Geophysical exploration

Geology (B.Sc) · Applied geology · free study notes for exams, placements and interviews

In short

Geophysical exploration measures contrasts in physical properties of rocks - density, magnetic susceptibility, electrical resistivity, elastic wave velocity, radioactivity - from the surface, air or boreholes, and inverts those measurements into a picture of the subsurface. It is the only non-destructive way to "see" rock below cover before drilling, so it decides where expensive boreholes go in mineral, groundwater, petroleum and civil projects. In India it is practised by the Geological Survey of India (airborne and ground surveys), NGRI Hyderabad, CGWB, ONGC, Oil India, AMD, MECL, NMDC, Coal India/CMPDI and private consultants. It carries heavy weight in GATE Geology & Geophysics (GG), UPSC Geologist/Geophysicist (Combined Geo-Scientist) and JAM/university Applied Geology papers, and almost every campus technical round on exploration starts with "which method would you choose and why".

Explained

Fundamentals: what geophysics actually measures

Principle: every method detects a contrast in a physical property between a target and its host. No contrast means no anomaly, however good the instrument.
Property - method pairing: density → gravity; magnetic susceptibility/remanence → magnetics; electrical resistivity and chargeability → resistivity, IP, SP, EM; elastic moduli and density → seismic; natural radioactivity → radiometrics; thermal conductivity → geothermal; dielectric permittivity → GPR.
Passive (natural-field) methods: gravity, magnetics, SP, radiometrics, magnetotellurics - measure an existing field.
Active (artificial-source) methods: seismic, resistivity, IP, controlled-source EM, GPR - inject energy and measure the response.
Anomaly: observed value minus a regional/background value. Regional (long wavelength, deep source) is separated from residual (short wavelength, shallow source) by trend-surface, upward continuation or filtering.
Ambiguity (non-uniqueness): infinitely many subsurface models fit one data set, so geophysics must be constrained by geology, drilling and other methods. Forward modelling computes the response of an assumed body; inversion fits a model to data.
Survey design terms: station/electrode spacing controls resolution; line spacing and orientation (lines across strike) control detectability; depth of investigation grows with array size, offset or frequency reduction; signal-to-noise ratio improves by stacking.
Workflow: reconnaissance (regional airborne/satellite) → prospecting scale (ground surveys on grids) → detailed/target scale → drilling → borehole logging → 3-D model.

Seismic methods

Basis: elastic waves - P (compressional, fastest), S (shear, no travel through fluids), and surface waves (Rayleigh, Love). Velocity V depends on elastic moduli and density; acoustic impedance Z = ρV controls reflections.
Typical P velocities (m/s): soil/weathered zone 300-900; dry sand 400-1,200; saturated alluvium 1,500-2,000; water 1,450-1,500; shale 2,000-4,000; sandstone 2,000-4,500; limestone 3,000-6,000; granite/basalt 4,500-6,500; steel ~5,900.
Refraction: uses head waves critically refracted along a high-velocity layer; needs velocity to increase with depth. Gives layer velocities and depths from a travel-time (t-x) plot; crossover distance separates direct and refracted arrivals. Spread length typically 3-5 times target depth.
Hidden layer / blind zone problems: a thin layer or a velocity inversion (low-velocity layer below a faster one) is missed by refraction - a standard exam point.
Reflection: records echoes from impedance boundaries; the workhorse of petroleum exploration. CDP/CMP gathers, NMO correction, stacking, migration, deconvolution give a structural image; multi-fold 2-D, 3-D and 4-D (time-lapse) surveys.
Sources: sledgehammer and plate, weight drop, explosives (shot holes), Vibroseis on land, air gun offshore. Receivers: geophones on land (vertical velocity sensors), hydrophones (pressure) in water.
MASW (multichannel analysis of surface waves) and downhole/crosshole seismic give shear-wave velocity Vs and Vs30 for seismic site classification - Vs30 is the NEHRP/IBC-type basis, while IS 1893 (Part 1) itself classifies soil types by SPT N-value, Vs data being used mainly in microzonation.
Advantages: best depth resolution and structural detail of all methods. Limitations: costliest, slow, needs skilled processing, poor under hard basalt cover (Deccan Trap "basalt problem" - tackled with low-frequency sources and magnetotellurics).

