In remote reference magnetotellurics one uses a reference magnetometer 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. The impedance tensor was determined as a function of the separation between this magnetometer and the magnetotelluric site, and compared with the tensor obtained simultaneously using the remote reference. This study yielded the surprising result that a separation of as little as 200 m 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 sec. It is concluded that a component of the noise arises from the non-plane wave nature of the magnetic and electric fields.--Modified journal abstract.

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