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.为绘制蓝黏土层（blue clay unit）的空间分布并探查疑似断层的存在，研究团队在科罗拉多州圣路易斯谷开展了瞬变电磁法（Transient Electromagnetic, TEM）探测工作。本次工作共在大沙丘国家公园和保护区及其周边完成147组瞬变电磁测深，另在阿拉莫萨国家野生动物保护区东缘的汉森崖附近增设6组测深。蓝黏土层是该区域关键的水文分界特征，可分隔无压地表含水层与深层承压含水层，明确其空间展布对区域水文模型构建具有重要意义。此前的测井曲线分析结果显示，蓝黏土层的电阻率不高于10欧姆·米，与该区域砂岩、砾岩及其他黏土层的高电阻率特征形成鲜明对比，使其成为瞬变电磁法探测的理想目标。探测发现蓝黏土层顶部存在显著起伏，表明其在沉积过程中或沉积后可能发生构造变形；而区域裂谷活动通常会引发此类变形，符合地质预期。针对航空磁测数据识别出的断层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）电性单元的地下高程起伏，此类特征均指向断层活动，但并非所有疑似断层都匹配此类异常模式。汉森崖剖面的电阻率与导体埋深变化特征，与东侧高地与西侧格兰德河泛滥平原之间的陡崖构造完全吻合。