40_03 - 3.0 Offset, Another Dimension E arlier chapters...

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3.0 Offset, Another Dimension Earlier chapters have assumed that the shot and the geophone are located in the same place. The reality is that there is often as much as a 3- km horizontal separation between them. The 3-km offset is comparable to the depth of many petroleum reservoirs. Offset is another dimension in the analysis of data. At the time of writ- ing, this dimension is often represented in field operations by about 48 chan- nels. No one seems to believe, however, that 48 channels is enough. Record- ing systems with as many as 1024 channels are coming into use. The offset dimension adds three important aspects to reflection seismol- ogy. First, it enables us to routinely measure the velocity of seismic waves in rocks. This velocity has been assumed to be known in the previous chapters of this book. Second, it gives us data redundancy: it gives independent meas- urements of quantities that should be the same. Superposition of the meas- urements (stacking) offers the potential for signal enhancement by destructive interference of noise. Third (a disadvantage), since the offset is nonzero, pro- cedures for migration take on another element of complexity. By the end of this chapter we will be trying to deal with three confusing subjects at the same time - dip, offset, and lateral velocity variation. Theoretically it seems that offset should offer us the possibility of identi- fying rocks by observing the reflection coefficient as a function of angle, both for P waves and for P-to-S converted waves. The reality seems to be that neither measurement can be made reliably, if at all. See Section 1.4 for a fuller discussion of converted waves, an interesting subject for research, with a large potential for practical rewards. See also Ostrander [1984] and Tatham and Stoffa [1976]. The reasons for the difficulty in measurement, and the resolution of the difficulty, are, however, not the goal of this book. This goal is instead to enable us to deal effectively with that which is routinely observ- able.
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OFFSET 3.0 O$set, Another Dimension Stacking Diagrams First, define the midpoint y between the shot and geophone, and define h to be half the horizontal offset between the shot and geophone: The reason for using half the offset in the equations is to simplify and sym- metrize many later equations. Offset is defined with g - s rather than with s - g so that positive offset means waves moving in the positive x direc- tion. In the marine case, this means the ship is presumed to sail negatively along the x-axis. In reality the ship may go either way, and shot points may either increase or decrease as the survey proceeds. In some situations you can clarify matters by setting the field observer's shot-point numbers to negative values. Data is defined experimentally in the space of (s, g ). Equation (I) represents a change of coordinates to the space of (y , h). Midpoint-offset coordinates are especially useful for interpretation and data processing. Since the data is also a function of the travel time t, the full dataset lies in a volume.
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This note was uploaded on 02/10/2011 for the course GEOL 7320 taught by Professor Stewart during the Spring '11 term at University of Houston - Downtown.

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40_03 - 3.0 Offset, Another Dimension E arlier chapters...

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