Annals of Earth Science and Geophysics

Application of Integrated Geophysical Techniques to Map Groundwater Potential Zones and Geological Structures at Borumeda, South Wollo Zone, North Ethiopia

Abstract

In this study to identify groundwater potential zones and geological structures, an integrated geophysical investigation utilizing magnetic and vertical electrical sounding approaches was carried out at BoruMeda. The primary goal of this research was to assess and evaluate the depth of the aquifers and the location of the most appropriate drill sites for Boru Med groundwater potential. The VES curve was interpreted qualitatively but geo-electric sections were interpreted quantitatively. Likewise, magnetic data quantitative interpretation was carried out using a 3D Euler deconvolution magnetic map, and plots of two-dimensional magnetic profiles. Using Win Resist and Resix-Ip modeling software to model the VES data, and by producing geo-electric sections along selected survey lines using the findings of various VES point interpretations and lithological information from a different borehole. Information from nearby boreholes were integrated to constrain the resistivity-sounding survey with geological layers and various types of magnetic data plots and 2D magnetic models to enhance and visualize the results. Additionally, the magnetic survey aided maps the basement structures and identified locations with significant groundwater reservoir potential, while the geo-electrical section helped to know the aquifer thickness & detect groundwater potential areas. The interpretation generally mentions two aquifers in the Borumeda: The upper Basalt aquifer, which is to some extent confined and flanked by alluvial deposits & moderately fractured Basalt & rhyolite. The higher aquifer depth varies between 20 to 80 meters, though the depth of the deeper aquifer exceeds 170 meters. The Borumeda geological structure has an orientation of NE-SW and NW-SE.

Keywords

Aquifer, Geo-electric section, Groundwater Potential, Pseudo depth

Abbreviations

VES: Vertical Electrical Sounding; AC: Alternative Current; DC: Direct voltage; N-S: North-South; IGRF International Geomagnetic Reference Field; GPS: Global Position System; NW-SE: Northwest-Southeast; SW-NE: Southwest-northeast; NE-SW: Northeast-Southwest; Mm: Millimeter; SE: South-East; NW: North-West; Masl: Meter Above Sea Level; MER: Main Ethiopian Rift; RMS: Root Mean Square

Introduction

Water is among a few valuable constituents and the primary importance of life which ever since the creation of life man, animals, and plants are dependent on. Other vital commodities such as food and oxygen are by-products of water. Hence, Water is the most fundamental component of life. It is the most basic resource that affects economic, agricultural, and municipal activity. Water is used for country growth, whether directly or indirectly. This precious resource is groundwater. Groundwater is the most valuable and extensively distributed resource on the planet, and like any other mineral resource, it is replenished by precipitation. Even though Ethiopia is a water-rich country in Africa, very little groundwater is exploited for commercial, domestic, and agricultural uses. Water is a priceless and essential resource that is used by households, farms, and businesses all around the world. It is also necessary for a country to develop and grow [1]. The majority of groundwater is trapped in and moves via the pore spaces between rock particles, as well as rock fractures and fissures. Groundwater is water that has saturated the pore spaces of sand and gravel. It flows in the aquifer layer, towards the point of discharge, which includes wells, springs, rivers, lakes, and the ocean [2]. Like most parts of the country, the community living in the River catchment gets their domestic water supply from groundwater. The types of wells are deep wells, shallow wells, hand-dug wells, and springs schemes from which groundwater are extracted. The application of applied geophysics for determining water quality and mapping groundwater resources significantly increased over the last ten years. This is mostly of the speedy developments in microprocessors and associated numerical modeling tools that quickly receive process and show data in an appropriate format [3]. The goal to reduce the possibility of digging holes dry & to reduce costs related to insufficient groundwater extraction has influenced the use of geophysics for groundwater research. The geophysicist currently offers helpful parameters for both hydrogeological modelings of fresh groundwater sources and for assessing contamination of groundwater.

