Electrical Resistivity Tomography Data along the Little Colorado River near Leupp, AZ 2019

The Little Colorado River alluvial aquifer near Leupp, Arizona was investigated as a possible source of irrigation water for the Leupp and Birdsprings Chapters of the Navajo Nation. The physical, chemical, and hydraulic characteristics of the alluvial aquifer were studied using geophysical surveys, installation of observation wells, water-level measurements, chemical analyses, groundwater pumping simulations, and review of previous investigations. Geophysical surveys and well borings were used to measure the thickness of the aquifer. Geophysical surveys included Electrical Resistivity Tomography (ERT) to better define the alluvial channel of the Little Colorado River near Leupp. ERT is a commonly used electrical geophysical technique for investigating the subsurface of the Earth. In this study it was tested in two separate areas to determine how effective the technique would be for determining the thickness of the alluvial sediments along the LCR. An Advanced Geosciences Inc. Supersting R8TM multi-channel resistivity/IP meter with a SwitchBox56® was used for the DC resistivity survey and all DC resistivity survey data was modeled using Advanced Geosciences Inc. Earthimager 2D software. For each survey line, 56 electrodes were placed on the ground with a spacing of 5 meters (except Line MB4 had a spacing of 6 meters) for the initial survey segment and when the segment was completed then the first 28 electrodes of the survey were moved to the end of the survey line in a roll-along technique. The survey line can be extended indefinitely in a leap-frog fashion even though each segment is only 56 electrodes at a fixed spacing of 5 meters. Resistivity measurements were made in different arrangements or arrays of electrodes, and for this survey dipole-dipole and strong-gradient arrays were used. For both array types, an electrical current is transmitted into the ground, and the resulting potential differences are measured at the surface (Sharma, 1997). Layers within the Earth that are electrically conductive or resistive will deflect or distort the normal potentials.针对亚利桑那州利普（Leupp）附近的小科罗拉多河冲积含水层，本研究将其作为纳瓦霍族（Navajo Nation）利普与伯德斯普林斯堂区（Birdsprings Chapters）的潜在灌溉水源开展调查。研究通过地球物理勘探、观测井布设、水位测量、化学分析、地下水抽水模拟及既往调研资料梳理，对该冲积含水层的物理、化学与水力特征展开探究。地球物理勘探与钻井取芯用于测定含水层厚度，其中电阻率层析成像（Electrical Resistivity Tomography, ERT）被用于更清晰地刻画利普附近小科罗拉多河冲积河道形态。ERT是一种常用于地球地下探测的电气地球物理技术，本研究在两个独立区域开展了ERT测试，以评估该技术在测定小科罗拉多河沿线冲积沉积物厚度方面的有效性。本次直流电阻率勘探采用先进地球科学公司（Advanced Geosciences Inc.）的Supersting R8™多通道电阻率/激电仪搭配SwitchBox56®开关箱，所有直流电阻率勘探数据均通过先进地球科学公司的Earthimager 2D二维建模软件进行反演建模。每条测线初始布设56个电极，电极间距为5米（MB4测线间距为6米）；当该段测线完成数据采集后，采用滚动布设技术将前28个电极移动至测线末端。即便每段测线仅以5米固定间距布设56个电极，仍可通过跳跃式布设方式实现测线的无限延伸。电阻率测量可通过不同的电极排列方式开展，本次勘探采用了偶极-偶极阵列与强梯度阵列两种布设方式。对于这两种阵列类型，均需向地下通入电流，并在地表测量由此产生的电位差（Sharma, 1997）。地球内部具有导电性或高阻性的地层会使正常电位场发生偏转或畸变。