Super Hewa Khola hydroelectric project is located at Super Hewa Khola in Sankhuwasabha District, Koshi Zone, Eastern Development Region of Nepal. The location map of the project area is shown in fig 1.
Geophysical exploration using two Dimensional (2D) electrical resistivity tomography (ERT) survey plays a significant role in determining the subsurface resistivity distribution by making measurement on the ground surface. From these measurements, apparent resistivity is recorded during the fieldwork, which is later converted to true resistivity using appropriate computer software. This true resistivity is then converted to subsurface geological condition. The 2D electrical resistivity profiling is capable of detecting boundaries between unconsolidated materials (overburden), rocks, identifying weathered rock from fresh rock and contact between lithology, water table etc.
Ⅰ.2D Electrical Resistivity Tomography Survey
Data Acquisition
The quality of data acquired in the field not only depends on quality of the equipment and accessories but also depends on the topography, geological setup and density of the measurement. Highly heterogeneous conditions are created by surface topography and geological variation. Planning of the fieldwork and layout of profiles is usually based on topographical map. Field data were gathered to obtain a continuous coverage of the subsurface along the line of investigation. As mentioned above Wenner electrode configuration was employed in the present study. Geologically the area under investigation lies in the Higher Himalaya rocks. Bed rock exposed at the study area is banded gneiss and schist and quartzite.
Data qualit
Field data are influenced by different types of noises of different origins. The degree of influences depends on the quality of the equipment and accessories, methods of data acquisition and geological and topographical setup. Selection of proper equipment and accessories, and data acquisition system helps to gather reliable field data. Proper selection of the orientation of the profiles or an additional cross profile helps to recognize noise due to thegeological and morphological setup of the area. The selection of the electrode arrangement is based on the requirement of depth of investigation and resolution of the subsurface. Some electrode arrangements produce high level of signal but poor resolution whereas some produce low level of signal but higher resolution of the subsurface. As the distance between transmitting and the receiving electrodes increase the level of the noise also increases dramatically. The type of the noises are capacitive coupling, induction and of telluric and cultural origin. To avoid this adverse effect on the receiving signal it is necessary to have high quality accessories and signal processing capacity of the receiver electronics.
It is necessary to understand that the geophysical methods look not only along vertical and lateral along the profile but also look sideways. Geological variations that come within the radius of influence in sideways also influence the data. Such influences could clutter the section and make difficult to meaningful interpretation. So the noises introduced into the data due to the geological setup that are not easily interpretable are known as geological noises.
Field Crew
The members of the field crew are geoscientist, assistants, surveyors and laborers. The main responsibilities lie on the geoscientist who should be involved in all stages starting from planning to report submission. Present field mission was carried out by an experienced geoscientist who was also involved for all stages of geophysical mission. Without good knowledge of the field condition it is difficult to make reliable interpretation of the model. Experienced assistants were involved during the field work and they were responsible for checking of the field layout and connection between shielded multi-core cables and the electrodes, preparation of the station for good grounding and supervising to laborers. Surveyor was responsible to read the profile preferred by geoscientist and to make topographic leveling needed for data inversion. Detail topographic leveling of the profiles is important in the study of slope.
Equipments used in the field study
In the surveys works the field data acquisition was made using the following equipments:
1)Multi function digital DC Electrical Resistivity (GD-10).
2)Multiplex electrode converter (Multi electrode switching equipment CS 60).
3)Multi core cable with each take out at every 5 m
The equipment has the following special technical features:
a) Capable of conducting DC Electrical Resistivity, Induced Polarization & Spontaneous Potential data acquisition in normal mode
b) Capable of conducting Electrical Resistivity and Induced Polarization Tomography (Imaging). Any survey length can be covered using roll along technique.
c) Capable of mass data storage of over 100,000 number of data in equipment memory
d) Capable of multiple functions, high accuracy, fast operation, high reliability etc) Data acquisition can be started from any electrode position.
g) Voltage maximum 144 V. j) All data acquisition operations are microcomputer controlled and user friendly menu driven
k) High anti interference performance and precision, integrated with multistage wave filtration and signal enhancement technology
l) Automatic cancellation of Self Potential, drift and electrode polarization.
n) Stored field data can be downloaded to the Personal Computer easily using RS 232 port. The data download can also be in RES2DINV recognizable format.
Ⅱ.Interpretation of Electrical Resistivity Tomography Results
Data Processing and Interpretation
The field data were filtered, processed and treated with the software, RES2DINV. However, to check the quality of the acquired data, preliminary processing of the data was carried out in the field itself by an experienced geoscientist. The software inverts the field data and calculates the appropriate model in term of resistivity and provides output in the form of resistivity contours. This inversion data is used to draw up the lithological and geological information. The basic principle behind the relation between resistivity data and lithology/geology are already dealt with in above sections. The inversion results showingresistivity model with interpretative cross sections of all 9 profiles are presented. Geological/ lithological information is extracted from the ERT result (resistivity contour value) & are marked in the respective ERT sections. The sections are prepared as per topographic undulations, as the electrodes are fixed at each 5m slope distance (not horizontal distance or plan distance).