Materials and Methods

Description of the study area

The research area is located in the Amhara National Regional State South Wollo Administrative Zone (Figure 1). Its approximate bounds are 11°12'37" to 11°13'04" North latitude and 39°37'06" to 39°37'05" East longitude, with an elevation between 2,600 and 2,611 meters above sea level. Dessie is situated 480 km from Bahirdar, the administrative center of the Amhara Regional State, and 401 km far from Addis Ababa, the capital of Ethiopia. Kutaber town is accessed through Bahirdar woldiya Dessie through asphalt road and from Dessie to Kutaber, it is 20 km through all weathered gravel road towards NW direction.

Data source

To collect data, the primary data was observations from the study area, and secondary data was obtained (using surveying 2010 E.C look at the Appendix). The Amhara Water Works Project and Supervision Enterprise selected an area North-South of Dessie City that must be carefully investigated for the potential based on previously comprehensive geological, hydrogeological, and VES data on groundwater resources assessments [4]. To comprehend groundwater potential zones, various lithological units, geological structures, and a detailed investigation of the area were intended to determine the physical features of the underlying rocks. A Schlumberger array with a current electrode spacing 500m (AB/2 = 500m) was generally used for the maximum depth of interest. A total of three (3) VES distributed on one line were surveyed in the study area with transect separation ranging from 0.5 km to 1 km. For this survey, magnetic and VES approaches are the two geophysical techniques that were selected. The location was chosen because of the presence of thick alluvial deposits along the upper and lower courses of the Borumeda and Borkena Rivers. All of the transect lines have been worked out in terms of geophysical observations.

Method of data analyses

To achieve these objectives, a number of steps have been followed. To obtain dependable and more precise subsurface information, secondary data from various sources will be combined with the field-collected primary data. The following are the activities: Preparing and evaluating by using ArcGIS to create a location map of the study region and display VES data points, magnetic data, and the location of previously drilled boreholes, primary geophysical raw data are presented. Oasis Montaj (V-7.0.1), Paint, and Microsoft Excel will be used to interpret magnetic data, whereas SURFER-16, used to analyze VES data.

Data Acquisition and Processing

Survey traverse selection

The Amhara Water Works Project and Supervision Enterprise selected an area North-South of Dessie City that must be carefully investigated for the potential based on previously comprehensive geological, hydrogeological, and VES data on groundwater resources assessments. To comprehend groundwater potential zones, various lithological units, geological structures, and a detailed investigation of the area were intended to determine the physical features of the underlying rocks. For this survey, magnetic and VES approaches are the two geophysical techniques that were selected. The location was chosen because of the presence of thick alluvial deposits along the upper and lower courses of the Borumeda and Borkena Rivers. All of the transect lines have been worked out in terms of geophysical observations.

Data Acquisition and Instrumentation

Vertical electrical sounding

Three transect lines approximately aligned east-west were used for the vertical electrical sounding (VES) survey, and observations were taken at intervals of about 1000m between sounding stations. To reach the selected depth of interest, a Schlumberger array with 500 m spacing current electrodes (AB/2 = 500m) was often used. The overall 3 VES data, distributed across lines, were collected in the Borumeda, with cut-across separations varying from 1 to 2.5 km. To have some control during the interpretation, of these VES, some were placed close to previously drilled boreholes. Although the VES observation locations were typically placed at 0.3 km intervals, these positions were shifted. In the VES, performed on intervals every 300m. The current electrode spacing for this survey was as follows: MN/2 meter: 1, 12, and 90; AB/2 meter: 1.5, 2.1, 3, 4.2, 6, 9, 13.5, 20, 30, 45, 66, and 100, 150, 220, 330. To avoid the uncertainty of inhomogeneity, Overlap point (AB/2; 20, 30, 150, and 220 m). The Italy-made electrical resistivity meter with a current output of 2 amper & 12 voltmeters, was the instrument used in the Vertical Electrical survey. VES point positions were recorded using a handheld GPS. In addition to recording the locality/locations of VES points, apparent resistivity for various electrode spacing was determined. For each data point, we plotted and examined. The spread directions were correctly recorded, and they were employed for anisotropic correction throughout the data processing session.

