Noise correlation lengths in remote reference magnetotellurics.

Abstract

In remote reference magnetotellurics one uses a reference magnetometer, located typically several km from the magnetotelluric site, to obtain estimates of the impedance tensor that are unbiased by random noise. Measurements have been made of the correlation length of this noise by using a third SQUID magnetometer as a local reference. This study yielded the surprising result that a separation of as little as 200m was sufficient to obtain an unbiased estimate of the impedance tensor. The magnitude of the magnetic noise was determined with two different techniques: one involving the impedance tensor, and the other involving a magnetic transfer tensor. The second method produced a substantially lower estimate of the magnetic noise at periods longer than about 1 second. It is concluded that a component of the noise arises from the non-plane wave nature of the magnetic and electric fields.-from Authors