Generating direct-S modes with simple, low-cost, widely available seismic sources

More robust seismic interpretation canbe donewhen an interpretation project uses both compressional (P) and shear (S)data rather thanusingonlyoneseismicmode,whether thatmodebeaPmodeoranSmode.Unfortunately, this fundamental interpretation principle is frustrated by the cost and difficulty of deploying S-wave sources and by the limited availability of direct-S sources. We introduce a new seismic interpretation option based on direct-P and direct-Smodes generatedbyvertical-force sources.Toexplain thepotential of this newmethod for acquiringdirectS data, we evaluate real-data examples that illustrate the physics of P and S body-wave radiations generated at vertical-force-source stations. First, a 3D model of direct-S radiation by a vertical-force source is tested. Next, we discuss a field experiment in which a horizontal vibrator create a series of radially oriented SV displacements at small azimuth increments to simulate the full-azimuth distribution of SV displacements created by a vertical vibrator. The resulting data are recorded by a VSP seismic array and show that for a far-field sensor, some source-generated SV displacements are received as a radial-S wavefield and other SV displacements are received as a transverse-S wavefield. We use data from a walkaround VSP to create map views of direct-P and direct-S radiations from a vertical vibrator. We then use data from a walkaway VSP to illustrate cross-section views of the illumination lobes of direct-P and direct-S propagating into the subsurface from a vertical-vibrator source station. Introduction It is essential to define the terminology that will be used to describe seismic wave modes and seismic sources in this discussion. Regarding wave mode terminology, the expression “direct-S mode” will be used numerous times. This term, direct-S, refers to any S mode that is produced directly at a source station or so close to a source station that negligible error is introduced by assuming the point of origin is directly at the source station. This type of S mode differs from a converted-S mode produced at an interface remote from a source station. The most common example of the latter is the P-SV mode, which is now becoming widely used in seismic interpretation. The focus of this discussion will be on land-based seismic data acquisition; thus, towed air-gun arrays will be ignored. For land-based data acquisition, sources can be categorized as being one of two types — either a horizontal-force source or a vertical-force source. Inclined impact sources apply both a horizontal force and a vertical force to the earth and can be placed in either source classification (horizontal-force or vertical-force) as needed. Likewise, shot-hole explosives apply both vertical and horizontal forces to the earth and can be viewed as a vertical-force source or a horizontal-force source depending on an interpreter’s point of view. In our work, we consider shot-hole explosives to be a vertical-force source. Vertical vibrators are currently the most popular source in land operations and are strictly vertical-force sources. The most popular horizontal-force source is the horizontal vibrator. Most of the discussion in this paper will focus on comparing data generated by horizontal vibrators and vertical vibrators. It can be argued that S-mode data are more valuable than P-mode data in seismic interpretation because S waves are more sensitive to fractures and other anisotropic features of rock systems than are P waves and because the combination of fast-S and slow-S modes provides more rock property information than do P waves. This last comment is stressed because P-waves exhibit azimuth-dependent fast/slow behaviors that tend to be much weaker than those exhibited by S waves. For these reasons, we at the Exploration Geophysics Laboratory practice the principle that directS modes are as valuable as direct-P modes for evaluating subsurface geology. However, we, like other seismic interpreters, are frustrated by the restricted availability, added cost, and operational limitations of conventional direct-S sources, which presently are of two types — either horizontal vibrators or inclined impacts. The University of Texas at Austin, Bureau of Economic Geology, Austin, Texas, USA. E-mail: [email protected]; [email protected]. Manuscript received by the Editor 25 June 2013; published online 22 April 2014. This paper appears in Interpretation, Vol. 2, No. 2 (May 2014); p.