Magnetic survey

Along the transect lines of the VES, observations were taken every 20 meters as part of a total field magnetic survey. A total of 223 magnetic data points were recorded, with transect separations that were between 1 km and 2.5 km, which is like to the VES. To detect any potential groundwater-related structure, the profiles were roughly oriented east-west and northwest-southeast. Only a small portion of this magnetic information was examined close to the already-dug boreholes. The magnetic data was measured three times and recorded the average, and base station measurements were taken at the start and end of each 1 km segment of the profile magnetic survey to account for diurnal fluctuation. In addition, to reduce the effect of diurnal variation, the data was collected from early morning at 12:30 a.m at 05:30. The GSM-19T, a flexible and durable tool for such a survey, was utilized to conduct the magnetic survey, and a portable GPS was used to record the location and time of the measurement site.

Data Reduction and Processing

VES data reduction and processing

VES data reduction: The values of the apparent resistivity are plotted on a translucent logarithmic paper. The collected data are processed using the apparent resistivity value on the ordinates and the electrode spacing (AB/2) on the abscissa. The potential electrode distance (MN) is gradually increased for the comparatively significant current electrode distance increased (AB/2), and the resistivity measurements were performed. This made it possible to quantify the influence and make necessary modifications, leading to the creation of a single smooth curve that could be processed using IP2WIN and later WIN RESIST software on a computer [5].

VES processing: Applying both quantitative and qualitative explanation techniques allowed for the presentation of the study field result. In the qualitative explanation approach, the field curve shape is investigated to give a qualitative knowledge of layer resistivity. The segmented curves were run to the small MN to ensure accurate interpretation. An electrical resistivity map slice stack, a geo-electric section, and pseudo-sections were among the results of this method of interpretation. To serve as the first model in the RESIST inversion software, the IP2WIN program parameters were set up & evaluated using the lithological units of existing boreholes. An acceptable RMS error range of 1.5-4.5%. Layer thickness and actual resistivity were obtained via the quantitative technique. The first curve of each profile and model parameter was obtained from the software and shown in Figure 2. The maximal depth, and sharpness, of the AMNB technique [6]. The apparent resistivity contour maps are shown in Figure 2 using the differential value of AB/2. Figure 3 show the plots for all pseudo-sections. Diagrams of stratified layers generated through drilling or electrical depth probing (resistivity), where strata are distinguished by their apparent resistivity, are known as geo-electric sections. These components are useful for figuring out the levels of the water table and if the water below is fresh or salty. The constraints of the resistivity and thickness models were used to produce. All of the parts produced by the inversion software RESIST.

Magnetic Data Reduction and Processing

Magnetic data reduction

The reduction and processing of data are done with great care in order to prevent both signals and noise from the data that isn't real but related to the site's geology. In other words, the data collection is reduced to only include signals that are relevant to the task before being interpreted.

The following is a list of these steps:

i. Editing and validating data: Getting eradication of the data noise and spikes is necessary for this. It was caused by the high-tension power wires and metallic material from the structure.

ii. Diurnal correction: By adjusting the survey data for the time-based change in the Earth's main field by subtraction of the time-synchronized signal and readings from a base station magnetometer.

iii. Eliminating IGRF: is to stay away from the powerful impulse. To do this, we subtracted the primary field result, which was computed by Geosoft, from the diurnal correction survey data.

Magnetic processing

The analysis of the magnetic data revealed a number of techniques, including IGRF correction, magnetic and geographic reference, and diurnal adjustment. When we interpret magnetic maps qualitatively, we hope to gain a comprehensive understanding of subsurface structures and estimate the relative depth of the magnetic anomaly sources. These quantitative interpretation techniques include two-dimension magnetic modeling and three-dimensional Euler deconvolution. Oasis Montaj V-7.0.1, a specialized computer program, carried out the analysis and processing.

