Annual/Seasonal Variation in Induction Vectors at Different Geological Locations in an Indian Sector

Magnetotelluric (MT) transfer functions, which carry information about the subsurface resistivity distribution, remain constant over time in geologically stable regions. However, recent results indicate the presence of significant seasonal changes in tipper/induction arrows irrespective of the geological processes. We have obtained the annual and seasonal variations in induction vectors at six observatories located in different geographical (island, inland, and coastal areas) and tectonic (cratonic, and passive/convergent margins) settings in India. Based on a 30-day period of data, the tipper/induction vectors are estimated by using 1-min magnetic variation data for 2 years at each site for a period range of 4–150 min. The average tipper amplitude variations range from 0.2 to 0.3 at coastal sites, from 0.1 to 0.2 at island sites, and < 0.1 at inland sites. The seasonal pattern at low-latitude sites is greatest at the summer solstice and smallest at the winter solstice, whereas at equatorial sites it is greatest at the winter solstice/autumnal equinox and smallest at the summer solstice. At low-latitude and equatorial sites, seasonal variation in the direction of the induction vectors is at the sharp conductivity contrast periods. The varying ranges of the seasonality of tipper amplitude and direction variations at different geological regimes are due to the interaction of the source field with the conductivity structure and the lateral changes in the conductivity around the particular site.