Potenzialverfahren
Geomagnetic measurements at Conrad Observatorium © Lammerhuber

Potential techniques are methods employed in the interpretation of conservative fields. They are used in gravimetry, magnetics and DC geoelectrics. Conservative fields can be described by the same mathematical formalism (Laplace or Poisson differential equations) irrespective of their source. The problem of ambiguity is inherent to all these methods, which means that from the field distribution it is impossible to unambiguously derive a source distribution. The interpretation therefore relies on a combination of other geophysical techniques (e.g. seismics) or geological reports.

Magnetics

Magnetics register anomalies in the Earth’s natural magnetic field. In a first approximation, this field can be treated as a dipole field onto which short-term variations originating from the ionosphere are superimposed. All materials experience an induced magnetisation in the Earth’s field depending on their magnetic susceptibility. The material’s magnetic field superposes the Earth’s field and thus creates an anomaly on the natural field, which can be very large for ferromagnetic materials. Measuring these anomalies using the appropriate equipment (magnetometers) allow magnetic bodies and objects to be found, delineated and modelled (position, depth, shape).

Magnetics is an aid for geological mapping, prospecting of reservoirs, finding and delineating landfill sites, locating ferromagnetic objects (barrels, tanks, ammunition, unexploded bombs, cables) and other underground structures and thus for archaeological investigations.

Measurements of the magnetic field are conducted using a magnetometer. Today, electronic or atomic systems are employed which either allow the absolute value (total intensity) of the field vector or its components to be registered. The unit of the magnetic flux density is Tesla (1T = 1Vs/m2). For historic reasons, the values are typically given in nano-Tesla (nT) e.g. magnetic field strength at Austria’s latitude is 48 000 nT.

An improved resolution can be achieved using gradiometers, where two magnetometers are used in a (usually vertical) spacing of 0.5m to record the difference in field values. Gradiometers therefore measure the approximate vertical gradient (of the vertical component) of the Earth’s magnetic field.

Gravimetry

Gravimetry is a discipline based on the Earth’s gravitational field. The basic research is concerned with the regional distribution of the gravitational field as well as the changes with time over all spatial (local, regional, global) and temporal (short- or long- term, secular, periodic) scales. Their interpretation contributes to the verification of global models (Earth, ocean) as well as to the understanding of dynamic processes (mass transports) on the Earth’s interior, in the oceans and the cryo- and atmosphere. The relevant physical parameters are similar to those in seismology: density, Lamé constants and viscosity. There are close links to physical geodesy and satellite geodesy, which today are able to provide information on the gravitational field to a great degree of accuracy.

In the field of applied research, the focus is placed on structural exploration of the lithosphere up to very small spatial sources (micro-gravimetry), which is of special interest during the prospection for raw materials. Micro-gravimetry is also used as a method for locating cavities non-destructively. The interpretation in this case is also based on methods of potential theory. The inherent ambiguity of potential field techniques can be overcome with the help of an interdisciplinary combination of other geophysical methods (especially seismic) or neighbouring disciplines (e.g. geology, mineralogy, petrology, rock geophysics). The basis of the quantitative interpretation is the Bouguer anomaly, which maps the density inhomogeneities inside the Earth. For Austria there is currently an updated and newly processed gravitational map available.

Modern instruments (superconducting gravimeters) are capable of resolving gravity differences at an order of magnitude of approximately 10-10 of the gravitational acceleration (in the time domain) and even 10-12 in the frequency domain. Spatial differences can be determined using an absolute or spring-based gravimeter with an accuracy of 10-9 of the gravitational acceleration. Transportable relative gravimeters can be calibrated along a baseline, where the gravitational acceleration at stations has been determined using an absolute gravimeter. In Austria, this baseline runs along the Hochkar (Lower Austria). Absolute gravimeters and superconducting gravimeters are used in geodynamic research. Austria currently has one superconducting gravimeter at the Conrad Observatory (Lower Austria) for the registration of temporal gravitational changes and natural oscillations of the Earth; absolute measurements of gravity were conducted by the Federal Office of Metrology and Surveying.