Transient electromagnetic (TEM) sounding data; San Luis Valley, Colorado, 2009

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.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，旨在绘制蓝色黏土层的分布位置，并调查疑似断层的存在情况。本次探测共在大沙丘国家公园及保护区内及其周边完成147组探测，另外在阿拉莫萨国家野生动物保护区东缘的汉森断崖附近完成6组探测。该区域内的蓝色黏土是重要的水文地质单元，其分隔了表层无压含水层与深部承压含水层。明确其分布位置对区域水文地质模型构建具有重要意义。此前的测井曲线分析显示，蓝色黏土的电阻率不超过10欧姆·米，与区域内砂岩、砾石及其他黏土层较高的电阻率形成鲜明对比，使其成为TEM探测的极佳目标体。探测结果显示，蓝色黏土的顶面存在显著起伏，表明其在沉积过程中或沉积后可能发生了变形。鉴于裂谷活动的存在，此类变形实属预期之中。在针对航空磁测数据识别出的断层开展的TEM剖面探测中，部分剖面显示出电阻率变化及（或）电阻率单元的地下高程起伏，这些特征均指示存在断层活动。但此类特征并未出现在所有疑似断层的对应剖面中。汉森断崖剖面显示出电阻率与导电体埋深的变化，该变化与东侧高地与西侧格兰德河泛滥平原之间的陡崖相吻合。