Transient electromagnetic (TEM) sounding technology is known for its capability in detecting resistive zones, especially those associated with fresh water. Nevertheless, to date, there have been no available studies that have employed TEM for riverbank filtration (RBF) application. Therefore, this study was conducted to investigate the capability of TEM technology in profiling the subsurface of RBF areas adjacent to Sungai Perak, Kota Lama Kiri, Kuala Kangsar. A total of 27 survey points of simple rectangular loop configuration with loop area of 100 m2 (10 m × 10 m) were executed in the study area. A suite of TerraTEM equipment was used for the data acquisition process. The inversion and interpretation processes were then conducted by using the modeling program of ‘TDEM Geomodel’ and Surfer 13. The results showed four 2-D resistivity cross-section (depth–distance) profiles with their corresponding geological units. The interpretation results indicated that the subsurfaces of the study area were identified as alluvial-type soils (with a range of 10 to 40 Ωm) up to 8 m due to the presence of silt, sand and gravel. Based on the good quality results produced as well as its convenience, it is recommended that the TEM method is used for RBF exploration in future work.
The transient electromagnetic (TEM) method, alternatively called time-domain electromagnetics (TDEM), is a widely used, non-intrusive, geophysical method for obtaining subsurface resistivity–conductivity data. It is an electromagnetic induction technique, with which the magnetic field due to a pulse of current in a transmitter loop is measured in the time domain (Lin et al. 2016). The electrical resistivity of the underground layers down to a depth of several hundred metres can be measured using TEM. This method was originally designed for mineral investigations, but over the last two decades, it has become increasingly popular for a variety of hydrogeophysical investigations as well as general geological mapping (Christiansen et al. 2006). TEM sounding has been successfully applied to investigate shallow and deep sedimentary basins (Bortolozo et al. 2014; Yogeshwar & Tezkan 2017), fault zones (Rödder & Tezkan 2013), and Quaternary valley structures (Jørgensen et al. 2003). For groundwater studies, work has been done on aquifer characterization, contamination and salinization issues (Auken et al. 2003; Levi et al. 2008; Metwaly et al. 2012; Shaaban et al. 2016). However TEM has not yet been applied in the prospecting of riverbank filtration (RBF) application.
The basic principal of all the electromagnetic geophysical methods is based upon the fact that a magnetic field varies in time – the primary field – and thus, according to Maxwell's equations, induce an electrical current in the surroundings – e.g. the ground, which is a conductor. The associated electrical and magnetic fields are called the secondary fields (Christiansen et al. 2006). TEM requires a specialized transmitter to drive a time-varying current into a transmitter loop, normally an ungrounded loop of wire laid on the surface (Figure 1). At some point, the loop current is quickly turned off, thereby causing a rapid change in the primary magnetic field generated by the transmitter. Faraday's law of induction states that the change of magnetic field will produce an electric field, which will then create an electric current. Hence, the rapidly changing magnetic field induces eddy currents to flow in nearby conductors producing small secondary magnetic fields. The secondary magnetic fields attenuate with time, thus the use of the terms ‘transient’ and ‘time domain’ to describe the method. The parameter typically sought is the time-rate-of-change of these magnetic fields – the voltages induced in a receiver loop. Because the magnitude and distribution of the current intensity depend upon the resistivity of the ground, the voltage gives information about the resistivity of the ground. As time passes and the location of maximum current intensity diffuses downward, the transient decays quickly with time, thereby giving information about deeper layers (McNeill 1980; Fitterman & Stewart 1986).
RBF is a process where pumping wells are located along the riverbanks to induce a portion of the river water to flow towards the wells (Ray et al. 2002). Induced infiltration refers to lowering the water table below the river water levels through pumping action, which consequently leads to entrance of the river water into the groundwater system (Gollnitz et al. 2004). During percolation through riverbeds into the aquifers, the infiltrated river water undergoes a series of physical, chemical and biological processes including filtration, dilution, sorption and biodegradation. This natural attenuation process significantly improves the raw water quality by efficiently removing or degrading any suspended particles, natural organic matter, inorganic pollutants, microbial pathogens and other contaminants (Bourg & Bertin 1993; Dillon et al. 2002; Ray et al. 2002; Tufenkji et al. 2002). RBF systems are predominantly positioned in alluvial valley aquifers, which have hydraulic conductivities higher than 1 × 10−4 m/s, and the thickness of the exploited aquifers ranges from 5 to 60 m (Grischek et al. 2002). Alluvial aquifers are among the key factor for water abstraction to succeed, due to their capacity to store water (Kim & Kim 2008). Unconfined aquifers have good water storability to produce high quantity and flow velocity of RBF (Chowdhury & Gupta 2011). The volume of the aquifer is critical in determining the quantity of water available for the abstraction well in the absence of recharge. The saturated thickness and the extent of the aquifer will be used to characterize the quantity of water available (Caldwell 2006). In most alluvial aquifers, sand and gravel predominate, but floodplain deposits also leave layers of silts and clays in the lithology (Tufenkji et al. 2002). The ideal conditions include coarse-grained, permeable water-bearing deposits that are hydraulically connected with riverbed materials. Furthermore, RBF systems can also be constructed in low permeability zones (clay and silt layers) within an alluvial aquifer (Hunt et al. 2002).
