The Wave Equation Modeled
We’ve looked at what happens to P-waves in the simple two-layer case...
The Wave Equation Modeled
…we get a direct P-wave, a refracted P-wave, and a reflected P-wave.
The Wave Equation Modeled
So, you can now easily find the three P-wave phases:
The Wave Equation Modeled
…but, as you saw in your lab field data, real data adds the complication of other phases–here, the air wave and ground roll (Rayleigh wave) are shown.
Multilayer Model
Remember, for refraction data you generally only need to know the first arrivals…
Distance
Time
Direct Wave
1st Layer Refraction2nd Refraction
Reflection/Refraction
Refraction Advantages:• Small number of sources/receivers can work• Little signal processing is needed• Interpretation straight forward
Reflection Advantages:• Small scale features can be resolved• No restriction on velocity-depth profile• Can be used in regions of complex geology• Uses entire reflected wavefield• Subsurface directly imaged
Seismic Reflection
Terms:Stacked Section/Seismic Section: A plot of seismic reflection data with substantial digital processing that represents a zero-offset profile.
Time Section: A seismic section in which the axes are offset and two-way travel time. Rarely represent correct spatial positioning of subsurface features.
Depth Section: A seismic section in which the time section has been converted to depth through a velocity model.
Seismic Reflection Data
Processing Steps
Unlike refraction data, there are manyprocessing steps to create an imageof the subsurface with seismicreflection data. Many of these are nottrivial, and we will look at some ofthem in lab.
Poorly processed seismic data canbe made to look like just aboutanything. You’ve been warned!
Seismic Reflection Data
The basis of common midpoint(CMP) processing is to create astacked profile representing azero-offset section. CMP processingis the current industry standard.
Seismic Reflection Data
Common Midpoint Processing
Step #1: Collect shot gathers (field data)
Each sequential shot is movedalong the profile at incrementscalled the shot interval.
Ideally, the receiver spreadwill be moved with the shotalong the profile. The intervalbetween the receivers is calledthe group interval.
This process (called "rolling")proceedes along the length ofthe profile (which depends onthe target).
Δ Δ Δ Δ*
Δ Δ Δ Δ*
Δ Δ Δ Δ*
*
**
** ** ** .etc
Seismic Reflection Data
Common Midpoint ProcessingStep #2: Resort data tocommon midpoint gathers
This process is done in thecomputer, and the shotgathers recorded in the fieldare split up and redisplayedas CMP gathers.
Notice that the intervalbetween CMP locations hereis 1/2 of the receiver/shotinterval.
The number of traces perCMP gather is called the fold,fold number = N/2 Δ /x Δ ,s
where N is the number of, channelsΔ x is the group
, interval andΔ s is the source. = 2.interval Here fold
Δ Δ Δ ΔΔΔ Δ
Δ Δ Δ ΔΔΔ Δ* *
Δ Δ Δ ΔΔΔ Δ**
Δ Δ Δ ΔΔΔ Δ**
**
Seismic Reflection Data
Processing Problems… Best Data Poorly Processed Data
0.1
0.2
0.0
1 15 30
0.1
0.2
0.0
Trace Number
Time (s)
No
unconformity
No faulting
Subhorizontal
bedding
0.1
0.2
0.01 10 20 30
0.1
0.2
0.0
Seismic Reflection Data
Examining Raw Data...
0.1
0.0
1 10 20 30 40 50 60 70 80 90
Trace Number
Time (s)
0.2
Noisy
Trace
Noisy
Trace
Noisy
Trace
Ground
Roll
Reflection
Air Wave
Reflection
Seismic Reflection Data
Examining Raw Data…
Be aware that seismic data is asvaried as geologicsettings, and thus canlook very different. 0.1
0.2
0.0
1 10 20
0.1
0.2
0.0
Trace Number
Time (sec)
Air
Wave
Noisy
Trace
Reflection
Reflection(?)
Reflection(?)
Refraction
Seismic Reflection Data
Digital filteringis a criticalprocessingstep...
0.1
0.0
1 20 40 60 80
Trace Number
Time (s)
AGC Gain
Gain + Low-Pass FilterGain + Med.-Pass Filter
Gain + V. High-Pass Filter
Gain + High-Pass Filter
Seismic Reflection Data
You datacan be analyzed inother domains as wellas the frequency domain.