0
Upcoming SlideShare
×

Thanks for flagging this SlideShare!

Oops! An error has occurred.

×
Saving this for later? Get the SlideShare app to save on your phone or tablet. Read anywhere, anytime – even offline.
Standard text messaging rates apply

# Reflection seismology 1

216

Published on

Published in: Education, Technology, Business
1 Like
Statistics
Notes
• Full Name
Comment goes here.

Are you sure you want to Yes No
Your message goes here
• Be the first to comment

Views
Total Views
216
On Slideshare
0
From Embeds
0
Number of Embeds
0
Actions
Shares
0
18
0
Likes
1
Embeds 0
No embeds

No notes for slide

### Transcript

• 1. Seismic wave propagation: Connected set of disturbance and wave motion is perpendicular to these wave fronts 3
• 2. When do we have a good reflection ??? Acoustic impedance: The acoustic impedance of a rock is determined by multiplying its density by its seismic wave-wave velocity, i.e., V. Acoustic impedance is generally designated as Z. 4
• 3. In practice, reflection seismology is carried out at comparatively small angles of incidence 5 More reflection coefficient better reflection.
• 4. 6
• 5. &#x2022; For SMALL INCIDENT ANGLES, all of the energy is the reflected or transmitted Pwaves there are essentially no S-waves. &#x2022; As the incident angle increases some of the energy goes into reflected and transmitted S-waves Conversion of P wave in to s wave at the interface 7
• 6. Reflected P- and S-waves and refracted P- and S-waves are generated from the incident P-wave.
• 7. CRITICAL ANGLE AND HEAD WAVES Critical Distance Critical angle Vp Lower Vp Higher The passage of the refracted wave along the Angle of refraction= 90 interface in the lower medium generates a plane wave traveling upward in the upper medium. Critically refracted waves are called head waves
• 8. Subcritical reflection: Angle of incidence less than the critical angle. Critical reflection: The ray that is incident on the boundary at C is called the critical ray because it experiences critical refraction. The critical ray is accompanied by a critical reflection. It reaches the surface at a critical distance (xc) from the source at O. Supercritical reflection: The seismic rays that are incident more obliquely than the critical angle are reflected almost completely. These reflections are termed supercritical reflections, or simply wide-angle reflections.
• 9. REFLECTION SEISMOLOGY finding the depths to reflecting surfaces and the seismic velocities of subsurface rock layers Principles: 1. A seismic signal (e.g., an explosion) is produced at a known place at a known time, and the echoes reflected from the boundaries between rock layers with different seismic velocities and densities are recorded and analyzed. 2. Compactly designed, robust, electromagnetic seismometers &#x2013; called &#x201C;geophones&#x201D; in industrial usage &#x2013; are spread in the region of subcritical reflection, within the critical distance from the shot-point, where no refracted arrivals are possible.
• 10. Reflection seismic data are most usually acquired along profiles that cross geological structures as nearly as possible normal to the strike of the structure. Reflection at a horizontal interface: &#x2022; d- Depth of the reflector below the shot point. &#x2022; x- Horizontal distance from the shot point to receiver at G &#x2022; The first signal received at G is from the direct wave that travels directly along SG (body wave). The travel-time t of the reflected ray SRG is (SR+RG)/V. However, SR and RG are equal and therefore
• 11. x S G &#x3B8; 2d R Vt V= velocity T= travel time S&#x2019; Hence in right angled triangle SS&#x2019;G we have x2 + (2d)2 = (vt)2 Equation of Hyperbola {(vt)2 / 4d2 }-{(x2)/4d2}=1 Hence, reflection travel time curve are hyperbola.
• 12. T= Reflection travel time &#x2022; At t=0, t=to (vertical travel time given by 2d/v) &#x2022; For large distances from the shot-point (x&gt;&gt;2d) the traveltime of the reflected ray approaches the travel-time of the direct ray and the hyperbola is asymptotic to the two lines tx/V A principle goal of seismic reflection profiling is usually to find the vertical distance (d) to a reflecting interface.
• 13. &#x2022; This can be determined from t0, the two-way reflection traveltime recorded by a geophone at the shot-point, once the velocity V is known. Determination of the velocity: &#x2022; One way of determining the velocity is by comparing t0 with the travel-time tx to a geophone at distance x. 1). laid out geophone close to the shot-point and the assumption is made that the geophone distance is much less than the depth of the reflector (x&lt;&lt;d). This can give us to approximately. And we have: Or, As d= Vt0 equ. 1 (Higher order terms have been neglected here
• 14. The difference between the travel-time tx and the shotpoint travel-time t0 is the normal moveout, &#x2206;tn = tx &#x2013; t0. By rearranging Eq. 1 we get The echo time t0 and the normal moveout time &#x2206;tn are found from the reflection data. The distance x of the geophone from the shot-point is known and therefore the layer velocity V can be determined. The depth d of the reflecting horizon can then be found by using the formula for the echo time.