In determining locations of RBF, work using other methods such as in boring exploratory wells can be done, but it is costly and time-consuming. New, non-intrusive methods such as electrical resistivity show some positive results (Razak et al. 2016). Coupled with remote sensing and geographic information system (GIS) techniques, site selection can be made within 80% accuracy (Razak et al. 2015). However, the method can be further improved if other similar non-intrusive methods are used. TEM sounding has several advantages compared with direct current (DC) resistivity techniques. For example, TEM has better depth-resolution than DC resistivity, particularly for mapping conductive zones in the resistive sections. The DC techniques have difficulty in mapping strata below a resistive layer, whereas TEM can easily map conductive strata beneath a thick resistive section. Furthermore, the latter does not require long electrode arrays and thus, is less sensitive to lateral changes in soils. They also do not need direct coupling to the ground and may be cost-effective (Georgsson & Karlsdόttir 2008; Zonge 2016). Therefore, this study was conducted to investigate the capability of TEM technology in profiling the subsurface areas in Kota Lama Kiri, Kuala Kangsar. The RBF area closer to Sungai Perak in Kota Lama Kiri was chosen as the subject for this case study to investigate the capability of the TEM survey.
The study area is located in Kota Lama Kiri, Kuala Kangsar, in the state of Perak, Malaysia, near to the water intake for Lembaga Air Perak (LAP), Kota Lama Kiri water treatment plant as shown in Figure 2, at 4°47′56″ to 4°48′49″ north latitude and 100°56′53″ to 100°57′24″ east longitude. The nearest town is Kuala Kangsar, which is approximately 4 km by road. Sungai Perak is 420 km long and has a catchment area of 14,834 km2, which covers about 70% of Perak state flows nearby the study area. The site was selected due to Sungai Perak being a main water supply source for Kuala Kangsar City and having been classified as a Class II River (DOE 2017). To meet the demand for clean water supply, a RBF well needed to be developed at the site area at coordinates of latitude 4°48′07″ north, longitude 100°57′06″ east. The land use and land cover of the surrounding areas can be categorized as rangeland, forest, built-up, agricultural, barren land and waterbody areas (Razak et al. 2015).
Acquisition of field data
A suite of TerraTEM time-domain electromagnetic surveying systems was carried out at 27 survey points as depicted in Figure 3, using a simple coincident rectangular loop configuration, in which the same loop transmits and receives signals. The loop area was 100 m2 (10 m × 10 m side length). To be able to construct an image of the subsurface, the survey was conducted along profiles with a sounding location spacing of 10 m. Each position of a survey point was taken with a handheld Garmin GPSmap 76CSx at the center of the loops. The TerraTEM produced currents of 8–10 A depending on the size of the transmitter loop (Payne & Teeple 2011). A minimum of four people were required in this field operation, which was able to take 15 to 20 soundings per day depending on the terrain and weather condition. Up to 100 stacks were used during data acquisition to increase the signal-to-noise ratio. At all stations, the measurement was repeated three or four times to ensure the data obtained was in a steady state. The data acquired later were stored in the instrument receiver console.
Data processing and interpretation
Prior to performing the modeling, the raw data were extracted from the instrument receiver and converted to USF (Universal Sounding Format). This process takes the raw data and formats it for import into a modeling program. The modeling program ‘TDEM Geomodel’ was used, which is a shareware web application designed for editing, inversion and interpretation of TEM data. Furthermore, the inversion is based on an algorithm with the support of CSIRO Division of Exploration and Mining and the Australian Mineral Institute Research Association (AMIRA) (Geomodel 2017).