Gravity and magnetic methods

Gravity basis: lateral density contrasts perturb g. Unit mGal (1 mGal = 10⁻⁵ m/s²); microgravity work uses µGal. Instruments: LaCoste-Romberg / Scintrex / Burris relative gravimeters, superconducting and absolute meters.
Gravity corrections (apply in order): drift and tide; latitude (normal gravity formula); free-air (+0.3086h mGal/m, h in m, for elevation above datum); Bouguer (−0.04193ρh mGal/m with ρ in g/cm³, removes the mass of rock between station and datum); terrain (topographic); Eötvös for a moving platform; isostatic for deep studies.
Free-air anomaly = observed − theoretical + free-air; Bouguer anomaly = free-air anomaly − Bouguer slab correction + terrain correction (the terrain correction is always positive, i.e. added). Positive Bouguer anomaly → dense body (chromite, iron ore, basic intrusive, basement high); negative → low density (salt dome, sediment-filled basin, cavity, kimberlite pipe).
Gravity targets in India: chromite (Sukinda), banded iron, Cambay/Krishna-Godavari basin geometry, Sohagpur/Son valley Gondwana basins, crustal studies of the Himalaya.
Magnetic basis: the geomagnetic field (declination, inclination, horizontal and vertical components, total field F) is distorted by magnetic minerals - chiefly magnetite, pyrrhotite, ilmenite. Unit nanotesla (nT); Earth's field ranges roughly 30,000-60,000 nT (about 40,000-48,000 nT over much of India; IGRF values change yearly).
Instruments: proton precession and Overhauser magnetometers, caesium-vapour (airborne), fluxgate gradiometers, SQUID for borehole/high-precision work.
Corrections: diurnal variation (base-station repeat readings or a recording base magnetometer), IGRF regional removal, heading/lag/levelling for airborne data. Reduction to the pole and analytic signal place anomalies over their sources; upward continuation emphasises deep sources.
Remanent magnetisation (especially in basalts and in reversely magnetised bodies) makes magnetic anomalies dipolar and asymmetric - the commonest interpretation trap.
Airborne geophysics in India: GSI/AMSE wing and NGRI fly aeromagnetic, time-domain EM and radiometric surveys; the National Aero-Geophysical Mapping Programme (NAGMP) covers Obvious Geological Potential areas, and heliborne surveys (NGRI, GSI, CGWB-supported aquifer mapping) are routine. Coverage figures keep changing - quote them as "ongoing national programme" rather than fixed numbers.
Advantages: fast, cheap per km², excellent reconnaissance; magnetics is the cheapest areal method. Limitations: strong non-uniqueness, depth poorly resolved, needs good topographic/elevation control (gravity especially - GPS heights to a few cm).

