Imaging buried Quaternary valleys using the transient electromagnetic method

Buried Quaternary valleys in Denmark are complex structures filled with various deposits consisting primarily of glacio-lacustrine clay, till and meltwater sand, and gravel. The valleys are important geophysical targets, because they often contain significant volumes of groundwater used for public water supply. About 700 km of buried valley structures have been imaged in the western part of Denmark by the transient electromagnetic (TEM) method. The ability to map the valleys depends primarily on valley geometry, infill architecture and the resistivity of the fill sediments as well as the substratum. One-dimensional (1-D) inversion models of the TEM soundings have been used to construct contour maps of 20 m average resistivities and depth to a good conductor, which provide images for geological interpretation. Images of buried valley morphology, fill properties, infill architecture, such as cut-and-fill structures, valley distribution and valley generations, are characterized for case studies from Hornsyld, Holstebro and the Vonsild/Agtrup areas of Denmark.     

Resolution studies for Magnetic Resonance Sounding (MRS) using the singular value decomposition

We present a new approach to analyse the subsurface water content distribution obtained by inversion of MRS data in terms of resolution and penetration depth. It is based on a singular value decomposition (SVD) of the MRS forward operator to derive the model resolution matrix including regularisation parameters, i.e. including noise conditions. The approach takes loop size, subsurface resistivity distribution and noise conditions as input parameters affecting MRS into account and allows an assessment on resolution and penetration.The application of the new approach shows that the loop diameter must be carefully chosen depending on the investigation site to obtain optimal resolution and penetration. Using the introduced resolution measures the quality and reliability of the estimated model can be assessed.     

Contribution of magnetic resonance sounding to aquifer characterization and recharge estimate in semiarid Niger

To improve the knowledge of the regionally important Continental Terminal 3 (CT3) aquifer in south-western Niger, fifteen magnetic resonance soundings (MRS) were carried out in December 2005 in the vicinity of wells and boreholes. The output MRS geophysical parameters, i.e. water content and decay constants versus depth, were compared to hydrogeological characteristics, i.e. water table depth, total porosity, specific yield and transmissivity estimated from direct measurements, pumping tests and transient groundwater modelling. The MRS-determined parameters were then used to estimate the rates of groundwater recharge.Contained in poorly consolidated Tertiary sandstones, the CT3 aquifer's water table has continuously risen by 4 m in total over the past four decades. Additionally, a significant portion of this increase has occurred in the past decade alone, with an annual rise now ranging between 0.1 and 0.3 m depending on the monitored well. Increase in groundwater recharge due to land clearance and deforestation explains this situation. According to previous estimations, the pre-clearing recharge ranged from 1 to 5 mm per year in 1950–60 s, while more recent recharge rates (1990s–2000s) range from 20 to 50 mm per year. These recharge values are directly affected by estimated aquifer specific yield value, while the spatial variation of rates of water table rise can be attributed to large scale hydrodynamic heterogeneities in the aquifer. However, few field measurements were available to confirm these assumptions.The main results of this study are: (1) The water table depth and aquifer transmissivity are estimated from MRS output parameters with an average accuracy of ± 10% and ± 9% respectively. (2) The MRS-determined water content is linked to both the total porosity and the specific yield of the aquifer, but no quantitative formulation can be proposed as yet. (3) Using the average MRS-determined water content over the investigated area, i.e. 13%, the groundwater recharge rates can be estimated to be ~ 2 mm per year in the 1950–1960s (pre-clearing period), and ~ 23 mm per year for the last decade. (4) The variations in specific yield and transmissivity cannot explain by themselves the spatial variability of the rise of the water table. (5) The ranges in transmissivity and water content obtained from MRS are more realistic than the groundwater modelling outputs. Therefore, MRS could be used to better constrain the aquifer parameters in groundwater modelling with a dense site network.Finally, this work illustrates how MRS can successfully improve characterisation and transient multi-year groundwater balance of commonly found sedimentary aquifers, particularly when integrated with well observations and pumping tests.     

Applications of hydrogeophysics in characterization of subsurface architecture and contaminant plumes

Geophysical methods have been applied to a wide range of hydrogeological problems. With improvement in geophysical inversion algorithms and measurement tools, significant achievements have been made in the characterization of subsurface architecture, time-lapse monitoring of hydrogeological process and contaminant plumes delineation. In this paper, we summarize the geophysical methods that are most widely used in hydrogeology including Electrical Resistivity Tomography (ERT), Induced Polarization (IP), Ground Penetrating Radar (GPR) and Electromagnetic Induction (EMI). Three examples including lab and field works are used to demonstrate current application of geophysical methods for characterizing subsurface architecture and contaminant plumes. Though great progress has been made in hydrogeohysics over the last few decades at home and abroad, challenges still remain in practical applications. More recently, hydrogeophysics continues to develop in the areas of establishment of hydrogeophysical models, large-scale architecture characterization, uncertainty analysis, biogeochemical process monitoring and ecosystem science.     

