Introduction to Geophysics

Ali Onceloncel@kfupm.edu.

saDepartment of Earth SciencesKFUPM

Seismic Reflection: Acquisiton, Processing, and Waveform Analysis

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Review: Travel-Time Curves

Measurable features in the shot record

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•Xcrit (Critical distance) - distance at which first head wave arrive at surface•Xcross (Cross-over distance) - is point at which head wave overtakes direct wave

• slope is 1/V1 for direct wave and 1/V2 for head wave• direct wave travels at velocity V1 and head wave at V2•Intercept time ti - the time at which back-extrapolated refractor curve intersects time axisIn

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Fig. 5.1 of Lillie

Seismic Reflection

•Refraction and reflection studies are complimentary.

•Refraction providing gross crustal velocities and thickness,

•Reflection showing finer details of structure and stratigraphy.

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Requirements for successful works by

Reflection and Refraction

Refraction requires that receivers extend to well beyond the Xcr, so that refraction spread lengths need to be five to ten times the depth to the deepest refractor of interest.

Reflections require usually vertically arriving which is less than the Xc. Then, reflection spread lengths therefore approximate the depth of the deepest reflector of interest.

Fig. 5.1 of Lillie

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Seismic Reflection

•Since it was developed in the 1920s and 30s as a tool for oil and gas exploration (Petty, 1976). •It has been used mostly as a Geophysical Technique for• Petroleum Application• In Conjunction with Refraction• Land and at Sea • On a Variety of Scales - 10’s of Meters to Crust and Mantle

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Objectives

• Visualizing high resolution of subsurface detail based upon the Seismic Velocity Inversion as:

•Structure and Stratigraphy

• Physical Properties of Rock/Sediment Layers

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Seismic Reflection - OnlandIn

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Kadif AreaIn

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Qatif Area

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FUPM Vibrator Truck

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Vibrator Truck

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At Sea

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Recording Truck

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Recording Truck

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Inside the Recording Truck

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Oncel, 2004

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Normal Incidence Seismic Section = Resulting from raypaths Striking reflectors at 90°.

What is a Seismic Section?In

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What is a Seismic Section?In

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Seismic Line Hundreds to Thousands of Seismic Traces

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• Composed of individual seismograms or seismic traces

• Reflector or Event has a two-way traveltime and amplitude

• Reflector or Event as a Wavelet

Nature of Seismic SectionIn

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•Acoustic Impedance

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Most geophones are designed to respond the motions that are the vertical. Reflected compressional waves (vertical particle motion) are enhanced, at the expense of events that produce horizontal motions at the surface (direct compressional waves; reflected shear waves).

Enhancement: Compressional WavesIn

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Single Channel Seismic ReflectionIn

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Seismic Profile• Produce Quasi-Cross-Section or 3-D Volume• Translation from 2-way Travel Times to Depth

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For next time read over

pages 100 -110 in Chapter 5

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