Questions: Induced Polarization and Frequency-Domain Response

Question 1

A geophysicist surveys a region looking for disseminated copper sulfide mineralization. Standard resistivity measurements show no anomaly, but an IP survey reveals a strong response. Why does IP succeed where resistivity fails?

• A. IP uses higher voltages that penetrate deeper, reaching ore bodies that resistivity cannot detect
• B. Disseminated sulfides have high polarizability from electrode polarization at grain boundaries, even though they are too sparse to significantly alter bulk resistivity
• C. Sulfide minerals are always highly resistive, which IP measures more accurately than standard methods
• D. IP averages readings over wider areas, smoothing out the small resistivity signature of disseminated ore

Answer: (B)

This is the key practical insight of IP: polarizability and resistivity are independent properties. A rock body containing 1-2% disseminated sulfide minerals by volume may barely change the bulk resistivity (there isn't enough metallic mineral to form continuous conductive pathways), but the electrode polarization at each grain boundary produces a strong, measurable chargeability signal. Resistivity measures how easily current flows through the rock; IP measures how much charge the rock can store and release — a different physical property entirely. This is why IP is uniquely effective for disseminated deposit types.

Question 2

In time-domain IP, when is the chargeability signal measured?

• A. While the current pulse is actively flowing — measuring the enhanced conductivity of polarizable minerals
• B. Simultaneously at two frequencies — comparing resistivity at high and low frequency
• C. After the current is abruptly cut off — measuring the slowly decaying residual voltage
• D. Before the current pulse — establishing a baseline that is subtracted from the active measurement

Answer: (C)

Chargeability in time-domain IP is the integral of the decaying voltage measured after the current is switched off. When current flows, charge accumulates at interfaces (electrode polarization at sulfide grain surfaces, membrane polarization at clay surfaces). When current stops, that stored charge slowly dissipates, producing a measurable voltage that decays over seconds. This delayed decay is the IP signal. The area under the decay curve (normalized to the applied voltage) gives chargeability. This is directly analogous to discharging a capacitor — polarizable materials act as leaky capacitors embedded in the rock.

Question 3

In time-domain IP, the chargeability signal is measured from the decaying voltage that persists after the injected current is shut off.

• T. True
• F. False

Answer: True

This is the defining measurement in time-domain IP. The slow voltage decay after current shutoff directly reflects the charge storage and release at interfaces in the rock. Non-polarizable rocks drop their voltage to near-zero almost instantaneously when current is cut; polarizable rocks (those with sulfide minerals, clay, or other interface-rich materials) maintain a decaying voltage for seconds. The shape and magnitude of this decay curve encodes information about the type and abundance of polarizable material in the subsurface.

Question 4

Electrode polarization in IP is caused by clay minerals partially blocking ion flow through narrow pore throats.

• T. True
• F. False

Answer: False

That description is membrane polarization, not electrode polarization. Electrode polarization (also called metallic or overvoltage polarization) occurs when ions moving through pore fluid encounter a metallic or sulfide mineral grain surface. The transition from ionic to electronic conduction creates a bottleneck where charge accumulates at the grain boundary. Membrane polarization is distinct — it arises from clay minerals and narrow pore throats where electrical double layers partially block ion flow. Both mechanisms produce IP signals, but electrode polarization is stronger and is the basis of sulfide mineral exploration.

Question 5

Explain why IP measurements reveal properties of subsurface materials that standard resistivity measurements cannot detect.

Model answer: Resistivity measures how efficiently current flows through rock, which depends mainly on pore fluid conductivity and porosity. IP measures polarizability — the ability of the rock to store and release charge at internal interfaces. These are independent properties. A rock can have normal resistivity but high polarizability (disseminated sulfides) or high resistivity with low polarizability (dry crystalline rock). IP is sensitive to the density of charge-storing interfaces (metal grain boundaries, clay surfaces, narrow pore throats) that leave no resistivity signature.

The physical basis of IP is charge storage, not charge transport. Resistivity methods measure the steady-state flow of current through a conductive medium; IP methods exploit the transient behavior when that flow is disrupted — specifically, the stored charge that dissipates after current is removed. Because charge storage depends on the density and geometry of interfaces within the rock, rather than bulk conductivity, IP detects features (disseminated sulfides, clay content, permafrost character) that appear invisible to resistivity. The two methods are complementary, and modern surveys often collect both simultaneously.