Utilisation of the refraction seismic method

Seismic refraction surveys have been carried out in the same traditional way since the 1950's. The development of seismics has been concentrated in improvement of the data processing techniques. The latest advance in the refraction method is the Generalised Reciprocal Method, GRM, developed in Australia during the last decade. The new method is actually a widening of the well-known ABC-method. The GRM-method makes it easier to determine the origins in soil layers or in the bedrock of the velocity anomalies found in refraction data. A FORTRAN-77 program is implemented on IBM-PC to calculate the time-depths and velocity analyses of the GRM-method. A FFT-routine needed in the wave form analysis is also programmed. These codes as well as plotting routines written in BASIC can be obtained from the Geotechnical Laboratory of the Technical Research Centre of Finland. In the traditional refraction survey only the first arrival of the elastic waves has been measured. Seismographs used in these surveys have usually an analog output on paper. Only two digital seismographs have been used in Finland, but at least three new digital systems will be bought this year. Digital seismographs are able to record the whole wave form, which gives the possibility to analyse also later arrivals. Examples from abroad show that the the quality of shallow bedrock can be interpreted quantitatively when the attenuation of the waves is studied. The bedrock reflection can also be recorded in digital seismograms. The integration of the refraction and reflection methods will increase the accuracy of the shallow seismic method significantly. In discussion with the interpreters of main Finnish companies running seismic surveys the most common problems met in the praxis and the needs for development work to improve the interpretation of digital refraction data were mapped