Huddle test measurement of a near Johnson noise limited geophone

TL;DR

This study accurately measured the sensor noise of two geophone configurations using improved huddle test techniques, demonstrating near Johnson noise limited performance in the 0.01 Hz to 100 Hz bandwidth.

Contribution

It introduces enhanced huddle test methods in quiet environments with many reference sensors, achieving measurements that closely match theoretical predictions.

Findings

Measured sensor noise matched predictions well

L-4C geophone achieved near Johnson noise limit

Effective noise measurement in the 0.01-100 Hz range

Abstract

In this paper the sensor noise of two geophone configurations (L-22D and L-4C geophones from Sercel with custom built amplifiers) was measured by performing two huddle tests. It is shown that the accuracy of the results can be significantly improved by performing the huddle test in a seismically quiet environment and by using a large number of reference sensors to remove the seismic foreground signal from the data. Using these two techniques, the measured sensor noise of the two geophone configurations matched calculated predictions remarkably well in the bandwidth of interest (0.01 Hz to 100 Hz). Low noise operational amplifiers OPA188 were utilized to amplify the L-4C geophone to give a sensor that was characterized to be near Johnson noise limited in the bandwidth of interest with a noise value of $10^{-11} \text{m}/\sqrt{\text{Hz}}$ at 1 Hz.