Synthetic modeling of EM34 in detecting the shallow clayey layer in aquifer system

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It is essential to get information on clayey layers, which may play a significant role in protecting shallow aquifers thanks to their low hydraulic conductivity from surface contamination. One of the simple geophysical instruments is EM34 which applies the electromagnetic principle to read directly and quickly the apparent electrical conductivity of the subsurface.

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Synthetic modeling of EM34 in detecting the shallow clayey layer in aquifer system Xây dựng mô hình tổng hợp để đánh giá phương pháp EM34 trong việc nhận diện tầng sét nông của hệ thống tầng chứa nước > THANH QUOC TRUONG1, 2, 4, TIN TRUNG NGUYEN1, 2, 3, THO TRUONG NGUYEN2, 3, PHUOC HOAI LE2, 3, KHANG LE DINH TRAN2, 3, HUNG TAN PHAM2, 3, MARC DESCLOITRES1, 4 1 Joint International Laboratory Lecz-Care, Ho Chi Minh University of Technology (HCMUT) 2 Faculty of Geology and Petroleum Engineering, HCMUT 3 Office for International Study Programs, Ho Chi Minh City University of Technology (HCMUT), 4 IGE, Institut Des Geosciences De L’environnement, University Grenoble-Alpes, France TÓM TẮT: ABSTRACT Việc thu thập thông tin về các lớp sét là rất cần thiết vì các lớp It is essential to get information on clayey layers, which may play sét có thể đóng vai trò quan trọng trong việc bảo vệ các tầng a significant role in protecting shallow aquifers thanks to their chứa nước nông khỏi sự nhiễm bẩn từ bề mặt. EM34 là một trong low hydraulic conductivity from surface contamination. One of the những phương pháp địa vật lý đơn giản bằng cách áp dụng simple geophysical instruments is EM34 which applies the nguyên lý điện từ để đọc trực tiếp và nhanh chóng độ dẫn điện electromagnetic principle to read directly and quickly the biểu kiến dưới bề mặt. Để đánh giá khả năng nhận diện sét của apparent electrical conductivity of the subsurface. To evaluate the phương pháp EM34, tổng cộng 42 mô hình tổng hợp đã được xây EM34’s detectability of clay, a total of 42 synthetic models were dựng tại các độ sâu và độ dày khác nhau của lớp sét đặt nằm built at different top depths and thicknesses of clay between sand giữa các lớp cát. Các mô hình này được chuyển thành dữ liệu và layers. Synthetic models were then transferred and added 2% thêm nhiễu ngẫu nhiên 2%, sau đó được minh giải theo quy trình random noise and interpreted followed to a procedure by using bằng cách sử dụng phần mềm IX1D. Từ việc so sánh các mô hình IX1D software. The comparison of synthetic models and new tổng hợp và các mô hình sau minh giải cho các kết quả tiềm năng interpreted models gives potential results that EM34 is well able rằng phương pháp EM34 có thể nhận diện tốt độ dày và độ sâu to detect the presence (the thickness and the top depth) of the của lớp sét nông nằm từ bề mặt đến độ sâu 30 mét. Tuy nhiên, shallow clayey layer from the surface to 30-meter depth. việc ước tính độ dẫn điện của đất sét và sự hiện diện của sét However, the estimation of clay’s conductivity and the presence of dưới độ sâu 30 mét được coi là kém hơn. Một tuyến đo thực địa clay under 30-meter depth are worse regarded. A field survey đã thực hiện với tổng cộng 47 điểm được minh giải với kết quả carried out with a total of 47 soundings was interpreted with a hai lớp cho thấy sự thay đổi địa chất của đất sét dưới bề mặt với two-layer solution revealing a lateral geologic change of clay in sự hiện diện rõ ràng của một “trũng” sâu. So sánh kết quả của the subsurface which shows an obvious presence of a deep dữ liệu thực địa với mô hình tổng hợp cho thấy có vấn đề với độ “valley”. The comparison with synthetic modeling indicates a chính xác của EM34 đối với lớp sâu. Mô hình tổng hợp chỉ ra rằng problem with the accuracy of EM34 for the deep layer. The EM34 có thể được sử dụng tốt để khảo sát các thành tạo sét ở độ synthetic modeling suggests the EM34 is well taken for surveying sâu nông (từ 0 đến 30 mét) clayey formations at shallow depth (from 0 to 30 meters). Từ khóa: EM34, phương pháp diện từ thời gian, xác định lớp sét, Keywords: EM34, Frequency Electromagnetic Method, Clay mô hình tổng hợp. detection, Synthetic Modeling. ISSN 2734-9888 10.2021 263
