A seismic survey is conducted over level ground consisting of two horizontal layers. Sketch a travel-time diagram that shows the arrival of the direct wave, the reflected wave and the double refracted wave. How can the seismic velocity of each layer be determine from the diagram?

Answer 1

To sketch a travel-time diagram for a seismic survey over level ground with two horizontal layers, we first need to understand the path of the seismic waves. The direct wave travels straight down to the first interface between the two layers, where it is partially reflected back to the surface and partially refracted and continues to travel through the second layer before reaching the surface again. The double refracted wave travels down to the first interface, refracts into the second layer, then reflects at the bottom interface before refracting back to the surface.

To determine the seismic velocity of each layer from the diagram, we need to consider the travel times of the direct wave, reflected wave, and double refracted wave. By measuring the arrival times of these waves at the surface, we can calculate the travel time for each layer and use the formula:

Velocity = Distance / Travel time

Using the travel time and known distances (e.g. thickness of each layer), we can determine the seismic velocity of each layer. Additionally, we can also use the reflection and refraction angles to calculate the seismic velocities and angles of incidence and refraction for each wave.