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Some requirements for PS-mode acquisition

CGG, Massy, France

Corresponding author: rgarotta@compuserve.com

The first 20% of the full text of this article appears below.

For land seismic surveys, it is still commonly thought that switching from conventional to multicomponent acquisition merely requires replacing vertical receivers with 3C receivers. This may work in some but not all cases. Fortunately, when it doesn't, the situation may be rectified by accounting for the physics of PS-mode propagation. In fact, this is often the prerequisite for obtaining comparable resolution of P and PS reflections at the target level—the condition for obtaining full benefit from the combined use of P and PS data for determining appropriate exploration attributes and seismic inversion.

Two major properties of PS mode, that have already been widely recognized and analyzed, are:

1. The critical nature of the receiver coupling pointed out by Thomsen (1997) and then studied by Meunier (1998).
   
2. The consequences of asymmetric PS propagation on survey design—in particular on the depth point sampling and target illumination as examined by Gaiser (1999) and Vermeer (1999).
   

This article will review some other physical phenomena—involving reflectivity, conversion, absorption, and intra-array statics—that should also be considered when field conditions require quantifying appropriate compensation for these negative effects.

	Comparison of P and PS waves

 
P and S reflectivities differ in two respects: the Poisson's reflectivity and the P or S relationships between reflectivity and angle, or offset.

Poisson's reflectivity is the difference between P- and S-velocity contrasts 2V_P/V_P - 2V_S/V_S. This relationship is important because the seismic amplitude of P-wave sections mainly reflects density and P-mode velocity contrast but PS-wave sections mainly reflect density and S-mode velocity contrast. Fortunately, for most geologic interfaces, Poisson's reflectivity is weak and P or PS events can generally be correlated.

However, Poisson's reflectivity is sometimes observed in sand/shale sequences in which P or S contrasts can be quite different. Perhaps most importantly, strong Poisson's reflectivity may exist in the presence of . . . [Full Text of this Article]