S-S imaging with vertical-force sources

We show examples of S-S images created from multicomponent seismic data generated by vertical-force sources that can be quite useful to seismic interpreters. Two source types are used: vertical vibrators and shot-hole explosives. We first discuss S-S images made from data generated by a vertical vibrator and recorded with vertical receiver arrays of 3C geophones. We next show images extracted from surface-based 3C geophones deployed around this VSP well as a 3D seismic grid. The energy sources used to generate these surface 3D seismic data were shot-hole explosives. In all data examples, we observe that each type of vertical-force source (vertical vibrator and shot-hole explosive) produces abundant direct-S energy on radial and transverse geophones. We find only minimal amounts of P-wave energy on transverse-receiver data. In contrast, radialreceiver data have significant P-wave events intermingled with radial-S events. The minimal amount of P-wave noise on transverse-receiver data makes it easier to study S-S wave physics and to create S-S images with transverse-S data. The data examples focus on transverse-S data created by vertical-force sources because interpreters will find it more convenient to process and use this S-mode. Subsequent publications will assign equal weight to radial-S and transverse-S data. Introduction Vertical-force sources produce P and SV displacements at the point where they apply their force to the earth. Because SV displacement vectors are oriented in every azimuth direction around the point of vertical-force application, a far-field geophone receives radial-S and transverse-S displacements regardless of where that sensor is positioned relative to a verticalforce source station (Hardage and Wagner, 2014). The attraction of using vertical-force sources to produce direct-S modes is that it will no longer be necessary for interpreters to deploy horizontal-vibrator sources or inclined-impact sources to generate directS data. The cost of S-S data acquisition will be reduced because full-elastic wavefield data can be acquired using only a simple, reliable, vertical-force source, and it will not be required to deploy two sources — a verticalforce source to generate direct-P data and a horizontalforce source to generate direct-S data. Perhaps more important is the fact that S-S seismic programs can now be implemented across any area where P-wave data can be acquired. This capability allows interpreters to acquire S-S data in swamps, marshes, desert dunes, dense timber, and across rugged mountainous terrains. These surface conditions are areas where the use of horizontal vibrators would not be considered. Although the source-side of S-S data acquisition is simplified by using a vertical-force source, the receiver-side of the data acquisition is unchanged. It will still be necessary to deploy 3C sensors to acquire S-S data when verticalforce sources are used. Numerous physical mechanisms have been proposed to explain why S waves are observed when verticalforce sources are used to generate illuminating wavefields. As will be explained in the following text, included in some of these proposals are assumptions that there has to be a subsurface interface local to a source station that causes P-to-SV mode conversion to occur near the source position, or that there must be a steplike variation in topography adjacent to the source station that creates a P-to-SV mode conversion close to the source location. Although these conditions do create converted SV modes, direct-S waves are created by vertical-force sources when these near-source physical anomalies are not present. The basic physics we have observed is that direct-S modes are produced at the point of application of a vertical force to the earth without the presence of local interfaces or close proximity to topographic variations. Geyer and Martner (1969) were among the first to recognize that a shot-hole explosive (one type of vertical-force source) produces S-wave energy. These authors came to this conclusion because they deviated from common industry practice of deploying only single-component vertical geophones during the decade of the 1960s, when they did their work, and use 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.