active seismic experiments

Among many research methods that give important insight in the structure of the lithosphere, seismic methods should be highlighted, especially deep seismic sounding. In seismic research different kinds of energy sources and different methods of interpreting the recorded data are used. In our Departament we mainly focus on modelling and interpretation of the data from active seimic experiments rather than passive recording.

The energy sources in land active seismic experiments are explosive charges (usually TNT) or Vibroseis ( a hydraulic device with a heavy steel plate hitting the ground with desired frequency, mounted on a truck). In marine experiments apart from explosives, airguns are used. Airguns are devices that are dragged behind the vessel and generate the seismic signal by sudden decompression of pressurized air. The energy generated in this manner is recorded by special equipment: ocean bottom seismometers (OBS) and/or land recording stations. In large regional-scale or bigger experiments a large number (usually several hundred) of one-channel (vertical-component) digital recording devices.

In passive experiments when the recording is continued over a longer period of time (a year or longer) we take advantage of the energy from natural (far, strong earthquakes as well as small local seismic events) and industrial sources (mines and quarries). Often all three components (Z,N and E) of the seismic signal are recorded.

In spite of significant increase in the number of passive experiments performed in the recent years and the spectacular research opportunities that they present, active seismics are still the most reliable when it comes to recognizing the crust and upper mantle structures. Active seismic methods can be divided into refraction and reflection methods.

Refraction method, also called deep seismic sounding (DSS) or wide-angle reflection and refraction (WARR), consists of recording and interpreting refracted and reflected waves. It allows to model the physical parameters (fields of P-wave and S-wave velocities as well as the Vp/Vs coefficient) of the Earth’s crust and upper mantle in the surveyed area and the geometry of the seismic boundaries. Depending on the scale and the design of the experiment and the quality of recorded data, one-, two- or three-dimensional models of the lithosphere structure in the area can be obtained.

Reflection method leads to very detailed imaging of reflecting properties of the layers in the crust and upper mantle in the seismic sections. The images allow for interpretation of the layers’ diversity, the dip of the boundaries, etc. This method does not determine the seismic velocities (except for the very shallow layers) and, in consequence, the exact depth of the reflecting horizons. The recording times in deep reflection soundings are several times longer than standard exploration reflection surveys, whose range is limited to a few kilometers of depth. The fieldwork and data processing requires appropriate technical equipment and software, so specialized geophysical companies need to be contracted.

The information obtained thanks to the two methods are complimentary. Joining them allows for much more detailed modelling in comparison to using anyone of the methods separately.