Electrical Resistivity Soundings Research Articles

Hydro-Geoelectric Study of Aquifer Potential in Parts of Ikot Abasi Local Government Area, Akwa Ibom State Using Electrical Resistivity Soundings

Hydro-Geoelectric Study of Aquifer Potential in Parts of Ikot Abasi Local Government Area, Akwa Ibom State Using Electrical Resistivity Soundings

Analysis of Electromagnetic Depth Sounding Responses over a Layered Earth: Investigating Oil & Gas Seeps in the Petroleum Provinces

The present work embodies the results of theoretical and practical investigations of electromagnetic depth sounding using central frequency sounding (CFS) system over a layered earth. Failure of conventional electrical resistivity sounding in the study of geological conditions under resistive overburden calls for variable frequency sounding techniques. Electromagnetic depth sounding which involves the measurement of variation in conductivity with depth is used for solving various geoengineering, hydrogeological and shallow cases of oil & gas seeps associated with stratified earth. The CFS, which is one of the depth sounding techniques involving the measurement of vertical component of magnetic field induced at the centre of a circular or square loop, is considered in the present study for obtaining theoretical responses over a layered earth and its interpretation with shallow oil and gas seeps. Because of some limitations of contour integration and numerical integration approaches, used earlier, a more rapid digital linear filter technique is adopted for evaluation of the integral involved in the CFS theoretical expressions. Theoretical expressions for frequency-domain soundings written for layered earth models are suitably transformed for evaluation through digital linear filter. Dimensionless normalized vertical magnetic field is computed for different frequencies and loop radii for layered earth models with different layer conductivities and thicknesses. The responses computed for these cases are analysed in terms of resolution characteristics and detectability effects. In frequency-domain sounding, amplitude response curves of layer-sequences show the effect of layer conductivity, layer thickness and loop radius. Separation between individual curves on the sets for amplitude responses normally gives sufficient indications for subsurface conductivity variations of the layered earth cases. The author explores the CFS applicability and feasibility in investigating shallow oil & gas seeps in oil & gas provinces, in particular on the flanks of the rifted grabens and basin margin areas, where sediment-basement contact areas are interpreted.

Role of Groundwater Protection Planning in Groundwater Governance: A Case Study from North India

Abstract: In the present case study, impact of urbanization and industrial development on the shallow groundwater regime of Saharanpur town of Uttar Pradesh in India is examined with the aim of planning groundwater protection for better governance. The hazardous physicochemical and bacteriological parameters and heavy metals detected in the shallow aquifer include harmful pathogens like fecal coliforms, heavy metals like cadmium, chromium, nitrates and sulphates. An assessment of ground water vulnerability using the well known DRASTIC method has confirmed that the shallow groundwater in some central and southern localities of Saharanpur town fall in the medium risk zones. Further, using field data of 32 electrical resistivity soundings, the protective capacity of the unconfined aquifer is assessed in terms of a ‘total longitudinal conductance’ of the semi-pervious to impervious sediments overlying the unconfined aquifer. However, some areas aligned along a northwest-southeast and in the western parts of the town seem to have relatively higher protective capacity against infiltrating waste pollutants. A ground water protection planning map prepared by combining the DRASTIC map and the ‘potentially hazardous pollutants’ map has brought out the need to install eleven new groundwater quality monitoring wells in the town at locations near the line sources and point sources of pollution. This approach can be readily employed by the decision makers in framing sound guidelines for groundwater protection and governance.

Vertical electrical sounding of soils and permafrost of marine terraces of Gronfjord (Svalbard archipelago)

Vertical electrical resistivity sounding (VERS) of soil-permafrost strata has been performed during the field work within the sea terraces of Gronfjord (Svalbard archipelago, West Spitsbergen island). Vertical electrical resistivity sounding of soil-permafrost strata was performed by portable device LandMapper. Then these data have been analyzed via ZondIP software (1d model). Apparent electrical resistivity values on the soil-permafrost strata usually change rapidly. It was established that studied soils with different origin and morphological properties are referred to 2 trunks, 2 orders, 4 types and 7 subtypes. Histic Gleysols, Cryosols, Gleysols and their subtypes have been investigated within the key plots (Grendasselva, Aldegonda rivers and catena on the sea terrace in surroundings of Barentsburg aerodrome). Several trends in profile distribution of electrical resistivity values have been distinguished. The main is connected with monotonous increasing of electrical resistivity values with a depth. Values of apparent electrical resistivity increase rapidly on the border of active layer-permafrost layer. The contrasts in profile distribution of electrical resistivity values are caused mainly by differences in water content, texture class and degree of strata heterogeneity (due to cryogenic processes). The depths of active layer-permafrost boundary have been distinguished using ZondIP software. Regional differences in this indicator may be explained not only by local differences in thermal regime of soil and permafrost layers, but also by different character of anthropogenic influence on key plots. Vertical electrical resistivity sounding method provides significant information for understanding soil electrical properties without any mechanical disturbances of soil cover. The data obtained is clearly coincided with field work data on soil morphology.

