24 July 2019

Rotation measurements have found applications in various fields of geophysics ranging from near-surface archaeological mapping to large scale upper mantle tomographic inversion. In the absence of a cost-effective sensor sensitive over the typical seismic frequency bandwidth, rotation measurement can be approximated by finite-differencing the response of the vertical particle velocity over a short distance. Accurate estimates of the horizontal gradient of the vertical wavefield can be obtained when perturbations associated with the measurement are minimised. These perturbations can be sensor related, for example geophone sensitivity and natural frequency, and/or deployment related such as tilt and coupling errors. In this E-Lecture, we show how perturbations can affect the gradient measurement (the tilt error turns out to be dominant) and discuss the minimum receiver spacing required to avoid the impact of perturbations dominating the gradient estimates. We also discuss a “sensitivity” chart ranking the perturbations according to impact and indicate which one of these needs to be minimised in order to obtain reliable gradient estimates.

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