General Discussion and Interpretation

An interpreted VES curve, a from depth slice map, apparent resistivity variation for qualitative interpretations, and a vertical geo-electric section are used to display the results of the resistivity sounding survey that allows for quantitative interpretations, as was discussed in the section before. Once more, various maps are used to display the magnetic survey data. For qualitative interpretation, the anomaly map, the analytic signal map, the TDR map, and the RTE map are used. Three-dimensional Euler deconvolution and a two-dimensional model are then used for quantitative interpretation.

Discussions and interpretation of VES

Interpreted VES curves : By further constraining it with the existing boreholes, the IP2Win is used to assess apparent resistivity versus electrode spacing plotted on a bi-log scale to determine the first model parameters (RESIST) to be entered into the inversion software. The interpreted field curves demonstrate that for all VES sites, there is a very valid relationship between the field data and the interpreted model sections. Three interpreted VES curves, one from each of the survey traverses, is generated as an illustration and are displayed in Figure 2. With the AB/2 of 500m used for the survey, three to five subsurface layers are shown too clearly in the three sounding curves to accurately depict the subsurface.

Profile one: Figure 3 shows the pseudo-depth section created for VES-1, 2, and 3, which are located on the survey traverse line-1. The Topmost portion of the section exhibits a lateral fluctuation in resistivity, with pronounced low resistivity top zones identified in VES-1, VES-2, and VES-3. Below a depth of 100 meters, a zone with a reasonably high resistivity can be found near the bottom section. Even though the larger area under the section displays extensive coverage of the low resistivity horizon (5 to 8.7 Ωm), it is encouraging for the presence of significant amounts of groundwater.

Geoelectric section: The geo-electric section that was created using the three VES lying on this traverse/profile and their interpreted layer parameters is provided in Appendix-1. The geoelectric section shown below (Figure 4) was essentially put together using the resistivity characteristics from each interpreted VES and the lithological description of the drill depth section (provided in Appendix). The geo-electric section is shown in three layers in Figure 4. Resistivity values in the uppermost portion of the cross-section range from 2-3 m. This layer most likely corresponds to Topsoil. In this instance, the top layer in Figure 4 has a thin thickness. The second layer resistivity value covers the following ranges from 4-6 Ωm, geologically represented by dark sticky clay. The three layers, which are geologically characterized by sandy gravely clay, have resistivity values that vary from 7 to 16 Ωm and beneath 30m depth, and with a thickness of 60.5m under VES-1 it has good groundwater potential to drill a borehole. The bottom layer is 70 meters deep and has a resistivity range of 21-30 Ωm at a depth of range from 80-100m with a mean thickness of about 30.6m geologic represented by the highest compared with the upper layers and is indicating weathered and fractured basalt, it has good groundwater potential to drill a borehole.

Litologic correlation of Borhole log (BH-1) and VES-1: In Figure 5, the geoelectric section for VES 1 (nearly to BH-1) is displayed. Four geoelectric layers, ranging from topsoil as the first layer with resistivities of 2-3 Ωm and thickness of 5m, the second layer is dark sticky clay with resistivities of 4-7 Ωm and thickness of 15 m while the third layer is water saturated sandy gravely clay with resistivities of 9-15 Ωm, whereas the aquifer targeted zone which is at the resistivity of 5-9 Ωm, is delineated in this VES location. Hence, a borehole can be recommended to be sink to a depth of 20-80m in this VES location. And bottom layer is weathered and fractured basalt with resistivities of 24-30 Ωm in this VES location. In general, in the geo-electrical section relatively the thickness of aquifers is larger in the southwest direction of the study area.

Interpretation of magnetic data: The observed magnetic field anomaly measurements were analyzed after IGRF and diurnal fluctuation were removed. Magnetic anomalies highly vary in shape and amplitude [7]. They are more or less constantly asymmetrical, occasionally appear complicated even from simple sources, and frequently indicate the combined effects of numerous sources. A particular anomaly can have an infinite number of possible causes, giving rise to the term ambiguity. By utilizing the proper data enhancement techniques: Geosoft (v7.0) was used for all magnetic data processing in this investigation.