PHÁT TRIỂN X ÂY DỰNG BỀN VỮNG TRONG ĐIỀU KIỆN BIẾN ĐỔI KHÍ HẬU KHU VỰC ĐỒNG BẰNG SÔNG CỬU LONG 1. INTRODUCTION geological model with the thickness and electrical conductivity of In sedimentary aquifer systems, the clayey layer plays an subsurface units. important role in offering a protection to the underlying aquifer In this study, the primary objective is using synthetic modeling from the surface contaminants which can infiltrate through the to check the interest and limitations of the FEM technique and ground, such as, pesticide, fertilizer and other chemical pollutant answer whether FEM, specifically EM34, can detect effectively from domestic uses. The clayey overburden which is characterized shallow clayey layers when applied in a particular subsurface by a good lateral continuity provides a good protective capacity [3]. environment. The theory of the FEM method and the principle of A small presence of a discontinuity in the clayey layer can facilitate EM34 is demonstrated in section 2. The way how to perform the infiltration of pollutants into the aquifer. As a result, it is numerical modeling will be described in detail in section 3. After essential to get information on clayey layers, which may play a that, the results aggregated and evaluated in EXCEL in section 4. significant role in protecting shallow aquifers thanks to their low Then a field data interpretation and comparison is presented in hydraulic conductivity that acts as a “barrier” to vertical fluxes. section 5 to apply the synthetic modeling work into a practical Fortunately, terrain conductivity meter (EM34) which is a simple field test. Some conclusions about the efficiency of EM34 in clay electromagnetic method allows the operator can read directly and detectability are stated at the end of this study. quickly the apparent electrical conductivity of the subsurface. Between the considerable amount of geophysical literature 2. METHODOLOGY dealing with FEM technique, most of them used one of the best Electromagnetic sounding methods include natural-field equipment available in the world, the EM34 (Geonics Ltd., Canada). methods such as Magnetotelluric (MT) and controlled-source Bartolino and Sterling (2000) have employed electromagnetic induction methods which generate the electromagnetic field in surveys (TDEM and EM34) to provide the presence of clay-rich which the frequencies are low enough (low induction number) layers in alluvium for quantifying the amount of water transmitted that the measurement distance is less than the free-space between aquifer systems that EM34 is used at a second phase in wavelength. In this study we use the controlled-source induction order to determine the distribution of hydrologically significant method, which is frequency domain electromagnetic induction or clay-rich sediment in the inner-valley alluvium. Triantafilis and FEM method. Theoretically, while FEM measurements made at Lesch (2005) have used Electromagnetic induction (with EM34) as large source-receiver separation are influenced by deep layers, an alternative method for providing spatial distribution of bulk soil shallow layers affect the measurements made at small separations. average clay content to a depth of 7 m instead of geostatistics. Hence, a set of such measurements made at various spacing Burrell et al. (2008) has used EM31 and EM34 conductivity data to contains data corresponding to the variation of conductivity with map the current and old stream channels and to determine the depth which is the fundamental working of the FEM method. In thickness of the stream sediments and the geological formation. this study, the FEM survey was carried out by using EM34 which is Zogala et al. (2009) carried out EM31 and EM34 surveys for the a simple-to-operate, cost- and time-effective instrument from investigation of oil-contaminated soils in a former underground Geonics. fuel base at Borne Sulinowo, NW Poland. Triantafilis and Buchanan Basically, the alternating current through the transmitter coil (2010) used EM34 surveys to map the spatial distribution of saline generates a primary magnetic field which induces a small current subsurface material to assess salinity hazards and manage in the earth. The secondary magnetic field (Hs) which is sensed environmental problems. With the possibility of mapping terrain with the primary field (Hp) is received