6.2 Geophysical Exploration Methods in Mining Geology

Geophysical exploration measures physical property contrasts between mineral deposits, host rock units, and structural zones. By detecting anomalies in Earth's electrical, magnetic, gravitational, radiometric, and electromagnetic fields, geophysics provides non-destructive 3D subsurface imagery that guides structural mapping and drill hole placement.

Electrical Methods: Induced Polarization & Resistivity

Electrical geophysical methods exploit contrasts in electrical conductivity ($\sigma$, measured in $\text{S/m}$) or electrical resistivity ($\rho = 1/\sigma$, in $\Omega\cdot\text{m}$) and chargeability ($M$, in $\text{mV/V}$ or milliseconds).

1. Induced Polarization (IP) Chargeability

Induced Polarization measures the transient decay of voltage in the ground after an applied direct current is interrupted. This charge storage phenomenon occurs via two principal mechanisms:

• Electrode Polarization: Occurs at interfaces between metallic mineral grains (e.g., chalcopyrite, pyrite, galena, graphite) and pore fluids. Current flow causes charge buildup across the metallic-electrolyte boundary.
• Membrane Polarization: Occurs in clay-rich rocks where narrow pore passages restrict ion movement.
• Exploration Application: IP chargeability is often useful for detecting disseminated polarizable minerals, including sulfides, but no geophysical method is universally most effective. Porphyry systems may produce chargeability anomalies where sulfides are disseminated, yet response magnitude depends on mineralogy, texture, alteration, clay, water, array, instrument, and processing; a fixed mV/V threshold is not diagnostic of ore.

2. Electrical Resistivity Tomography (ERT)

ERT measures apparent resistivity ($\rho_a$) by injecting current through ground electrodes and measuring potential differences across receiving pairs.

• High-Resistivity Targets ($> 1,000\text{ }\Omega\cdot\text{m}$): Quartz veins, silicified breccia zones, unweathered crystalline intrusive rocks, and massive limestone units.
• Low-Resistivity / Conductive Targets ($< 50\text{ }\Omega\cdot\text{m}$): Water-saturated fault zones, clay-altered argillic halos, graphite beds, and metallic sulfide bodies.
• Array Geometries: Dipole-Dipole arrays provide high horizontal resolution for vertical structures; Pole-Dipole arrays offer deeper penetration depth ($> 300\text{ m}$) with superior signal-to-noise ratios.
• Philippine Laterite Application: ERT effectively maps the depth of weathered nickel laterite profiles, differentiating conductive water-saturated saprolite/clay zones ($10\text{--}50\text{ }\Omega\cdot\text{m}$) from underlying high-resistivity unweathered ultramafic bedrock ($> 500\text{ }\Omega\cdot\text{m}$).

Magnetic Surveying: Total Magnetic Intensity & Reduced to Pole

Magnetic surveys measure local perturbations in the Earth's main magnetic field caused by spatial variations in rock magnetic susceptibility ($\chi$). Magnetic susceptibility depends primarily on magnetite content ($Fe_3O_4$) and, to a lesser extent, pyrrhotite ($Fe_{1-x}S$).

1. Total Magnetic Intensity (TMI) & Dipolar Complexity

Ground magnetometers or airborne sensor arrays record Total Magnetic Intensity (TMI) in nanoTeslas ($\text{nT}$). At non-polar geomagnetic latitudes, Earth's inclined magnetic field induces dipolar anomalies (a paired magnetic high and magnetic low) offset laterally from the causative subsurface body.

2. Low-Latitude Magnetic Transformations

Reduced-to-Pole (RTP) recalculates an induced magnetic response as if the inducing field were vertical. For a simple body with known induced magnetization this can reduce dipolar asymmetry, but conventional RTP becomes numerically unstable as inclination approaches zero and can amplify noise. Remanent magnetization, anisotropy, topography, and interacting sources also prevent a unique “peak equals source” interpretation.

For Philippine low-latitude data, test field direction and remanence and compare stabilized RTP, reduction-to-equator, total gradient or analytic-signal products, tilt derivatives, and forward or inverse modeling. Choose the method from synthetic tests and geology; never make RTP a mandatory preprocessing step before drilling.

3. Application in Porphyry & Skarn Systems

• Potassic Core Highs: High-temperature potassic alteration cores of porphyry deposits feature hydrothermal magnetite-biotite assemblages, producing prominent RTP magnetic highs.
• Phyllic Magnetite-Destructive Lows: Outlying quartz-sericite-pyrite (phyllic) alteration destroys primary magmatic magnetite, converting it to non-magnetic pyrite and producing distinct magnetic low rings surrounding the intrusive core.
• Magnetite Skarns: Intrusions contacting carbonate rocks form massive magnetite skarns (e.g., Larap, Camarines Norte), generating extreme magnetic anomalies ($> 5,000\text{ nT}$).

Gravity Surveys & Bulk Density Contrasts

Gravity surveys measure variations in Earth's gravitational acceleration ($g$, in $\text{mGal}$, where $1\text{ mGal} = 10^{-5}\text{ m/s}^2$) resulting from subsurface bulk density ($\rho$) contrasts.

1. Bouguer Gravity Anomaly & Corrections

Field observations are corrected for latitude, elevation (Free-Air correction), slab mass (Bouguer correction), and surrounding topography (Terrain correction) to yield the Complete Bouguer Anomaly: $\Delta g_{\text{Bouguer}} = g_{\text{obs}} - g_{\text{theoretical}} + \Delta g_{\text{FA}} - \Delta g_{\text{Bouguer slab}} + \Delta g_{\text{Terrain}}$

2. Density Contrasts ($\Delta \rho$) & Mineral Applications

• Massive Sulfides & Chromite: Chromite ($\rho = 4.2\text{--}4.8\text{ g/cm}^3$) and Volcanogenic Massive Sulfides (VMS, $\rho = 4.0\text{--}5.0\text{ g/cm}^3$) exhibit strong positive density contrasts ($\Delta \rho > +1.0\text{--}1.5\text{ g/cm}^3$) relative to host volcanic or ultramafic rocks ($\rho = 2.7\text{--}3.2\text{ g/cm}^3$). High-precision gravity surveys detect positive mGal anomalies overlying concealed chromite lenses in ophiolite terranes (e.g., Zambales Ophiolite Complex).
• Barite & Iron Ore: Direct detection of dense hematite/magnetite bodies and massive barite veins.

Radiometric & Electromagnetic (EM) Methods

1. Radiometrics (Gamma-Ray Spectrometry)

Radiometric surveys measure gamma radiation emitted during the radioactive decay of Potassium ($^{40}K$), Uranium ($^{238}U$), and Thorium ($^{232}Th$) in the upper $30\text{ cm}$ of soil and rock.

• Potassic Alteration Mapping: Hydrothermal potassic alteration introduces secondary K-feldspar and biotite, sharply elevating potassium concentrations relative to thorium ($K/Th$ ratio anomaly). This provides a direct geochemical-geophysical footprint for porphyry centers.

2. Airborne Electromagnetic Methods (EM & VTEM)

Airborne Versatile Time-Domain Electromagnetic (VTEM) systems induce secondary eddy currents in conductive subsurface bodies using a transmitter loop towed by a helicopter.

• Massive Conductors: VTEM rapidly screens large areas for highly conductive, connected massive sulfide bodies (VMS, nickel-copper sulfides) buried beneath conductive overburden.

Matrix of Geophysical Methods by Exploration Target