Pitfalls of the processing and interpretation
Every geophysical method has some advantages and limitations. The limitations are usually posed by data density, quality of the instrument and accessories, signal resolving capacity, geological, hydrogeological and topographical setup of the area, and by the physics of the particular methods. Data obtained from the complicated geological and topographical setup, and noise in the data combined with the processing techniques could result in artifacts. There could be possibility of the over interpretation of such artifacts.
In the interpretation of resistivity tomograms following factors have been taken into account:
Artifacts of inversion code are usually due to the poor data coverage. To sample a subsurface target of interest with higher degree of reliability we need to collect data with more coverage. The equipment used in the present study has facility which helps to collect considerable number of data. High density data coverage and measurement helps to minimize and recognize the artifacts which are usual in scanty data coverage.
Proximity of any unusually high or low resistivity and their relative thickness: In our case sheared zone, fractured and jointed rock mass zone are highly conductive than the intact bedrock. In smooth model inversion technique, which is generally applied to identify the geological boundaries low resistivity zones (if they are thin) have tendency to inflate in thickness.
Non-uniqueness of the geophysical interpretations, also called principle of equivalence in electrical resistivity method. The resistivity and thickness of the thin layers are distorted. Thickness of low resistivity zone is usually inflated than the actual thickness. The resistivity values obtained for such thin zones are also distorted but they are indicative.
Since there are no sufficient data points in the start and end of 2D electrical resistivity tomography profiles the reliability of the start and end of the profile largely depends on localgeological variation. Model section could be distorted at the start and end of the profile if there is significant level of variation in the geology in these parts.
The model sections obtained from data inversion are presented as resistivity tomogram sections. These tomogram sections show the variation of modeled electrical resistivity in depth and along the line of investigation. These variations in modeled physical properties have relation with the subsurface geological and hydro geological set up. Representative resistivity tomogram sections for each section and their interpretations are presented in Figure 4 to 10.
Resistivity Tomogram and Lithological interpretation of ERT -1(Across the river at dam site)
Figure 4: ERT 1 Across the river at dam site
The upper portion of this profile contains the saturated overburden except from chainage 35- 45 m. Within this chainage (35-40m) big rock boulder is present. The depth of this overburden ranges from 5 m in the periphery to 15m in the central part. The big rock boulders have been encountered on the way from chainage 20-40 m below 5m from the surface. At the central part of the profile from chainage 50-70 m and below the depth of about 10 m from the surface contains fractured rock. The thickness of this fractured rock is more than 10 m
Resistivity Tomogram and Lithological interpretation of ERT -2 (Left bank along the river (Dam site)
Figure 5: ERT2 Left bank along the river (Dam site）
Upper Layer of this profile contains saturated overburden from chainage 0 to 30 m. Thickness of this saturated overburden ranges from 5 to 10 m below the surface. From chainage 30 m to 120 m, it is covered by dry overburden, which is even more dry from chainage 85-95 m. The depth of this overburden ranges from 15 m to 20 m in the left part of the profile (chainage 30-50 m) and is around 10 in the central part of the profile where it overlies saturated water bearing zone. This saturated water bearing zone extends from chainage 40 m to 90 m. The thickness of this saturated zone is around 10 m in the central part (from chainage 40-75m) and is around 5m from chainage 85 to 95 m.
Resistivity Tomogram and Lithological interpretation of ERT -2a (Left bank along the river at settling basin）
Figure 6: ERT -2a (Left bank along the river at settling basin)
Upper Layer of this profile contains saturated overburden from chainage 0 to 30 m. Thickness of this saturated overburden ranges from 5 to 10 m below the surface. From chainage 30 m to 120 m it is covered by dry overburden. The depth of this overburden ranges from 15 m to 20 m along the profile (chainage 30-50 m and chainage 70-85 m) and is around 10 in the central part of the profile where it overlies saturated water bearing zone. This saturated water bearing zone extends from chainage 40 m to 70 m. The thickness of this saturated zone is around 10 m in this part (from chainage 40-75m) and is less than 5m from chainage 85 to 90 m. A trace of competent bedrock has been identified at 20m depth below the surface at chainage 70 m.
Resistivity Tomogram and Lithological interpretation of ERT -3(Right bank along the river, Downslope, dam axis)
Figure 7: ERT -3(Right bank along the river, Downslope, dam axis)
Upper Layer of this profile contains saturated overburden from chainage 0-45 m. The thickness of this overburden is less than 5 m. From chainage 45 to 120 m, the upper part of contains dry overburden which extends up to the depth of 15m from the surface. Below the saturated overburden from chainage 0 to 50 m there exists dry alluvium having a thickness of about 15m. At a depth of 10-15m from the surface, saturated fractured rock is encountered from chainage 40 to 90 m.
