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Coal reserve volume in Fujian Province, China, ranks third among the nine provinces in southern China. Over the past decades, most coal mines in uncovered and semi-covered coal strata have been explored and developed. Present exploration mainly focuses on deep coal strata, which are generally covered by older strata and volcanic strata. Because of the terrain and deep targets, seismic prospecting is difficult.
The major coal-containing strata that can be excavated in Fujian are within the Tungtzeyan Formation of the Lower Permian Series. The thickness of this formation is greater than 680 m. The CSAMT (controlled source audio frequency magnetotelluric) method is used to delineate four deep coal strata units. The aim of this prospecting is to delineate the upper and lower boundaries of the stratum containing the coal units. The prospecting target depth is approximately 1,200 m, which required a frequency range between 8,192 Hz and 1 Hz to provide sufficient data coverage. The distance from the transmitter to the survey line was 8 km, with a transmitting current of 10 amps and voltage of 600 volts. The time synchronization between the transmitter and receiver was controlled by a global positioning system. Major static problems were encountered during the CSAMT survey. Both terrain and static corrections were applied during data processing, which smoothed the inverted results for better interpretation. Comparison of the CSAMT interpreted results with available engineering geology information shows a clear difference in electrical properties between coal stratum, upper old stratum, red-beds and volcanic strata. The CSAMT method is useful in identifying the coal strata under these geological conditions.