Seismic Methods

Geoph 465/565 ERB 4127 Lecture 6 – Feb 19, 2014

Lee M. Liberty Associate Research Professor Boise State University

Homework review

1a) Plot the 3 shot gathers from 0-0.2 s with a gain that displays the first motion (as positive amplitudes) on each trace (use suwind, sugain, suop) 1b) 2 sources of noise dead trace on 2001 double hit on 2008 traffic noise on 2110 1c) center frequency  ~120 Hz

Homework review

1d) Filter and display the shots (sufilter) with 1 octave (10 Hz side lobes)

sufilter f=70,80,160,170

and 3 octave (1 octave side lobes) filters around the center frequency.

sufilter f=15,30,240,480

Homework review

• 2-6) Pick first breaks, perform 3-layer refraction analysis, sketch the model, calculate/sum error Critical distance vs. crossover distance

Critical distance vs crossover distance

Cross over distance

e.g. v1=100, v2=200, h=1 Xcrit=4/sqrt(3) =2.3 m Xcross=12 m

Homework review

7) Sources of refraction error picking error, assumption of 1-d model, sampling error, air wave is faster than v1 v1 250 386 390 205 239 308 250 289.7143

h1 v2 0.5

500 2.6

991 2.6

800 0.5

593 0.9

631 1.1

896 2.25

1.492857

600 715.8571

h2 v3 2 2000 3.34

2660 3.15

3333 2.4

2441 2.77

2680 1.4

1867 1.94

2.428571

2000 2425.857

Homework review

8) Calculate the hyperbola for each of the 2 reflecting boundaries Plot the direct wave, refraction, and reflections for each of the 3 shot locations 9) Assume a 20 degree dip to the south on the deepest layer. Redo step 8 with this assumption.

0 0 0,01 0,02 0,03 0,04 0,05 0,06 0,07 0,08 0,09 5 10 15 20 25 30 V1 reflector V2 reflector First Break Picks dipping reflector

Homework review

•

Temporal resolution

Frequency=100 Hz (cycles/sec) • • velocity=2000 m/sec Wavelength=20 m/cycle • ¼ wavelength (Widess) criteria = 5 m • Spatial (Fresnel) resolution - F = v *sqrt(t/f

dom

) =2000 *sqrt(.02/100) ~28 m (pre-migration)

F d = λ /4 = V avg /4 F – post-migration ~5 m

Borehole homework

• To add source depth values to traces, use sushw, but sx is an integer value sushw key=sx a=4000 b=0 c=25 j=2 SUSHW - Set one or more Header Words using trace number, mod and integer divide to compute the header word values or input the header word values from a file Optional parameters (): key=cdp,... header key word(s) to set a=0,... value(s) on first trace b=0,... increment(s) within group c=0,... group increment(s) d=0,... trace number shift(s) j=ULONG_MAX,ULONG_MAX,... number of elements in group

Borehole homework

• To remove bad shots, use suwind reject=

Friday’s Lab

• Vertical seismic profiles using hydrophones, 3c geophone, sledge hammer and shear wave hammer

Caveats of Refraction

• Only works if each successive layer has increasing velocity • Cannot detect a low velocity layer • May not detect thin layers • Requires multiple (survey) lines • Make certain interfaces are horizontal • Determine actual dip direction not just apparent dip

• To determine if interfaces are dipping…

Dipping Interfaces

• Shoot lines forward and reversed • If dip is small (< 5 o ) you can take average slope • The intercepts will be different at both ends • Implies different thickness Beware: the calculated thicknesses will be perpendicular to the interface, not vertical

• • •

Dipping Interfaces

If you shoot down-dip • Slopes on t-x diagram are too steep • Underestimates velocity • May underestimate layer thickness Converse is true if you shoot up-dip In both cases the calculated direct ray velocity is the same.

• The intercepts t int will also be different at both ends of survey

The Low Velocity Layer

• If a layer has a lower velocity than the one above… • There can be no critical refraction • The refracted rays are bent towards the normal • There will be no refracted segment on the t-x diagram • The t-x diagram to the right will be interpreted as • Two layers • Depth to layer 3 and Thickness of layer1 will be exaggerated

The Low Velocity Layer

• Causes: • Sand below clay • Sedimentary rock below igneous rock • (sometimes) sandstone below limestone • How Can you Know?

• Consult geologic data!

• Boreholes / Logs • Geologic sections • Geologic maps

The Hidden Layer

• Recall that the refracted ray eventually overtakes the direct ray (cross over distance).

• The second refracted ray may overtake the direct ray first if: • The second layer is thin • The third layer has a much faster velocity

Geophone Spacing / Resolution

• Often near surface layers have very low velocities • E.g. soil, subsoil, weathered top layers of rock • These layers are likely of little interest • But due to low velocities, time spent in them may be significant • • • To correctly interpret data these layers must be detected Decrease geophone spacing near source This problem is an example of…?

Undulating Interfaces

• • Undulating interfaces produce non-linear t-x diagrams There are techniques that can deal with this • delay times & plus minus method • We won’t cover these techniques…

Detecting Offsets

• Offsets are detected as discontinuities in the t-x diagram • Offset because the interface is deeper and D’E’ receives no refracted rays.

Fan Shooting

• Discontinuous targets can be mapped using radial transects: called “Fan Shooting” • A form of seismic tomography

Ray Tracing

• All seismic refraction techniques discussed thus far are inverse methods • One can also fit seismic data to forward models using Snell’s law, geometry, and a computer • Initial structure is “guessed” and then the computer uses statistical versions of “guess and check” to fit the data.

• Model generates synthetic seismograms, which are compared to the real seismograms

Before starting the interpretation, inspect the traveltime-distance graphs • • As a check on quality of data being acquired In order to decide which interpretational method to use: - simple solutions for planar layers and for a dipping refractor - more sophisticated analysis for the case of an irregular interface i ) ii ) iii ) iv ) v ) vi )

Travel time anomalies

Isolated spurious travel time of a first arrival, due to a mispick of the first arrival or a misplot of the correct travel time value Changes in velocity or thickness in the near-surface region Changes in surface topography Zones of different velocity within the intermediate depth range Localised topographic features on an otherwise planar refractor Lateral changes in refractor velocity

Travel time anomalies and their respective causes A) Bump and cusp in layer 1 B) Lens with anomalous velocity in layer 2 C) Cusp and bump at the interface between layers 2 and 3 D) Vertical, but narrow zone with anomalous velocity within layer 3