by the receiver coil. The conductivity rapidly; furthermore, the sample volume is averaged secondary magnetic field is a complicated function of the in such a manner as to yield unexcelled resolution in conductivity. operating frequency f, the ground conductivity r, the intercoil The EM34 is employed in a variety of investigations and this spacing s. Under a certain constraint “low induction number”, the potential equipment can be applied in this study. secondary magnetic field function is reduced to a simple Electromagnetic (EM) soundings are commonly used in the expression which is linearly proportional to the ground exploration of several resources in terms of hydrogeological and conductivity. These constraints are incorporated in the design of environmental studies [4]. The EM soundings are normally used to the EM34-3 whence the secondary magnetic field is shown to be: [1] map structures or variations in lithology, and delineation of 𝐻𝐻� 𝑖𝑖𝑖𝑖𝑖𝑖� 𝜎𝜎𝜎𝜎 � resources. Moreover, EM soundings are also used in engineering � �1� 𝐻𝐻� 4 studies to determine material properties of rock or indicate where conductive features, anomalies. EM soundings are theoretically 𝐻𝐻� = secondary magnetic field at the receiver coil made to determine the electrical conductivity, which is the inverse 𝐻𝐻� = primary magnetic field at the of electrical resistivity, of the subsurface with depth. The 𝑖𝑖 = 2f with f = frequency (Hz) frequency-domain electromagnetic method (FEM), especially, 𝑖𝑖� = permeability of free space EM34 instrument measures the apparent conductivity of the 𝜎𝜎 = ground conductivity (mS/m) underlying earth by changing both parameters: intercoil spacing s = intercoil spacing (m) and dipole orientation. After the measurements, the measured 𝑖𝑖 � √�1 conductivity soundings are interpreted into a multilayer system for In an inhomogeneous environment which the Earth is not confirming the presence of a clayey layer (high conductivity layer) assumed to be horizontally stratified. If Hs/Hp is given, the at shallow depth by using IX1D software. This work is known as apparent conductivity indicated by the instrument is defined from Inversion Modeling in which the multilayer parameters: the the equation as: electrical conductivity and thickness of each geologic unit are calculated from a raw dataset of the measured apparent 4 𝐻𝐻� conductivity. In contrast, Forward Modeling is able to compute the 𝜎𝜎� � �2� 𝑖𝑖𝑖𝑖� 𝜎𝜎 � 𝐻𝐻� apparent conductivity instrument readings given a known 264 10.2021 ISSN 2734-9888
At the beginning of the profile survey, make an initial set-up measurement at some outcrops. The standard tabular model is set with the instrument EM34, such as, connect cables to the up as base geometries by increasing the depth and thickness to a transmitter and receiver console, check battery indicator, clayey layer between sand layers. The typical resistivity or electronic nulling to remove any offset in the output circuitry. conductivity for each component layer is also defined respectively Then, the procedure of taking a measurement at one station 20 Ωm or 50 mS/m for clays and 200 Ωm or 5 mS/m for sand. These includes 5 main steps. First of all, the transmitter operator stops at resistivity values are measured averagely and observed from the the measurement station then the receiver operator moves the borehole log database near the second EM34 survey line and an receiver coil backwards or forwards until his meter indicates additional resistivity database from the outcrop at the first EM34 correct intercoil spacing. Secondly, at the transmitter console, the survey line. operator turns the switch ON and adjusts the certain parameters The clayey unit thickness which needs to be tested was which we want to investigate, such as the intercoil spacing (10m, formerly determined from thin clayey layer 0.5 meters to thick 20m or 40m) and the sensitivity of the measurement (High/Low). clayey layer 20 meters: 0.5 m; 1 m; 3 m; 5m; 10 m; 15 m; 20m. The Afterward, the receiver operator reads the average value of the increasing depth of the clayey layer is at the surface (0 m); 3 m; 10 resistivity from the console of the receiver and writes it in a m; 20 m; 30 m; 50m. Thus, there are 7 thicknesses and 6 depths of prepared sheet. Measurements at each station are made in both clayey layer which