The method uses tomographic data analysis for enabling us to handle lateral velocity changes in a layer. To expand the applicability of the seismic refraction method to such complicated sites, we have developed a new method for analyzing seismic refraction data. For example, these methods analyze refraction data supposing that the earth is a layered medium. However, these conventional methods have limitations in the application to sites where surface topography and subsurface geological structure are complex, because they make some assumptions for these surface and subsurface features. There are various methods for analyzing seismic refraction data, for example, Hagiwara's method, the Plus–minus method, and the generalized reciprocal method. 1 shows the example of measured refraction profile and traveltime curves.
![seisimager seisimager](https://rayfract.com/mdr8high.jpg)
In the typical seismic refraction method in Japan, geophones are uses for receivers and dynamite is used for the sources with the receiver spacing of 5 or 10 m and the source spacing of 30–100 m. For example, in tunneling sites, the seismic velocity model has been used for the excavation plan and the design of supporting steel intervals. The seismic refraction method provides subsurface seismic velocity models, which are very important for the design and the construction phase of these projects. In Japan, the seismic refraction method has been widely used in many civil engineering projects in mountainous areas, which include railway and highway tunnels, dam constructions and landslide protections.
![seisimager seisimager](https://www.geomatrix.co.uk/cms/resources/product-thumbs/seisimager2d-1.jpg)
It can also shorten the data processing time drastically and reduce the survey cost. Numerical and actual field examples have shown that the method can greatly improve the accuracy and the reliability of the seismic velocity models obtained by the seismic refraction method as compared to the conventional analysis methods. (3) In order to apply the seismic refraction technique to a site that has a more complex structure, the method can use the data obtained from sources and receivers on the surface, as well as those within boreholes simultaneously.
![seisimager seisimager](http://ika638.com/rwd/work/oyo_hp01/1-geo_survey/1-seismic/image/seisimager_2d1-l.jpg)
(2) The method is applicable to both layered velocity structures and those showing a velocity gradient with depth. The main features of the method we developed are as follows: (1) The algorithm of analysis is based on the tomographic reconstruction technique, and refraction data can be analyzed almost automatically after traveltime picking. The authors have developed a new method for the analysis of seismic refraction data, which can handle both surface and borehole data simultaneously.