Electrical and electromagnetic methods

Resistivity (DC): current injected through electrodes A, B; potential measured at M, N. Apparent resistivity ρa = K·ΔV/I; K is the geometric factor of the array (units of metres), and ρa is in Ω·m.
Arrays: Wenner (equal spacing a, K = 2πa) - good vertical resolution, slow; Schlumberger (short MN, long AB) - deeper sounding, fewer moves, best for VES; dipole-dipole - good lateral resolution, used in IP and imaging; pole-dipole, gradient, Lee partition, square array.
Vertical Electrical Sounding (VES): expand electrodes about a fixed centre to get resistivity versus depth - the standard groundwater tool, interpreted by curve matching (Orellana-Mooney) or software inversion; curve types H, K, A, Q for three layers.
Resistivity profiling (constant separation traversing) maps lateral changes - dykes, fracture zones, contacts. Electrical Resistivity Tomography (ERT) with multi-electrode cables gives 2-D/3-D sections.
Typical resistivities (Ω·m): clay 1-50; saline/brackish water-saturated sand <10; fresh-water sand/gravel 50-500; weathered granite 50-200; fresh granite/charnockite 1,000-10,000+; massive basalt 500-5,000; metallic sulphides <1.
Principle of equivalence and suppression: different layer combinations give identical curves; a thin layer may be suppressed - a classic limitation question.
Induced polarisation (IP): measures chargeability (time domain, mV/V) or percent frequency effect/metal factor (frequency domain). Responds to disseminated sulphides and to clay (membrane polarisation vs electrode polarisation) - the key method for porphyry/disseminated base metals where resistivity fails.
Self potential (SP): natural voltages from sulphide/graphite bodies (mineralisation potential, typically negative over the body), streaming and diffusion potentials; cheap, used for seepage detection in dams and canals.
Mise-à-la-masse: electrode planted in an intersected conductor in a borehole to trace its extent.
Electromagnetic (EM): a primary alternating field induces eddy currents in conductors, whose secondary field is measured. Frequency-domain (Slingram/HLEM, VLF, ground conductivity meters) and time-domain/TEM (good for deep, strong conductors; heliborne TEM for aquifer mapping). Skin depth controls penetration - lower frequency, deeper.
Magnetotellurics (MT)/CSAMT: uses natural EM fields for crustal-depth resistivity - used in India under the Deccan Traps, in geothermal (Puga, Tatapani) and basin studies.
GPR: high-frequency radar (25 MHz-2 GHz) for very shallow, high-resolution work - utilities, rebar, pavement, archaeology, shallow stratigraphy; penetration collapses in clay and saline ground (often <1-2 m).
Advantages: electrical methods are cheap, rapid and directly sensitive to water, salinity and sulphides. Limitations: shallow depth compared with seismic, sensitive to cultural noise and to contact resistance in dry/rocky ground.

Radiometric, well-logging and other methods

Radiometrics: gamma-ray spectrometry measures K, U (via Bi-214), Th (via Tl-208) channels and total count; used by AMD for uranium (Cuddapah basin, Singhbhum shear zone, Meghalaya), and for lithological mapping and placer heavy minerals. Penetration is only tens of centimetres - essentially a surface method.
Geophysical well logging (borehole geophysics):
- Electrical: SP log (shale/permeable bed discrimination), resistivity and induction logs, microresistivity for invaded zone - used with the Archie equation for water saturation.
- Nuclear: natural gamma (shale indicator, uranium), gamma-gamma density, neutron porosity, neutron activation.
- Acoustic/sonic: transit time → porosity and Vp; used for seismic-to-well tie and synthetic seismogram.
- Others: caliper (hole diameter, washouts), temperature and fluid-conductivity logs, flowmeter, dipmeter/image logs (FMI), NMR, cement bond log, deviation survey.
- Logging gives in-situ properties at high vertical resolution but only around the hole - complementary to surface geophysics.
Cross-hole and downhole seismic tomography, seismic refraction tomography for dam foundations and tunnels.
Geothermal methods: heat-flow and temperature-gradient measurements (Puga, Manikaran, Tattapani, Bakreshwar).
Marine geophysics: multibeam bathymetry, side-scan sonar, sub-bottom profiler, marine magnetics and gravity - used by NIO, NCPOR and the Deep Ocean Mission for polymetallic nodules and gas hydrates.
Mineral-type to method guide: magnetite/iron ore → magnetics + gravity; massive sulphide → EM + SP + gravity; disseminated sulphide → IP + resistivity; kimberlite → magnetics + gravity low + EM (Panna, Raichur fields); bauxite → resistivity/GPR; coal → reflection seismic + logging; groundwater → VES/ERT + heliborne TEM; placer heavies → radiometrics + shallow seismic.

