# Integration of ERT, IP and SP Methods in Hard Rock Engineering
## 3 Materials and Methods
SP anomalies at this site are mainly controlled by groundwater flow through the weathered/fractured zones [47]. The SP surveys measure positive and negative anomalies depending on the subsurface materials properties [48; 49]. The groundwater flow system is mainly associated with negative SP anomalies [9]. In order to map the groundwater flow path in the study area, the SP measurements were obtained along with the ERT and IP data  
Geophysical data acquisition of ERT, IP and SP was performed along the same profile using the same (constant) electrode spacing and the same number of measurements. All electrodes are connected to the resistivity meter via multi-core cables. The lateral and vertical resolution of the subsurface depends on the spacing between the electrodes. The resolution of a model increases with decreasing electrode spacing; however, this will reduce the depth of investigation. Therefore, electrode spacing should be selected considering a balance between the investigation depth and model resolution. The ERT, IP and SP data were obtained by the ABEM Lund Imaging System (ABEM Terrameter (SAS 4000)) along with ten profiles (A, B, C, D, E, F, G, H, I, and J) (Figure 1). A power amplifier, receiver and transmitter are all integrated into the imaging system. The system, connected with a PC, collects the main survey parameters including injected electric current, voltage, apparent resistivity, chargeability and self-potential. The ERT, IP and SP measurements were obtained using non-polarizing electrodes. Such electrodes are a requirement for IP and SP surveys to reduce contact resistances in electrically resistive mediums. A measurement of apparent resistivity, chargeability, and self-potential is performed for each electrode combination. This reading is displayed on the computer output at the lateral midpoint of the electrode locations and a prescribed depth level. Three parameters (electrical resistivity, chargeability and self-potential) are acquired at each of these points. First, the ERT data were obtained via Imaging System on resistivity mode. Afterwards, IP and SP measurements were obtained by using the Imaging System on IP and SP modes, respectively. The geophysical measurements (ERT, IP and SP) were acquired using the pole-dipole array with 5 m electrode spacing. The asymmetrical nature of this array was removed via the integration of two independent measurements (forward and reverse). Geophysical data of ERT, IP and SP along ten profiles were acquired for a total of 6150 m profile length, 1240 electrodes and 31,000 measurements, including 25 layers (depth levels) along each profile with the first layer at 7.5 m depth and last layer at 127.5 m depth. Data acquisition parameters of ERT, IP and SP along each profile are given in Table 1. Prior to exporting geoelectrical data (apparent resistivity, chargeability and self-potential) to an inversion package, PROSYSII (IRIS Instruments) was used to display raw data in graphical plots,and to separate and remove incorrect data points (low/very high potential readings compared with surrounding readings). Furthermore, the signal strength was also enhanced through the use of stacking, i.e., the repeated measurement of transfer resistance through a number of cycles of current injection. The geophysical data obtained in the field were further processed. The apparent electrical resistivity, IP and assigned data weights were inverted by RES2 DINV [37]. ZondSP2D program was used for SP data inversion. In SP inversion, the medium model is created through a number of cells, and each cell has the electrical value and type of conductivity (ion or electronic) properties. RES2 DINV provides the 2D resistivity/IP model via the nonlinear optimization procedure [37; 50]. The robust constraint was used to enhance the data quality by removing the bad data (data values that cause spikes when compared with the surrounding data points and/or data with standard deviations over the average).