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Earth Observation

 

 

Earth Observation in the frame of EO-MINERS - Earth Observation methods

Geophysical

Geophysics is the practical application of physical methods to determine the properties of rocks, and in particular, to detect physical differences between different geological materials, whether in their natural context or worked over by man. Many of these methods were originally developed for exploration purposes, but can also be applied e.g. in a mine site remediation context.

Most of the techniques, in principle, give point measurements, but can be combined to construct subsurface profiles or even three-dimensional representations of the distributions of the properties of interest. Modern computing technology allows the construction of tomographic images and three-dimensional representation.

The main geophysical methods include:

  • Seismic techniques
  • Gravity techniques
  • Magnetic techniques
  • Geoelectrical techniques.
  • Electromagnetic techniques
  • Radar techniques
  • Radiation techniques

 

Some of these techniques can be deployed from the surface or in boreholes, commonly called borehole geophysics.


Seismic techniques - seismic waves can be used in different ways for analytical purposes. An energy source for generating the waves is needed, either explosives charges, compressed air or a mechanical vibrator (a lorry with a heavy weight that is agitated by the engine, VibroseisTM). Different types and frequencies of waves are generated, shear waves, compression waves and head waves. The waves are reflected or refracted at interfaces between geological materials of different density. The waves are detected by seismometers or geophones. These are usually layed out in profiles. Analysis of the arrival times of the different types of waves at the various geophones allows conclusions as to the location of reflecting or refracting interfaces in depth. Advanced numerical processing techniques nowadays allow the construction of tomographic images of the detected underground structures. The penetration depth and spatial resolution depends on the type of energy source and number of geophones, and can range from a few metres to several kilometres. Sensitive site properties, such as built-up areas or unstable slopes, may put restrictions onto the energy sources and their strength that may be employed.

SONAR (in waters) and SODAR (in air) are acoustic techniques that follow the same physical principles as seismics, but are more commonly considered remote sensing techniques. Scanning SONAR techniques can produce 3-D maps of e.g. the sea or lake bottoms and are used to identify objects at the bottom of surface waters.

Seismic reflection diagram Seismic refraction diagram
The principles of the seismic reflection (left) and refraction (right) methods (from www.geologicresources.com).

 

Gravity techniques - The gravity field of the earth is locally distorted by the distribution of rocks of different densities. The local gravity is determined as the deviation from the theoretical gravity ellipsoids. Correction factors to be considered inter alia are the topographical elevation, the surrounding topography, the relative position of sun and moon, etc. Gravity can be modelled by a tensor and the components of gravitational acceleration can be measured with a variety of instruments. The classical instrument is a cast iron ball that is suspended from springs in a frame. The elongation of the springs can be measured with high accuracy and thus the gravity tensor be calculated. Modern gravity gradiometers use a variety of techniques to measure acceleration in space and can be deployed inter alia airborne, on satellites and in boreholes. Gravity measurements are useful over a wide variety of scales, ranging from determining the thickness of the Earth's crust beneath mountains, determining the thickness of sedimentary graben structures, to detecting voids underground such as tunnels and bunker

Gravity diagram
The principle of gravimetric measurements (from media.tiscali.co.uk).

 

Geoelectrical techniques - The resistance of soils and rocks as measured between two electrodes is a function of the water contents, porewater salinity, conducting minerals and conducting man-made features. As the application of a current between two electrodes results in polarisation and electrolysis, the measurement are typically carried out in four-electrode arrangements. The outer electrodes are used to apply the current, while the inner electrode pair detects the response of the material. However, the decay of induce polarisation (IP) can also be detected and utilised. Electrodes are typically staked out in a line of several or many electrodes. Different quadrupoles can be connected for individual measurements, giving different resolutions along the line and into depth. Repeating the measurements along different lines or staking out the electrodes on a grid allow the construction of tomographic images of the resistivity distribution over a particular soil or rock volume. Since only relative resistivities are measured, identification of particular material requires the determination of specific conductivities on samples recovered from boreholes. Resistivity measurements can also be performed in boreholes. In this case either probes with several electrodes are driven into the ground or the electrode quadrupole is moved down and up a borehole.

geoelectric resistivity measurments diagram
The principle of geoelectric resistivity measurments (from www.arctic-geophysics.com)

 

Electromagnetic techniques - Coronal mass ejections (CME) on the sun and lightning induce telluric currents in the Earth's crust. These currents deform the Earth's magnetic field and the deformations can be recorded by magnetometers. The induced currents cover a broad spectrum of frequencies, ranging from a few thousandth of a Hz to tens of kHz. The penetration depth of the currents depends on their frequencies. Due to the skin effect, higher frequencies are more suitable to near-surface investigations, while lower frequency currents may penetrate thousands of kilometres. The interpretation of the signals is similar to the ones for magnetics. The method is mainly used to identify high electrical resistivity zones, such as hydrocarbon filled sedimentary rocks, or low resistivity zones, such metal ores.

Telluric currents can also be induced by a conductive loop and the decay of the induced magnetic field be observed. This method of transient electromagnetics can be used to detect underground conducting or magnetic features, such as metal objects, or with higher powered systems ore bodies at depth of up to several hundred meters. Systems can be deployed hand-held, on board of vehicles or airborne using helicopters.

aeromagnetic surveys diagram
The principle of aeromagnetic surveys (from www.bgr.bund.de)

 

Radar techniques - As in  conventional radar applications electromagnetic radiation in the microwave bands is sent into the ground and signals reflected by object or interfaces are detected. The strength of the reflected signal depends on the dielectric impedance between the reflector and the surrounding geological material. Higher frequencies/ shorter wavelengths give better spatial resolution, while the opposite gives deeper penetration at the expense of resolution. Geological materials with good electrical conductivity allow deeper penetration. Modulating the signal duration and frequency increases the analytical scope. A good contact of the antennae increase the efficiency of ground penetrating radar (GPR), but the technique can also be deployed airborne. The technique is typically used to map underground structures and detect buried objects, such as pipes, drums or unexploded ordnance. It is also a commonly employed borehole geophysical technique to map permeability. Surface investigations proceed in profiles that can be later processed into tomographic images of the reflecting surfaces and interfaces.

ground-penetrating radar image
The principle of ground-penetrating radar (from www.malags.com).

 

Radiation techniques - Many geological materials contain radionuclides in varying concentrations. The radionuclides may be target themselves, e.g. when prospecting for uranium, or the radionuclide content may be used to distinguish between different rock types. Releases of the radioactive gas radon can also be used to map gas-conducting underground features. Numerous techniques to detect ionising radiation exist. In field applications either gaseous ionisation detectors (Geiger-Müller counters, proportional counters) or solid-state detectors (scintillation counters, semi-conductor detectors, Cherenkov detectors) are used. In principle all of these can be deployed from all platforms, but size and other technical requirements impose certain limitations. Thus, for instance, scintillation counters using large NaI crystals and the need for cooling with liquid nitrogen make their airborne application impractical.

Gamma ray logging is commonly used method to distinguish between different geological formations in boreholes. The gamma ray signal varies according to the respective concentrations of 40K, U or Th. In oil exploration larger signals often indicate clay-rich and, hence, less permeable formation as often these radionuclides are associated with the clay content. This industry also uses man-made radiation sources (e.g. 60Co) to determine the bulk density of formations by the amount of Compton-scattering.

using airborne radiometrics
Mapping wetland contamination by U-tailings using airborne radiometrics, West Rand, South Africa.


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