Discussion on monitoring and characterising group drilling pumping test within a massive thickness aquifer using the time-lapse transient electromagnetic method (TEM)

To investigate underground hydrogeological conditions and to obtain accurate aquifer parameters, the time-lapse transient electromagnetic method (TEM) is applied to image groundwater drawdown within a massive thickness sandstone aquifer (thickness over 100 m) in Inner Mongolia, China. 2D geophysical models are established based on the hydrostratigraphy of the survey site and, then, the induced electromotive forces (EMF) pre- and post-pumping test are analysed. Multiple aquifers can be detected by 2D forward modelling and the unsaturated region can be distinguished by the variation of electromagnetic responses. Furthermore, the measured electromotive force curves before (pre-) and after (post-) pumping deviate from each other and the groundwater drawdown has a positive correlation with the deviation degree, namely relative variation. The raw EMF data are processed with Occam's inversion method and two 2D resistivity data volumes are obtained. The resistivity-depth cross-sections from the data volume show that the groundwater drawdown is larger in the east of the TEM area due to the large permeability and water content. The centre of the depression cone is located at east Pumping Well 1 and the drawdown is 70 m, which is basically coincident with the group drilling pumping test result.