Department of Environmental Engineering, Technical University of Denmark, Miljøvej, Bldg 113, DK-2800, Kgs Lyngby, Denmark.
Ground Water. 2013 May-Jun;51(3):385-97. doi: 10.1111/j.1745-6584.2012.00974.x. Epub 2012 Aug 14.
Salt water intrusion models are commonly used to support groundwater resource management in coastal aquifers. Concentration data used for model calibration are often sparse and limited in spatial extent. With airborne and ground-based electromagnetic surveys, electrical resistivity models can be obtained to provide high-resolution three-dimensional models of subsurface resistivity variations that can be related to geology and salt concentrations on a regional scale. Several previous studies have calibrated salt water intrusion models with geophysical data, but are typically limited to the use of the inverted electrical resistivity models without considering the measured geophysical data directly. This induces a number of errors related to inconsistent scales between the geophysical and hydrologic models and the applied regularization constraints in the geophysical inversion. To overcome these errors, we perform a coupled hydrogeophysical inversion (CHI) in which we use a salt water intrusion model to interpret the geophysical data and guide the geophysical inversion. We refer to this methodology as a Coupled Hydrogeophysical Inversion-State (CHI-S), in which simulated salt concentrations are transformed to an electrical resistivity model, after which a geophysical forward response is calculated and compared with the measured geophysical data. This approach was applied for a field site in Santa Cruz County, California, where a time-domain electromagnetic (TDEM) dataset was collected. For this location, a simple two-dimensional cross-sectional salt water intrusion model was developed, for which we estimated five uniform aquifer properties, incorporating the porosity that was also part of the employed petrophysical relationship. In addition, one geophysical parameter was estimated. The six parameters could be resolved well by fitting more than 300 apparent resistivities that were comprised by the TDEM dataset. Except for three sounding locations, all the TDEM data could be fitted close to a root-mean-square error of 1. Possible explanations for the poor fit of these soundings are the assumption of spatial uniformity, fixed boundary conditions and the neglecting of 3D effects in the groundwater model and the TDEM forward responses.
盐水入侵模型常用于支持沿海含水层的地下水资源管理。用于模型校准的浓度数据通常稀疏且空间范围有限。通过航空和地面电磁调查，可以获得电阻率模型，从而提供地下电阻率变化的高分辨率三维模型，这些模型可以与地质和盐分浓度在区域尺度上相关联。以前有几项研究使用地球物理数据校准盐水入侵模型，但通常仅限于使用反演电阻率模型，而不直接考虑测量的地球物理数据。这会引起与地球物理和水文模型之间不一致的尺度以及地球物理反演中应用的正则化约束相关的一系列错误。为了克服这些错误，我们进行了耦合水文地球物理反演（CHI），其中我们使用盐水入侵模型来解释地球物理数据并指导地球物理反演。我们将这种方法称为耦合水文地球物理反演状态（CHI-S），其中模拟的盐浓度被转换为电阻率模型，然后计算地球物理正演响应并将其与测量的地球物理数据进行比较。该方法应用于加利福尼亚州圣克鲁斯县的一个现场，在那里收集了时间域电磁（TDEM）数据集。对于该位置，开发了一个简单的二维横截面盐水入侵模型，我们估计了五个均匀含水层的属性，包括作为所采用的岩石物理关系一部分的孔隙度。此外，还估计了一个地球物理参数。通过拟合由 TDEM 数据集组成的 300 多个表观电阻率，可以很好地解决这六个参数。除了三个探测位置外，所有的 TDEM 数据都可以拟合得接近均方根误差为 1。这些探测拟合不良的可能解释是空间均匀性的假设、固定边界条件以及在地下水模型和 TDEM 正演响应中忽略 3D 效应。
