Groundwater exploration using Advance Magneto Telluric (ADMT-600S-X) in Tsahuda Road Campus Adamawa state University Mubi, Adamawa State

Introduction

Groundwater development has increased greatly in developed countries. Today groundwater availability is determined by the presence and hydraulic qualities of groundwater bearing units, while portability is determined by hydro geochemical properties (Obiadi, et. al, 2016). Water is regarded the most important natural resource in the world for survival; hence its significance cannot be overemphasise. It is however disturbing if this all-important resource is becoming more and more scarce or difficult to explore (Christopher, 2006). (Rosen and Vincent, 2014). Water scarcity is acute especially in the developing countries, which suggest that 67% of the rural population lacks access to safe drinking water. This is due to the fact that people in rural regions rely on surface water from lakes, streams, ponds, and rivers to survive. Surface water bodies, on the other hand, are unreliable due to high evaporation rates, which are common in high temperate regions, and they are often polluted and infected with waterborne diseases (Gyau-Boakye, et. al., 2008). Treatment is required to supply water from these sources, primarily for domestic use towns (Rosen and Vincent, 2014). People that have access to this good drinking water are vulnerable to water borne diseases which may lead to guinea worm infestation and bilharziasis. Because groundwater is difficult to locate, a range of geophysical methods are required to offer data on its occurrence and location. (Thapa et. al, 2008) proposed that many criteria such as lithology, slope, lineaments, hydro geomorphology, land use, and land cover should be understood in order to estimate groundwater potential zones of a region. Soil information is also crucial for determining groundwater potential zones. Coarse soils, for example, are generally permeable, but fine-grained soils have a lower permeability hindering recharge of groundwater (Amaresh, et. al, 2006).

For instance, geophysics can be used to map groundwater resources as well as assess water quality (Hewaidy, et. al, 2015). Gravity, magnetics, seismic, electrical resistivity, electrical resistivity tomography, and electromagnetic approaches are just a few of the geophysical techniques that have been used to prospect for groundwater (Reynolds, 1997). The electrical and electromagnetic surveys have shown to be very useful in groundwater studies (Soupios, et. al, 2005). Many of these geophysical approaches have since been employed to characterize groundwater, but the electrical method has once again proven to be the cheapest and successful (Eke and Igboekwe, 2011). That is why the study seeks to use the Advance Magneto- Telluric (ADMT) or popularly known as electric resistivity tomography in the search for groundwater potential zones Basement complex area in Mubi Tsahuda Road Campus, Adamawa state University, Mubi South Local government area, Nigeria. In groundwater resource mapping, geophysical technique is used for mapping out subsurface geological structure in which groundwater water occurs (Araffa, 2012). Mubi and Environs is known to have a lots of failed (aborted) boreholes which are often shallow as a result of the Basement complex terrain. The application of ADMT geophysical technique before drilling of borehole becomes necessary in Tsahuda road campus of Adamawa State University Mubi.The presence of basement complex formation in this area is arbitrary difficult to drill a productive borehole while the hand dug wells are always shallow due to the difficulty in digging deeper wells. (Salem, 2001, Soupios, et. al, 2005 and Ekwe, et. al, 2006) the vertical electrical sounding (VES) approach has been effectively employed in groundwater exploration and the computation of hydraulic properties such as hydraulic conductivity and transmissivity, with very effective and efficient results. (Ojo, et. al, 2007).The vertical electrical sounding (VES) technique was proven to be useful in achieving good lateral coverage for mapping aquifer units and drilling productive boreholes, according to the submission, (Onimisi, et. al, 2013) the electrical resistivity approach is a useful tool for identifying locations with good groundwater and development potential. (Igboekwe, 2005 and Igboekwe, et. al, 2006) vertical electrical sounding (VES) is a geophysical technique for determining subsurface geology. It's also been frequently utilized for determining aquifer potential in borehole drilling. (Emenike, 2001, Onwuemesi and Egboka, 2006 and Okoro, et. al, 2010) have been successful with the vertical electrical approach and used an integrated array of geophysical approaches to prospect for groundwater in a fractured shale aquifer. Also, (Nejad, et. al, 2011) geoelectrical survey was conducted utilizing the electrical resistivity method to study subsurface layers and determine aquifer features. The work is aimed at carrying out geophysical survey using Advance Magneto-Telluric (ADMT) by running some profiles (2D survey) to delineate optimal drilling point in the area for groundwater production. During the course of this work, twenty profile were conducted and the result presented as 2D images in the result section.