Indian practice, standards and regulation

Agencies: GSI (national geophysical mapping, airborne surveys), NGRI-CSIR (deep crustal, MT, heliborne), ONGC and Oil India (seismic), CGWB (groundwater and the National Aquifer Mapping programme), AMD (radiometrics), MECL and CMPDI (exploration drilling with logging), NMDC, Hindustan Zinc, state DGMs.
Legal framework: the Mines and Minerals (Development and Regulation) Act, 1957 as amended (notably 2015, 2021 and 2023) governs reconnaissance permits (RP), prospecting licences (PL) and composite licences; exploration-stage reporting follows the UNFC classification and the Minerals (Evidence of Mineral Contents) Rules, 2015; the National Mineral Exploration Trust funds regional exploration; the Geoscience Data Repository / National Geoscience Data Repository archives survey data. Offshore petroleum acreage is awarded under HELP/OALP with data from the National Data Repository (DGH). Deep-seated and critical-mineral exploration licences came in with the 2023 MMDR amendment.
Standards and codes: IS 15681 covers geophysical exploration by resistivity for groundwater-type investigations; IS 1892 (subsurface investigation for foundations) and IS 13063-type codes refer geophysical methods for civil sites; IS 1893 (Part 1) classifies sites by SPT N-value (Vs30/shear-wave data are used in microzonation and in NEHRP/IBC-type classification); ASTM D5777 (seismic refraction) and D6431 (DC resistivity) are commonly cited in tender specifications. IS numbers and editions are revised periodically - verify the current edition of any IS before quoting it in a report.
Safety and permissions: explosive sources need licences under the Explosives Act/Rules and district permission; the four Labour Codes in force from 21 November 2025 govern field-camp working conditions and safety (replacing the earlier Factories, Minimum Wages, Industrial Disputes and BOCW Acts); forest and wildlife clearance is required for surveys inside notified forest areas; radioactive sources in logging are regulated by AERB.
Deliverables: station data sheets, corrected anomaly maps (contour/image), interpreted sections, depth-to-basement or depth-to-bedrock maps, drill-target recommendations with depth and confidence - all georeferenced, usually in a GIS.

Key points

Contrast rule - a method works only if the target differs in the property it senses; zero contrast gives zero anomaly.
Non-uniqueness - many models fit one data set; geophysics constrains geology, it does not replace drilling.
Passive vs active - gravity, magnetics, SP, radiometrics, MT measure natural fields; seismic, resistivity, IP, EM, GPR use a source.
Acoustic impedance Z = ρV - reflections come from impedance contrasts, not from velocity alone.
Refraction needs increasing velocity with depth - a low-velocity layer below a faster one is invisible (hidden/blind layer).
Reflection seismic gives the best structural resolution and is the petroleum industry's primary tool (CMP, NMO, stack, migration).
mGal and nT - gravity in milligal (10⁻⁵ m/s²), magnetic field in nanotesla; free-air +0.3086 mGal/m, Bouguer −0.04193ρ mGal/m.
Bouguer anomaly = observed g corrected for drift, tide, latitude, elevation, slab mass and terrain; positive over dense bodies.
Magnetite dominates magnetic response; remanence makes anomalies dipolar and shifted - reduce to pole before interpreting.
Diurnal correction is mandatory in magnetic surveys; drift and tide corrections in gravity surveys.
ρa = K ΔV/I - Wenner K = 2πa; Schlumberger K = π(L² − l²)/(2l) using half-separations.
VES gives layering, profiling gives lateral change, ERT gives a 2-D section - choose by the question asked.
Equivalence and suppression limit resistivity resolution; thin layers and resistivity-thickness products trade off.
IP for disseminated sulphides, EM/SP for massive conductors - the standard base-metal pairing.
Skin depth falls with increasing frequency and conductivity - low frequency for depth, high frequency for resolution.
GPR is highest resolution, shallowest reach; clay and salinity kill it.
Well logs (SP, resistivity, gamma, density, neutron, sonic, caliper) give in-situ properties at centimetre resolution near the hole.
Indian regulation - MMDR Act, 1957 (amended 2015/2021/2023), UNFC reporting, Minerals (Evidence of Mineral Contents) Rules 2015, NMET funding, NGDR/NDR data repositories.

