Geophysics lecture 5- Seismic refraction and reflection methods

Question 1

Active geophysical technique

Answer 1

You generate the signal being measured

Question 2

Exploitation of seismic (acoustic waves0 for geophysical surveying

Answer 2

Requires a controlled source (or sources) and receivers

Question 3

What technique would you use for deep exploration

Answer 3

Reflection

Question 4

Sources examples

Answer 4

Explosives, hammer, weight drop, airgun

Question 5

What is a source

Answer 5

Anything that generates vibration/shockwave

Question 6

What are receivers often in the form of ?

Answer 6

An array or ‘takeout’ of geophones

Question 7

Seismic reflection

Answer 7

• 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)

Question 8

Seismic refraction

Answer 8

*Shockwaves sent into subsurface
* You measure the “headwave” of seismic energy that is
refracted along the subsurface
boundary

Question 9

The only method useful for deep subsurface probing

Answer 9

Seismic reflection

Question 10

Features of seismic reflection

Answer 10

• Seismic array is horizontally short c/w surveying depth
• Sources have to be big
• Expensive

Question 11

Why doesn’t seismic refraction work for deep surface?

Answer 11

• Returned signals too weak to detect

Question 13

What technique do you use for shallow probing?

Answer 13

Seismic refraction

Question 14

Features of seismic refraction

Answer 14

• 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.)

Question 15

Seismic reflection tools

Answer 15

Machine airgun, vibroseis, explosives

Question 16

Seismic refraction tools

Answer 16

• Hammer and plate
• Weight drop

Question 17

Two categories of seismic waves

Answer 17

• Surface waves
• Body waves (P and S waves)

Question 18

What wave type does seismic refraction use?

Answer 18

P waves

Question 19

P waves and propagation

Answer 19

P waves propagate faster than S waves
- Particle motion is parallel to direction of propagation

Question 20

S waves and propagation

Answer 20

• Particle motion is perpendicular to direction of propagation

Question 21

What does geophone measure?

Answer 21

Measures vertical component of motion rate
- Respond to rate and not amount of ground movement

Question 21

Critical refraction

Answer 21

At r=90, ray travels just below and parallel to interface in faster medium

Question 21

Receiving array

Answer 21

• 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

Question 22

Geophone

Answer 22

• 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

Question 23

What frequency are geophones tuned to

Answer 23

> 20 Hz

Question 24

How to use a geophone

Answer 24

–> Measure travel-time for first arrival at each geophone
* Close to source – first arrival is direct wave
* Further away – first arrival is refracted wave

Question 25

Dipping layers

Answer 25

• 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

Question 26

Crossover distances and itnercept times

Answer 26

Different for forwards and reverse shots but total travel time is the same

Question 27

Multiple layers=

Answer 27

Same principles more equations; requires v1<v2,v3 etc

Question 28

Hidden layer

Answer 28

When there’s a layer of lower velocity beneath one of higher velocity. Invisible on seismic refraction record and causes problems.

Question 29

Seismic velocity

Answer 29

Affected by porosity, joints and rock type etc but plenty of overlapping values

Question 30

Multi method investigation

Answer 30

Seismic techniques often combined with other methods e.g. resistivity, EM and borehole

Question 31

Tomography

Answer 31

Velocity structure as a function of position, not just layers of constant velocity

Question 32

Ambient seismics

Answer 32

Collecting noise data and turning it into a source at one stattion and recorded at another