need to be tested in this synthetic modeling the vertical and horizontal dipole position to give an indication of section. Therefore, a total of 42 synthetic data sets created from 42 the conductivity variation. As a result, when the first coil synthetic models is named from SD01 to SD42. For instance, the orientation (such as HMD) operation is finished, the operators of SM14 structure represents a 20-meter thickness of clayey layer two coils change coil orientation to the other (VMD). The value of bounded by a top 3-meter thin sand and sand substratum and its conductivity is read and recorded again in a sheet, then they move conductivity value ranges from 5 mS/m (200 Ωm) to 50 mS/m (20 the instrument to the next station. The location of the center Ωm) and 5 mS/m (Ωm) respectively. All the geophysical synthetic between two coils contemporarily is obtained from a GPS modeling was carried out with the use of the IX1D software to instrument by another operator at each station. The UTM generate 6 synthetic apparent conductivity values of VMD and (Universal Transverse Mercator) coordinate is recommended to be HMD of 10-meter, 20-meter, 40-meter for each synthetic model. A used in the measurement because the meters’ base unit of UTM total of 252 synthetic data points (VMD-10, VMD-20, VMD-40, coordinate makes conversions and measurements easier and the HMD-10, HMD-20, HMD-40) are shown in Table1 from 42 synthetic system is convenient for mapping small profiles. Each station models. Subsequently, 2% of random noise was added to synthetic normally takes approximately from 20 to 30 seconds for both data to take into account more realistic field conditions. It’s then dipole modes HMD and VMD. This workflow is the same for all next named from SDn01 to SDn42. station points. 3. NUMERICAL MODELING The workflow of synthetic modeling is simply described as a followed diagram: Figure 1 The simple workflow of synthetic modeling Figure 3 All 42 synthetic models with increasing in depth and thickness of clayey layer The inversion procedure of EM34 synthetic datasets were also carried out using the IX1D-EM Conductivity Sounding section Figure 2 The synthetic modeling workflow for detecting shallow clayey layer with 10 which considers a 1D horizontal model. The loading procedure of meter thick at different depths raw datasets into IX1D software is represented in Appendix. From Initially, a variety of synthetic models were built to approach the raw dataset, a prior model is created manually based on the the intention of synthetic modeling. The synthetic modelling aims apparent conductivity figuration and it’s called the “Starting to answer a typical question as: “Is EM34 able to detect a clay layer Model” (StM). There are then repeated steps that the interpreted of various thicknesses at various depths?”. A synthetic model (SM) model’s data is minimized closely to the raw data in order to get essentially consists of the thickness and electrical conductivity of the best fit result. To select the suitable StM for inversion of each component units. Firstly, synthetic modeling of EM34 was synthetic dataset, the comparison depends on two factors: the conducted based on the resistivity ranges obtained from the fitting error value and the similarity of the result, i.e. the borehole log database and direct electrical resistivity “Interpreted Model”, IM, to the original synthetic model SM. ISSN 2734-9888 10.2021 265
PHÁT TRIỂN X ÂY DỰNG BỀN VỮNG TRONG ĐIỀU KIỆN BIẾN ĐỔI KHÍ HẬU KHU VỰC ĐỒNG BẰNG SÔNG CỬU LONG The first model, StM1, based on the increasing or decreasing of layers. From 42 interpreted models, the preliminary evaluation of conductivity data against the intercoil spacing increase of HMD or the clayey layer is carried out and the electrical conductivity, the VMD measurement. The interpretation of StM1 with every simple thickness of the clay layer, and the depth to top of the clayey layer dataset is used when there is no information about the geology of are gathered in EXCEL file in order to make different comparisons subsurface or any additional geophysical data, and it is also a between interpreted clayey layer and synthetic clayey layer. useful model for the beginner of EM34 interpretation. It is Results showing various responses of EM34/FEM in detectable proposed that when 6 