Geophysics lecture 5- Seismic refraction and reflection methods Flashcards
Active geophysical technique
You generate the signal being measured
Exploitation of seismic (acoustic waves0 for geophysical surveying
Requires a controlled source (or sources) and receivers
What technique would you use for deep exploration
Reflection
Sources examples
Explosives, hammer, weight drop, airgun
What is a source
Anything that generates vibration/shockwave
What are receivers often in the form of ?
An array or ‘takeout’ of geophones
Seismic reflection
- Field and data processing maximises
energy reflected along near-vertical ray
paths - You measure (pretty much) the direct
return of shockwaves off a subsurface
boundary (e.g. change from sediment
to bedrock)
Seismic refraction
*Shockwaves sent into subsurface
* You measure the “headwave” of seismic energy that is
refracted along the subsurface
boundary
The only method useful for deep subsurface probing
Seismic reflection
Features of seismic reflection
- Seismic array is horizontally short c/w surveying depth
- Sources have to be big
- Expensive
Why doesn’t seismic refraction work for deep surface?
- Returned signals too weak to detect
What technique do you use for shallow probing?
Seismic refraction
Features of seismic refraction
- Seismic array has to be wide c/w survey depth – usually geophone array ~4-5 x depth to boundary of interest
- Method reliant on seismic velocities
increasing with depth - Sources smaller (cheaper equipment, staff training etc.)
Seismic reflection tools
Machine airgun, vibroseis, explosives
Seismic refraction tools
- Hammer and plate
- Weight drop
Two categories of seismic waves
- Surface waves
- Body waves (P and S waves)
What wave type does seismic refraction use?
P waves
P waves and propagation
P waves propagate faster than S waves
- Particle motion is parallel to direction of propagation
S waves and propagation
- Particle motion is perpendicular to direction of propagation
What does geophone measure?
Measures vertical component of motion rate
- Respond to rate and not amount of ground movement
Critical refraction
At r=90, ray travels just below and parallel to interface in faster medium
Receiving array
- Typically 12 or 14 geophones in a spread
- Can stack several shots to
improve signal-to-noise
ratio - Besides forward and
reverse shots, can also
have a split spread by
shooting from the centre
Geophone
- Magnet moves up and down with
respect to the coils - Springs top and bottom damp movement
so they don’t keep ‘ringing’ - Respond to rate, not amount, of ground
movement - Output is a voltage proportional to rate
of movement - Spike for good ground coupling
- Sensitive to wind/traffic noise vibrations
Geophone
What frequency are geophones tuned to
> 20 Hz
How to use a geophone
–> Measure travel-time for first arrival at each geophone
* Close to source – first arrival is direct wave
* Further away – first arrival is refracted wave
Dipping layers
- Shoot in forward and reverse directions – asymmetry of time-distance plots
in the two directions indicates a dipping layer - Get apparent up-dip and down-dip velocities from gradient
- Can resolve into actual second layer velocity and dips
Crossover distances and itnercept times
Different for forwards and reverse shots but total travel time is the same
Multiple layers=
Same principles more equations; requires v1<v2,v3 etc
Hidden layer
When there’s a layer of lower velocity beneath one of higher velocity. Invisible on seismic refraction record and causes problems.
Seismic velocity
Affected by porosity, joints and rock type etc but plenty of overlapping values
Multi method investigation
Seismic techniques often combined with other methods e.g. resistivity, EM and borehole
Tomography
Velocity structure as a function of position, not just layers of constant velocity
Ambient seismics
Collecting noise data and turning it into a source at one stattion and recorded at another
