Seismic Data Processing Lecture 16

Kirchhoff Migration & land survey

Prepared by Dr. Amin Khalil

School of Physics, USM2014

Why 3D?!!!

Because:1- The earth change its properties in three

dimensions.

2- In 2D we may get reflections and diffraction from points outside the plane. Thus all our migration will fail or become misleading.

Comparison

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3D migration

Downward Continuation

Artifact

Velocity is needed for applying 3D migration. In the present case we need to know the velocity change in 3D meaning with x, y and z direction which is not feasible. In addition using velocity which change spatially will also complicate the problem.

Land Seismic Survey

Survey Elements covered

1- Sources

2- Geophones

3- Geophone interval and arrays

4- Spread length

5- Recording time

6- Sample rate

sources

Many factors affect the choice of the proper source for given experiment. One of this factors are the maximum depth of investigation. Deeper exploration requires more energy thus sometimes explosives might be used, in shallow surveying a hammer or weight drop may be used. The seismic sources should have narrow wavelet. A perfect wavelet is the spike or the delta function.

Why narrow or finite width is preferred?

To increase resolution and ability to detect thin beds.

Source types

1- Dynamite

2- Air Shooting

2- Land air gun

3- vibrator or vibroseis

4- Sozie

5- Weight drop and hammer

Dynamite

The most common type of impulsive source is a subterranean explosive source (dynamite). Typically an explosive source is used in the following manner: a hole (usually between 6-30 m) is drilled at the desired location; the charge is placed in the hole with a loading pole (to insure it achieves the proper position); and the charge is detonated with an electrically ignited blasting cap. The frequency of waves from small charges is higher than that of large charges and the depth of the charge affects noise generation. Standard charges expend their energy in all direction. A special type of explosive charge is used when a focused charge direction is desired.

Dynamite movie

Dynamite Problems

1- Dynamite can not be used in cities or in an easy accessible areas.

2- Dynamite can cause damages and requires high security measures.

3- Dynamite must be fired away from water area or porous lithology.

Air Shooting

Air shooting is another seismic impulse generating technique. Explosive charges are also used in air shooting but rather than being placed in boreholes they are placed atop an array of sticks and detonated above the ground. The expanding air wave created by the explosions creates the impulse as it strikes the ground.

Land Air Gun

This device consists of a water pan with an expandable diapragm and an air gun. This device is placed beneath a vehicle and is held against the ground by the vehicle's weight. The gun fires compressed air into the water pan, forcing the pan's diaphragm into the ground, creating an impulse which is transmitted into the earth.

Vibrator

The most common type of nonimpulsive source is a vibrator. A vibrator is composed of a base plate connected to a piston inside a large mass. Oil is either pumped in or out of the piston chamber, causing the mass to oscillate. The oscillations are transmitted through the plate into the ground. Vibrators are generally mounted on large trucks which move together over the prescribed area. Vibrators are useful in cities and other easily-damaged areas due to their low energy density.

Sozie or Whacker

Another non-impulsive source is the Sozie, or the "whacker". In the Sozie method, an impactor strikes the ground 5-10 times/sec for about three minutes and the results are recorded on detectors.

Weight Drop

Weight droppers also generate impulses. The operation of a weight dropper consists simply of a dropping a 3 ton weight from a 3 m height, and recording the impulses. The impulse generated is not particularly strong and weight droppers are generally only good in deserts, where the waterless conditions are best suited for this technique

Thank you