Lecture 3b - Refraction Seismics

Flashcards based on a lecture about near surface investigations using refraction surveys.

What are the necessary components for a near surface refraction survey?

A source and a set of geophones.

What is a suitable source for a site scale investigation enabling imaging down to about 10 meters?

A large hammer on a metal plate.

How are geophones arranged in a near surface refraction survey?

A profile line with maximum offsets between each one of about a hundred meters.

What happens when a 'shot' is taken in a near surface refraction survey?

The source is activated, often by using a hammer to hit a plate, and the geophone record is taken for a brief duration of time to capture the data.

Why are shots taken from both ends of the geophone profile (forward and reverse)?

To test the assumption of horizontality and to relax it.

What does the data collected from any one shot represent?

The time series of oscillations recorded from each geophone.

What is the key task when interpreting shot records for refraction seismic?

To identify when the seismic waves from the source first arrive at each geophone.

How are first breaks usually manually identified on a shot record?

The point where the background seismic level is exceeded due to the vibrations from the source arriving at that geophone.

What are the two types of ray paths to consider for first breaks?

Direct ray paths and refracted ray paths.

Which layer do direct wave paths travel through?

The upper layer only.

Why do geophones nearer the source usually receive direct rays first?

The direct rays arrive first due to the shorter path taken.

How is a time distance plot created?

By plotting the first break times against the distance of the geophone from the source.

In a time distance plot, which segment represents arrivals that have traveled along a direct ray path?

The segment nearest the associated source location.

How is seismic velocity calculated from a time distance plot?

The distance divided by the change in time (reciprocal of the gradient).

What does the velocity calculated from the first breaks of the direct wave represent?

The velocity of the material in the upper layer.

What are the two methods to estimate the depth to the interface?

Using the intercept time or the crossover distance.

What is the crossover distance on a time distance plot?

The shortest distance along the profile at which the first break is from the refracted wave path.

Why is seismic velocity important in identifying subsurface materials?

Seismic velocity is related to the material strength and density.

What happens when buried low velocity areas are encountered in refraction seismic surveys?

They can't be detected.

What is the main advantage of refraction surveys compared to reflection surveys?

Refraction surveys have the advantage that they can be relatively straightforward to process.