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Suppose you wanted to drill a well for water extraction. How could you know ahead of time how deep you needed to drill? How might you know what material you are drilling through? These questions are important to estimating whether or not the project will be feasible and how much it will cost.
A similar question would be, how thick is the sediment layer above bedrock (bedrock = lithified sedimentary rock, or other igneous/metamorphic rocks). This question is relevant to, for example, civil engineering but also to general questions about geologic structure. It's not hard to imagine extending this question to something like: "is there are buried fault here? How deep is it?" Etc.
What the above two questions have in common is: how deep is it to a geologic contact, or interface.
In the case we will consider in our field-lab activity the "contact or interface" is unsaturated sediment over saturated sediment (i.e. we want to find the depth to groundwater). So, the question again is:
How can we detect the presence of subsurface geologic interfaces, as well as their depths or thicknesses?
The fundamental aspect of seismic refraction surveys is to note that when a seismic P-wave encounters a geologic contact the plot of P-wave arrival time vs. distance to receiver will change slope: Animation. Note: this is only true if the P-wave velocity in the lower layer is larger than in the top layer (this is because of Snell's Law).
We will collect seismic refraction data in the field, and then see how to use the collected data to determine the layer velocities and consequently the depth to the water table below our site via an in-class activity.
In both cases our hypothesis of the water table having receded below the base of the (dry) creek bed was verified via (1) the seismic velocities were consistent with an upper layer of dry sand for layer 1, and saturated sand for layer 2, and (2) the depth the the second layer was larger than the depth to the dry creek bed (which was 6 ft = 1.8 m).