Applied geophysics

The Applied Geophysics is that branch of geophysics , where all practical and economically be summarized meaningful process. Their common goal is to explore the rocks and layers of the earth's crust . An important sub-area is the exploration of deposits , which is called exploration . Applied geophysics operates at the interface between natural and engineering sciences .

Methodical overview

The Geophysics knows (like other geosciences ) a variety of methods - in particular because of the diverse shape of the earth, its rocks , underground structures , its fluids and escaping gases . The methods of geophysics are usually divided according to their 6–8 most important measurement and evaluation methods :

Potential methods - especially gravity and magnetic fields in the earth's crust

Wave method - exploration of the subsurface with seismic waves (natural and artificial earthquakes )

electromagnetic methods , especially geoelectrics and ground penetrating radar

radiometric methods

geochemical- physical processes and gas measurement

Geothermal

In-situ methods ( borehole geophysics and soil mechanical investigations)

Method groups 1 and 2 extend to great depths, but in principle deliver ambiguous results in each individual group (see also the inversion problem of potential theory ). The same often applies to group 3 and some in-situ procedures.

As a rule, however, gravimetry , magnetics and seismics complement each other and with the laboratory methods. In order to achieve unambiguous interpretations, as much geological data as possible is usually included - which is primarily done in outcrops and, among other things. a. the recording of the rock types encountered there , their density (approx. 2.0 to 3 g / cm³) and the position of their layers in space ( brushing , falling ).

Potential method

They use the peculiarities of physical fields (in the technical jargon of potential theory : vortex-free vector fields ) and their effects on the measuring points on the earth's surface . This allows you to determine differences in density or layers in depth:

Gravimetry

Exact measurements of the earth's gravity field (gravity and possibly also gravity gradients ) allow the localization of horizontal and vertical differences in rock density in the subsurface. The ascertained gravity anomalies indicate deposits , cavities, rock loosening ( rock appearance ), etc.
In theoretical geophysics gravity measurements are also used to determine the crust thickness , the large-scale structure of the shell Earth and its density compensation ( isostasy ).

The measurements are made with so-called gravimeters , which work on the principle of an extremely fine spring balance and are also used in geodesy ( earth measurement ). There are relative and absolute gravimeters , and in the past the Eötvös rotary balance was also used , which can measure horizontal gravity gradients .

Geomagnetics

A detailed measurement of the earth's magnetic field on the ground ("terrestrial") or from an airplane or helicopter (" aeromagnetics ") reflects the existence of magnetic and magnetizable rocks in the subsurface. In modeling the associated potential cross-connections are to gravity potential possible gravimetry.

Geoelectrics (see below)

Some of the geoelectric methods also work with potential fields , but are mostly combined in a separate group (see Chapter 4).

Wave method ( seismic )

allow the exploration of the earth's crust and possibly the earth's mantle with natural and artificial earthquakes. When these vibrations propagate, the mechanical waves are divided into

longitudinal waves ( shock waves , also called P waves)
transverse waves or shear waves (S waves)
and special wave types (e.g. waves guided at an interface, seam waves )

The reflection or refraction of the waves in the earth's interior allows conclusions to be drawn about its stratification, whereby the depth of penetration depends on the strength of the earthquake or blast. The measurement and computation effort is considerable, but it can (with certain uncertainties) deliver three-dimensional models.

seismology

Measurement and interpretation of natural earthquakes . These methods are more often used for general than applied geophysics. Since the 1920s it has been possible to determine the depth of the earth's mantle and core , and in recent decades it has also been able to determine finer subdivisions, especially in the upper mantle .

Geoseismics

Measurement and interpretation of artificial earthquakes (impact and explosive seismic ) and artificially generated vibrations. The earthquake waves are bent or reflected at the boundaries of geological formations when the density or elasticity of the rock changes there. A distinction is made between refraction seismics and deeper, but more complicated reflection seismics .

Geoseismics is especially important for the exploration (exploration) of oil and natural gas , because these hydrocarbons collect in typical, arched structures. The shock and shear waves are recorded by geophones that are laid out in profiles or planar and connected with long cables. The artificial tremors are triggered in different ways:

Blasting in deep, dammed-up boreholes

Vibroseis - heavy trucks with vibrating floor panels

Percussion hammer ; A heavy manual hammer with electrical contacts may be sufficient for relatively thin layers, such as for measuring the depth of glaciers or for looking at rock formation under sediments (“hammer-blow seismic”).

Electromagnetic process

Some of the geoelectric methods work with potential fields (see Chapter 2), but are usually combined in a separate group.