data points (VMD and HMD) or 3 data points clayey layers are presented below. (VMD/HMD) show an increasing pattern of apparent conductivity with penetration depth, the model structure of the StM will also have 3 three layers which are increasing conductivity with depth and vice versa. The conductivity value of each layer in the model is chosen equal to the apparent conductivity from the dataset. For example, the first model StM1 of a slight decrease of apparent conductivity with depth can be 5-meter thin layer (50 mS/m) at the surface, a thick layer (5 mS/m) at the bottom, and a 15-meter layer (with a middle value between 5 mS/m and 50 mS/m, such as 27 mS/m) in the middle. The second StM2 model based on the presence of a clayey layer at shallow depth according to the objective of this study. This consists of a top 5-meter unsaturated sand (5 mS/m), 15-meter clayey layer (50 mS/m), and a saturated sand layer (15 mS/m) at the bottom. As a result, the model StM will Figure 4 Comparison of true thickness and interpreted thickness of clayey layer be similar to field conditions and have a better capability to detect The first graph shows the comparison between the interpreted clay zones or clayey layers. thickness and true thickness of clayey layer at different true depth Some SM models are chosen within 42 SM models to apply two of clayey layer. While the horizontal axis is the true thickness of the StM models in the interpretation for selecting the best starting clayey layer in meters, the vertical axis is the interpreted thickness model. The selected StM model will be applied for all SM models. in meters. Different line’s colors illustrate the true depth of the top Some SM models are compared with their new IM models of the clayey layer in meters: at surface (0m), 3m, 10m, 20m, 30m, produced using successively StM1 and StM2 with two multiple 50m. The red dash line with the coefficient 1 which means that the iterations. Two multiple cycles of iterations are indeed enough to true thickness is equal to the interpreted thickness or the EM34 obtain a convergence of the RMS to a stable minimum value. The method is able to quantify the thickness of the clay. It is clear from results indicate that the StM2 models will effectively give a better the chart that clay is not detectable at the 30-meter (blue) and 50- fit and close interpreted models which reveal a presence of clay meter depth (green). For example, the interpreted synthetic model than StM1 models. Subsequently, the best starting model StM2 is of clayey layer at 50-meter depth firstly indicates that there is no described as a 15-meter thick clay with the conductivity of 50 layer of roughly 50mS/m and an infinite thickness layer of clay at mS/m (20 Ωm) bounded by a top 5-meter thick unsaturated sand ≈32 mS/m if clay’s conductivity is assumed to be nearly 20mS/m. (5 mS/m or 200 Ωm) and saturated sand (15 mS/m or 66.6 Ωm). Moreover, the thickness of a clay layer situated at depths of 10 and The interpreted models are obtained by the inversion with the 20 meters is systematically overestimated by approximately 50%. StM2 for the 42 synthetic datasets with a constant number of At the depth at 3 meter and at the surface, the interpreted multiple iterations. The interpreted results were considered thickness is roughly the same as the true thickness but when the acceptable when the root mean square factor (RMS) of fitting error clayey layer is thicker than 10 meter, the method is slightly between raw data and calculated data is less than 1.5% due to 2% underestimated. As a result, the method can detect the shallow noise factor. In addition, the preliminary interpretations of all clayey layer’s thickness which coincides with the interpreted datasets have significant 1st and 2nd RMS values. Therefore, we thickness. In particular, for clayey layers which are thin or thick in decided to increase the number of multiple iterations at 3 instead the first 10 meter, its thickness is very well defined and if the clay is of 2, that means the inversion will be stopped at the 3rd multiple situated deeper from 10 to 20 meter, its thickness is slightly iterations with the start model in order to get the interpreted overestimated, and this confirms that shallow clayey layer is well thickness and conductivity range value of geologic units from each detected by EM34. dataset. The workflow of synthetic modeling summarized below describes the procedure of evaluating the detectability of a 10- meter thick clay layer at different depths (at surface, 3m, 10m, 20m, 30m, 50m). 