Magnetic field anomaly map

The difference between the determined value of the IGRF and the diurnally corrected total magnetic field constitutes the magnetic anomaly map that is generated. The primary magnetic field of the Earth affects the magnetic field that the American-made GSM-19T proton precession magnetometer records in the field, the external field arising from solar activities, and the anomalous magnetic field that arises from variations in subsurface geology. Low magnetic field intensity and a horizontal ambient field direction make it challenging to interpret magnetic data obtained there. At low latitudes, magnetic anomalies top magnetically susceptible entities exhibit negative rather than positive values, and the anomalies are highly influenced by azimuthal orientation [8]. As shown in Figure 2 the study area indicates high magnetic anomaly over the north-eastern, north-western, and south-western parts of the map which is caused by high magnetic susceptibility of rocks (basalt) which are expected to show high remanent magnetization as compared to other felsic volcanic rocks, reveals high magnetic anomaly and the central, eastern, and western, and south-eastern parts are distinguished by low magnetic anomalies, which could be the result of low magnetic susceptibility of rocks or thick accumulation of sediments.

Reduction to the equator of residual

The reduction to the Equator of the map (Figure 6), was produced from the residual map (with IGRF = 36878.39 nano Tesla, Inclination = 7.93, and Declination = 2.31), and is characterized by high and low magnetic anomalies. It ranges from -180.6 - 17.4 nano-Tesla. The intensity value increases because the reduction to the Equator filter is an amplitude amplifier filter. This map is exemplified by a southwest trending anomaly that, due to the latitude and direction problem, is not visible on the residual map. The maximum in the residual map becomes the minimum in the RTE.

Magnetic analytical signal map

The analytic signal map (Figure 7) was produced from the residual map. Since the variation of Earth's magnetic field intensity in low latitude areas is affected by the magnetization direction, enhancement techniques that do not consider the direction of magnetization are appropriate. The frequency of the analytic signal map is high around the North, South East, and Central parts where the maximum and minimum boundaries exist in the residual map. This map displays the distribution of the magnetization edge. It has amplitude ranges from 0.03-0.82 nano-Tesla per meter. The analytical signal map is highly correlated to the geology of the area and low peaks within the map are attributed to the response of alluvial sediments. In comparison to the nearby sediments and low magnetic susceptibilities rock units like ignimbrite, rhyolite, and other pyroclastic volcanic rocks, the susceptibility contrasts in the contact basaltic units produce large analytic signal gradients.

Magnetic Tilt derivative map

By utilizing Geosoft Oasis Montaj software to apply magnetic a tilt derivative filter to the magnetic signal map, the TDR map of the research area is created. The map's purpose is to make it simple to identify faults, contacts, and geological structures in the Borumeda. Compared to the AS Sginal map, the TDR map (Figure 8) displays more precise structural contacts and boundaries. It displays positive readings compared to magnetic sources that cross through zero at or near the locations of the fault/contact, and negative readings outside source zones. As a result, a set of various faults in the N-S, NW-SE, and NE-SW directions are used to classify the research region. The research area's widespread N-S and NE-SW trending structures are associated with these directions by the region's general tectonic setting which is directly correlated to the geological structures.

Euler deconvolution magnetic map

The 3D Euler deconvolution method can be used to estimate the depth and location of the magnetic source in a magnetic anomaly. Each model in the common Euler deconvolution process has solutions of a specific structural type identified by a structural index. The geological properties of contact, vertical pipe/horizontal cylinder, and sphere, respectively, are represented by the structural index values 1, 2, and 3 according to the Euler deconvolution principle. Figure 9 is a magnetic map of the research area created using Euler deconvolution with SI = 1. The map shows magnetic sources at various depths, each of which is indicated by a symbol of a distinct colour. A green, radish, and pink color circle denote magnetic sources that are located less than 50 meters beneath the surface. The 50 to 150m deep magnetic sources are shown by the pink circle. The 150 to 300m deep sources are shown by the green circle. The 300-400m deep sources are shown as the blue circle. Sources that are farther than 450 meters are represented by the light green circle. This representation has been regarded as a present because the northeast to the southwest edge of the current Borumeda area has structures (weak zones) that contain highly magnetized bodies, as illustrated in the map provided (Figure 9). Therefore, from the perspective of groundwater, it is suggested that a shallow depth flank be present on all of the area boundaries.

