Seismology is the science of earthquakes and studies the causes and effects from minute pulsations to the most catastrophic natural phenomenon inside Earth. The methods are classified into two divisions based on energy source of the seismic waves. Earthquake seismology is caused by natural shock waves of earthquakes and derives information on physical properties, composition, and the gross internal structure of Earth.Explosion seismologyis the product of artificial blasts: (1) detonating a charge of dynamite (land) and (2) nonexplosive vibroseis or com- pressed air (marine) at selected sites to infer information about regional/local structures. This is extensively being applied to interpret the interfaces of rock boundaries, layered sedimentary sequences, location of water tables, and oil and gas exploration.
The survey works on the mode of propagation of waves in elastic media; more precisely, travel in rock media. The subsurface unit is assumed to be homogeneous and isotropic to simplify wave propagation resulting in basic interpreta- tion of the measured effects at the plane of discontinuity.
TABLE 6.1 Geophysical Surveying Methods With Parameters and Properties Suitable for Type Deposits Method Measured Parameters
Operative Physical
Property Suitable Deposit Type Seismic Travel time of reflected and
refracted seismic waves
Density and elastic mode Coal, oil and gas, groundwater, layered sedimentary basin, NieCuePGE in volcanic basal flows (Dentith and Mudge, 2014) Velocity, acoustic
Gravity Gravity (measuring variation in Earth’s gravitational field)
Density contrast between the surrounding host rocks
Massive sulfides, chromite, NieCuePGE, salt domes, barite, kimberlite pipes, concealed basins
Magnetic Measuring spatial variation in Earth’s magnetic field
Magnetic susceptibility Magnetite-, ilmenite-, pyrrhotite-rich sulfides, NieCuePGE
Electrical
1. Resistivity Earth’s resistance Electrical conductivity Groundwater, sulfide, NieCuePGE 2. Induced
potential
Polarization voltage/frequency development of ground resistance
Electrical capacitance Large sulfide dissemination, NieCuePGE, graphite
3. Self-potential Electrical potential Electrical conductivity Sulfide veins, graphite, ground water, NieCuePGE
Electromagnetic Response to electromagnetic radiation
Electrical conductivity and inductance
Sulfide, CreNieCuePGE ore, graphite
Radiometric Gamma radiation Gamma ray Thorium, uranium, radium
Borehole geophysics and mise-a`-la-masse
Downhole probe All types Continuity of sulfide, NieCreCuePGE in
strike and depth
PGE, platinum-group elements.
6.2.2 Stress and Strain
The propagation of seismic waves causes redistribution of internal forces and deformation of geometrical shapes within a rock mass. The concepts of stress and strain are a result of these changes.
The internal forces appear inside a body and simulta- neously try to recover the original configuration when it is deformed or strained. This balancing internal force or restoring force per unit area across a surface element“A” within the material created due to deformation is called stress. Stress is the force per unit area acting on a plane within a body.
Stress¼(Internal or restoring force)/(Area)¼F/A The unit is dyne/cm2.
Stress isnormalwhen F is perpendicular to the plane of the area element. Stress istensileorcompressivedepending on its direction from or into the object on which it acts.
Stress isshearingwhen F is tangential to the area element.
Strain is the fractional change in length, area, and volume associated with deformation of Earth by tectonic stresses or passage of seismic waves. Strain is defined as the fractional changes in dimension being deformed per unit original dimension. It is unitless.
Strain¼(Change in dimension)/(Original dimension) The strain that causes only a change in shape with no change in volume is called ashear or distortion strain. A change in volume without a change in shape isdilatation or contraction strain. The strains that are associated with relative changes in length in the directions of respective stresses are called normal stress.
6.2.3 Elastic Moduli
The elastic properties of a material are described by certain clastic constants that quantitatively specify the relationships between different types of stress and strain. The velocity of an elastic wave traversing in homogeneous medium de- pends on a number of factors: Young’s modulus, bulk modulus, rigidity or shear modulus, and Poisson’s ratio.
Young’s modulus (E) is the ratio between longitudinal stress and longitudinal strain.Bulk modulus(K) is the ratio of uniform compressive stress to fractional change in vol- ume.Rigidity stress(m) is the measure of the stress/strain ratio in the case of a simple tangential stress. Poisson’s ratio (s) is a measure of the geometrical change in the shape of a clastic body.
6.2.4 Seismic Waves
Seismic waves from natural or artificial sources propagate outward as pulses. There are two groups of seismic waves:
(1)body waveand (2)surface wave.
6.2.4.1 Body Wave
Body waves propagate through the internal volume of an elastic solid medium, and there are of two types:
compressional wavesandshear waves.
Compressional waves (longitudinal, primary, P-waves of earthquake seismology) are the fastest of all seismic waves.
