Transient electromagnetic (TEM) sounding data; San Luis Valley, Colorado, 2007

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.为圈定美国科罗拉多州圣路易斯谷内蓝粘土层的空间分布，并调查区域内疑似断层的存在，研究团队在此开展了瞬变电磁（Transient Electromagnetic, TEM）探测。本次探测共完成147个有效测点，覆盖大沙丘国家公园及周边区域；另有6个测点布置于阿拉莫萨国家野生动物保护区东缘的汉森崖附近。蓝粘土是该区域关键的水文地质单元，可分隔表层无压含水层与深部承压含水层，明确其空间分布对区域水文地质模型的构建具有重要意义。此前的测井资料分析显示，蓝粘土的电阻率不超过10 Ω·m，与区域内砂岩、砾岩及其他粘土层的高电阻率特征形成鲜明对比，因此成为瞬变电磁探测的优质目标体。探测结果表明，蓝粘土顶面存在显著起伏，暗示其在沉积过程中或沉积后可能发生了地层变形。鉴于该区域存在裂谷活动，此类地层变形本属预期现象。在针对航空磁测数据识别的断层开展的瞬变电磁剖面测量中，部分剖面呈现出电阻率变化及（或）电性单元的地下高程起伏，这些特征均指向断层活动的可能性，但此类电性异常并非所有疑似断层均有体现。汉森崖剖面的电阻率及导体埋深变化，与东侧高地与西侧里奥格兰德河泛滥平原之间的陡崖地貌高度吻合。