2D Magnetic model

The GM-SYS modeling program is used to create the two-dimensional magnetic model, which is crucial for estimating the physical properties of the subsurface geological units. Based on the depth/lithology of (Appendix), the interpreted layer constraints of the VES (geo-electric section of VES), and the magnetic susceptibility constraints taken into consideration for the original model, the two-dimensional magnetic models (Figure 10) created near to profile Four consists of four layers. The measured and calculated magnetic values are fit by further adjusting these parameters, as illustrated in Figure 10. The near-surface topsoil products in the final magnetic model, which corresponds to the first layer, have an average magnetic susceptibility value of roughly 0.000001 cgs and a relatively low magnetic susceptibility. This layer typical thickness ranges from 1 to 20 meters. The second layer contains to be weathered basalt because of its average magnetic susceptibility value of 0.078 cgs. The third layer is considered a depth of 60 to 105 meters of thick, moderately weathered basalt. The fourth layer is considered to consist of massive weathered basalt rocks that range in thickness from 105 to 180 meters. The model indicates that there are faults down to a depth of 20 meters. The alluvial deposit and basalts are visible in the geoelectric section, and they exhibit varied degrees of weathering and fracture. Again, the basalt persistence varies in each of the layers. Highly weathered and fractured Horizons and geological formations in the area govern the groundwater system.

Conclusions

Based on using a combination of data presentation techniques, the following general conclusions have been reached about the findings, discussions, and interpretations:

• Borumeda is dominantly covered by basaltic rocks and alluvial sediments on the ridge and along the catchment. Among these basalt and ignimbrite are the main outcrop rock unit of the area.

• The primary aquifers in the Borumeda are alluvial deposit has a thickness of 118 m. A confined aquifer is the type of aquifer present in the study region.

• The Borumeda water table ranges in depth from 20 to 80 meters and increases towards the southwest of the research area.

• BoruMeda is a valley between the mountains that were full of transported material from the surrounding highlands. The filled sediment deposit grain size ranges from clay to gravel, and the coarse-grained deposits at BoruMeda exhibit strong transmissivity and good water-bearing zones.

• Basalts and alluvial deposits are significant geologic units exposed over the survey area that are most likely to contain groundwater (based on the degree of weathering and fracturing).

• In the Borumeda area, the geologic structures (faults, fractures, and contacts) have a significant impact on the occurrence and transport of groundwater. Groundwater recharge and flow through weak zones and faults.

• A comparison of the geophysical results with borehole lithologic data shows that the results of geophysical interpretations are well correlated with the borehole lithologic log. The geological translation of the VES survey in the geoelectric section (Figure 4) is attributed to the same subsurface stratigraphy within the 2D magnetic modeling (Figure 10). The integrated geophysical method has facilitated data interpretation and greatly reduced the ambiguity in each method.

Recommendations

The expected scope of the study for the future has been forwarded below:

• For an additional and further deep groundwater exploration the area should be studied in detail by having maximum current electrode separation and survey for a detailed investigation into the expansion of low resistivity zones, the existing profiles should be oriented horizontally and perpendicularly.

• I highly recommended another researcher who has willing to research this area to undergo in detail geologic structural studies to map the strike, dip, and extension of faults, weak zones, and their orientation as fractures and faults. Because those Geological features have a role in the position and configuration of the aquifer system.

• Agricultural processes, as well as the removal of industrial and home waste, may pose possible hazards to the basin water sources, so serious consideration must be given to these activities in the future.

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