(100e200 m above ground) avoiding excessive monsoon, peak summer, and foggy weather. The depth of penetration of an airborne survey will depend on the capacity of the instruments. In a ground survey, traverses are designed across a strike of the formations at a line interval less than the width of the expected causative body. Magnetic anomalies caused by shallow objects are more easily detectable than deep-seated targets. Airborne magnetic and geomagnetic surveys with advanced configuration systems, both in frequency and time domain, with high penetration capacity can identify deep-seated metallic bodies. Appli- cation of the system requires a considerable increase in bandwidth of both helicopter-borne frequency-domain electromagnetic and fixed-wing time-domain electromag- netic (TDEM) systems.
Depth estimates from aeromagnetic data can determine values for broad areas, such as the thickness of the sedi- mentary section in an oil and gas reservoir basin or at a limited number of points within the basin.
Interpretation of 2D and 3D isocontour maps of cor- rected magnetic data provides a qualitative existence of orebodies. The approximate location and horizontal extent of causative bodies can be determined by studying the relative spreads of the maxima and minima of anomalies. A comparison of gravity and magnetic interpretation of rich sulfide orebodies is given inFig. 6.14. Similar studies will be applicable for the Ni-PGE-Cu belt, Sudbury Camp, Canada.
6.5 ELECTRICAL SURVEY
6.5.2 Resistivity Method
6.5.2.1 DefinitionThe property of electrical resistance of a material is expressed in terms of its resistivity. It is defined as the resistance between opposite faces of a unit cube. Resistivity is one of the extremely variable physical properties of rocks and minerals. Certain minerals, native metals, and graphite conduct electricity via the passage of electrons. Most rock- forming minerals are insulators. Hard compact rocks are usually bad conductors of electricity. The electric current is carried through a rock by the passage of ions in pore wa- ters. Porosity and degree of saturation govern the resistivity of rocks. Resistivity increases as porosity decreases.
An artificially generated electric current is introduced into the ground and resulting potential differences (volts) are measured at the surface. Deviations from the back- ground pattern of potential differences indicate in- homogeneities and the presence of anomalous objects in the subsurface.
6.5.2.2 Electrode Configuration
Various electrode configurations are designed for field practices using different linear arrays. In a Wenner array, four electrodes (current, C1eC2, and potential, P1eP2) are kept along a straight line at an equal array spacing,“a.”In a Schlumberger array the distance between potential elec- trodes (2l) is small compared to the distance between the outer current electrodes (2L). In a dipole/dipole array
configuration the potential probes, P1eP2, are kept outside the current electrodes, C1eC2, each pair having a constant mutual separation,“a.”The current source is treated as an electric dipole if the distance between the two pairs,“na,”is relatively large (Fig 6.15).
6.5.2.3 Field Procedure
There are two types of resistivity surveying: vertical elec- trical sounding and constant separation traversing.
Vertical electrical soundingor electrical drilling re- tains current and potential electrodes along a straight line at the same relative spacing around afixed central point. It presumes that current penetrates continuously deeper with increasing separation of current electrodes. The electrical sounding infers variation of resistivity with depth from a given point on the ground for near-horizontal layers of formation below. The method is useful for determining loose horizontal overburden thickness over hard rocks in river valleys and groundwater projects.
Constant separation traversing is obtained by pro- gressively moving an electrode spread withfixed electrode separation along a traverse line, the electrodes’configura- tion being aligned either in the direction of traverse (lon- gitudinal) or at right angles to it (transverse).
6.5.2.4 Resistivity Survey Instruments
Resistivity survey instruments measure very low levels of resistance with high accuracy. Most resistivity meters
FIGURE 6.15 Schematic diagram showing common types of electrode configuration in resistivity surveying: (A) Wenner, (B) Schlumberger, and (C) dipole/dipole.
employ a microprocessor-controlled low-frequency alter- nating current source (between 100 Hz for shallow probes around 10 m and less than10 Hz for 100 m penetration).
The direct current source along with nonpolarizing elec- trodes is suitable for greater depth penetration of hundreds of meters in favorable ground conditions. The unit of re- sistivity is the ohm-meter (ohm m orUm).
Electrical resistivity equipment is light, portable, inex- pensive, flexible, and available at minimal fixed expense.
The qualitative interpretation of the data is rapid and straightforward.
6.5.2.5 Applications
The resistivity survey was not initially favored during reconnaissance due to the slow process of manually planting electrodes before each measurement. This restric- tion could be complemented by increasing the availability of noncontacting conductivity measuring devices (electro- magnetic survey). This is widely used in hydrogeological
investigations covering subsurface structure, rock types, and water resources for drilling, engineering geological investigation sites before construction, and exploration of sulfide deposits (Fig. 6.16).
6.5.3 Induced Polarization Method
6.5.3.1 DefinitionInduced polarization is an imaging technique that identifies electrical chargeability of subsurface materials (orebodies).
The electrochemical voltage does not drop to zero instantly when externally applied direct current, connected through a standard four-electrode resistivity spread, is switched off abruptly (t00). The voltage dissipates gradually to zero at t03 after many seconds with a large initial decrease (Fig. 6.17).