They propagate by compressional and dilatational uniaxial strains in the direction of wave travel through solid, liquid, and gas media. Body waves cause the compression of rocks when their energy acts upon them. A rock expands beyond its original volume when P-waves move past the rock, only to be compressed again by the next P-wave (Fig. 6.3A).
Shear waves (transverse, secondary, S-waves of earth- quake seismology) carry energy through the earth in a very complex pattern. The particles of the medium vibrate about their fixed mean position in a plane perpendicular to the direction of wave propagation. They move more slowly than P-waves and cannot travel through the outer core because they cannot exist in fluids, e.g., air, water, and molten rock (Fig. 6.3B).
6.2.4.2 Surface Wave
Surface waves (Rayleigh and Love waves) travel only along a free surface or along the boundary between two dissimilar solid media. Rayleigh waves are formed when the particle motion is a combination of both longitudinal and transverse vibration giving rise to an elliptical retro- grade motion in the vertical plane along the direction of travel.Love wavesare a major type of surface wave having a horizontal motion that is shear or transverse to the direction of propagation.
The velocity of propagation of any body wave in any homogeneous, isotropic material is determined by the elastic moduli and densities of the material through which it passes. The traditional seismic survey uses only
FIGURE 6.3 Propagation of (A) compressional or primary and (B) shear or secondary waves showing type of elastic deformation of ground particles.
compressional waves due to easy detection of the vertical ground motion in the detectors that becomes fast because of high-speed wave velocity. The recording of stress and surface waves provides greater information about the sub- surface, but at a cost of greater data acquisition and consequent complex processing. The multicomponent sur- vey is becoming more popular and useful.
6.2.5 Seismic Reflection and Refraction Method
Seismic reflection and refraction are frequently practiced methods for mapping subsurface structure in sedimentary formation in connection with coal, oil, and gas exploration.
They follow the laws of reflection and refraction of optical waves in contact with two different media. Similarly, P- and S-seismic waves move uniformly from the source
and reflect and refract on the boundary of a second medium with different elastic velocity. The energy is partly reflected and partly transmitted in a second medium (Fig 6.4).
A wave travels from source“S”and reflects at a point
“R”of the interface at a thickness of“h1”and arrives at the geophone“G”at a time interval of“Tx.”The velocity“V1” of the upper layer and depth “h1”to the interface can be obtained mathematically by recording the reflection times at two distances (x, x0).
The information obtained by a single reflected pulse at one detector position is not enough to establish the ex- istence of a reflecting horizon. In practice, stepwise shifting of the entire shot-geophone with a series of multitrack geophones placed at short intervals is used. A continuous mapping of the reflecting horizon is possible in this way (Figs. 6.5e6.7).
FIGURE 6.4 Method of seismic reflection profiling by time versus distance curve at media interface.
FIGURE 6.5 Multichannel seismic profiling between central shot and multiple detectors on either side.
The generation of artificial seismic waves involves explosion of a dynamite charge in a hole or a weight dropping. The truck-mounted mechanical vibrators (vibro- seis) are used to pass an extended vibration of low ampli- tude into the ground with continuously varying frequency between 10 and 80 Hz. In marine seismic studies an electric or gas spark or air gun shot is used as energy source. The seismometerorgeophoneis a device to detect and receive seismic ground motion. It is an electromechanical device that converts mechanical input (seismic pulse) into elec- trical output, and finally produces a continuous graph (seismograph). The modern seismic survey simultaneously records ground motions received from all directions due to combinations of transverse and longitudinal waves by a three-component geophone (Fig. 6.8).
6.2.6 Applications
The seismic survey can explain subsurface discontinuities, layering, and probable rocks/structures. It is suitable for the investigation of coal, oil and gas, groundwater, and massive metallic deposits. A 3D seismic survey outlined the basin configuration and resource estimate at Krishna- Godavari Basin, India, and a 2D seismic section map- ping and establishment of a major structure for basement faults was applied successfully at Zeegt lignite coal mine in Mongol Altai coal basin. The other areas covered in metallic minerals are Munni Munni platinum-group element (PGE) deposit, Australia, Kevitsa NieCuePGE deposit, Finland, goldfields of Witwatersrand Basin, South Africa, and Bathurst zinc-gold Mining Camp, Canada. The oceanfloor, otherwise unknown, was mapped precisely by marine seismic survey in the mid-20th century. The Mid- Atlantic Ridge at an average water depth of 5 km, and deep oceanic trenches in the Western Pacific, were discovered.
FIGURE 6.7 Schematic diagram of marine base underwater seismic reflection profiling.
FIGURE 6.6 Seismic reflection profiling of subsurface geological formation.
FIGURE 6.8 A three-component vertical 14 Hz“geophone”device for detection and recording of seismic reflection and refraction from subsur- face interface.