Transient electromagnetic (TEM) sounding data; San Luis Valley, Colorado, 2011

Transient electromagnetic (TEM) soundings were made in the San Luis Valley, Colorado, to map the location of a blue clay unit as well as to investigate the presence of suspected faults. A total of 147 soundings were made near and in Great Sand Dunes National Park and Preserve, an additional 6 soundings were made near Hansen Bluff on the eastern edge of the Alamosa National Wildlife Refuge. The blue clay is a significant hydrologic feature in the area that separates an unconfined surface aquifer from a deeper confined aquifer. Knowledge of its location is important to regional hydrological models. Previous analysis of well logs has shown that the blue clay has a resistivity of 10 ohm-meters or less, which is in contrast to the higher resistivity of sand, gravel, and other clay units found in the area, making it a very good target for TEM soundings. The top of the blue clay was found to have considerable relief suggesting the possibility of deformation of the clay during or after deposition. Because of rift activity deformation is to be expected. Of the TEM profiles made across faults identified by aeromagnetic data, some showed resistivity variations and (or) subsurface elevation relief of resistivity units suggestive of faulting. Such patterns were not associated with all suspected faults. The Hansen Bluff profile showed variations in resistivity and depth to conductor that coincide with a scarp between the highlands to the east and the floodplain of the Rio Grande River to the west.

为绘制蓝粘土层的空间分布并调查疑似断层的存在，科研人员在科罗拉多州圣路易斯谷开展了瞬变电磁（Transient Electromagnetic, TEM）探测。本次探测总计在大沙丘国家公园及保护区内及其周边完成147次探测，另在阿拉莫萨国家野生动物保护区东缘的汉森崖附近开展了6次探测。蓝粘土是该区域重要的水文地质特征，其分隔了表层无压含水层与深部承压含水层。明确该层的空间分布对区域水文模型的构建具有关键意义。此前的测井曲线分析显示，蓝粘土的电阻率不超过10欧姆·米，与区域内砂岩、砾岩及其他粘土层的高电阻率形成显著差异，使其成为瞬变电磁探测的理想目标。探测发现蓝粘土顶界面存在显著起伏，暗示其在沉积过程中或沉积后发生了变形；鉴于区域存在裂谷活动，此类变形本在预期之内。在通过航空磁测数据识别出的断层区域开展的瞬变电磁剖面探测中，部分剖面呈现出电阻率变化以及（或）电阻率地层的地下高程起伏，这些特征均指向断层活动的存在，但并非所有疑似断层都对应此类异常模式。汉森崖剖面的电阻率与导体埋深变化特征，与东侧高地与西侧里奥格兰德河漫滩之间的陡崖地貌完全吻合。