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<br />~ <br />OOamplifying and timing circuits and an electric writing oscillograph which <br />cY) <br />0Jprovides a permanent record of seismic waves received. The instrument features <br /> <br />. signal enchancement by stacking repeated signals in a digital memory. <br /> <br />On a time-distance graph plotted from the data, each subsurface layer is re- <br />presented by a straight line segment.of constant velocity. Compressional wave <br />velocities of these layers are equal to the inverse slopes of the line segments. <br />Depths to increasingly higher velocity material can be calculated from the 10- <br /> <br />cations of the velocity breaks on the graph. <br /> <br />Method Limitations: The organization of seismic refraction surveys and the in- <br />terpretation of the data are generally well established and straightforward. <br /> <br />There are, however, no inflexible approaches to interpreting the data and <br /> <br /> <br />ambiguities and uncertainties are common. There are also two major potential <br /> <br /> <br />problem areas inherent in the method: velocity reversals and blind zones. <br /> <br />The velocity reversal problem exists when higher seismic velocity material <br /> <br />overlies lower velocity material. Refraction data analysis is based on the <br />assumption that seismic velocity increases with depth. The blind zone prob- <br />lem is the inability of the method to discern the existence of layers because <br />of insufficient velocity contrast or thickness of media. <br /> <br />Results obtained from shallow refraction investigations are useful because <br />they rapidly provide information on the bedrock configuration and can be used <br />as a guide for subsequent drilling. To remedy the problems mentioned above, it <br /> <br />is normal practice to use exploratory drilling in conjunction with the seismic <br /> <br />survey wherever possible. Also, it should be remembered that the results of <br />refraction analysis are depths to velocity interfaces and not necessarily depths <br /> <br />to lithologic boundaries. <br />