Formulas and examples

ρa = K · ΔV / I — apparent resistivity (Ω·m); ΔV in volts, I in amperes; K in metres
K(Wenner) = 2πa — a = electrode spacing (m)
K(Schlumberger) = π (L² − l²) / (2l) — L = AB/2, l = MN/2 (m)
Snell's law: sin i / V₁ = sin r / V₂; critical angle ic = sin⁻¹(V₁/V₂)
Refraction, two layers (shot-point intercept): z₁ = (ti · V₁V₂) / (2√(V₂² − V₁²)) — ti = intercept time (s), z₁ = layer thickness (m)
Refraction, crossover distance: xc = 2 z₁ √((V₂+V₁)/(V₂−V₁))
Reflection (flat layer, CMP): t² = t₀² + x²/V²; t₀ = 2z/V — x = offset (m), t = travel time (s)
Reflection coefficient R = (ρ₂V₂ − ρ₁V₁)/(ρ₂V₂ + ρ₁V₁); Z = ρV (kg·m⁻²·s⁻¹)
Newton: g = G M / r²; G = 6.674 × 10⁻¹¹ N·m²/kg²; 1 mGal = 10⁻⁵ m/s²
Free-air correction = +0.3086 h (mGal, h in m); Bouguer slab = −0.04193 ρ h (mGal, ρ in g/cm³, h in m)
Bouguer anomaly = g(obs) − g(theoretical) + 0.3086h − 0.04193ρh + terrain correction
Buried sphere, maximum half-width: depth z ≈ 1.305 x½ (x½ = half-width of the anomaly profile)
Total magnetic field F = √(H² + Z²); tan I = Z/H — I = inclination, H and Z in nT
Magnetic induction in a body: J = κ H — κ = magnetic susceptibility (dimensionless, SI)
Skin depth δ = 503 √(ρ / f) metres — ρ in Ω·m, f in Hz
Archie: Sw = [ (a · Rw) / (φ^m · Rt) ]^1/n — Sw = water saturation, φ = porosity, Rw = formation water resistivity, Rt = true resistivity, a ≈ 1, m ≈ 2, n ≈ 2
Chargeability (time domain) M = (1/(t₂−t₁)) ∫ V(t) dt / Vp (mV/V)
Seismic velocity in a rod/solid: Vp = √((K + 4µ/3)/ρ) ; Vs = √(µ/ρ)

Example 1 - Seismic refraction depth to bedrock (dam site). A 12-geophone refraction spread over weathered alluvium gives direct-wave velocity V₁ = 600 m/s, refractor velocity V₂ = 2,400 m/s, and an intercept time ti = 0.030 s.

√(V₂² − V₁²) = √(5,760,000 − 360,000) = √5,400,000 = 2,324 m/s.
z₁ = (0.030 × 600 × 2,400)/(2 × 2,324) = 43,200/4,648 = 9.3 m depth to bedrock.
Crossover distance xc = 2 × 9.3 × √((2,400+600)/(2,400−600)) = 18.6 × √1.6667 = 18.6 × 1.291 = 24.0 m - so the spread must be well over 24 m long to record the refracted arrival; a 5 m geophone spacing (55 m spread) is adequate.
Critical angle ic = sin⁻¹(600/2,400) = sin⁻¹(0.25) = 14.5°.

Example 2 - Apparent resistivity, Wenner and Schlumberger. (a) Wenner array, a = 10 m, I = 0.5 A, ΔV = 0.040 V.

K = 2π × 10 = 62.83 m; ρa = 62.83 × (0.040/0.5) = 62.83 × 0.08 = 5.03 Ω·m → clay or saline-saturated material, poor for a borehole.

(b) Schlumberger, AB/2 = 100 m, MN/2 = 5 m, I = 1.2 A, ΔV = 0.018 V.

K = π(100² − 5²)/(2 × 5) = π(10,000 − 25)/10 = π × 997.5 = 3,133.9 m.
ρa = 3,133.9 × (0.018/1.2) = 3,133.9 × 0.015 = 47.0 Ω·m → at about 25-35 m effective depth, a likely saturated sandy/weathered zone - a reasonable drilling target.

Example 3 - Bouguer anomaly reduction. Observed g = 978,520.40 mGal at a station of elevation 420 m; theoretical (latitude) value = 978,540.00 mGal; assumed crustal density ρ = 2.67 g/cm³; terrain correction = +1.20 mGal.

Free-air correction = +0.3086 × 420 = +129.61 mGal.
Bouguer slab correction = −0.04193 × 2.67 × 420 = −0.11195 × 420 = −47.02 mGal.
Bouguer anomaly = (978,520.40 − 978,540.00) + 129.61 − 47.02 + 1.20 = −19.60 + 129.61 − 47.02 + 1.20 = +64.19 mGal.
A large positive residual after regional removal would point to a dense body (basement high or iron/chromite-bearing mass).

