The CSMT (controlled-source magnetotellurics) and TDEM (time-domain electromagnetics) methods adopting an artificial electromagnetic source have been used, as electromagnetic exploration methods for relatively deep geological targets. These methods transmit an electromagnetic signal from the source which is placed away from signal measurement sites. By measuring weak electromagnetic fields induced in ground at various frequencies, we can evaluate a subsurface resistivity distribution from the surface to several kilometers.
 Even though these methods use the artificial and controllable source, periodic electromagnetic noises generated by power lines or electrical appliances become fatal obstacles for receiving the weak electromagnetic fields, especially near urban areas. In order to overcome this periodic noise problem, we have examined some efficient wave-analysis schemes. Consequently, we arrived at the conclusion that it is a key factor to improve the resolution of frequency spectrum because it can avoid diffusion of the periodic noises in the process of frequency-spectrum analysis. To improve the resolution, it is required to extend the sampling length of time-series data for the frequency-spectrum analysis. For dealing with long sampling time-series data, accurate time synchronization is crucial. In this study, we have developed a deep electromagnetic exploration instrument with high frequency spectrum resolution by using GPS synchronization. A field test of the instrument has been carried out in the Nanki Sirahama Hot Spring area, in the central western part of Japan. As a result, we obtained better quality data in comparison with the data evaluated by a conventional stacking process, even if there are many noises in a town.