Subsurface Exploration - Geophysical Methods

Profiling

Lateral changes in resistivity by fixing the electrode separation and moving the whole electrode array between each resistivity

Electrical sounding

Vertical changes in resistivity are measured by progressively moving the electrodes outwards with respect to a fixed central point

Conventional site investigation

determining subsurface conditions by examining the material taken from various depths or continuously with depth at a number of points over the site INFO FROM CORE

Limitations of Geophysical Methods:

difficulty in correct interpretation when the strata are not well defined and horizontal - need to be confirmed

seismic limitations

- low velocity contrast -Velocity inversion (blind zone) -Hidden layer problem -Continuous increase in velocity with depth -WT -Ground-coupled air waves -Velocity anisotropy - noise

Electrical Resistivity Method

- extensive employment -based on measuring subsurface variations of electrical current flow manifested by an increase or a decrease in electrical potential between two electrodes -map lateral and vertical changes in geological materials - Also to: asses quality of rock/ soil masses in eng terms det WT depth map saline/ fresh water interface econ deposits of sand and gravel locate buried features - quantitative interpretation with curve matching techniques

Seismic methods

- seismic waves, measure time, with geophone - seismic E generated with sledge hammer and plate, falling weight, detonator, and explosives - receptor= geophones - 2 types of info: continuity and location of discontinuity elastic modulus and degree of fracturing from velocities - relies on the differences in velocity of elastic or seismic waves through different geological or man-made materials (P WAVES) - continuity, rock condition, geological properties, RM deformability - combined travel times - quantitative depth determination and profiling - rockhead= nuisance -qualitative info

GPR

-Ground penetrating radar -electromagnetic sounding method which uses radio frequencies -transmission antenna sends electromagnetic pulses through a medium and the receiving antenna records the waves reflected back from the electric interfaces in that medium while rest proceeds fwrd - in surface of inside it -effective tech, locating non conductive utilities and UG anomalies -CS or subsurface utilities - #D in right conditions , data to estimate depth

4 types of elastic waves

-Longitudinal waves (P-waves): fastest, Ground motion in same direction of propagation. -Transverse or Shear Waves (S-Waves): ground motion is perpendicular to the direction of propagation -Rayleigh Waves: along the surface only -Love waves: horizontal and transverse

Magnetic methods

-based on the measurement of local variations in the Earth's magnetic field -data cannot be readily interpreted in a quantitative method - useful to locate features of interest :abandoned mine shafts, sink holes, and buried services

steps for implementation

-geo assesmet of RM conditions -min 2 insitu for rock deformability -stress field in test A -continuity of RM conditions - RQD essential

Advantages of Geophysical Methods:

-large As at low C during reconnaissance - not hampered by boulders or coarse gravel

Gravity Methods

-lateral changes in the Earth's gravitational field -ambiguous data are common and hence the interpretation of field data is qualitative -rapid near surface changes in density limit its use for large scale structures - measured values are function of density, lat, elevation, topo, tidal effect can be id and corrected

2 main paths of seismic energy

-refracted or head wave beneath interface btw 2 rock types MOST IMPORTANT, P waves diff materials and velocities when less than critical angle -reflected wave from the interface btw the two rock types depth analysis accurate profiling geo structures adv of permitting mapping of horizons from 1 shot, not decrease with depth

Geophysical methods

allow subsurface conditions to be examined indirectly, providing information of continuous nature INFO FROM AREA

seismic method equipment

energy source, geophones and recording device (seismograph) geophones at regular intervals in straight line

electrical resistivity

resistance offered by a unit cube of that material to the flow of electrical current between two opposite faces - Rocks and soils conduct electricity by electrolytic conduction w/in water in pores, fissure, joints

P wave velocity include

▪ Well shooting: borehole is drilled and accurately logged ▪ Cross hole shooting: two boreholes to be drilled and accurately logged ▪ Continuous velocity logging: use of one or two pulse generators and two or four detectors

Corrections of gravity method

◼ Latitude correction ◼ Elevation correction ◼ Free air correction ◼ Bouguer correction ◼ Terrain correction ◼ Instrument drift and tidal correction

Errors in apparent resistivity measurements c

➢ Electromagnetic coupling between potential and current electrode cables ➢ Interference from high (or low) tension electrical cables or electrified railway lines ➢ Highly heterogeneous ground