Basic Reflection Theory

Question 1

What are the three wave types? Describe them

Answer 1

P-waves (longitudinal): move parallel, backwards and forwards in direction of wave propagation, waves travel twice the speed of S-waves

S-waves (transverse): move perpindicular to seismic wave propagation, waves travel half the speed of P-waves

Boundary waves: low velocity, low frequency waves that travel in complex motion

Question 2

Name and describe two ways of viewing seismic waves

Answer 2

Raypaths: line in the direction of wave propagation (i.e. radially from source)

Wavepath: line connecting seismic waves doing the same thing at the same time (usually a spherical shape)

With one can predict the other

Question 3

Describe the processes involved to carry out the reflection seismic method ( 5 steps )

Answer 3

• Series of impulses of energy are put into ground at several locations
• Some energy reflected back from rock interfaces beneath
• Detectors receive the energy and measure changes in the response characteristics of different rocks
• Conducted along a series of seismic lines arranged in a grid
• After intensive processing, able to estimate nature and distribution of the geology

Question 4

Name and describe the difference between the two reflection types of P-wavelets received by detectors at the surface

Answer 4

Compressional reflection: positive polarity, push (compression) occurs before pull (dilation)

Dilational reflection: negative polarity, where pull occurs before push

Question 5

What are the 2 pros and 3 cons of colouring half of the wavelet oscillations black?

Answer 5

Pros: Easier to see, easier to colour

Cons: Makes it harder to identify strong reflectors and key gas/liquid reflections, and it is the opposite convention to density logs

Question 6

What is the equation for acoustic impedance?

Answer 6

AI = p x V

Where:
AI = Acoustic Impedance
p = density
V = P-wave velocity

Question 7

Under what circumstances do a compressional wavelet, and a dilational wavelet reflect back to the surface? What are the boundaries called?

Answer 7

Compressional wavelet reflected back to the surface with no polarity change where a layer of low impedance overlies a layer of higher acoustic impedance
Boundary is called a positive reflection coefficient

Dilational wavelet is reflected back to surface where a high acoustic impedance over lies a layer of lower impedance
Boundary is called a negative reflection coefficient

Question 8

What do the reflection values of + 0.1 and - 0.2 mean in terms of the proportion of energy reflected from a boundary?

Answer 8

+ 0.1 : boundary will reflect 10% as compressional wavelet

• 0.2 : boundary will reflect 20% as dilational wavelet

Question 9

What will happen to the remainder of the seismic energy that is not reflected back? What will their polarity be?

Answer 9

Snell’s Law states that seismic energy will be refracted down into the underlying layer

The refracted p wavelets will have the opposite polarity to the downwards reflected waves, if they are the same then they are multiples

Question 10

What is meant by a period of oscillation?

Answer 10

One cycle consists of a peak and trough and the time taken for the complete cycle is known as the period of oscillation

Question 11

Describe when a period of oscillation may look like a sine curve, a cosine curve and what will happen if it does not appear like these?

Answer 11

Sine curve: minimum phase, begins at zero amplitude and rises to peak

Cosine curve: zero phase, begins at peak amplitude

Any cycle not beginning at these consists of a combination, can also be represented with shifted cosine curve

Question 12

What is meant by the term ‘frequency analysis’? When and how is it done?

Answer 12

A composite wavelet can be broken down into component frequencies with each frequency having a specific phase angle and amplitude

This is done during seismic processing using a mathematical technique known as a Fourier transform

Question 13

What is meant by deconvolution?
What does it involve?
How will it appear if done well?

Answer 13

Deconvolution is the conversion of mixed phase wavelet to an approximate zero phase wavelet

Involves cross-correlating an approximation of the input wavelet with the actual wavelet

If deconvolution works well then a cross correlation spike or narrow zero phase wavelet will be produced that represents the coefficient of the originating boundary

Question 14

Why are ray paths constructed?

What must be assumed to do this?

Answer 14

To predict the part of the subsurface that will be sampled by a particular combination of source and receiver location

Assumed that the reflector is horizontal and hence angle of incidence and reflection are equal

Question 15

What is the CDP?

Answer 15

Common Depth Point: can be related to point on the reflector vertically below, therefore at each source position there will be a large number of CDP’s sampled as defined by different receiver positions

Question 16

What is the CMP?

Answer 16

Common Mid-Point: point on surface equidistant between the source and the receiver

Question 17

What is multiplicity?

Answer 17

The number of times that a particular CDP is sampled

Achieved by spacing out source and receiver positions over short regular distances chosen so that different CDP’s are sampled more than once

Question 18

How can multiplicity be used?

Answer 18

After corrections made, traces from the same CDP can be stacked to enhance reflection info and reduce noise

Question 19

What are stacking diagrams used to show?

Answer 19

Visually show the build up of CMP coverage for 2D and 3D surveys to shot and detector positions

Question 20

What is digital data sampling? What is used to do this?

Answer 20

Sampling is performed to convert analogue data from the earth into discreet digital data values using a multiplexer

Question 21

Describe the three stages to digital data sampling

Answer 21

• Each input channel of data representing a geophone is sampled in turn at regular short time intervals so the oscillating analogue waveform can be later reconstructed
• Process can be pictured as the switch rapidly rotates making contact with the first and then each successive channel before returning to make the second sample from the first channel
• Each analogue sample is converted to a digital number which is written to a data recorder

Question 22

What are the disadvantages of sampling?

Answer 22

Must be at high enough rate to allow reconstruction of the original sesimic frequencies from the sampled values

Question 23

What is the Nquist rate and frequency?

Answer 23

Nyquist rate: Max defined rate of a frequency

Nyquist frequency: max defined frequency

Question 24

How is the Nyquist rate used to ensure faithful recovery of the max required frequency?

Answer 24

Sampling will be performed at twice the Nyquist rate