4. RESULT The inversion of 42 synthetic datasets commonly result in a three-layer structure which consist of electrical conductivity (EC1) and thickness (T1) of first layer, electrical conductivity (EC2) and thickness (T2) of second layer, and electrical conductivity (EC3) of third layer [2]. Occasionally, if the EC of a layer is approximately EC of an adjacent layer, we decide to give a two-layer solution. The new layer is a layer whose thickness is the sum of two thicknesses and the electrical conductivity is the average EC value of both Figure 5 The comparison of true depth and interpreted depth of clayey layer 266 10.2021 ISSN 2734-9888
The second graph was established to evaluate the efficiency of horizontal axis is the true thickness of the clayey layer in meters, the EM34 method in detecting the top depth of a clayey layer. This the vertical axis is the interpreted electrical conductivity of the line chart compares the interpreted top depth in terms of true top clayey layer in meters. Different line’s colors describe the true top depth of clayey layer at various true thickness. The horizontal axis depth of the clayey layer in meters: at surface (0m), 3m, 10m, 20m, represents the true top depth of the clayey layer in meters, and the 30m, 50m. The red dash line with the interpreted conductivity of vertical axis is the interpreted top depth in meters. A wide range of clay is 50 mS/m which means that the interpreted conductivity line’s colors illustrate the true thickness of the clayey layer in coincides with the true conductivity of clay or the EM34 method is meters: 0.5m, 1m, 3m, 5m, 10m, 15m and 20m. The red dash line able to effectively quantify the electrical conductivity of the clay. with the coefficient 1 meaning the true top depth is equal to the The clay is truly interpreted as the true value of conductivity or interpreted top depth of the clayey layer or the EM34 method is resistivity when it’s well defined that the clay is more than 5-meter- able to effectively quantify the top depth of the clay. It can easily thick and close to the surface from 0 to 3-meter depth. If the clay is be seen that as the clayey layer is thinner, the method cannot deep, the conductivity is strongly underestimated. detect the presence of the clay at lower depth. Additionally, the interpreted depth is slightly equal to true depth from 0 to 30 meter and the result is strongly underestimated when the clayey layer’s top depth is over 30 meter. Eventually, the EM34 method is able to detect efficiently the depth to the conductive substratum (clayey layer) up to 30 meter. Figure 7 The comparison of VMD and HMD with 5 meter thick clayey layer with EM34 in the range apparent conductivity from 3 to 7 mS/m Figure 6 The comparison of VMD and HMD data with 5-meter thick clayey layer The third line chart represents the comparison of synthetic apparent conductivity data of a 5-meter thick clayey layer between VMD and HMD of 3 three intercoil spacing at different depths. While the vertical axis indicates the synthetic apparent conductivity data in mS/m in the range from 2.0 to 32 mS/m, the horizontal axis shows the contribution of true top depth of the clayey layer to the dataset in meters. The different colors of solid Figure 8 The comparison of interpreted and true conductivity of clayey layer lines represent the horizontal mode dipole’s data of 10-meter, 20- In general, although the EM34 method is not chosen for good meter, 40-meter intercoil spacing, and the dash lines are estimating conductivity value of clay, it’s well able to detect the conversely vertical mode dipole’s data. This line chart initially presence: the thickness and the top depth of shallow clayey layer illustrates the fact that the sensitivity of the method is the best from surface to 30-meter depth. In contrast, the inaccuracy and between 0-meter and 10-meter depth of the clayey layer because uncertainty of the EM34 method moderately increases with depth of the difference in apparent conductivity data of components: when the clayey layer is situated deeper than 30 meter. VMD and