Moving magnetotellurics - experimental setup and case studies from Sweden

The idea of measuring three-component magnetic fields on board an aircraft or on a moving device in a frequency band from 30 Hz to 1000 Hz was tested. A rigid frame was constructed carrying the three magnetic sensors together with accelerometers and angle sensors. Two test measurements with the frame moving were carried out by putting it on a trailer behind a car and on board an aircraft. From both data sets we observed strong engine noise. Three stability test measurements were made by putting the magnetic sensors in parallel and we concluded that uncorrelated signals are generated during movements even when moving the frame at lower walking speed. It was finally decided to measure the three magnetic field components when the frame is at rest on the ground. Additionally two short electric field sensors were set up and five components electromagnetic fields were measured. The new data acquisition technique is based on the traditional magnetotelluric (MT) method but more efficient in several ways. We name it Moving Magnetotellurics (MMT) and did tests in different areas with different earth conditions. We found that the MMT method suffers from the same noise disturbances as the other MT methods do. Man-made noise such as power lines or railways reduce the data quality to a high degree, but its influence can be reduced using data analysis. Recordings at night time generally are less disturbed from the man-made noise. The data quality is much better when the measurement is carried out in areas with low resistivity. The MMT method is proven to be a applicable method in shallow depth studies, especially in areas where normal MT measurements are inconvenient and/or too expensive to carry out.