Each receiver station commonly is not a single instrument, but rather a array of several receivers, connected electronically and a centered around a point. ARRAY or GROUP: Receiver stations of connected geophones. Geophone responses for a group add together as a single seismic trace. Response of one geophone group. If the group length is equal to the wavelength of a Rayleigh wave, positive signals from geophones moving up cancel negative signals from those moving down, the Rayleigh wave is thus attenuated. A reflected P-wave at close distance to the source moves the group of geophones up or down in unison, enhancing the arrival. The geometry of an array ( or a geophone group) is designed to cancel certain unwanted signals, while enhancing reflected waves. GROUNDROLL: Rayleigh waves which are example of unwanted noise.
CMP method provides redundancy of information that enhances signal-to-noise ratio and facilitates the determination of seismic velocities used in processing and interpretation. EXAMPLE: During the first shot, the midpoint between the source (S1) and first receiver (R1) is the same point as S2/S3 and S3/S5, for shots 2 and 3 respectively.
SHOT GATHER: Each record of six seismic traces is known as a shot gather. CMP Gather: A display of traces corresponding to reflections around a each source, is known as CMP or CDP. NMO: The differences in travel time are called normal moveout. PRIMARY REFLECTION= Resulted from energy that undergoes only one reflection before returning to the surface.
ÖNCEL AKADEMİ: INTRODUCTION TO GEOPHYSICS
Introduction to Geophysics Ali Oncel [email_address] Department of Earth Sciences KFUPM Seismic Reflection 2: Acquisiton, Processing, and Waveform Analysis Introduction to Geophysics-KFUPM
Previous Lecture <ul><li>What Seismic Reflection Provides? </li></ul><ul><li>What requires for a successful study of Reflection? </li></ul><ul><li>What provides Seismic Reflection on subsurface geological Model? </li></ul><ul><li>On Land- Seismic Reflection </li></ul><ul><li>On Sea- Seismic Reflection: Photos from Seismic Crew </li></ul><ul><li>Equipments: Vibrator Track and Receiver Truck, Geophones, Battery </li></ul><ul><li>What is a seismic section? </li></ul><ul><li>what is a Seismic Line? </li></ul><ul><li>Nature of Seismic Section? </li></ul><ul><li>Acoustic Impedance </li></ul><ul><li>Enhancement the quality for Compressional waves? </li></ul><ul><li>Single Channel Seismic Reflection? </li></ul><ul><li>What is a seismic Profile? </li></ul>Introduction to Geophysics-KFUPM
Reflection Seismology Introduction to Geophysics-KFUPM
Seismic Methods Introduction to Geophysics-KFUPM
Introduction to Geophysics-KFUPM Angle Incidence = Angle Reflection Multifold Reflections Four fold coverage of the CMP
Receiver Arrays Introduction to Geophysics-KFUPM
Common Depth Point Common Midpoint Many Source Receiver Offsets Sample Same Depth Point Introduction to Geophysics-KFUPM CMP Method Figure 6.4 of Reynolds Sixfold coverage of the CDP
Introduction to Geophysics-KFUPM CMP Method Sample Same Subsurface Point – Common Depth Point 4 Times
Introduction to Geophysics-KFUPM Processing Fold of Stack Coverage Redundancy Improve Signal to Noise Ratio
=Travel time from source, to interface, to receiver X= horizontal distance from source to receiver V 1 = seismic velocity of layer above distance T 0 = T-axis intercept (time directly down to interface, straight back up to shot location h = thickness of layer above interface V 1 = seismic layer of above interface Introduction to Geophysics-KFUPM T NMO = T f - t 0 Normal moveout is the difference between the travel time and hyperbola (T f ) and the intercept time (t 0 ). Normal Moveout See EQ 4-10 of Robinson and Coruh, 1988