# Groundwater Detection Using the Pseudo-3D Resistivity Method: A History of Case Studies
## 4 Discussion  
ERM is proven to be the most effective technique in mapping groundwater resources, as the groundwater resource is largely localized and difficult to be mapped [26]. Thus, this technique is of great importance in providing the initial result of groundwater detection. However, the hydrological interpretation of geoelectric results can be very ambiguous due to the extremely wide resistivity ranges which characterize both freshwater bearing aquifers as well as overlying and underlying strata [3].  
In electrical resistivity surveys, high resolution and reliable imaging depends on the choices of electrode configuration, which is normally known as an array [27]. We have presented a pseudo-3D resistivity method which uses a dipole-pole multi-point source electric method exploration device. Notice that the survey line should be placed perpendicular to the strike of the targeted structure, which overcomes the limitation of the 2-D survey method [28].  
In the case of groundwater geophysical surveys in Zixing, according to profiles K8 and K9, the resistivity image from the pseudo-3D resistivity detection (in Figure 7b) provided a nice subsurface resistivity distribution map with clear anomalies. In the study of [PERSON] and [PERSON], the high resistivity anomaly was located in a shallow part, but it was not explained very well [24]. According to the data, we speculated that there is a northwest-trending fault nearby, dipping southwest at an angle of about 50 degrees.  
Another groundwater geophysical survey in Huarong provided a similar successful case study. In this survey, the 2D DC resistivity inversion results indicate that the layers were discrete (i.e., the high resistivity anomalies were not connected in Figure 9a) because it could not clearly divide layers in deep areas due to the inhomogeneous properties of the soil material [26]. The faults and broken zones were filled up with groundwater and are characterized as low resistivities in the pseudo-3D resistivity survey (in Figure 9b). Because the groundwater reservoirs are typically found in saturated sand, saturated sandy, and saturated silt, clay, and sand [17], there was a steeper resistivity contact surface, which was between argillaceous slate and sandy slate. The resistivity values of the two lithological contact surfaces were about 400 \(\Omega\).m. It was used to infer the position of the contact surface. Compared with the drilling results, the resistivity interpretations had high authenticity.  
The groundwater reservoir was normally related with the faults or broken zones [7]. When it came to the Xiaojiashan Dam leakage surveys, we inferred a low-resistivity anomaly between the station of the \(-120\)\(\sim\)\(-40\)th measurement location in profile 1 to be the location of seepage. Moreover, we inferred that the water source comes from a mountain crevice on the east side of the dam by using pseudo-3D resistivity inversion. The second channel of the dam leakage path, clear from the inversion result, is characterized by an obvious low-resistivity anomaly in the superficial view of the 0-40 measurement location in profiles 2 and 3. The water came from the east side of the dam, and the groundwater seeps into the dam through the fissures and flows through the weak layer of the dam. The source of seepage was related to the reservoir and the groundwater of the mountain on the east side of the dam.  
The pseudo-3D method has a better resistivity exploration effect than the 2D resistivity method due to better resolution. However, we note that the limitation of the pseudo-3D resistivity is the high computational cost compared with the 2D resistivity method.