Hydrogeophysical investigation of groundwater potential and aquifer vulnerability prediction in basement complex terrain – A case study from Akure, Southwestern Nigeria

Abstract This study provides a model for the prediction of groundwater potential and vulnerability of basement aquifers in parts of Akure, Southwestern Nigeria. Hydrogeophysical surveys involving very-low-frequency electromagnetic (VLF-EM) profiling and electrical resistivity (ER) sounding, as well as evaluation of hydraulic gradient using three-point method, were carried out. Ten VLF-EM reconnaissance survey traverses, with lengths ranging from 55 m to 75 m, at 10 m station separation, and 12 vertical electrical sounding (VES) stations were occupied. Two-dimensional map of the filtered real component reveals areas of high conductivity, indicative of linear features that can serve as a reservoir or conduit for fluid flow. Interpretation of the VES results delineates three to four geoelectric units. Two aquifer zones were identified, with resistivity values in the ranges of 20 Ωm to 310 Ωm and 100 Ωm to 3,000 Ω m, respectively. Transverse resistance, longitudinal conductance, coefficient of anisotropy and hydraulic gradient have values ranging from 318.2 Ωm2 to 1,041.8 Ωm2, 0.11 mhos to 0.39 mhos, 1.04 to 1.74 and 0.017 to 0.05, respectively. The results of this study identified two prospective borehole locations and the optimum position to site the proposed septic system, based on the aquifer’s protective capacity and groundwater flow properties.

Geomorphology of the Alluvial Sediments and Bedrock in an Intermontane Basin: Application of Variogram Modeling to Electrical Resistivity Soundings

The study describes a methodology used to integrate legacy resistivity data with limited geological data in order to build three-dimensional models of the near subsurface. Variogram analysis and inversion techniques more typically found in the petroleum industry are applied to a set of 1D resistivity data taken from electrical surveys conducted in the 1980s. Through careful integration with limited geological data collected from boreholes and outcrops, the resultant model can be visualized in three dimensions to depict alluvium layers as lithological and structural units within the bedrock. By tuning the variogram parameters to account for directionality, it is possible to visualize the individual lithofacies and geomorphological features in the subsurface. In this study, an electrical resistivity data set collected as part of a groundwater study in an area of the Peshawar basin in Pakistan has been re-examined. Additional lithological logs from boreholes throughout the area have been combined with local outcrop information to calibrate the data. Tectonic activity during the Himalayan orogeny has caused uplift in the area and generated significant faulting in the bedrock resulting in the formation of depressions which are identified by low resistivity values representing clays. Paleo-streams have reworked these clays which have been eroded and replaced by gravel–sand facies along paleo-channels. It is concluded that the sediments have been deposited as prograding fan-shaped bodies and lacustrine deposits with interlayered gravel–sand and clay–silt facies. The Naranji area aquifer system has thus been formed as a result of local tectonic activity with fluvial erosion and deposition and is characterized by coarse sediments with high electrical resistivities.

Hydro-Geoelectric Study of Aquifer Potentialin Parts of Ikot Abasi Local Government Area, Akwa Ibom State Using Electrical Resistivity Soundings

Geoelectric survey employing Vertical Electrical Sounding(VES) was carried out in parts of Ikot Abasi Local Government Area, Akwa Ibom state, Nigeria. In this area there is no borehole that has been drilled until now, the main objectives of this study is to be a guide for the suitable position of borehole in order to avoid wild cat drilling and also to avoid salt intrusions using the VES geophysical techniques. Schlumberger electrode configuration was used in acquiring the data. Four to five geoelectric layers were delineated from the interpreted results. It shows the topsoil having resistivity range of 80.64-2810.23 Ωm, with a thickness and depth ranging from 0.13-2.26 m. The second layer has resistivity range of 12.48-2802.61 Ωm, the third layer shows a resistivity range of 9.08-2534.06 Ωm, whereas the fourth layer with a resistivity of 0.53-1483.27 Ωm, harbours most of the aquifers in the study area and is highly resistive. The geoelectric parameters obtained were also used to estimate the longitudinal conductance and transverse resistance. This result shows that most parts of the study area have aquifer with poor protecting capacity (<0.1 mhos) resulting in high salt intrusion. The groundwater potential is high in VES 3 and 4,and moderate in others. Curve types: HAA, KHKH, KHK and HKH, were obtained.

