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.