Resistivity Tomogram and Lithological interpretation of ERT -3a (Right bank along the river up Slope (Dam site)
Figure 8: ERT -3a (Right bank along the river up Slope (Dam site)
Upper Layer of this profile contains partly saturated overburden from chainage 0-55 m. The thickness of this overburden is about 5 m. From chainage 55 to 120 m, the upper layer of this profile contains dry overburden which extends up to the depth of 15m from the surface. Below the partly saturated overburden from chainage 0 to 50 m there exists dry alluvium having a thickness of about 15m. Below the dry alluvium, at a depth of 10-15m from the surface saturated fractured rock having thickness of 2-10 m is present from chainage 40 to 90 m
Resistivity Tomogram and Lithological interpretation of ERT -4 (Left bank along the river (Powerhouse site)
Figure 9: ERT -4 (Left bank along the river (Powerhouse site)
Upper Layer of this profile contains dry overburden of high resistivity value. This overburden has a thickness of up to 15 m. From chainage 50 to 65 m there might be big rock boulders at a depth of about 5 m. Some rock boulders are exposed to the surface at chainage 75 to 80 m. From chainage 40 to 70 m there exists saturated fractured rock at a depth of 10 m from the surface. The thickness of this saturated part is around 10 m. Similarly, a saturated alluvial deposit is encountered below a depth of about 5m from the surface from chainage 75-85 m. The thickness of this saturated part is also around 10 m. There might be presence of bedrock at a depth of 10 m below the surface from chainage 20-40 m and 85-95 m.
Resistivity Tomogram and Lithological interpretation of ERT -4a (Left bank along the river upslope, powerhouse site)
Figure 10: ERT -4a (Left bank along the river upslope, powerhouse site)
Upper layer of this profile contains dry overburden from chainage 0 to 120 m. The thickness of overburden ranged from 10 m to 20 m. From chainage 20 to 30 m the thickness of dry overburden is about 10 m composed of dry alluvial terrace deposit. And below this deposit there exists bed rock. Thick overburden of residual soil is found from chainage 40 m to 80 m which overlies the dry alluvial deposits. From chainage 75 to 90 m saturated alluvial deposits of 10 m thickness are encountered at a depth of 10m from the surface. A small trace of bed rock can be found at a depth 10 from the surface from chainage 85 to 100 m. The rock type in this area is higher Himalayan gneiss.
Resistivity Tomogram and Lithological interpretation of ERT -4b (Left bank along the river lower downslope, powerhouse site)
Figure 11: ERT -4b (Left bank along the river lower downslope, powerhouse site)
Upper layer of this profile contains different types of overburden. There is partly saturated overburden from chainage 0-45 m, saturated overburden from chainage 50-65 m, dry overburden from 70-85 m and saturated overburden from chainage 100-120 m. The thickness of partly saturated overburden ranged from 10 m to 20 m. From chainage 20 to 30 m the thickness of dry overburden is about 10 m composed of dry alluvial deposit which overlies the bed rock. Thick overburden of residual soil is found from chainage 40 m to 90 m which overlies the saturated alluvial deposits of 10 m thickness at a depth of 10m from the surface. A small trace of bed rock can be found at a depth 10 from the surface from chainage 90 to 100 m. The rock type in this area is higher Himalayan gneiss.
Resistivity Tomogram and Lithological interpretation of ERT – 5 (Left bank perpendicular to the river (North) upslope, Powerhouse Site)
Figure 12: ERT 5 (Left bank perpendicular to the river (North) upslope, Powerhouse Site)
Upper Layer of this profile contains dry overburden from chainage 0 to 120 m. The thickness of this layer ranges from 5-10 m. From chainage 55 to 115 m, at a depth of 5m from the surface contains bedrock of high resistivity value.
Resistivity Tomogram and Lithological interpretation of ERT -5a (Left bank perpendicular to (South) the river upslope, Powerhouse site)
Figure 13: ERT -5a (Left bank perpendicular to (South) the river upslope, Powerhouse site)
Upper Layer of this profile contains dry overburden from chainage 0 to 120 m. The thickness of this layer ranges from 5-10 m. From chainage 55 to 115 m, at a depth of 5m from the surface contains bedrock of high resistivity value. The bedrock is gneiss and schist.
1.Detail information of the outcome of the interpretation is presented with figures in the previous chapters. The general conclusions are as follows:
2.The surveyed area of headworks consists of bended gneiss and schist. Subsurface geology of left bank of the weir axis is stable and consists of thick alluvial deposits. However, some fractures and partly saturated rocks are present below depth 15-20 m. In some parts, highly saturated zone is present. In right bank, there is thick alluvial terrace deposits is present up to depth 15 m. The bedrock is present below the depth of 15 to 20 m. so exploratory core drilling is recommended to confirm the depth of the alluvial deposits and saturated zone.
3.Surface geology of the powerhouse area resembles with that of headworks. The overall subsurface geology of powerhouse is favorable for geological point of view, however some dry overburden (alluvial deposits) up to 15 and below to this depth bedrock is encountered which is probably gneiss. So, core drilling work is recommended to confirm the subsurface condition and evaluate the overburden and saturated zones and rock boundaries.
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