One of the effective tools used in exploration geophysics is magnetic surveying. It is based on the principle, that magnetic field of the Earth induces secondary magnetic field in ferrous rocks.
Measuring the magnetic field can help in searching of deposits with ferrous minerals and man-made iron and steel objects. The targeted structures or items need to be of detectable size and have a right orientation to the primary Earth’s magnetic field.
Measuring the gravity field is a non-destructive way of calculation the subsurface material density. Since gravity field is not the same in different parts of the Earth, it depends on the distribution of masses in subsurface.
Dense materials show stronger localized gravitation field.
How it works
Magnetometers can use only one sensor to measure total magnetic field. In this case in order to consider variations of magnetic field during the day (caused by magnetic storms in upper atmosphere) base stations are used. If magnetometer uses two or more sensors (such magnetometers are called gradiometers), it measures the gradient of magnetic field between those sensors. In such a case the usage of base station is not required.
All objects and different strata in subsurface cause local disturbances in Earth’s magnetic field. Measuring such disturbances with magnetometer can help in mapping geological structures or in delineating buried ferrous objects.
Different objects in survey area (such as buildings, fences, wells, underground utilities, etc.) can interfere with signals from target objects. Wearing magnetic objects during the survey can also contaminate data.
Shape of measured anomalies can tell several facts about the target object – how deep it is, its orientation, size, etc.
As a result of magnetic surveying, one can obtain various maps:
- maps of defined objects;
- contour maps;
- interpretation maps with anomalous objects’ delineation.
Gravity is the force of attraction between two bodies, that depends on their masses and the distance between them.
Specialized gravity meters are used in order to measure the Earth’s gravity field. A simplified gravimeter consists of measured mass suspended by spring. Placing the gravimeter over dense material results in expanding the spring due to additional force. For correct measurements the elevation should be known with good accuracy.
Several mathematical corrections are used, to correct for:
- the elevation of measurement point;
- spatial location of gravity meter;
- density of subsurface material;
- topography and tides.
Various maps can be a result of gravity surveying:
- contour maps of reduced gravity values;
- residual anomaly separation maps;
- final anomaly maps;
- inversion of the anomaly values to a causal geologic body.
Various rock types differ in density resulting as density anomalies.
- deposits detection;
- archaeological objects search;
- mapping of fractures, intrusions and other structures;
- lithological division of rocks;
- finding correlation between rocks based on their magnetic properties;
UXO and military echo objects search.
- regional and global tectonics tasks;
- geological mapping (detection of geological boundaries, cavities, subsurface karst features, subsoil irregularities, landfills);
- glacial studies;
- oil, gas and coal detection;
- bulk mineral deposits search (minerals, sands, gravels);
- exploration of underground water supplies;
- environmental and engineering tasks (location of buried voids, mine shafts and wells);
- military tasks.