Remote‐reference Magnetotellurics: Equipment and Procedures*

A bstract During the past 3 years, major advances in the magnetotelluric technique have improved the quality of magnetotelluric data to the point where random errors in the impedance tensor and tipper are generally smaller than the uncertainty in their interpretation. The major factor in this improvement has been the introduction of the remote‐reference technique, although the use of ultrasensitive magnetometers and minicomputers for in‐field data processing has also been important. After a review of the remote‐reference technique, this paper describes the equipment and procedures used for remote‐reference magnetotellurics by the authors. Magnetometers using d.c. Superconducting Quantum Interference Devices typically have a sensitivity of 10 −14 T Hz −1/2 , a dynamic range of 10 7 in a 1 Hz bandwidth, and a slewing rate of 3 × 10 −5 .T s −1 at 10 kHz. The electric field measurements use conventional Cu‐CuSO 4 electrodes. The remote magnetic reference signals are transmitted to the base station using FM analog telemetry. The data are collected and processed by a minicomputer based on an LSI‐11 microprocessor; the essential results—for example, the apparent resistivities and the tipper components, with their probable errors—are available in the field. Practical details are given of the handling of superconducting devices, low temperature cryostats and liquid helium in the field. Various spurious noise sources are mentioned, and techniques for minimizing their effects are described.