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Geophysical survey

The Downhole Test is a method which determines soil stiffness properties by analyzing direct compressional and shear waves along a borehole down to about 30 m.

The aim of the downhole testing is to derive elastic rock properties such as Poissons ratio or YOUNGs modulus. Shear waves have to be generated at surface. A shear wave source (sledge hammer hit sidewise) is used at surface and a coupled receiver system is moved in the borehole.Traveltimes of the seismic waves are analysed and seismic velocity is calculated. Shear wave velocity can be transformed to soil stiffness

The measurements can be performed below and above the groundwater table. A grouted casing (e.g. PVC) with a diameter between 3 and 6 inch needs to be prepared.

The seismic crosshole test provides dynamical soil parameters down to depths of 100 or 150 m based on the determination of wave velocities. Seismic waves are generated in a source borehole and recorded in a receiver borehole.

The focus of the crosshole test is on the determination of the shear wave velocity (s-wave velocity). Therefore a s-wave source has to be applied (e.g. a s-wave sparker or a borehole hammer). A borehole geophone is installed in the receiver hole and tightly coupled to the borehole walls by means of a pneumatic packer.A good coupling is needed since s-waves cannot be transmitted by the groundwater only.

Source and receiver are alwaysinstalled at same depths and moved parallel along the boreholesto achieve a velocity profile with depth. The usual vertical spacing between two measuring points is 1 to 2 m. The shear modulus (i.e. the dynamic stiffness) can be calculated if a value of the material density is assumed.

The measurements can be performed below and above the groundwater table. A grouted casing (e.g. PVC) with a diameter between 3 and 6 inch needs to be prepared. Typical surface distances between source and receiver borehole are between 5 and 20 m.

The Multichannel Analysis of Surface Waves (MASW) method is one of the seismic survey
methods evaluating the elastic condition (stiffness) of the ground for geotechnical engineering purposes. MASW first measures
seismic surface waves generated from various types of seismic sources—such as sledge hammer—analyzes the propagation
velocities of those surface waves, and then finally deduces shear-wave velocity (Vs) variations below the surveyed area that is most
responsible for the analyzed propagation velocity pattern of surface waves. Shear-wave velocity (Vs) is one of the elastic constants
and closely related to Young’s modulus. Under most circumstances, Vs is a direct indicator of the ground strength (stiffness) and
therefore commonly used to derive load-bearing capacity. After a relatively simple procedure, final Vs information is provided in 1-D,
2-D, and 3-D formats

Microtremor is a low amplitude (in the order of micrometres) ambient vibration of the ground caused by man-made or atmospheric disturbances. Observation of microtremors can give useful information on dynamic properties of the site such as predominant period and amplitude. Microtremor observations are easy to perform, inexpensive and can be applied to places with low seismicity as well, hence, microtremor measurements can be used conveniently for seismic microzonation.

Continous seismoacoustic profiling is done by use of two channel digital equipment to solve the flowing tasks:

  • Image of sea bottom relief;
  • Rock thickness in upper portion of section;

Acquired data are processed by use of "Sonar WIZ5" and "Delph Seismic" software packages. "CSP-S6000" and"CSP-S12000" are aplied as a power sources depending on exploration depth. Exploration depths of equipment vary within 50-750 m ranges.