SV-P: An ignored seismic mode that has great value for interpreters

Abstract

We show that SV-P reflectivity closely matches P-SV reflectivity; thus, in concept, an SV-P image should be as informative and as valuable as a P-SV image for seismic interpretation purposes. If the dip of rock layering is not severe, the length of the SV raypath involved in SV-P imaging is approximately the same as the length of the SV raypath in P-SV imaging; thus, the important lithology-sensitive V P ∕V S velocity ratio determined with SV-P data should be approximately the same as the V P ∕V S velocity ratio determined with P-SV data. We compare velocities used in P-SV imaging and SV-P imaging to emphasize the equivalence of P-SV and SV-P stacking velocities, and therefore seismic-derived V P ∕V S velocity ratios, obtained with both converted-wave modes. We compare images of P-SV and SV-P data to illustrate the high-quality images that can be made with a SV-P mode. The SV-P data used in these comparisons are recorded by vertical geophones, whereas the P-SV data are recorded by horizontal geophones. In the real-data examples we present, the energy sources that produced the downgoing SV wavefield are vertical-force sources, not horizontal-force sources. A vertical vibrator is used in the first case, and shot-hole explosives are used in the second case. The interpretation technology described here thus introduces the option of extracting valuable S-wave information and images from legacy P-wave data generated by a vertical-force source and recorded with only 1C vertical geophones. We discuss several principles involved in constructing SV-P images from VSP data because of the importance that VSP technology has in calibrating depth-based geology with surface-recorded SV-P data. We emphasize that cautious and attentive data processing procedures are required to segregate SV-P reflections and P-P reflections in VSP data.

Modelling S‐wave radiation for SP‐waves in isotropic and anisotropic media

“…) and that the amplitude of the outer illuminating S‐wave lobe can exceed the amplitude of the P‐wave by a factor of two to five (Hardage et al . ).…”

“…Hardage et al . () suggest that P‐wave multiples should be viewed as potential SP‐wave primary reflections. Fortunately, amplitude variation with offset may help differentiate the two.…”

“…Furthermore, the amplitude of these S‐waves can exceed the amplitude of the P‐wave by a factor of 2 to 5 for illuminating the subsurface (Hardage et al . ). Also, H&W discuss ‘full azimuth radiation of direct‐S modes away from a vertical‐force source’.…”

“…Motivation for Karr () and Li et al . () are two publications, which promote the use of SP‐waves: Hardage and Wagner () (H&W) and Hardage, Sava and Wagner (). The impact of these SP‐waves could be significant for multicomponent exploration geophysics: obviating the need for three component (3C) sensors and indicating that P‐wave multiples should be viewed as potential SP‐wave primary reflections.…”

“…They suggest 'that vertical-force sources are as efficient as horizontal-force sources for producing direct-S modes, and they are perhaps even more efficient' for illuminating the subsurface with S-waves, and illustrate this point at essentially an arbitrary polar angle (here denoted as θ with respect to vertical). Furthermore, the amplitude of these S-waves can exceed the amplitude of the P-wave by a factor of 2 to 5 for illuminating the subsurface (Hardage et al 2014). Also, H&W discuss 'full azimuth radiation of direct-S modes away from a vertical-force source'.…”