Experimental study on the electrical resistivity characteristics of sand under different testing conditions
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摘要:
砂土电阻率特性的室内试验研究成果被广泛用于野外高密度电法的成果解译中, 但测试条件的影响及电阻率与砂土特性关系曲线的野外应用需要进一步研究。为此研制了室内二电极电阻率测试装置, 以分析电流类型、交流电频率和电压及砂样不同粒径对电阻率-含水饱和度(
ρ -S r)、电阻率-孔隙水盐浓度(ρ -n )两对关系的影响。结果表明: 进行交流电电阻率试验时, 为减小电阻测试误差, 可选择设置电流频率10 Hz、输入电压1 V;ρ -S r和ρ -n 在交流电与直流电下均满足幂函数关系, 但直流电会使砂柱两侧电极在孔隙水盐质量浓度较高时发生电解反应, 建议选择交流电法开展电阻率试验; 当含水饱和度超过50%时, 砂样粒径引起的电阻率差异较小, 且电阻率不再随含水饱和度增大而明显减小, 保持相对稳定, 指示这一电阻率相对稳定的界面可能分布于含水饱和度约50%的包气带中而非潜水面; 当孔隙水盐质量浓度超过2 g/L后, 5种粒径的饱和砂样电阻率随孔隙水盐质量浓度增大不再明显减小。研究结果证实电阻率法易区分淡水和微咸水, 难以进一步细分盐质量浓度大于2 g/L的微咸水、咸水和卤水。Abstract:Objective The results of laboratory tests on the electrical resistivity of sand are widely used in the interpretation of field electrical resistivity tomography. However, the influences of testing conditions and the field implementations of relationship curves between the electrical resistivity and properties of sand need further research.
Methods First, the electrical resistivity test device was developed by using the two-electrode method. Then, the influences of current type, alternating current (AC) frequency, voltage and grain size of sand on the relationship between electrical resistivity and water saturation (
ρ -S r), as well as electrical resistivity and salinity of the pore water (ρ -n ), were studied.Results The results showed that an AC current frequency of 10 Hz and an input voltage of 1 V were recommended to reduce the test error of resistance. There was a power function between the electrical resistivity and water saturation (
ρ -S r), as well as electrical resistivity and salinity of pore water (ρ -n ), under AC and direct current (DC). As DC might cause an electrolysis reaction of the electrodes of the sand column under the high salinity of pore water, the AC electrical method was suggested. When the water saturation is greater than 50%, the electrical resistivity difference caused by the grain size of sand becomes small. Moreover, the electrical resistivity did not decrease significantly with increasing water saturation, which suggested that a relatively stable interface of the electrical resistivity might exist in the unsaturated zone with 50% saturation rather than in the water table. When the salinity of pore water is greater than 2 g/L, the electrical resistivity of all five types of saturated sands did not decrease obviously with increasing salinity.Conclusion Research results comfirm the electrical resistivity method easily distinguishes fresh water from brackish water but has difficulty further subdividing brackish water, salt water and brine.
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Key words:
- electrical resistivity /
- two-electrode method /
- AC frequency /
- water saturation /
- salinity
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图 2 粒径[0.3, 0.6) mm砂样不同含水饱和度的接触电阻
Figure 2. Contact resistance of sand with grain sizes of [0.3, 0.6) mm with different water saturations
图 3 交流电频率(a)与输入电压(b)对电阻率的影响
Figure 3. Impact of alternating current frequency (a) and voltage (b) on the electrical resistivity
图 5 5种砂样电阻率与含水饱和度关系
Figure 5. Relationship between the electrical resistivity and water saturation for five sand types
图 6 5种砂样下电阻率与孔隙水盐质量浓度关系
Figure 6. Relationship between the electrical resistivity and salinity of pore water for five sand types
表 1 5种砂样特性
Table 1. Properties of the five sand types
砂样粒径/mm [0.1, 0.25) [0.25, 0.4] [0.3, 0.6) [0.6, 1) [1, 4] 孔隙度/% 41.5 39.1 40.5 40.1 38.5 干砂密度ρd/
(g·cm-3)1.510 1.535 1.654 1.837 1.730 
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表 2 电阻率测试因素及设计值
Table 2. Experimental factors and design values of the electrical resistivity test
测试因素 设计值 电流频率f/Hz 10，102，103，5×104 电压U/V 0.1，0.5，1 砂样粒径/mm [0.1, 0.25)，[0.25, 0.4]，
[0.3, 0.6)，[0.6, 1)，[1, 4]含水饱和度Sr/% 10，20，30，50，70，100 孔隙水盐浓度n/(g·L-1) 0，0.5，1，2，5，10 砂柱长度/cm 7，10.5，14，17.5
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