Deep seismic surveying

Seismology (from the Greek seismos = earthquake and logos = knowledge) is the scientific study of earthquakes and the propagation of elastic waves through the Earth. In general, one distinguishes between active seismic, where a signal is generated (e.g. by explosions) and passive seismic, where natural sources are used (e.g. earthqakes).
Buoys marking airguns during seismic surveys in Andfjorden in Nordland.

Seismic waves are produced by large movements of earth: events like earthquakes, volcanic eruptions, landslides and man-made explosions can produce seismic waves. These waves can be travel through the bedrock, and can be used to image both sources and structures deep within the Earth.

P- and S-waves

There are three basic types of seismic waves in solids: P-wave and S-wave (both body waves) and surface waves. The two basic kinds of surface waves (Rayleigh and Love), can be fundamentally explained in terms of interacting P- and/or S-waves.

Using seismic tomography with earthquake waves, the interior of the Earth has been completely mapped to a resolution of several hundred kilometres. This process has enabled scientists to identify convection cells, mantle plumes and other large-scale features of the inner Earth.

Measuring Earthquakes

The passive seismic method can also be used to image the deep crust and upper mantle recording teleseismic arrivals along semi-permanent networks. A network of seismographs, instruments that sense and record the travel time of seismic waves through the Earth, are positioned along a profile or outline an area to record the response of earthquakes at great distances (almost the other side of the world) by measuring their travel time through the Earth.

Differences in travel-times from different directions allow us to image regional structures.Seismic waves produced by explosions or controlled vibrating-sources are the primary method of underground exploration. Controlled-source seismology, often called active seismics, has been used to map faults, anticlines and in general the structure of the crust below southern Norway.

This type of seismic registration is often very labour-extensive, and dozens of people can be involved in the data acquisition, as a large number of seismographs is usually deployed.

Seismic Refraction (SR)

Splashes of seismic measurements.

Seismic Refraction (SR) is based on the refraction of seismic ('sound') waves from subsurface layers and rocks or soil.  Models are constructed of the seismic velocity and the thickness of the subsurface layers. One of the NGU Geophysics department's predecessors, Geofysisk Malmleting A/S, had began using seismic refraction in geophysical surveys already in 1953. Later is the method used by survey mineral resources, the environment, and geotechnical surveys and for quaternary mapping. The method is commonly used to determine the quality of bedrock, and the most reliable geophysical method for determining the depth of surficial deposits. In cooperation with the Nordic universities NGU has used Seismic Refraction techniques for various research studies on the earth's crust.

Seismic Refraction (SR) tomography

Development of the seismic refraction method has been made possible through access to greater computer processing power.  By running calculations on a large number of tomography measurements more detailed models constructed.  In cases where great lateral heterogeneity is the suspected, tomographic inversion approach should be chosen.

Seismic Reflection

Seismic reflection surveys involve measuring the two-way travel time of seismic waves transmitted from surface and reflected back again. Refleksjonsseismikk er standardverktøy ved oljeundersøkelser, og metoden benyttes også til tektoniske studier på land. Seismic reflection profiling is standard by oil research, and the method is also used to tectonic studies on land. For geoengineering survey, the method has demanded great amount of resources, but now that PC and digital seimographs can easily purchased, the method is now widely used.  The advantage of seismic reflective profiling lies in its ability to identify structure in surficial deposits thereby providing useful geologic knowledge. Today the technique is routinely used in determining feasibility studies for projects like submarine tunnels in the fjord.

Acoustic logging of drill wells

NGU has equipment for logging of seismic velocities (P-and S-wave) in the drill wells. See borehole logging.