Example 4 - Reflection coefficient and NMO. Shale (ρ = 2.40 g/cm³, V = 2,500 m/s) overlies limestone (ρ = 2.65 g/cm³, V = 4,000 m/s).

Z₁ = 2,400 × 2,500 = 6.00 × 10⁶; Z₂ = 2,650 × 4,000 = 1.06 × 10⁷ kg·m⁻²·s⁻¹.
R = (10.60 − 6.00)/(10.60 + 6.00) = 4.60/16.60 = +0.277 - a strong, positive (peak) reflection.
If the interface is at z = 1,000 m in the shale, t₀ = 2 × 1,000/2,500 = 0.80 s. At offset x = 600 m: t = √(0.80² + 600²/2,500²) = √(0.64 + 0.0576) = √0.6976 = 0.8352 s, so NMO = 0.8352 − 0.80 = 35.2 ms.

Example 5 - Skin depth and Archie saturation. (a) An EM system at f = 1,000 Hz over ground of ρ = 50 Ω·m: δ = 503 √(50/1,000) = 503 × 0.2236 = 112 m. Dropping to f = 10 Hz: δ = 503 × √5 = 503 × 2.236 = 1,125 m - depth needs low frequency. (b) A sandstone log gives φ = 0.25, Rt = 20 Ω·m, Rw = 0.5 Ω·m, a = 1, m = n = 2.

F = 1/φ² = 1/0.0625 = 16; Ro = F·Rw = 16 × 0.5 = 8 Ω·m.
Sw = √(Ro/Rt) = √(8/20) = √0.40 = 0.632 → about 63% water, 37% hydrocarbon saturation.

Method	Property sensed	Typical depth reach	Main targets	Cost/speed
Reflection seismic	Acoustic impedance	100 m to >10 km	Oil, gas, coal, basin structure	Very high cost, slow
Refraction seismic	P velocity	5-100 m (engineering)	Bedrock depth, rippability, Vs30	Moderate
Gravity	Density	10 m to crustal	Chromite, iron, basins, cavities	Low cost, slow stations
Magnetics	Susceptibility, remanence	Surface to basement	Magnetite, kimberlite, dykes, basement	Cheapest areal
Resistivity/ERT	Resistivity	1-300 m	Groundwater, salinity, foundations	Low cost
IP	Chargeability	50-500 m	Disseminated sulphides	Moderate, slow
EM/TEM	Conductivity	10 m to >500 m	Massive sulphides, aquifers, saline zones	Moderate
SP	Natural potential	<100 m	Sulphides, graphite, dam seepage	Very cheap
Radiometrics	K, U, Th	Centimetres	Uranium, lithology, placers	Cheap, airborne
GPR	Dielectric permittivity	0.5-20 m	Utilities, rebar, shallow layers, cavities	Moderate

Array	Geometric factor K	Strength	Weakness
Wenner	2πa	Strong signal, good vertical resolution	Needs all four electrodes moved; slow
Schlumberger	π(L² − l²)/2l	Deep sounding, fewer moves, standard for VES	Weak signal at large L; MN must be increased
Dipole-dipole	πn(n+1)(n+2)a	Excellent lateral resolution, used for IP	Low signal, high noise sensitivity

Where it is used

Groundwater: CGWB and state groundwater departments run VES/ERT and heliborne TEM under the National Aquifer Mapping (NAQUIM) programme to site borewells and map saline-fresh interfaces in Rajasthan, Gujarat and coastal Tamil Nadu; consultants bill per sounding.
Mineral exploration: GSI, NMDC, Hindustan Zinc, Vedanta and MECL use aeromagnetics plus ground IP/EM for base metals in the Aravalli belt, iron ore in Bailadila/Bellary-Hospet, chromite in Sukinda, gold in Hutti and Kolar, and kimberlites in Panna and Raichur.
Petroleum: ONGC, Oil India and private operators run 2-D/3-D seismic in Cambay, Krishna-Godavari, Mumbai Offshore, Assam-Arakan and Rajasthan basins; job roles include seismic data processor, interpretation geoscientist, petrophysicist and QC geophysicist.
Civil and infrastructure: refraction, MASW and ERT for dam foundations, tunnels (Himalayan rail and road tunnels), metro alignments and bridge piers; GPR for utility detection and pavement/rebar condition in municipal and NHAI work.
Coal and lignite: CMPDI and Coal India use reflection seismic plus geophysical well logging to map seam continuity and faults; logging crews work for MECL and private service companies.
Hazard, environment and archaeology: site-response and liquefaction studies for seismic microzonation (Delhi, Guwahati, Jabalpur, Bengaluru), landslide investigation, leachate and contamination mapping at landfills, dam-seepage SP surveys, and ASI-linked GPR/magnetic surveys of buried structures.

