微動アレイ探査を用いた被覆層厚の推定 ―チリ共和国斑岩銅鉱床の例―

抄録

In a porphyry copper district in Chile where surficial cover layers composed predominantly of sand and gravel are widely distributed, we conducted a microtremor array survey to estimate the thickness of the cover. One-dimensional S-wave velocity (Vs) profiles were derived from observations at 24 sites within an area of approximately 2.0 km (E–W) by 2.5 km (N–S), yielding S-wave velocity structures to investigation depths of 300–1,000 m. Assuming Vs = 550 m/s as the S-wave velocity at the top of bedrock, the cover thickness was estimated to be 7–29.5 m. In addition, within the bedrock beneath the cover, distinct S-wave velocity contrasts were also identified near 1,500 m/s, 3,000 m/s, and 4,000 m/s. Among these, the 4,000 m/s isovelocity surface forms a NW–SE-trending depression in the central part of the study area, which is consistent with the inferred fault strike in the region and broadly coincides with the mineralization center proposed in existing ore-deposit models. These results suggest that the microtremor array survey may be applicable not only to estimating cover thickness but also as a geophysical tool for porphyry copper exploration. Future work should include comparison with drill cores and integrated interpretation with multiple geophysical datasets that account for three-dimensional effects, in order to evaluate the geologic and ore-deposit significance of the S-wave velocity boundaries.