Protective Capacity Assessment of Vadose Zone Material by Geo-Electrical Method: A Case Study of Pakistan

Rapid and poorly managed agricultural landuse in Punjab plain is a major contention in groundwater quality deterioration. This study is focused on the determination of groundwater vulnerability potential of vadose material using geo-electrical method. Total of eight electrical resistivity soundings with Schlumberger electrode configuration are taken near Khizarabad Dairy Form, district Sargodha. The instrument ABEM Terrameter SAS 300 is used for data acquisition. The quantitative interpretation is done by 1D modelling of resistivity soundings by using IX1D software. The results characterize the area into three geo-electrical layers as dry alluvium (15.7 - 32.4 Ωm), saturated alluvium with high resistivity (10 - 20 Ωm) and low resistivity (>10 Ωm). Aquifer vulnerability is assessed by calculating Dar Zarrouk parameter (Longitudinal Conductance) and thickness of vadose zone. Resultant vulnerability is divided into three zones as low (>3.8 siemens) moderate (1.6 - 2.5 siemens) and high (<1.6 siemens). It is concluded from this study that Vertical Resistivity Sounding (VES) can be used for vulnerability assessment of an unconfined sandy alluvial aquifer from agricultural contaminants.

Integrated geophysical-geological model for seismic local site response: the Caldes alpine slope case (Southern Alps, NE Italy)

We present the seismic local response analysis of an alpine slope based on an integrated geophysical and geological model. The studied site is located in Caldes, in the lower Val di Sole valley in the Trentino region, Southern Italian Alps. Caldes represents the typical alpine valley formed by glacial and fluvio-glacial processes characterized by complex geomorphological, lithological and tectonic settings. A detailed reconstruction of the valley slope geology (subsoil geometry and stratigraphy) was obtained using electrical tomography resistivity soundings, controlled source multi-channel analysis of surface waves, microtremor single station measurements, borehole investigations, and geotechnical analyses on undisturbed samples. The geological model was shaken with real accelerometric series to obtain 1D (linear equivalent approach) and 2D (finite element approach) local seismic responses. The results, considering the uncertainties of the geophysical measurements, show different seismic responses between 1D and 2D approaches due to relevant geometrical amplifications, with clearer variations in the 2D approach. The comparison of the results with the building code design spectra shows the limits of the simplified approaches, suggesting that detailed characterizations are necessary, especially for such populous alpine environments.

Groundwater freshening following coastal progradation and land reclamation of the Po Plain, Italy

Many coastal areas historically were inundated by seawater, but have since undergone land reclamation to enable settlements and farming. This study focuses on the coastal unconfined aquifer in the Po Plain near Ravenna, Italy. Freshwater is present as isolated thin (1–5 m) lenses on top of brackish to saline water. Historical maps show large areas of sea inundation until approximately 150–200 years ago when coastal progradation and construction of the drainage canals began. Since then, the aquifer has been freshening from recharge. A three-dimensional SEAWAT model is used to simulate a 200-year freshening history, starting with a model domain that is saturated with seawater, and applying recharge across the top model layer. Calibration to the observed concentrations for discrete depths within many monitoring wells is remarkably good. The current distribution of freshwater is largely controlled by the drainage network. Within and adjacent to the drains, the groundwater has high salinity due to up-coning of salt water. Between drains, the surface layers of the aquifer are fresh due to the flushing action of recharge. The modeling results are consistent with cation exchange processes revealed in the groundwater chemistry and with freshwater lenses identified in electrical resistivity soundings.

Determination of thе soil-permafrost border in two maritime Antarctic regions on thе base of vertical electric sounding data

The deepness of soil-permafrost layer was measured bу vertical electrical resistivity sounding. It has been revealed that the deepness of active layer оn the Point Thomas oasis , King George lsland is in average about 100-150 cm, while оn the Argentine lslands it is 40-50 cm, which is caused bу climatic conditions. Weak soils of the Argentine Islands are connected to permafrost via the massive rock, while in case of Point Thomas oasis (King-George Island) fine-coarsy gravel soils are underlayed directly bу permafrost.

