Discrete deep targets are a significant challenge for most surface-based geophysical techniques, even when considering high property contrasts. In general, surface-based geophysical methods lose lateral and vertical resolution with depth as a result of the limited acquisition geometry and increased signal attenuation. The former can be overcome through use of cross-borehole methods, but lateral localization is still needed for optimal borehole placement. As such, a relatively small, deep void located near the maximum depth of investigation (DOI) is very unlikely to be detected. Yet, secondary features associated with these voids can be exploited for enhanced detection performance. When voids are located below the groundwater table, a significant amount of dewatering and pumping is required to make them a functional passageway. This dewatering not only removes water from the void space but also the surrounding formation, resulting in a much larger, if more diffuse, secondary target: an induced groundwater table gradient. Many geophysical sensing methods are sensitive to subsurface moisture content. We have implemented a 2D joint-petrophysical mixing model (JPM), using inverted electrical resistivity tomography (ERT) and inverted seismic refraction models to sense changes in the groundwater table gradient. Our results are validated using the depth to bedrock, groundwater-surface water information, ground-penetrating radar, and time-domain reflectometry methods. Our initial proof of concept is applied to a shallow area with a significant soil moisture gradient, through different surface soil types and bedrock. The 2D JPM results are used to generate an estimate of air, moisture, and matrix percent fractions in the investigation area, providing a clear delineation of the groundwater surface and associated gradient. This refined hydraulic gradient estimate can then be used to laterally locate a void at or below the DOI of ERT and seismic refraction.

中文翻译：

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通过二次脱水特征的地球物理特征定位深层空洞

对于大多数基于地面的地球物理技术而言，离散深目标是一项重大挑战，即使考虑到高对比度时也是如此。通常，由于有限的采集几何形状和增加的信号衰减，基于地面的地球物理方法会随着深度而损失横向和垂直分辨率。前者可以通过使用跨孔方法来克服，但是为了获得最佳的钻孔位置，仍然需要侧向定位。因此，非常不可能检测到位于最大调查深度（DOI）附近的相对较小的深空洞。然而，可以利用与这些空隙相关的次要特征来增强检测性能。当空隙位于地下水位以下时，需要大量的脱水和抽水才能使其成为功能通道。这种脱水不仅可以从空隙中除去水分，而且可以从周围的地层中除去水分，从而导致更大，甚至更分散的次要目标：诱发的地下水位梯度。许多地球物理传感方法对地下水分含量敏感。我们已经实现了二维联合岩石物理混合模型（JPM），使用反向电阻层析成像（ERT）和反向地震折射模型来感测地下水位梯度的变化。我们的结果通过使用基岩深度，地下水-地表水信息，探地雷达和时域反射法进行了验证。我们的初步概念证明适用于通过不同表层土壤类型和基岩的土壤湿度梯度较大的浅层区域。2D JPM结果用于生成空气估算值，研究区域中的水分和基质百分率，清楚地描绘了地下水表面和相关的坡度。然后，可以使用这种改进的水力坡度估算来将孔隙横向定位在ERT和地震折射的DOI处或以下。