A proposed polarity standard for multicomponent seismic data

Abstract

This paper proposes a multicomponent acquisition and preprocessing polarity standard that will apply generally to the three Cartesian geophone components and the hydrophone or microphone components of a 2‐D or 3‐D multicomponent survey on land, at the sea bottom, acquired as a vertical seismic profile, vertical‐cable, or marine streamer survey. We use a four‐component ocean‐bottom data set for purposes of illustration and example. A primary objective is a consistent system of polarity specifications to facilitate consistent horizon correlation among multicomponent data sets and enable determination of correct reflectivity polarity.The basis of this standard is the current SEG polarity standard, first enunciated as a field‐recording standard for vertical geophone data and hydrophone streamer data. It is founded on a right‐handed coordinate system: z positive downward; x positive in the forward line direction in a 2‐D survey, or a specified direction in a 3‐D survey, usually that of the receiver‐cable lines; and y positive in the direction 90° clockwise from x. The polarities of these axes determine the polarity of ground motion in any component direction (e.g., downward ground motion recording as positive values on the vertical‐geophone trace). According also to this SEG standard, a pressure decrease is to be recorded as positive output on the hydrophone trace. We also recommend a cyclic indexing convention, [W, X, Y, Z] or [0, 1, 2, 3], to denote hydrophone or microphone (pressure), inline (radial) geophone, crossline (transverse) geophone, and vertical geophone, respectively.We distinguish among three kinds of polarity standard: acquisition, preprocessing, and final‐display standards. The acquisition standard (summarized in the preceding paragraph) relates instrument output solely to sense of ground motion (geophones) and of pressure change (hydrophones). Polarity considerations beyond this [involving, e.g., source type, wave type (P or S), direction of arrival, anisotropy, tap‐test adjustments, etc.] fall under preprocessing polarity standards. We largely defer any consideration of a display standard.