After the USF file data were imported into the program, all 27 stations were selected for the inversion process and generated a 1-D inversion model (models of apparent resistivity as a function of apparent depth) for each station. For each sounding, apparent resistivity values were calculated from the raw voltage values measured. When plotted in time these apparent resistivity values yield a decay curve representing the subsurface electrical lithology. Data points that deviated substantially (from the viewpoint of the author at the site) from the decay curve (and therefore represented suspect data) were deleted before inverse modeling. The inversion values were then exported as a .txt file and by using the Kriging interpolation method in Surfer 13 software, four (4) lines of 2-D pseudosection subsurface profiles were produced. Each line consisted of a combination of several stations as follows (refer to Figure 3):
Line 1: S1-S5-S9-S13-S17-S21-S25
Line 2: S2-S6-S10-S14-S18-S22-S26
Line 3: S3-S7-S11-S15-S19-S23-S27
Line 4: S4-S8-S12-S16-S20-S24
The derived 2-D subsurface apparent resistivity–depth sections were interpreted with borehole data logging in the study area as shown in Table 1 to represent the geologic unit type with depth.
RESULTS AND DISCUSSION
The TEM data collected throughout the selected area were represented as four 2-D resistivity cross-section (depth–distance) profiles with their corresponding geological units after being compared with borehole data (in Table 1). The output of each profile line resulted from a combination of several survey station data, which was generated using Surfer 13 software. The length of each profile was 70 m with 10 m spacing (the distance to each survey station) except for profile 4 (total length of 60 m).
The results from all profiles in Figure 4 indicated an approximately equivalent pattern of resistivity distributions. These profiles can be summarized to consist of two main zones of different resistivities. The first, upper zone is of relatively high resistivity (10–40 Ωm) and extends to about 8 m depth, corresponding to the saturated sand and gravel. The second zone extends to 30 m depth, showing a decrease in resistivity (<5 Ωm) due to the presence of saturated weathered rock in the subsurface. These data fit well with the lithology log (Table 1) extracted from one of the boreholes near the existing RBF well.
The top layer of the first zone (0–4 m depth) can be attributed to wet silt with resistivity values ranging from approximately 5 to 10 Ωm. The soil is considered wet (water present in the soil) because of the low resistivity value, as an effect of data acquisition during the rainy season. In the second zone (>8.2 m depth), a very low resistivity (high conductivity) value was detected probably due to the presence of fresh water within the soil, which came from surface water seepage through the riverbed to the deeper layers. This second zone is suspected to have good potential as a groundwater source.
From Figure 4, there are some areas in the profile that appear to have negative resistivity values. Bortolozo et al. (2016) pointed out that the negative values obtained have no physical meaning and the problems encountered are related to inaccurate calculation of the apparent resistivity. Furthermore, because of the low resolution from these values, there is increased ambiguity and reduced stability of the inversion processes.
As a whole, the presence of silt, sand and gravel in the first zone matched the characteristics of alluvial-type soil as stated by Schubert (2002), thus meeting the criteria for a potential RBF area. As reported in the previous study by Razak et al. (2016), the results of ground resistivity surveying at the same location showed that layers with extended depth to 8 m were also identified as alluvial aquifer zones. However, the range of resistivity values acquired by the TEM sounding (in the current study) is much lower than the ground resistivity (from the previous study), indicating that the zones are saturated. According to Tsiboah (2002), the TEM results can be said to approximate fairly well with the real situations compared with ground resistivity results, particularly in terms of its layer thicknesses. Due to its high sensitivity to conductivity, the TEM soundings are very useful in detecting the resistive zones which may be associated with fresh water as compared with the ground resistivity method. Therefore, the use of TEM for future RBF exploration is recommended.
The TEM method is a very useful tool for determination of the alluvial aquifer zone for RBF as demonstrated by this study. The interpretation results showed that: (a) the first upper zone in the study area (up to 8 m depth) was identified as saturated sand and gravel (resistivity value of 10–40 Ωm); (b) the second zone (extended to 30 m depth) was characterized as saturated weathered rock (resistivity value <5 Ωm); and (c) the top layer of the upper zone (0–4 m depth) can be attributed to wet silt (resistivity value of 5–10 Ωm). Hence, the first upper layer was considered as alluvial-type soils in the presence of silt, sand and gravel. Based on the good quality results produced as well as its convenience, it is recommended that the TEM method is used for RBF exploration in future work.
The author would like to acknowledge the Ministry of Education Malaysia for providing LRGS Grant on Water Security ‘Protection of Drinking Water: Source Abstraction and Treatment (203/PKT/6720006)’ for sub-project 1.4: ‘Site Characterization of Surface and Subsurface Spatial Data in RBF’.