oa Characterisation of cultural noise in the AMT band

As part of a project to investigate long-term exposure of low-level earth currents on dairy cows, an unusual multiple-site, remote-reference audio magnetotelluric (AMT) data set was acquired on four Wisconsin dairy farms and two remote sites in the summer of 2001. At each dairy, several five-component sites were recorded along with a remote located in a wildlife refuge, at a distance of roughly 30 miles from the farms. EMI MT24 systems were deployed under power line transformers, above and below three-phase transmission lines, in close proximity to machinery such as compressors, pumps, electric fences, and large fans. Data were recorded at the dairies with 20 m dipoles to capture the natural signal while not overloading the instrument electronics with high cultural signals. Sampling frequencies of 48, 9.6 and 2 kHz were employed.

To accurately determine the characteristics of the earth's fields, including the magnitude of the very narrow band-nature of 60 Hz signal and its harmonics, normal impedance estimation techniques using relatively short segments of the time series were found inadequate. Thus, the data were processed with a Fast Fourier Transform of length 524,288 applied to each of the 500,000 data point segments. Electrical resistivity data were acquired with Wenner array spacings of 1 to 10 m. Earth current densities were estimated from the 1 m resistivity values and the electric field data from the MT24.

Short recording times were used to determine earth currents under different conditions, not to estimate the impedance. Hence, we present characteristics of various cultural noise sources to improve the knowledge base of what practitioners must deal with in noisy environments. We are not evaluating processing technique, but have applied some robust processing to observe if it is possible to attain impedance estimates in the presence of coherent noise and low signal strength.