Integrated electrical resistivity and hydrogeological studies for delineating the Miocene aquifer at Eastern side of Nile Delta, Egypt

East Nile Delta area was subjected through its geologic history to complex series of tectonic activities. These activities were affected on the distribution of groundwater aquifer of this area. Oligocene, Miocene, and Pleistocene aquifers were detected in the east Nile Delta region. Among them, Miocene aquifer is characterized by slightly saline water. Delineating the contact boundary and distribution of Miocene aquifer will assist in managing drilling and development programs of the area. For achieving this purposes, 21 vertical electrical resistivity sounding were measured, processed, and interpreted. Also 23 water samples and lithologic information were collected and chemically analyzed from the drilled wells in the study area. The results revealed that the Miocene aquifer seemed to be structurally controlled by some faults. These faults played an important role in the distribution of this aquifer relative to other aquifers and changed its depositional style from sand to limestone. Hydrochemical analysis showed that Miocene groundwater samples are not suitable for drinking and agricultural purposes but can be used only in industrial projects.

A Computational Intelligence Approach to Solve the Inverse Problem of Electrical DC Resistivity Sounding

Electrical methods have been widely used in geophysical surveying to obtain high-resolution information about subsurface conditions, since the last few decades. Resistivity is an important parameter in judging the ground properties, especially detecting buried objects of anomalous conductivity. Electrical DC (i.e. Direct Current) resistivity sounding is the commonly used technique to obtain the apparent 2-D resistivity of the region under investigation. Acquiring the true resistivity from collected data remains a complex task due to nonlinearity particularly due to contrasts distributed in the region. In this work, a radial basis function neural network (RBFNN) metamodelling approach is proposed to solve the 2-D resistivity inverse problem. The model was trained with synthetic data samples obtained for a homogeneous medium of 100Ω.m. The neural network was then tested on another set of synthetic data. The results show the ability of the proposed approach to estimate the true resistivity from the 2-D apparent resistivity sounding data with high correlation. The proposed technique, when executed, appears to be computationally-efficient, as it requires less processing time and produces less error than conventional method.

An investigation of the potential environmental contamination from the leachate of the Rasht waste disposal site in Iran

This research deals with detecting the underground layer geometrics and conditions, probable faults and crushed zones, thickness of alluvium, determination of pollutant plume development, and groundwater quality in the study area. The Visual HELP and HYDRUS programs were used to evaluate hydrologic properties and contaminant transport, respectively. All required data which consisted of the geotechnical study (drilling the borehole and dug well), the geophysical study (geoelectrical sounding), and geochemical analysis of water and leachate samples were collected in the field operations. According to the results of ten years of hydrologic data evaluation of this site by using the Visual HELP program, the mean rate of leachate percolation from this site was equal to 1.095 m per year. From the interpretation of the vertical electrical resistivity soundings curves which was calibrated by the geotechnical borehole log, a four-layer geoelectrical model was recognized. These layers consisted of surface layer (loose and fine grained topsoil), medium grained alluvium, saturated layer, and high resistivity bedrock. The physico-chemical analysis of ten surface and groundwater sample indicated that these samples were not contaminated by leachate yet. Based on 1D, 2D and 3D modeling of contaminant transport using the HYDRUS 1D and the HYDRUS 2D/3D program, during the ten years of the onset of leachate seepage, contaminant plume migrated a short distance (<200 m) due to high compaction of fine grain sediments. In addition, the high compaction of a third layer (M3) in a depth of more than 40 m introduces a natural barrier to infiltration and movement of contaminant to the deep subsurface.

