The techniques of geophysical exploration involve the remote sensing of some physical property of the ground using instruments, which in most cases remain on the ground surface; therefore non-intrusive investigation. Passive methods accurately measure ground properties and search for minute anomalies (local distortions within the overall pattern).
These include gravity and magnetic surveys (and radioactivity, which is limited in applications for ground investigation). Induction methods send a signal into the ground and pick it up again nearby. These include seismic, electrical and electromagnetic surveys; latter include GPR (radar), NMR (nuclear magnetic resonance) and others.
USES IN GROUND INVESTIGATION
Geophysics is low cost compared to multiple boreholes. It can be cost-effective in ground investigations within certain terrains – where anyone, or more, individual types of a geophysical survey may be appropriate; there is no single geophysical system applicable to all problems. It is essential to select the best method for any given situation, so it is often beneficial to take advice from a consultant who is independent of commercial operators of the individual methods.
Geophysical surveys have three main applications in ground investigations: • Reconnaissance assessments; • Filling in detail between boreholes; • Searching a large area for anomalies before drilling.
Modern geophysical techniques produce increasingly successful data that have been applied effectively to various specific problems in the ground investigation. • Search for unknown cavities: GPR if depth >10 m, or gravity survey if cover depth the expected diameter. • Search for suspected mine shafts: a magnetic survey. • Trace lateral contrasts, notably between sand and clay, in shallow drift: GPR, electromagnetic survey. • Rockhead profiling between boreholes: refraction seismic survey. • Estimate rock fracturing ahead of new tunnel drive; seismic tomography is optimum if site conditions allow.
Interpretation of geophysical data invariably requires boreholes, to calibrate profiles or to test-drill anomalies. Magnetic searches for buried mineshafts are simple enough for operation and interpretation by untrained personnel with low-cost rented equipment. All other geophysical surveys require operation and data interpretation (only possible by computer inversion of the raw data) by specialists who are working as part of a ground investigation team.
GEOPHYSICAL SURVEY COSTS
Comparisons of costs are tenuous because each method is best applied to only certain ground problems. Unit costs decline on larger surveys because set-up costs are applicable only on the first day.
Very rough guide is given by the coverage achievable within a day at a cost of about £2000 at 2008 prices: Microgravity survey 0.25 ha on 5 m grid Magnetic survey 1.5 ha on 1 m line spacing Electromagnetic survey 2.0 ha on 2 m line spacing Ground probing radar 1.0 ha on 1 m grid Seismic refraction 8 spreads along 1 km Seismic tomography 2-D profile to 40 m depth (between available boreholes) Borehole 1 cored hole 20 m deep
Non-contacting terrain conductivity meter creates an electromagnetic field in the ground and measures field distortion some metres away; low-cost equipment, simple to use, similar in principle to a large metal detector. Measures mean conductivity in a hemisphere of the ground reaching to depths of 6–30 m. Can be continuous reading, two-man operation.
High conductivity of clay, basalt and water, contrasts low conductivity of sand and limestone. Can use to map shallow lateral changes: clay-filled fissure zones, filled sinkholes, rockhead steps, alluvial channel fills, highly-permeability fracture zones.
GROUND PROBING RADAR (GPR)
Trolley-mounted transmitter and receiver record the microwave electromagnetic radar signals reflected from ground contrasts. A ground cross-section is produced as computer output; outputs are complicated by reflection interference, but some can be realistic displays. Calibrate depth and materials with borehole. Limited depth penetration is the main restriction: 10–20 m in dry sand, only a metre or so in wet clay. Can tow-behind car at 5 km/h for a continuous profile. Can use to map shallow drift profiles, filled sinkholes, shallow voids.
Numerous methods, both contacting and inductive, are applied successfully to mineral exploration and are also used widely in ground investigations. Resistivity or conductivity surveys, generally with arrays of four ground electrodes, can be used to map lateral and vertical changes in ground conditions. Require specialist interpretation with computer inversion.
Record distortions of Earth’s magnetic field. Proton magnetometer measures total field; low cost, robust equipment. Measures to 1 nanotesla. Simple to use, 10 seconds per station, one-man operation. Dipole anomalies, easily recognized, lie over vertical linear features, e.g. buried mine shafts. Unlined shafts with a fill of wall rock may go undetected. Fences drains, power lines, iron-rich fill prohibit use.
Record minute variations in Earth’s gravitational force. Gravimeter measures the length of an internal weighted spring; high-cost, very delicate instrument. Measures to 0⋅01 gravitational unit. Ten minutes per station, one-man operation. Negative anomalies are created by underground voids (caves or mines) or low-density soil or rock (in buried valleys or sinkholes); both significant to engineering.
The limit is set by background noise, but microgravity surveys with computer analysis of closely spaced data points can trace small mines to depths of 20 m, and larger limestone features too much deeper. Depth and size of void may be interpreted from the shape of an anomaly, but normally drill all negative anomalies.
Shock waves, produced by hammer blows or explosions, are reflected or refracted at geological boundaries. Reflection Seismic waves reflected from deep strata boundaries. Successfully used for all oil exploration. Some applications in a shallow ground investigation. Refraction Seismic waves refracted at shallow geological boundaries and returned to surface.
Drop-hammer or 3 kg sledgehammer adequate for 20 m penetration; deeper with explosive shock source; small geophones detect wave arrivals; low-cost equipment and two-man operation. Refraction relies on the faster layer at depth: rockhead is an ideal boundary to detect, with slow soil over fast rock. Graphical plot of first wave arrivals reveals depths to boundaries and velocities (hence strength properties).
Surface Wave Seismic Vertical Rayleigh waves are used to assess variations in ground stiffness at successive depths to about 50 m. Seismic velocity (speed of shock wave through rock) increases with the strength of rock and decreases with more fracturing – related to RQD.
Rock or soil Seismic velocity (P wave)
Drift and soil 500–1500 (m/s)
Shale and sandstone 1500–4000
Fractured rock V (unfractured) x RQD/100
Transmitters and detectors in adjacent boreholes acquire data for intervening ground. With signals to and from all combinations of depth positions integrated by computer, a tomographic image of the ground properties is generated, in 2-D profile between two boreholes, or in the 3-D model between multiple boreholes. Mainly used with seismic signals or electrical resistivity, and can produce spectacular results that identify voids or anomalies within the unseen ground.