Geophysical Monitoring of the Southwest Florida Coast

Water management decisions that impact Everglades restoration efforts require high quality data and reliable hydrologic models. Traditionally these data for hydrologic models have been obtained through observation wells. In the Everglades, this approach is limited by the difficult access due to water which covers most of the area and to the limited number of roads. Airborne geophysical techniques provide a means of accessing large parcels of land and developing three-dimensional resistivity models of the area. The overall objective of this project is the collection of geophysical data that can be used to develop ground-water flow models of the area capable of modeling saltwater intrusion. This objective includes mapping of subsurface electrical properties of the aquifer and correlation of lateral variation in these properties to aspects of aquifer geometry and water quality that are pertinent to hydrologic model development. Completion of combined ground and airborne geophysical surveys in Everglades National Park and Big Cypress National Preserve has shown the utility of these methods to map saltwater intrusion and provide geological information needed to develop ground-water flow models. The strategy that has been used is to interpret the HEM data as layered-earth resistivity models that slowly vary from place to place. Surface geophysical measurements (time-domain electromagnetic soundings) have been used to assist in this interpretation and provide an independent check on the HEM data. Borehole data in the form of formation resistivities and water quality sampling have allowed us to develop relationships for converting the interpreted resistivity-depth models into estimated water quality given as specific conductance (SC) or chloride concentration. This information is of great value to hydrologic modelers. These data will be used to develop a ground-water flow model which is bounded on the north by the Tamiami Trail, on the south by Florida Bay, on the east by the Atlantic coastal ridge, and on the west by the Gulf of Mexico. Completion of a combined ground and airborne geophysical study in the southern portion of Everglades National Park has shown the utility of these methods to map the extent of saltwater intrusion and provide geological information needed to develop ground-water flow models. The same approach should prove equally useful in the development of hydrologic models in the region to the west where little subsurface information exists. The approach requires three components: ground-based, airborne, and borehole electrical geophysical measurements. In combination these measurements can provide detailed information on the location of geologic and hydrologic boundaries essential for ground-water model development. The mapping of saltwater intrusion in coastal aquifers has traditionally relied upon observation wells and collection of water samples. This approach may miss important hydrologic features related to saltwater intrusion in areas where access is difficult and wells are widely spaced, such as the Everglades. To map saltwater intrusion in Everglades National Park, a different approach has been used. We have relied heavily on helicopter electromagnetic (HEM) measurements to map lateral variations of electrical resistivity, which are directly related to water quality. The HEM data are inverted to provide a three-dimensional resistivity model of the subsurface. Borehole geophysical and water quality measurements made in a selected set of observations wells are used to determine the relation between formation resistivity and specific conductance of pore water. Applying this relation to the 3-D HEM resistivity model produces an estimated water-quality model. This model provides constraints for variable density, ground-water models of the area. Time-domain electromagnetic (TEM) soundings have also be used to map saltwater intrusion. Because of the high density of HEM sampling (a measurement point every 10 meters along flight lines) models with a cell size of 100 meters on a side are possible, revealing features which could not be recognized from either the TEM or the observation wells alone. The very detailed resistivity maps show the extent of saltwater intrusion and the effect of former and present canals and roadbeds. The TEM survey provides a means of quickly obtaining a synoptic picture of saltwater intrusion, which also serves as a baseline for monitoring the effects of Everglades restoration activities.影响大沼泽地（Everglades）修复工程的水资源管理决策，离不开高质量数据与可靠的水文模型（hydrologic model）。传统上，水文模型所需的这类数据均通过观测井（observation wells）获取。但在大沼泽地，该方法受限于两大瓶颈：其一，该区域大部分区域被水体覆盖，通行难度极大；其二，道路数量有限。航空地球物理技术（airborne geophysical techniques）则为大面积区域勘探提供了可行途径，可用于构建研究区的三维电阻率模型（three-dimensional resistivity models）。本项目的总体目标为采集地球物理数据，以此开发适用于该区域的地下水流模型（ground-water flow models），以实现海水入侵过程模拟。这一目标涵盖两方面内容：一是绘制含水层（aquifer）的地下电性特征（subsurface electrical properties）分布图，二是将这些电性特征的横向变化与水文模型开发所需的含水层几何形态及水质参数相关联。 在大沼泽地国家公园与大柏树国家保护区（Big Cypress National Preserve）完成的地面与航空联合地球物理勘探，已证实此类方法可有效绘制海水入侵范围，并为地下水流模型开发提供所需的地质信息。本次研究采用的策略为：将直升机电磁（HEM）数据解译为逐点缓慢变化的层状地球电阻率模型。地面地球物理测量（time-domain electromagnetic soundings）被用于辅助该解译过程，并对HEM数据提供独立验证。以地层电阻率（formation resistivities）与水质采样形式获取的钻孔数据，则帮助我们建立了转换关系，可将解译得到的电阻率-深度模型转换为以比电导（SC）或氯离子浓度表征的估算水质参数。这类信息对水文模型开发者具有极高价值。 本次采集的数据将用于构建地下水流模型，该模型的北界为塔米亚米步道（Tamiami Trail），南界为佛罗里达湾（Florida Bay），东界为大西洋海岸脊（Atlantic coastal ridge），西界为墨西哥湾（Gulf of Mexico）。 在大沼泽地国家公园南部完成的地面与航空联合地球物理研究，同样证实了此类方法可有效绘制海水入侵范围，并为地下水流模型开发提供所需的地质信息。该方法在西侧区域（当前地下信息极度匮乏）的水文模型开发中，同样具备应用潜力。 该研究方法包含三大核心组成部分：地面、航空与钻孔电性地球物理测量。三者结合可提供详细的地质与水文边界信息，这正是地下水流模型开发的关键依据。 传统上，海岸含水层的海水入侵制图依赖观测井与水样采集，但在通行困难、井网稀疏的区域（如大沼泽地），这类方法可能遗漏与海水入侵相关的重要水文特征。为绘制大沼泽地国家公园的海水入侵范围，研究团队采用了截然不同的方案：大量依赖直升机电磁（HEM）测量来绘制与水质直接相关的电性电阻率横向变化。通过反演HEM数据，可得到地下三维电阻率模型。 在选定的观测井中开展的钻孔地球物理与水质测量，用于确定地层电阻率与孔隙水（pore water）比电导之间的定量关系。将该关系应用于三维HEM电阻率模型，即可得到估算水质模型，该模型可为区域变密度（variable density）地下水流模型提供约束条件。 时域电磁（TEM）测深同样被用于海水入侵制图。得益于HEM采样的高密度（飞行线路上每10米设置一个测量点），可构建边长为100米的网格模型，从而揭示出仅通过TEM测量或观测井无法识别的细节特征。这些高精度电阻率图清晰展现了海水入侵的范围，以及既往与现存运河、路基的影响。TEM测量可快速获取海水入侵的概览图像，同时作为监测大沼泽地修复工程影响的基线数据。