36 Component Seismic Data: Investigating Translational and Rotational Components in Exploration Seismology

Abstract

In conventional multicomponent seismic exploration, the wavefield is recorded by measuring translational motions in three directions using three-component sensors. A novel approach in land seismic acquisition is to additionally record the rotational components of the wavefield around the various Cartesian axes and to combine them with translational measurements. Additional rotational measurements provide the opportunity to locally extract valuable information on the propagating wavefield, that either cannot be obtained from conventional translational measurements alone or are challenging to extract. For example, rotational data facilitate wavefield separation, shear-wave (S-wave) imaging, and ground roll suppression because of the direct link between rotation and the S-wave component, but also enable local instantaneous phase velocity estimation. At the Earth’s free surface, rotational motions can be expressed in terms of spatial seismic wavefield gradients. Wavefield gradients can be estimated by differencing the outputs of closely spaced three-component translational sensors. The same approach can be adapted to source arrays: differencing of recordings from closely spaced translational (directed) sources can be used to simulate rotational sources that primarily emit S-waves. The combination of three components of translation and three components of rotation on both the source and the receiver side leads to a total of 36 measurable seismic components. In this thesis, I first verify that array-derived rotational rates, estimated using spatial seismic wavefield gradients, correspond to direct rotation measurements from rotational sensors. Then, I investigate the value of 36-component seismic data using synthetic as well as real field data. I show that rotational components around the vertical axis mainly contain horizontally polarized S-waves. I found that rotational components around the crossline (transverse horizontal) axis mainly contain ground-roll and vertically polarized S-waves and that these data can be combined with translational data to suppress ground roll. I show that the amplitudes of rotational components are dependent on the angle of incidence of the wavefield and that source-sided rotational components are reciprocal to receiver-sided rotational components. To accelerate multicomponent acquisition, I furthermore present a new multicomponent seismic vector-source, which uses the Galperin configuration to obtain orthogonal vector sources of equal impact patterns and constant source-coupling. This source allows a fast multicomponent dataacquisition in engineering and environmental exploration seismology.