ADMT method is a non-destructive, non-invasive, portable, and environmentally benign technique with a wide range of applications in engineering, environmental science, and subsurface geology (Metwaly, 2012). Two-dimensional electrical resistivity tomography has proven to be effective. (Loke, 2013) even in the presence of geological and topographical difficulties, The ADMT results provides a more accurate 2-D resistivity model of the subsurface, where resistivity variations in the vertical as well as horizontal directions along the survey line are recorded continuously. (Yang, et. al., 2002, Hauck, et. al., 2003, Cheng, et. al., 2008, Crook, et. al, 2008) 2D ADMT approach is a more effective and powerful way for studying both shallow and deep subsurface electrical structures in a variety of situations. (Hossain, 2000, Suzuki, et. al., 2000, Demanet, et. al, 2001, Daily, et. al, 2004, Adepelumi, et. al, 2006, Gokturkler, et. al, 2008 and Andrade, 2011) groundwater exploration and prospecting, engineering geophysics, and environmental site evaluations have all made substantial use of Advance Magneto- Telluric (ADMT).

The Study Area

The Adamawa State University Sahuda Road Campus is siyuated in Mubi South Local Government Area of Adamawa Sate and lies between Latitudes 10°14'00"N and 10°15'30"N of the Equator and between Longitudes 13°17'30"E and 13°21'00"E of the Prime Meridian (Figure 1), covering a total area of 6.49 km2 with a perimeter of 11.41 km.

The area forms part of the northeast basement complex and comprises of metamorphic (migmatitic gneisses, amphibolites, mylonites) and igneous (intrusive granitoids and volcanics) rocks. The study area also formed parts of Nigerian Basement Complex. The drainage pattern in the area is typically dendritic and characteristic of igneous-metamorphic terrains with streams emanating from areas of high relief and draining into low relief areas. The presence of major road (Mubi to Cameroon through Tsahuda, Madanya to Cameroon through Moduguva) and foot paths makes the area quite accessible. (Islam et al, 1986) differentiated the undifferentiated north eastern Nigerian basement rocks into major and minor types based on study of aerial photographs, field and laboratory studies, while (Islam et al 1989) mapped and divided the north-eastern Nigerian basement complex into four; the Mandara Mountain, the Alantika Mountain, the Shebshi Mountain and the Adamawa Massif. (Toteu, 1990) investigated the geochemical characters of the main petrographical and structural units of northern Cameroun and assessed its implication for Pan African evolution.

The occurrence, field relationships and petrography of different rock types have been described on a regional scale by some workers (Carter et al, 1963 and Mc Curry, 1971). The study of the rocks of Mandara Hills have been made by (Islam and Baba, 1990, Baba et al, 2006 and Siddig, 2012), where they documented the petrogenesis and geochemistry of rocks from Gwoza and Madagali areas (located at the northern tip of the study area). Also, (Abaa and Najime 2006) studied the occurrences of some ore minerals such as cassiterite, wolframite, galena, chalcopyrite, barite and gem minerals in the Oban-Obudu-Mandara-Gwoza area in the eastern part of the Nigerian Basement complex. The Mubi area like most basement terrains in Nigeria have experienced extensive tectonism and metamorphism leaving behind imprints, structures and the emplacement of large volumes of granitoid in the Pan-African (600 ±150 Ma) but because of lack of enough geologic data in the area these events were not adequately documented. This work is aimed at delineating groundwater potential in Adamawa State University Mubi, Tsahuda road Campus in Mubi.

