Basic Reflection Theory Flashcards
What are the three wave types? Describe them
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
Name and describe two ways of viewing seismic waves
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
Describe the processes involved to carry out the reflection seismic method ( 5 steps )
- 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
Name and describe the difference between the two reflection types of P-wavelets received by detectors at the surface
Compressional reflection: positive polarity, push (compression) occurs before pull (dilation)
Dilational reflection: negative polarity, where pull occurs before push
What are the 2 pros and 3 cons of colouring half of the wavelet oscillations black?
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
What is the equation for acoustic impedance?
AI = p x V
Where:
AI = Acoustic Impedance
p = density
V = P-wave velocity
Under what circumstances do a compressional wavelet, and a dilational wavelet reflect back to the surface? What are the boundaries called?
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
What do the reflection values of + 0.1 and - 0.2 mean in terms of the proportion of energy reflected from a boundary?
+ 0.1 : boundary will reflect 10% as compressional wavelet
- 0.2 : boundary will reflect 20% as dilational wavelet
What will happen to the remainder of the seismic energy that is not reflected back? What will their polarity be?
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
What is meant by a period of oscillation?
One cycle consists of a peak and trough and the time taken for the complete cycle is known as the period of oscillation
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?
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
What is meant by the term ‘frequency analysis’? When and how is it done?
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
What is meant by deconvolution?
What does it involve?
How will it appear if done well?
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
Why are ray paths constructed?
What must be assumed to do this?
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
What is the CDP?
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
What is the CMP?
Common Mid-Point: point on surface equidistant between the source and the receiver
What is multiplicity?
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
How can multiplicity be used?
After corrections made, traces from the same CDP can be stacked to enhance reflection info and reduce noise
What are stacking diagrams used to show?
Visually show the build up of CMP coverage for 2D and 3D surveys to shot and detector positions
What is digital data sampling? What is used to do this?
Sampling is performed to convert analogue data from the earth into discreet digital data values using a multiplexer
Describe the three stages to digital data sampling
- 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
What are the disadvantages of sampling?
Must be at high enough rate to allow reconstruction of the original sesimic frequencies from the sampled values
What is the Nquist rate and frequency?
Nyquist rate: Max defined rate of a frequency
Nyquist frequency: max defined frequency
How is the Nyquist rate used to ensure faithful recovery of the max required frequency?
Sampling will be performed at twice the Nyquist rate