Similarly, the initial voltage jump at t0and a slow incre- ment take place over a time interval (t1 e t2) before the steady-state value is reached (t3) when the current is switched on. The ground acts as a capacitor, storing
FIGURE 6.16 Geophysical interpretation of self-potential, induced potential, and resistivity survey confirmed by drill testing of rich sulfide orebody in Rajasthan.
electrical charge, and becomes electrically polarized (t01to t02). The measurement of a decaying voltage over a certain time interval is known as a time-domain induced potential survey. Alternatively, if a variable low-frequency alternate current source is used for measurement of resistivity it is observed that the measured apparent resistivity of subsur- face rocks decreases as the AC frequency is increased. This is known as a frequency-domain induced potential survey.
6.5.3.2 Induced Polarization Measurement The induced polarization method works on externally imposed voltage causing electrolyticflow in the porefluid of rocks. Thus negative and positive ions build up on either side of minerals and are charged during imposed voltage.
The minerals return to their original state over a finite length of time on removal of the impressed voltage causing gradual decay of voltage. Metallic minerals, if present, become additionally charged during external currentflow and the voltage gradually decays on switching off the source. This effect is known asmembraneorelectrolytic polarization in the case of nonmetallic minerals and electrode polarization or overvoltage in the case of metallic minerals. Metallic sulfides, oxides, and graphite are good conductors and respond better to this effect. The magnitude of electrode polarization depends on both in- tensity of impressed voltage and concentration of conduc- tive minerals. The ubiquitous disseminated sulfide orebody provides a larger surface area available for maximum ioniceelectronic interchange, and hence is extensively suitable for induced polarization surveys.
The measurement of decaying voltage (M), shortly after switching off the polarization current, is the area“A”of the decay curve over a specific time interval (t01et02). The area
“A” is determined within the measuring instrument by analog integration. The measured parameter is charge- ability. The unit is milliseconds (ms). Different minerals
are distinguished by unique chargeability, such as pyrite (13.4 ms) and magnetite (2.2 ms).
M¼A/V orDV/V
6.5.3.3 Applications
The induced polarization method is extensively and effec- tively used in base metal mineral exploration (Fig. 6.16) for locating low-grade ore deposits, e.g., disseminated sulfides.
It has other applications in hydrogeophysical surveys, environmental investigations, and geotechnical engineering projects.
The use of induced polarization in groundwater explo- ration is growing in prominence. The induced polarization sounding (time-domain) method was applied near the Sauk- Soo River area in Crimea, Ukraine. The alluvial deposits in the river basin area clearly distinguished three horizons by their polarizabilities. The section consisted of a top layer of weak polarizability that represents a dry loamy layer; a second middle layer of strong polarizability representing a clayey sand layer saturated with fresh water; and a third lower layer of weak polarizability that represents imper- vious siltstones.
6.5.4 Self-Potential Method
6.5.4.1 Definition and MechanismSelf-potential or spontaneous polarization is based on natural potential differences resulting from electrochemical reactions in the subsurface. The process is unique because it is passive, nonintrusive, and does not require the applica- tion of an electric current. The causative body has to exist partially close to the water table to form a zone of oxida- tion. The electrolytes in the porefluids undergo oxidation and release electrons that move upward through the ore- body. The released electrons cause reduction of electrolytes at the top of the orebody. An electronic circuit is thus created in the orebody so that the top of the body acts as a negative terminal. The self-potential anomaly is invariably centrally negative over metallic ore deposits (Fig. 6.16).
The bulk of the orebody exists below the water table, en- dures no chemical reaction, and simply transports electrons from depth generating stable potential differences over long periods at the surface.
6.5.4.2 Equipment and Field Procedure
Standard self-potential field survey equipment utilizes a pair of nonpolarizing porous pot electrodes, connected by insulated cable via a millivoltmeter. The electrodes are composed of simple metal spikes immersed in a saturated solution of its own salt (Cu in CuSO4) in a porous pot that
FIGURE 6.17 Schematic diagram elucidating the principles of the induced potential method of survey by means of voltage versus time curve passing through conductive mass.
allows slow leakage of solution into the ground. The measurements of potential differences are conducted by shifting successive electrodes or byfixing one electrode at the base station in barren ground (up to 50 mV) and by moving the second mobilefield electrode in steps across the expected target area (1000 mV). The spacing between electrodes is 10, 20, and 30 m apart. Typical spontaneous polarization anomalies may have amplitudes of several hundred millivolts with respect to barren ground, and exceed several thousand millivolts over deposits of metallic sulfides, magnetite, and graphite.
6.5.4.3 Applications
The self-potential method is traditionally used:
1. As a mineral exploration tool for massive sulfide orebodies.
2. For downhole logging in the oil industry.
3. For hydrogeological investigation, and assessing the effectiveness of water-engineering remedial measures.
4. To identify seepage in dams and reservoirs.
5. Forfinding leaks in canal embankments.