Geoelectrics

The measurement of natural and artificial electric fields mainly reveals changes in resistance . This can be used to determine subterranean stratifications and some rock parameters, as well as to explore water-containing strata ( groundwater and deep water) and pore structures . The methods can be broken down as follows:

natural currents and artificial currents ( electrical earth currents )
Magnetotellurics
VLF
Spectral Induced Polarization
Nuclear magnetic resonance (NMR, surface SNMR), see also Chapter 5

Georadar

The ground penetrating radar or "Ground Penetrating Radar" (GPR) is mainly used to locate smaller irregularities and metal-containing structures in the underground, for example when examining garbage dumps or already covered landfills , but also in archeology to find old foundation walls, etc.

Radiometry and radioactivity

(A brief description would have to be added.)

Geothermal energy

Measurement of geothermal heat or heat flow , interpretation with regard to thermal conductivity and temperatures in the subsurface.

Geochemical-physical methods

Geochemical distribution measurements
Methods with radon and other gases
... ...

In situ methods

Their measuring principles are partly identical to the methods listed above. A distinction is made between the measurements taken directly on the rocks:

Laboratory measurements on handpieces and samples

For quick and reliable assessment, " handpieces " are often picked up from typical rocks in the field, broken out of the rock or obtained by core drilling . An experienced geologist can already make important statements.

In the laboratory , important rock parameters are then examined more closely: specific density , pore and water content , modulus of elasticity , electrical resistance, grain size of the components, etc. When determining density - which can be decisive for potential methods and seismics (see Chapters 2 and 3 above) - must one can distinguish exactly between the mountainous condition and the dry density .

Borehole geophysics

In addition to the surface measurements include this measurement probes in boreholes, such as a density - Log , for electrical resistance for thermal conductivity and gamma radiation (see mass spectrometer ). Further, acoustic emission sensors and magnetometer used.

Aero geophysics

This group - which also uses the above-mentioned principles - includes all measurements from the air (aircraft, helicopter), with which the geophysical surface and borehole measurements are supplemented. Above all, aerogravimetry and aeromagnetics are frequently used in order to carry out large-scale initial examinations so that areas to be examined in greater detail can be separated out later.

In addition to the known geophysical legislation, special height and rotation corrections must be observed.

Cross connections

Almost all of the methods and groups of methods listed above have links to other geoscientific subjects. Examples are:

Geodesy - for gravimetry, potential theory and geodynamics
Hydrology - v. a. to geoelectrics
Geotechnics - seismics and soil mechanics
Meteorology - radon and other gases, temperature effects, etc.
Aeronomy and ionospheric research - for example in magnetics .

But the results of applied geophysics also have an effect on the other areas within geophysics - above all

on general geophysics , which concerns the structure and composition of the earth's body ,
the theoretical Geophysics which the potential of physical fields researched or the propagation equations of seismic waves , the question of the reference frames or the basis and used coordinate systems , as well as the cross-connection to astronomy manufactures
the experimental geophysics that the lab determined that rock parameters required by the geophysicist "in the field" to the interpretation of his measurements. Examples are the speed of sound of different materials, the modulus of elasticity , the specific density, the porosity and pore radius distribution, permeability , thermal conductivity and magnetic susceptibility .

All of these relationships make it easier for applied geophysics to successfully search for underground structures and site analyzes, as well as for deposits , deposits of water or ores . However, the many dependencies also complicate the theory and the software .

Civil engineers and organizations

Especially the sedimentary basins - on which the majority of humanity lives - are easily accessible to geophysics; For their applied research there has been a priority program in the FRG since 2002. Geophysical methods - along with other specialist areas - are also used to determine potentially suitable final storage sites for waste, nuclear waste and landfills . In practice, civil engineers often work independently and in cooperation with engineering geologists . Only in larger, mainly of research serving projects give institutes of higher education institutions , from colleges or departments of (country) - Governments set the tone.

The wide range of environmental protection has led many, especially younger geophysicists , to specialize in these newer fields. Almost all major construction projects are preceded by detailed investigations of the subsoil (stability, water conditions , etc.) and, more recently, methods of "agrogeophysics" in agriculture .

The different areas each have their own forms of organization on professional and regional levels - such as the technical tasks and resource exploration (see also geophysical prospecting )
The transnational study and research subjects are more likely during the IUGG (International Union of Geodesy and Geophysics) and its seven members of the association, which holds its general assembly every four years and brings together more than 5000 experts in large congresses . In between, several hundred conferences take place every year for special areas, for example within the framework of the European EGU and the American AGU .

literature

László Egyed : Solid Earth Physics , 370 p., Akadémiai Kiadó, Budapest 1969
Friedrich Bender: Applied Geosciences , Volume II: Applied Geophysics , 766 pages, Enke-Verlag, Stuttgart 1985