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NEAR SURFACE RESOLUTION AND TURNOFF TIMES IN AIRBORNE TEM INVESTIGATIONS

机译：近近地表分辨率和机载TEM调查中的岔路时间

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Aquifer vulnerability is a key issue in groundwater resource management. The airborne TEM method has demonstrated the potential to provide detailed information about the shallow subsurface resistivity distribution to constitute a valid background for vulnerability estimation. Combined with its ability to delineate aquifers in a variety of geological settings, the airborne TEM method is an efficient tool in large scale hydrogeological investigations. However, in sedimentary environments with moderate to high resistivities in the shallow subsurface it can be a challenging task to provide satisfactory near-surface resolution applying airborne TEM measurements. It is well known that the near-surface resolution is highly dependent on the ability of a specific instrument to measure the early time part of the transient earth response, although the bandwidth of the receiver and the applied current waveform also have a significant impact on the resulting near-surface resolution. Focusing on the applied current waveform, most impulse response type TEM systems employ a linear current turn-off ramp. We show that the effect of a long linear ramp is to make the early time response similar to a step response, while a short linear ramp retains the impulse response character. The sensitivity of the step response to the shallow resistivity distribution is inferior to that of the impulse response, indicating that a short ramp is preferable for environmental investigations. In order to quantify the effects of the ramp length and of the unavoidable measurement dead-time on the near-surface resolution we have performed linearized inversion parameter analyses of ID layered earth models for a generic airborne transient EM system. The analyses clearly demonstrate that specialized instrumentation, such as employed by the dual moment SkyTEM system, S0rensen and Auken (2004), is necessary if aquifer vulnerability is to be estimated from airborne transient EM data in these types of sedimentary environments.
机译：含水层漏洞是地下水资源管理的关键问题。空中TEM方法证明了提供有关浅层地下电阻率分布的详细信息的潜力，以构成漏洞估计的有效背景。结合其在各种地质环境中描绘含水层的能力，机载的TEM方法是大规模水文地质调查的有效工具。然而，在浅层地下中适中到高电阻的沉积环境中，它可以是一个具有挑战性的任务，以提供令人满意的近表面分辨率，施加空气传播的TEM测量。众所周知，近表面分辨率高度依赖于特定仪器测量瞬态地球响应的早期时间部分的能力，尽管接收器和施加的电流波形的带宽也对此产生了重大影响导致近表面分辨率。专注于施加的电流波形，大多数脉冲响应型TEM系统采用线性电流关闭坡道。我们表明，长线性斜坡的效果是使类似于阶跃响应的早期时间响应，而短线性斜坡保持脉冲响应特征。对浅电阻率分布的阶梯响应的灵敏度不如脉冲响应的响应，表明对环境调查优选短斜坡。为了量化斜坡长度和近表面分辨率上不可避免的测量死区时间的影响，我们已经对通用空气传播的EM系统进行了ID分层地球模型的线性化反转参数分析。分析清楚地证明，如果要从这些类型的沉积环境中的空气传播瞬态EM数据估计含水层漏洞，则需要采用双力矩Skytem系统，S0Rensen和Auken（2004）所采用的专业仪器。

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