HMD. Moreover, if the clay is situated between 10 meter and 30 5. FIELD EXAMPLE meter, it still shows a sharp difference between VMD and HMD. However, the difference between values of VMD and HMD is very small when the top depth of the clayey layer is lower than 30 meter. As can be seen from the Figure 7 which describes more detail with small-scale or apparent conductivity data, if a deep clay is from 20 to 30 meter, the EM34 method has to measure the apparent conductivity data from 4.5 to 7 mS/m but the variation of noise is very high in the field. Subsequently, it’s difficult to get data and interpretation of deep clayey layers because the range of apparent conductivity is very small. Hence, the method needs to Figure 9. A general view and location of study area in Google Earth showing the measure good data and limit the noise effect in order to be able to profile EMLINE1 contains 47 data stations. obtain the possibility of interpretation of deep clay. In this study, an example of field survey is presented to An additional fifth line chart is made to compare between the illustrate the efficiency of EM34 and to compare with synthetic interpreted conductivity and true conductivity of clayey layer at modeling statements. The survey EMLINE1 was carried out at a different thickness and true top depth of clayey layer which means large paddy field in Cu Chi area, Ho Chi Minh city and near Saigon to check the detectability of correct conductivity value of clay. The River where it is convenient to take a sounding measurement with ISSN 2734-9888 10.2021 267
PHÁT TRIỂN X ÂY DỰNG BỀN VỮNG TRONG ĐIỀU KIỆN BIẾN ĐỔI KHÍ HẬU KHU VỰC ĐỒNG BẰNG SÔNG CỬU LONG all 10, 20 and 40-meter intercoil spacing. The profile ranges 40 to 80 meters) which the method gives a strongly poor approximately 1,170 km long with 47 soundings (from estimated result at the range over 30-meter depth. As a result, EM01...EM39, EM41, EM43, EM45, EM47, EM49, EM51, EM53, EM55) segment B should provide a better result than segment A in terms which are 20 meter separately. Moreover, the location does not of electrical conductivity and top depth of clay. appear a strong effect of noise due to the absence of nearby powerlines or metallic overburden objects. Although some fittings 6. CONCLUSIONS do not comply with an acceptable RMS of less than 5% because of It is essential to get the information of the presence of shallow the discrepancy of HMD and VMD fitting in the interpretation, the clayey layers for checking the continuity of clayey overburden the data is acceptable due to the calibration of the instrument EM34. aquifer system from infiltration of surface contaminants. The EM34 The data is demonstrated in a graph of measured conductivity instrument which applies frequency electromagnetic method is (apparent conductivity) and effective depth which is correlated selected to make an experiment because it can measure directly with the selection of intercoil spacing and coil orientation in figure the apparent electrical conductivity of the subsurface. This below. The purple square marks show the HMD (vertical coil) data, instrument is a cost-effective and not a time-consuming while the blue ones are the VMD (horizontal coil) data. geophysical technique for make a sounding or mapping in the field. A total of 42 synthetic models were built at different top depths and thicknesses of clay between sand layers to test the efficiency of EM34 in detecting the accurate presence of the shallow clayey layer. In the workflow, synthetic models were then transferred and added 2% random noise and interpreted followed to a procedure by using IX1D software. The comparison of synthetic models and new interpreted models emphases that EM34 is well able to detect the presence (the thickness and the top Figure 10 EM34 field data and interpreted result (2D model) of EMLINE1 survey, depth) of the shallow clayey layer from the surface to 30-meter respectively showing (A) the presence of “valley”; (B) intermedia depth of clay at roughly depth. However, the evaluation of clay’s conductivity and the 30 meter presence of clay under 30-meter depth suggest that EM34 should Along the profiles, there is no evidence of shallow clayey layers not be selected for making an investigation to estimate the (0-15-20 m). We know from the synthetic modelling