Methods

SEISMIC MEASUREMENT PROCEDURES – THE METHODOLOGY

Seismic measurement systems are used in many different areas – for example in research, natural resource exploration or subsurface exploration.

The basic principle of a seismic measurement is always similar: during a seismic subsurface investigation, vibrations are generated at the surface with the help of seismic sources. These vibrations propagate in the subsurface at a material-dependent velocity and are reflected and refracted by the individual geological layer boundaries. With the help of geophones placed on the surface, the travel times and amplitudes of the individual waves are measured and displayed in a seismogram. Based on the data obtained in this way, an image of the subsurface can be generated.

For each seismic measurement, the method used must be adapted to the individual target of the investigation. This includes the choice of refraction and/or reflection seismic measurements and the dimensions of the seismic profiles, including source and receiver positions.

In reflection seismic measurements, elastic waves are observed that are reflected back to the surface at the individual layer boundaries and discontinuities in the subsurface. From the travel time of the seismic wave between its excitation point at the surface and its arrival back at the surface, and from the material-dependent wave velocity, the depth and course of the geological layer boundaries in the subsurface can be determined. The set-up of the measurement devices at the surface must be adapted to the respective task.

APPLICATION EXAMPLES

• Determination of soil stratification and fault zones
• Subsoil investigation
• Mapping of landfills
• Exploration of reservoirs (natural gas, oil, geothermal energy)

Refraction seismics yields the different, material-dependent wave velocities of the individual layers in the subsurface. For this purpose, the first arrivals of the seismic waves at the geophones along the measurement line are analysed. Using the velocities calculated from the travel times, the thicknesses of the individual layers can be derived. A precondition for a refraction seismic survey is that the seismic velocity of the lower layer is greater than that of the layer on top.

APPLICATION EXAMPLES

• Determination of bedrock depth or overburden thickness
• Determination of elastic soil properties, e.g. shear strength and modulus of elasticity
• Determination of the thickness of weathered layers
• Determination of rock edges and rock classes

POSSIBLE FIELDS OF APPLICATION OF GEOPHYSICAL MEASUREMENT SYSTEMS

Construction projects / site stability

• Detection of unstable areas (loosened zones), e.g. in mining areas
• Measurement and evaluation of soil stability in the run-up to construction projects
• Investigation of subrosion phenomena and destabilisation processes
• Risk assessment of already built-up areas and used sites

Earthquake zones

• Soil evaluation for earthquake-resistant construction
• Measurement data for damage assessments after earthquakes

Dyke areas

• Evaluation of dyke conditions
• Dyke investigation for stable flood protection
• Efficient investigation to determine dyke stability
• Determination of the rough structure of the dyke
• Identification of extensive zones of weakness
• Investigation of the ground beneath and beside the dyke

Archaeology

• Checking the ground conditions at historical excavation sites
• Optimisation of exact excavation positions

Geothermal energy

• Ground investigations prior to drilling
• Determination of location and depth of geothermal reservoirs

Glaciology

• Measurements in permafrost or on glaciers
• Investigating the composition of the rock mass under the ice
• Effects of glacier melt and changes in permafrost areas
• Verification of system reliability at high altitudes and at very low temperatures