Hydrogeological Implication of Electrical Resistivity Sounding in Ita-Eku, Ado Ekiti, Nigeria

Electrical resistivity prospecting method was used in evaluating the groundwater potential of Ita-Eku, southeastern part of Ado-Ekiti, Ekiti state, Nigeria using the vertical electrical sounding technique. A total of seven electrical soundings were conducted using the Schlumberger electrode array. The data acquired were interpreted qualitatively and quantitatively and were presented as tables, charts and section. The interpreted models revealed three to four geo-electric layers namely; topsoil, clay/lateritic soil, weathered/fractured layer and fresh basement. The topsoil which is relatively thin has thickness ranging between 0.8 and 2.5m while the resistivity values range from 26 to 171 �-m. The second layer has thickness ranging from 2.5 ‐ 5.9 m and resistivity values from 5 to 19 �-m. The third layer is the weathered/fractured layer with thickness ranging from 5.9‐35.3 m and the resistivity values range from 171 to 404 �-m. The basement resistivity values range between 711 and 3466 �-m. First and second order (Dar Zarrouk parameters) data were used to generate the geo-electric section, overburden thickness chart and longitudinal conductance chart. These section and charts were considered in generating the groundwater potential table. The groundwater potential table shows that about 28% of the entire study area falls within zone rated as having high groundwater

Volcano-sedimentary stratigraphy in the Valsequillo Basin, Central Mexico inferred from electrical resistivity soundings

Initial results of an electrical resistivity survey of the volcano-sedimentary sequence of the Valsequillo basin in central Mexico are presented. The volcano-sedimentary deposits preserve rich paleontological, paleoclimatic and paleoenvironmental records, which include extinct megafauna remains associated with human artifacts. The report of possible 38 ka old human footprint tracks in the Xalnene tuff attracted renewed interest in the basin stratigraphy. We examine the shallow stratigraphic sequence in the Xalnene tuff outcrop plain northwest of Cerro Toluquilla volcano using vertical resistivity soundings (VES). Inversion models of VES soundings show a layered structure of high and low resistivity units, which characterize the Xalnene tuff, lacustrine and fluvial sediments and volcanic rocks. 2-D resistivity cross sections document three major units corresponding to the Xalnene tuff and sediments filling a<30m deep basin lying on volcanic rocks. Resistivity models provide further support for the association of Xalnene tuff with the Toluquilla volcano and emplacement of the pyroclastic deposits on a shallow lacustrine environment. The resistivity cross sections constrain the thickness of the tuff layers and underlying lacustrine sediments. Observations during the data acquisition field work provide insight on the possible origin of the apparent tracks, which seem to develop from erosion processes acting on quarrying marks. Further analysis and experimental evidence is required to understand the morphology and weathered patterns. The tuff layers are being removed by quarrying operations and the outcrops significantly altered. Adequate conservation measurements should be implemented to preserve the deposits for scientific research.

Estimation of Aquifer Hydraulic Characteristics from Surface Geoelectrical Methods: Case Study of the Rima Basin, North Western Nigeria

Forty vertical electrical resistivity soundings (VES) were acquired, using a Schlumberger array. Three soundings were additionally made at existing boreholes in the vicinity of the study area for comparison. Since the area is a basement complex terrain, a maximum current electrode spacing of 200 m which is believed to probe down to the fresh basement in the area was used. This is believed to enable fresh basement in the area to be observed. The concept of Dar Zarrouk parameters (transverse unit resistance and longitudinal conductance in porous media) was used to determine aquifer hydraulic characteristics in the sedimentary formation underlying the area. The results of the interpreted VES data using a combination of curve matching technique and computer iterative modeling suggest that the area is underlain by between 3–5 geoelectric layers. The geologic sections derived from the equivalent geoelectrical sections revealed lithologic sequence of mostly five layers. However, there are minor isolated cases of three layers. The geoelectrical sections showed that the resistivity of the aquifer varies between 14.1 and 128.8 Ω m. The thickness of the aquifer ranges from 1.0 to 48 m while its depth from the surface lies between 1.0 and 50 m. Relationships between aquifer characteristics and the geoelectrical parameters revealed the hydraulic conductivity K and transmissivity T variations to be 3.10–28.34 m/day and 10.0–796.6 m2/day, respectively. The maximum K and T occur around VES 4 and 14; thus the maximum yield could be expected around VES 4 and 14.

Mapping Precambrian Aquifer Architecture in Fractured Hydrogeological Setting with Vertical Electrical Sounding

