A Proposal of optimum calculation settings of continuous wavelet transform in magnetotelluric data processing
Ogawa, Hiroki; Hama, Yuki*; Asamori, Koichi; Ueda, Takumi*
In the magnetotelluric (MT) method, so as to identify the subsurface resistivity structure, the apparent resistivity and phase profiles are calculated by transforming time-series data into spectral data. The continuous wavelet transform (CWT) is well known as a new method of time-frequency analysis instead of the short-time Fourier transform. The CWT is superior in processing non-stationary wideband signals like the MT signal by adjusting the size of the wavelet according to the value of frequency. However, the calculation settings of the CWT, such as the type of basis function and the wavelet parameter, are often determined empirically because of the arbitrariness of the shape of the wavelet. Although there might be differences between the calculated MT responses and the true responses due to improper settings of the CWT, there are no detailed studies considering the effect of numerical errors derived from spectral transforms on MT data. In this study, focusing on the frequency band between 0.001 Hz and 1 Hz, we examined the optimum calculation settings of the CWT in processing MT data in terms of suppressing the numerical errors caused by the spectral transform of time-series data. We also show the validity of the proposed calculation settings by applying the CWT to MT survey data of different types. Superiority of the CWT with proposed settings is suggested especially when the signal-to-noise ratio of observed data is low. Consequently, the proposed calculation settings were confirmed to strike a balance between the resolutions of the time and frequency domains well and will therefore be effective in obtaining reliable MT responses.