that this range electrical conductivity or to identify the deep clayey layer. of depth is the highest range of sensitivity of the method, so we From a total of 47 soundings with a two-layer solution of the can have confidence in this result. The section can be divided into field survey, there is no evidence of shallow clayey layers in 0-20 two segments A (530m to 1,030m) & B (the other distances). meters but there is a clayey layer at an intermediate depth of Segment A & B both contain conductive substratum (blue zone) nearly 30 meters which the method is sensitive at this range of underlain by a resistive layer (orange zone). Segment B situated at depth proved by synthetic modeling section. These results may the edge of the section is a total of 670m long. It shows that there additionally indicate that there is a lateral geologic change of clay is a clayey layer that has a top depth of approximately 30 meters in the subsurface which shows an obvious presence of a deep and does not change much laterally. However, the top depth of “valley”. In conclusion, the EM34 is a very effective instrument for clay is nearly 40 meters at a distance of 350-meter distance. the use of surveying quickly the clayey formations at shallow Segment B has a light blue color which indicates the lower depth (from 0 to 30 meters). It also can be used for further studies, conductivity of clay nearly 30 mS/m. In contrast, segment A shows such as, mapping the continuity of the shallow clayey layer or a deeper clayey substratum and has a pattern of a valley. This mapping promptly the variation of apparent conductivity with “valley” has the deepest top depth of clay is nearly 75 meters at an depth. 845-meter long distance. However, the conductivity value of clay is very high, roughly 100-200 mS/m, but the conductivity value of the ACKNOWLEDGEMENT: upper resistive layer remains constant in range from 2.5 to 3.7 This study was financed by different sources: the LECZ-CARE International Joint mS/m similar to segment A. Some attempts in fixing the Laboratory, the French Embassy in Vietnam, the CARE laboratory and the Faculty of conductivity value in segment A at lower value (30-50 mS/m) have Geology (GEOPET) of the Ho Chi Minh University of Technology (HCMUT), the IGE been done and the results are showing that the top depth of the laboratory. References clayey layer is more shallow (50-65 m) than the old result, but the [1] McNeill, J.D. (1980), Electromagnetic Terain conductivity mearuement at low RMS value increases from 4 to 6%. induction numbers, Technical TN-6, Geonics LimitedGeonics Limited. These results are then compared with the synthetic modeling [2] McNeil, J.D. (1980), EM34-3 Survey Interpretation Techniques, Technical Note TN- to examine the effective range with the FEM method and to 8, Geonics Limited. answer whether these results are potentially accurate concerning [3] James R. Bartolino, J. M. (2000). Electromagnetic Surveys to Detect Clay-rich the real subsurface. At segment B, the top depth of the clayey layer Sediment in the Rio Grande Inner Valley, Albuquerque Area, New Mexico. US Department is approximately 30 meters and it can be trusted to be slightly of the Interior, US Geological Survey. equal to the true depth in real subsurface because EM34 effectively [4] Ogungbemi, O. S., Badmus, G. O., Ayeni, O. G., & Ologe, O. L. U. W. A. T. O. Y. I. N. detects conductive substratum at this range of depth. However, (2013). Geoelectric Investigation of Aquifer Vulnerability within Afe Babalola University, the conductivity value of clay is underestimated at this depth, Ado–Ekiti, Southwestern Nigeria. IOSR Journal of Applied Geology and Geophysics, 1(5), 1- which means the true electrical conductivity has a much higher 7. value than the interpreted value (27-31 mS/m). The “valley” at [5] Shiraz, F. A., Ardejani, F. D., Moradzadeh, A., & Arab-Amiri, A. R. (2013). segment A is considered to be not a good result according to Investigating the source of contaminated plumes downstream of the Alborz Sharghi coal synthetic modeling conclusions. The top depth and electrical washing plant using EM34 conductivity data, VLF-EM and DC-resistivity geophysical conductivity of the clayey layer at this segment is very deep (from methods. Exploration Geophysics, 44(1), 16-24. 268 10.2021 ISSN 2734-9888