m/s. The critical angle is _____ degrees.