Seismic refraction methods pdf

The technique is basically to inspect the T-X diagram and identify? Values are then picked off the T-X diagram and converted into structure parameters such as depth, etc using the assumed geometry of the ray path. Thus we need to know how T-X diagrams arise. A refraction T-X diagram is based on the first arrival at each geophone. This is either picked off the geophone output manually or in software or is automatically recorded by a cut-off timer.

Horizontal interfaces provide a simple introduction to the construction of T-X diagrams. Close to the source, the first arrival is due to the direct ray travelling in layer 1.

This plots as a straight line on the T-X diagram. The slope of the line is the reciprocal of the layer 1 velocity assuming distance is on the X-axis. The intercept is zero. When the critical distance is exceeded, refraction occurs and some energy enters layer 2. A refracted ray then travels at V2 sending return rays back to the surface as it does so. At some point the cross-over distance the refracted ray being the faster will overtake the direct ray and the return rays will become the first arrivals, despite their longer travel distance.

It is these that are now plotted on the T-X diagram. The T-X diagram thus develops an upper branch due to the refracted ray. This is again a straight line, whose slope is the reciprocal of V2. There is now an intercept time T1 whose value is determined by the layer 1 thickness and the two velocities The intercept time is an example of a delay time sum, composed of the separate times taken by the signal to descend to the interface and then to return to the surface.

Since, in this case, the ray path is symmetrical, the intercept time is the sum of two equal delay times By a similar argument, a third layer introduces a third branch into the T-X diagram.

The slope is the reciprocal of V3 and the intercept is a composite of the layer 1 and layer 2 delay times. The presence of a dipping interface is recognised if the reversed profile is not the mirror image of the forward profile The analysis of a dipping interface introduces three new issues: There is an additional unknown the dip angle The T-X diagram is no longer symmetrical and so the updip and downdip intercepts are not equal The updip and downdip velocities in layer 2 are not equal.

The asymmetry arises because the return ray has a successively shorter updip or longer downdip path as the distance from the shot point increases. This is expressed by saying that the apparent velocity in layer 2 the reciprocal slope of the upper branch is greater flatter slope in the updip than than in the downdip directions.

It is necessary to analyse both the forward and the reverse profiles to solve for V1, V2, Z and dip angle. Since it is not known in advance whether or not an interface is dipping - and most usually are! The dip will very probably be an apparent dip in the geological sense, since the profile is unlikely to follow the line of true dip.

Thus a second, perpendicular, profile is required to allow the true dip to be found. The T-X method smooths off interfaces by fitting a straight line through the data and so irregularies are not analysed. They are however visible as deviations from the best fit line and can be analysed using a different method. It is possible to analyse these deviations by using the so-called plus-minus method. This simply uses the previously-measured arrival times: a new survey is not required.

We return to the idea of a delay time. The delay time is the time taken to reach the lower layer minus the time taken to travel the horizontal distance. We now state that the arrival time between any two stations say A and B is the horizontal transit time at the fastest velocity plus the sum of all the delay times along the ray path For the simple two-layer example.

If we now take the sum of the forward and reverse times to any intermediate geophone and subtract the overall travel time, we can find the delay time at the intermediate geophone and hence the local depth. Since we have used both the forward and reverse profiles, the value obtained is an average depth around the position approx smoothed over a distance of one-third depth.

Hence it is called the plus-minus method. It is also known as the intermediate geophone method. Seismic refraction is a useful tool for the general investigation of bedrock structure, particularly at depth. The T-X method averages out depth variations, although the plus-minus method will show them from the same data It is incapable of fine detail, especially if the bedrock is irregular or lacks internal elastic contrasts.

It assumes that the velocity increases in each successive layer. If it doesnt, the lower velocity layer is missed. The velocity can be obtained from the T-X plot but is often measured in the field. Open navigation menu. Close suggestions Search Search. User Settings. Skip carousel. Carousel Previous. Carousel Next. What is Scribd? Explore Ebooks. Bestsellers Editors' Picks All Ebooks.

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Flag for inappropriate content. Download now. Related titles. Carousel Previous Carousel Next. Jump to Page. Search inside document. Hajji Assamo Padil. Arun Goyal. The dynamic geotechnical parameters of the foundation layers are revealed by seismic refraction method for utilizing the velocities of P-waves and SH-waves at the study area. This study proposes a classification to foundation rock material in the investigated region for engineering purposes based on all the calculated moduli and parameters.

This study divided to the following zones Fig. High competent materials characterize this zone suggesting suitability for any engineering purposes. This zone consists of moderate competent materials which is suitable for limited construction purposes.

This zone consists of low competent materials not suitable for limited construction purposes. This zone is suggesting non-suitability for heavy engineering purposes that must be keeping away from any constructions or loudness and watery activities.

Finally, the study delineates clearly the main structural trends in the study area. These trends should be carefully avoided when the planer and the officials plan the city and its roads. This shininess will protect roads to be broken by any overload produce from cars and vans or sudden earthquakes.

The findings of the study show the nature and shape of the boundaries of the basic materials that compose the subsurface layers and their geotechnical properties. It is clear that these results are reasonable and acceptable and have a significant impact in the field of civil engineering and other purposes, achieving the main objectives of the study. References i. Abd Elrahman, M. Rock material competence assessed by seismic measurements with emphasis on soil competence scales and their applications in some urban areas in Yemen, EGS.

Inferring mechanical properties of the foundation materials at the 2nd industrial zone, Sadat City, from Geophysical measurement, EGS. Of the 10th. Adeep, S.

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