An acquisition polarity standard for multicomponent seismic data
R. James Brown, Robert R. Stewart, James E. Gaiser, Donald C. Lawton
This paper deals with the question of a polarity standard to apply generally to 3D multicomponent seismic data, or any subset thereof. The primary goal is to provide a field recording standard according to which the designations normal polarity and reverse(d) polarity may be used on any or all of the three or four components normally acquired in a multicomponent seismic survey. Since this includes three geophone components and a possibly a hydrophone or microphone component, this polarity standard should - and does - also apply in a consistent way to 2D or 3D land 3C, 3C VSP, vertical cable, and streamer seismic data.
Recommendations or guidelines are given on how to proceed, both in acquisition and preprocessing, to determine the polarity for any particular data component. The basis of this standard is the SEG polarity standard, which was first enunciated as a field-recording standard for vertical-component land data and marine hydrophone data. It is founded on a right-handed coordinate system, with z positive downward, x positive in the positive line direction in 2D, or some specified principal direction in 3D, usually that of the receiver-cable lines, and y positive in the direction 90 degrees clockwise from x. The polarities of these axes determine the polarity of ground motion in any component direction.
The present recommendations are for a field-recording or acquisition standard (which is taken to include certain preprocessing steps) rather than a final-display polarity standard. A primary objective has been an internally consistent system of polarity specifications, encompassing all of the recorded components, in order to facilitate, among other things, consistent horizon correlation among multicomponent datasets and determination of correct reflectivity polarity. We also recommend a cyclic indexing convention for multicomponent seismic data, namely (W, X, Y, Z) such that, when used e.g. as subscripts, W would denote hydrophone (pressure), X inline geophone, Y crossline, and Z vertical. The (x, y, z) symbols denote the Cartesian distance coordinates of the right-handed system specified above.