Interview questions

Q: Why do geophysical interpretations need drilling confirmation? A: Because the inverse problem is non-unique - many density, resistivity or velocity models reproduce the same observations - so drilling and geology fix the correct one.
Q: Which method would you choose to find a disseminated copper sulphide body, and why? A: Induced polarisation with resistivity, because disseminated grains give a strong chargeability response even when the bulk resistivity contrast and conductivity are too low for EM.
Q: What is the main limitation of seismic refraction? A: It needs velocity to increase with depth; a low-velocity layer or a thin hidden layer is not detected, and depths are then wrong.
Q: Difference between a free-air anomaly and a Bouguer anomaly? A: The free-air anomaly corrects only for elevation; the Bouguer anomaly additionally removes the mass of rock between the station and the datum plus terrain effects.
Q: Why is diurnal correction essential in a magnetic survey? A: The geomagnetic field varies tens of nanotesla through the day from ionospheric currents, which is comparable to or larger than many targets, so base-station or repeat readings must be subtracted.
Q: What causes a negative Bouguer anomaly? A: A mass deficiency - thick sediments, salt, a cavity, a weathered or fractured zone, or a low-density intrusive such as some kimberlites.
Q: Why is Schlumberger preferred over Wenner for deep sounding? A: Only the current electrodes move for most of the sounding, the potential dipole stays short, so it is faster and gives a smoother curve to greater depth, though signal strength falls.
Q: What do equivalence and suppression mean in resistivity sounding? A: Equivalence - different layer resistivity/thickness combinations give identical curves; suppression - a thin layer makes too small a change to be resolved. Both limit how much detail a VES can prove.
Q: Why does GPR fail in clayey or saline ground? A: High electrical conductivity attenuates the radar wave rapidly, cutting penetration to well under a metre or two.
Q: How is depth of investigation controlled in a DC resistivity survey versus an EM survey? A: In DC resistivity by electrode separation (roughly a fraction of AB); in EM by frequency (and conductivity) through skin depth, or by time gate in TEM.
Q: Which logs would you run to decide if a sandstone is an aquifer or a hydrocarbon reservoir? A: Gamma for shale content, neutron and density for porosity, resistivity with Archie's equation for water saturation, SP for permeable beds, and a sonic/caliper for velocity and hole condition.
Q: Why is exploration under the Deccan Traps difficult, and what is done about it? A: The high-velocity, heterogeneous basalt scatters seismic energy and masks sub-trappean sediments; low-frequency seismic sources, long-offset acquisition and magnetotelluric or gravity surveys are used alongside.
Q: Does a magnetic anomaly always sit over its source? A: No - at non-vertical inclinations and with remanent magnetisation the anomaly is dipolar and offset; reduction to the pole or an analytic signal transform is applied before locating the body.

Common mistakes

Treating a geophysical anomaly as proof of ore, water or hydrocarbon - it only indicates a physical-property contrast that needs testing.
Applying the Bouguer correction with the wrong sign or forgetting the terrain correction, and omitting drift/tide or diurnal corrections altogether.
Using the full electrode separations in the Schlumberger geometric factor instead of the half-separations L = AB/2 and l = MN/2.
Interpreting refraction data without checking for a hidden or low-velocity layer, or using a spread shorter than the crossover distance.
Mixing units - mGal with m/s², nT with gauss, Ω·m with Ω, metres with feet - and quoting apparent resistivity as true resistivity.
Ignoring remanent magnetisation and assuming every magnetic anomaly is a symmetric high directly above the body.
Separating regional from residual arbitrarily, so the "anomaly" is an artefact of the trend-removal choice; and running a single method where two complementary ones would resolve the ambiguity.