58 the difference between the times taken to travel

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58 the difference between the times taken to travel QR at velocity V2 and QGR at velocity V1. thickness, d , of the upper layer and F is equal to V 1 , 2. If there are more interfaces, F is a composite of all the velocities involved, weighted according to the layer thicknesses. At the short shots 2 D/ F = ti (the intercept time) and the F values can be calculated. The ways in which F varies between these points may be very complex, but linear interpolation is usually adequate in the field (Example 3.1). Although t R can be measured directly, it is more convenient to calculate it from the equation above using the intercept times. This can be done provided that geophones are located at the short-shot points when the long shots are fired (so that t A + t B at those points can be measured). The estimates of t R made using the data from the two ends should agree within the limits of error of the method (i.e. within 1–2 msec). If they are not, the raw data and the calculations should be thoroughly checked to find the reason for the discrepancy. Short-shot reciprocal times are measured directly if short-shots are fired at end-geophone positions, and the fact that they should be equal may help in picking arrivals. However, they have little interpretational significance. 4.0 Conclusion The science of seismology dates from the invention of the seismograph by the English scientist John Milne in 1892. Its name derives from its abilityto convert an unfelt ground vibration into a visible record. The seismograph consist of a receiver and a recorder. The ground vibration is detected and amplified by a sensor, called the seismometer or, in exploration seismology, the geophone. In modern instruments the vibration is amplified and filtered electronically. The amplified ground motion is converted to a visible record, called the seismogram. 5.0 Summary Early seismographs were undamped and reacted only to limited band of seismic frequencies. Seismic waves with inappropriate frequencies were barely recorded at all, but strong waves could set the instrument into resonant vibration. In 1903, the Germen seismologist Emil Wiechart substantially increased the accuracy of the seismograph by improving the amplification method and by damping the instrument. These early instruments relied on mechanical levers for
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59 amplification and recording signals on smoked paper. This made them both bulky and heavy, which severely restricted their application. 6.0Tutor Marked Assignment Q1. What are intercept times? Q2. Explain by the use of diagram the reciprocal time interpretations Q3 Differentiates between refractor relief and true velocity. 7.0References/ Further Readings Kearey, P., Brooks, M. and Hill, I. (2002) An Introduction to Geophysical Exploration (Third Edition), Blackwell Science, Oxford, 262 pp.
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