利用分段时频峰值滤波法抑制磁共振全波信号随机噪声

磁共振信号极其微弱,容易受到周围环境中各种电磁噪声干扰.其中随机噪声,由于频带宽、不规则、无规律、与有效信号混叠,难以抑制.近年来,采用数量级为10⁴~10⁵ Hz采样频率收录的全波磁共振信号,以其携带丰富全面的信息量,为数据处理及解释提供了更多的潜能.然而,只要随机噪声的幅度大于信号幅度,拟合得到的信号特征参数准确度就会降低.目前普遍采用的数据叠加方法仅能抑制部分随机噪声,且需要多次采集信号,探测效率低.本文针对全波磁共振信号采样点数多和信号非线性强的特点,提出采用分段时频峰值滤波（STFPF）法消噪,将全波磁共振信号分成若干段,编码为解析信号的瞬时频率,采用短窗长PWVD计算解析信号的时频分布,利用时频分布沿瞬时频率集中的特性,通过提取时频分布的峰值获得信号的无偏估计,达到抑制全波磁共振信号随机噪声的目的.为了验证消噪效果,与传统叠加法进行对比分析,仿真结果表明,对于单次采集信号,信噪比低至-5 dB时,STFPF方法依然能有效抑制信号中的随机噪声,消除随机噪声后信噪比提高23.19 dB,信号的初始振幅拟合误差为3.03%,平均横向弛豫时间拟合误差为2.7%,消噪效果优于传统叠加法,且由于无需多次采集磁共振信号,可有效提高探测效率.模型数据的反演解释进一步验证了STFPF方法的有效性,本文研究结果为实际数据处理奠定了良好的基础.     

Extreme sensitivity of crosshole electrical resistivity tomography measurements to geometric errors

The effects of geometric errors on crosshole resistivity data are investigated using analytical methods. Geometric errors are systematic and can occur due to uncertainties in the individual electrode positions, the vertical spacing between electrodes in the same borehole, or the vertical offset between electrodes in opposite boreholes. An estimate of the sensitivity to geometric error is calculated for each of two generic types of four-electrode crosshole configuration: current flow and potential difference crosshole (XH) and in-hole (IH). It is found that XH configurations are not particularly sensitive to geometric error unless the boreholes are closely spaced on the scale of the vertical separation of the current and potential electrodes. However, extremely sensitive IH configurations are shown to exist for any borehole separation. Therefore, it is recommended that XH configurations be used in preference to IH schemes. The effects of geometric error are demonstrated using real XH data from a closely spaced line of boreholes designed to monitor bioremediation of chlorinated solvents at an industrial site. A small fraction of the data had physically unrealistic apparent resistivities, which were either negative or unexpectedly large. However by filtering out configurations with high sensitivities to geometric error, all of the suspect data were removed. This filtering also significantly improved the convergence between the predicted and the measured resistivities when the data were inverted. In addition to systematic geometric errors, the measured data also exhibit a high level of random noise. Despite this, the resulting inverted images correspond reasonably closely with the known geology and nearby cone penetrometer resistivity profiles.     

Inversion of resistivity in Magnetic Resonance Sounding

Magnetic Resonance Sounding (MRS, or Surface Nuclear Magnetic Resonance - SNMR) is used for groundwater exploration and aquifer characterization. Since this is an electromagnetic method, the excitation magnetic field depends on the resistivity of the subsurface. Therefore, the resistivity has to be taken into account in the inversion: either as a priori information or as an inversion parameter during the inversion process, as introduced in the presented paper. Studies with synthetic data show that water content and resistivity can be resolved for a low resistive aquifer even using only the amplitude of the MRS signal. However, the inversion result can be significantly improved using amplitude and phase of the MRS signal. The successful implementation of the inversion for field data shows that the resistivities derived from MRS are comparable to those from conventional geoelectric methods such as DC resistivity and transient electromagnetic. By having information about both the resistivity and the water content, MRS inversions give information about the quality of the water in the aquifer. This is of utmost interest in hydrogeological studies as this specific information cannot be determined solely by geoelectric measurements, due to the nonunique dependence of resistivity on water content and salinity.