Seismic Refraction Flashcards
What 3 assumptions are made about the ray path geometry in a refraction survey?
That the subsurface is composed of a series of layers separated by planar and potentially dipping interfaces, velocities are constant within layers and increase with depth, and ray paths are restricted to a vertical plane containing the profile line (no cross dip)
What is the total travel time for a refracted ray path?
The time it would have taken to travel the total range at the refractor velocity plus additional time allowing for travel down to the refractor and back up to the surface (delay time)
What is crossover distance of a ray path?
The distance at which travel times of direct and refracted rays are equal
When is apparent velocity higher than refractor velocity?
When recording along a profile line updip from the shot point
When is apparent velocity lower than refractor velocity?
When recording velocity along a profile line downdip from shot point
What method is used to deal with the possibility of refractor dip?
Shots are fired at each end of the profile line and seismic arrivals are recorded with forward and reverse travel time plots differing in gradients and intercepts in the presence of dip
How does fault displacing affect travel-time plots?
The segments of the plot on opposite sides of the fault are displaced, with 2 intercept times
How does the minus term derive velocity?
Plotting minus term against distance (2x-l) yields a slope of 1/v
How does the plus term derive perpendicular depth?
The term can be inserted into the delay time refractor depth equation when the refractor is overlain by one layer only
What is ray tracing and how does it work?
A model is created to calculate travel-times of refracted rays to compare with observed travel times. Adjustments are made iteratively to determine ray path
What requirements are there for a layer to be detected in a first arrival refraction survey?
The layer must be underlain by a layer of higher velocity to create head waves, and must be thick enough that head waves are the first arrival
What is a hidden layer?
A layer that produces head waves but does not give rise to first arrivals, either due to thinness or closeness in velocity to the overlying layer
What is a blind layer?
A layer with a low enough velocity that rays cannot be critically refracted, creating no head waves
What is the weathering correction and why is it applied?
The weathering correction replaces a weathered layer (with varying thickness) with material of velocity equal to that of the underlying layer to mitigate variable delay introduced by weathering
What is the elevation correction and why is it applied?
The elevation correction removes the effect of differences of elevation of shots and detectors to reduce travel times to a common datum plane
