Conventional Sensors: Is There Anything Useable Below Their Natural Frequency?

Summary

It is often believed that conventional 10 Hz geophones are inadequate to recover low-frequency signals. Using the low-frequency signal generated by tectonic, teleseismic earthquakes, we demonstrate that both phase and amplitudes are properly recovered, provided that the instrumental responses are correctly accounted for. Instead of focusing on sensor’s sensitivity and bandwidth, we emphasize on the importance of signal-to-noise ratio in the analysis of such type of waves, with a particular consideration for self-noise at low frequencies.

Introduction

Low frequencies are being increasingly sought for in seismic imaging as it can serve many purposes, such as large wavelength starting model in full waveform inversion (FWI) or increased resolution of deep targets. Beaten et al. (2013) demonstrate decisive FWI improvement when starting the inversion from 1.5 Hz. These low frequency waves are recorded using common 28.8 V/m/s, 10 Hz geophones. It is however often believed that these sensors are unable to properly record signal below their natural frequency. Yet their ease of use and robustness make these sensors the standard in the industry. In the present paper, we show that these sensors are able to record this low-frequency part of a signal provided that (1) their frequency response is correctly accounted for and (2) that the signal-to-noise ratio is high enough. To this effect, we will first discuss the too often overlooked signal-to-noise ratio notion, and show how low sensitivity, 10 Hz geophones can record the low frequency part of a seismic wavefield. We illustrate this statement with the continuous recordings of the Mw 9 Tohoku great earthquake at a teleseismic distance.