Electrical resistivity sounding was employed to delineate different water bearing layers and their architectural parameters. Sixty Vertical Electrical Soundings (VES) were obtained in the built-up area of Olabisi Onabanjo University campus to map out variations in subsurface resistivity, which was used to determine aquifer parameters favourable for groundwater development. Interpretation of electrical sounding data helps in determining the resistivity and thickness of aquifers, aquifer and overburden thicknesses and depth to the basement. We employ a lithology-based hydrogeological model with definite boundary to classify the apparent resistivity. In this model, range of electrical resistivity values were assigned to different layers based on field observation and knowledge of the geology of the area. In all, five litho-units were delineated besides the topsoil, which are: clay with resistivity range of 25–53 ; clayey sand/sandy clay with resistivity range of 122–440 ; weathered layer with 62–119 ; fractured basement with 208–667 and resistivity above 720 were taken as fresh basement. From the initial lithologic-based model, we developed 2D model that conceptualized aquifer architecture and bedrock topography along the major traverses obtained in the area. Also, Isopac map reveals that the overburden is thicker in the western half while the 2D model and structural map clearly show undulating bedrock topography made up of bedrock ridge with elevations of 127.0–170.0 m and bedrock depressions with elevation of 97.0–123 m. Derived Geo-electric parameters revealed that aquifer quality increases as we move northward, though recommended aquifers cut across all the area survey. Based on the study, we recommended VES stations with good overall qualities to be developed to productive water-supply tube wells.

Hydrogeologic-structure and groundwater-movement imaging in tideland using electrical sounding resistivity: a case study on the Ariake Sea coast, southwest Japan

Groundwater flow exerts a crucial control on the boundary between the sea and freshwater and is thus a key factor for preserving groundwater resources and preventing seawater intrusion in coastal areas. Although it is highly probable that geological faults in coastal areas affect groundwater flow patterns, the effect has not been described yet in detail. This study is aimed at detecting and imaging groundwater flow and its temporal change around a fault in a coastal area through resistivity and chargeability distributions using electrical sounding. The Okoshiki area in central Kyushu, southwest Japan, was selected as a case study area, because of the presence of Kamiouda Fault. The measurements were conducted along six lines of both parallel and perpendicular orientations to the coastline. A feature suggesting a fault zone was evident on two lines. Through the temporal change of resistivity, movement and mixing processes of the seawater and freshwater during the ebb, low and flood tides were interpreted. A conceptual model of the processes was constructed in which a fault zone and the configuration of bedrock are dominant elements by acting as a selective path and a barrier to the groundwater flow, respectively.

Shallow subsurface stratigraphy and alluvial architecture of the Kosi and Gandak megafans in the Himalayan foreland basin, India

The Kosi and the Gandak are two major Himalayan tributaries of the Ganga River in the north Bihar plains India. With a large hinterland in the Nepal Himalaya, both these rivers have generated megafans in the plains over the Quaternary time scale. Both these rivers are known to be highly dynamic and sediment-charged. A few conceptual models and limited field data suggested that these megafans have produced thick sand sheets over Late Quaternary period but these ideas have remained speculative and there is no data on the size and dimension of these sand bodies. This paper attempts to reconstruct the subsurface stratigraphy and alluvial architecture for the upper ~100m of the megafans based on electrical resistivity soundings, borehole data and drill cores. Alluvial architecture of the Kosi megafan shows significant variability from proximal to medial parts of the fan in terms of sediment grain size and layer thicknesses. While the medial part shows ~20–30m thick medium to coarse sand sheets which are laterally stacked, the proximal part of the fan has a dominantly gravel unit below ~15m depth that is underlain and overlain by medium to coarse sand units. Further, the medial fan also shows significant vertical and lateral variability in alluvial stratigraphy. The near-surface (<20m depth) deposits from the Kosi megafan have pockets of clay and silt within large amalgamated sand bodies whereas the shallow sub-surface (50–100m depth) sediments are largely sandy and devoid of clay and silt pockets. Alluvial architecture of the Gandak megafan shows two major lithounits; the upper fan succession has a higher stacking density of smaller sand bodies perhaps reflecting the migratory behavior of the river whereas the lower succession shows narrow but thick sand fills reflecting incised channels. The western part of the Gandak megafan has more abundant sand bodies compared to the eastern side of the river along both transects. There are no significant differences between proximal and medial transects across the Gandak megafan. The absence of gravel deposits in the shallow subsurface of the Gandak megafan may be attributed to the presence of a prominent intermontane valley in the hinterland of the Gandak river which has acted as a ‘sediment filter’ thereby trapping most of the coarser fraction. On the other hand, the Kosi river exits directly through the mountain front, and therefore, has been able to transport gravels into the plains in pre-historic times. Our study thus suggests significant variability in subsurface stratigraphy of the Kosi and the Gandak megafans even though they are located in similar geographic region. Such differences are attributed to the geomorphic diversity of the mountain exits of these megafans and their sediment transport history.