1. Applied Geophysics Fall 2016 Umass Lowell
Lecture 10: Class 12 Seismic Reflection Applications, Acquisition, and Interpretation
Applied Geophysics
Fall 2016
Umass Lowell

2. Previously…
Over the past two lectures, we looked at the equations that allow an interpreter to obtain information about the subsurface from field records
We determined how to calculate the thickness of a layer, how to assess the velocity of layered media, and how to make general statements about subsurface conditions based on preliminary observations of shot records

3. What comes next?
The acquisition of seismic reflection data requires a site-specific approach
Acquisition parameters are often based on the targets of interest (not always finding bedrock!)
After acquisition, “sorting” the shot records in to “gathers” can allow one to observe particular characteristics of the subsurface
Finally, although we are able to make basic calculations from shot gathers and time-distance plots, we can analyze more realistic (worse) data more quickly with computer processing software

4. Acquisition: Equipment
We try to use higher frequency energy to study the earth with reflection, both to allow for greater resolution and to reduce the chances of interference from low-frequency surface waves – to accommodate this, we use higher-frequency geophones and reduce the sample interval
Typical equipment consists of:
to 100 Hz geophones
Sledge hammer, weight drop, or seisgun source
Acquisition units
Typical sampling interval = 62.5 us (compare to refraction, where typical sampling interval = ms, and MASW where sampling interval = ms) to capture higher frequencies WITHOUT ALIASING

5. Acquisition: The Optimum Window
Real seismic reflection surveys in the shallow subsurface are obscured by the air wave and ground roll at early times in the recordThe “optimum window” is the range of source-receiver distances over which the reflections are clearly distinguishable from the slower air and ground roll arrivals
The “near side” of the optimum window is where the arrivals from the low- velocity waves break away from the arrivals of the reflected waves (minimum source-receiver distance where reflections can be clearly observed)
The “far side” of the optimum window is constrained by interference between low-velocity reflections from shallower layers and high-velocity reflections from faster layers at depth, the water table refraction, and phase changes that become an issue by making it difficult to correlate arrivals from trace to trace when source-receiver distance > depth to bedrock (same restriction as Dix equation)
NOTE: in practice, optimum window can shift if your bedrock surface is very irregular…

6. Near side

7. Far side

8. Acquisition: Determining the optimum window in practice
First resource is ground-truthed data! (well logs)
Second resource could be interpretation of depth to bedrock from another method (refraction, GPR, resistivity)
Third resource is what usually happens with limited budget and order of investigations…the walkaway experiment
Keeping the array stationary, choose a source-receiver separation to start with and collect a reflection record – determine if the reflections are visible
Move the shot point back some distance (10 foot intervals can work well) and collect another record – determine if the appearance of reflections has improved
Continue moving the shot point away from the array until the reflections are clearly discernable from the air wave and surface waves and use this as the reflection shot point

9. Potential Issues
In addition to the difficulty often experienced in identifying the reflected arrivals and separating them from the air wave and surface waves, the user should also consider
the potential for multiples (bounces) within the layers and their effects on the records – short path multiples appear close to the actual reflection event, but long-path multiples may appear as a second reflection
interference from diffractions, which also appear as hyperbolic patterns in travel-time records but won’t have the same NMO

10. Acquisition: Resolution
Both horizontal and vertical resolution are improved with higher frequency energy/geophones to detect it because waves spread spherically from a point
Vertical resolution is typically up to 𝜆 4 for deep applications, but up to 𝜆/2 for shallow studies
Horizontal resolution is determined by the first Fresnel zone, which is the radius of the area on the reflector that is primarily responsible for the generation of the reflection signal:
𝑅 1 = 𝜆 ℎ 1

11. Acquisition: Geometry
Choices of geometry:
Split-spread – take a shot at the middle of a spread and move the entire spread + shot point to a new location to collect data along a line to form a profile
Common offset – take a shot at the optimum offset from geophone 1; freeze geophone 1 so the data remains in place and clear the rest of the channels. Take a second shot at the same distance from the second geophone, freeze it and clear remaining channels, and do the same for the third geophone. Continue collection until all geophones have data. Doesn’t require NMO correction and produces an “instant” profile, but does create a stretching distortion since shot point is always behind geophone and may lead to misinterpretation of data
Common depth point – sample each subsurface point several times to stack out unwanted signals by moving shot point and receiver out along line (Figure 4.38)
You can shoot in common depth point mode and then rearrange traces later to simulate common offset collection!

12. Stretching of Data (Water Table Refraction Similar to Reflection)

13. Gathers

14. Applications: Stratigraphy

15. Seismic Reflection at the Exploration Scale

16. Applications: Void Detection