Materials and Methods

Magneto telluric method allows the determination of an electric conductivity earth model from the measurements of natural variation of the surface electric (E) and magnetic field (H) over wide frequency range (Vozoff, 1972). Magneto telluric field that moves through the earth is dependent on the resistivity of the geologic materials and frequency of the equipment (Agyemang, 2020). Detailed explanation on magneto telluric technique is found in (Vozoff, 1972, Kaufman and Keller 1981, Berdichevsky and Zhdanov 1984). The inversion was done using Bostic, 1977 with the inversion generated and represented as continues resistivity progression versus depth. The instrument used is ADMT 600 - SX it is considered cheap but it has proved useful and effective in delineating natural magneto telluric field within the earth for mineral and groundwater study. ADMT 600 – SX geophysical instrument comprises of two electrodes, connecting cable and main frame with touch screen. Ground electromagnetic waves in the earth and soil follows the Maxwell equation as presented in equations 1 - 8. Assuming that most of the sub-surface geologic formation are non-magnetic and uniformly conductive macroscopically, therefore, no charge is accumulated, then, the Maxwell equation can be simplified as:

𝛻2𝐻+𝑘2𝐻=0 (1)

𝛻2𝐸+𝑘2𝐸=0 (2)

Where k is called the wave number (or propagation coefficient)

K= [𝜔2𝜇𝜀−ⅈ𝜔𝜇𝜎]1/2 (3)

Considering that the propagation coefficient k is a complex number, let k = b + i a

Where: ‘a’ is called the phase coefficient and b is called the absorption coefficient.

In the electromagnetic frequency range measured by the ADMT series of natural electric field geophysical instruments (0.1Hz to 5 kHz), the displacement current can usually be ignored, and k is further simplified as:

K = −ⅈ𝜔𝜇𝜎 (4)

Wave group resistance and resistivity

A magnetic field with a change in the Helmholtz equation induces a changing electric field, and the magneto electric relationship is:

𝐸𝐻=−ⅈ𝜔𝜌𝑘 (5)

The surface impedance Z is defined as the ratio of the surface electric field and the horizontal component of the magnetic field. In the case of uniform earth, this impedance is independent of the polarization of the incident field and is related to the earth resistivity and the frequency of the electromagnetic field:

Z = 𝐸𝐻= √ⅈ𝜔𝜇𝜌 iπ/4 (6)

(5) The formula can be used to determine the resistivity of the earth:

𝜌=1/5𝑓*(𝐸/𝐻)*(E/H) (7)

Skin depth

In non-magnetic media, the skin depth formula is:

𝛿≈503√𝜌/𝑓 (8)

It can be seen from the above equation that the penetration depth of electromagnetic waves is related to frequency and resistivity. The frequency is certain, the higher the resistivity, the greater the penetration depth, the higher the resistivity, and the lower the frequency, the greater the penetration depth. Through multi-channel simultaneous input measurement, large data with high-density measurement can be obtained, which breaks through the depth’s limitation of traditional high density electrical method and enables the maximum exploration depth. The equipment used is shown on Fig.2.

ADMT 600S-X 300 was used for the investigation, ADMT geophysical instrument comprises of two electrodes, connecting cable and main frame with touch screen. The principles of the instrument is based on electrical resistivity method which is measured through the potential electrode and the result presented as 2D imageTwo methods are involved in carrying out the measurement of resistivity of the subsurface using this equipment, these includes;

Method 1: Electrode measurement

The measuring points is at the midpoint of the MN electrode as demonstrated in Fig. 3 below:

Method 2: Wireless probe measurement

The measurement point of the wireless probe is at the midpoint of the wireless probe when the probe is continuously moved at 2m intervals as presented in Fig. In this research Method 2 was adopted because of its simplicity ( One person can can do measurement)

Results

The result of the investigations conducted are presented as 2D image, Y axis representing depth in meters (M) ( vertical values) while the X-axis is representing the distance on ground in meteres (M) used as Profile line. The Blue colour represents loose sediment/weathered materials which are conductive in naturewith low resistivity values while the ( light green to red colour) represent geologic formation that are consolidated (resistive) in nature with higher resistivity values.The black rectangular lines on the profile on some certainpoints on the ground represent where Borehole should be located. The 2D images of ADMT 600S-X profiles acquired in the study area defines subsurface geologic formation in Tsahuda road Campus, Adamawa State University Mubi (Fig.5-11)

Discussion of Results

The aim of the magneto-telluric survey is to ascertain the resistivity distribution of the surrounding rock materials to delineate potential groundwater sources, low frequencies are generated and supplied into the sub surface resulting potential difference were measured and the results obtained are processed and presented as resistivity pseudo sections (Teoh, et al, 2018), known and presented as 2D images, where Y axis representing depth while the X-axis is representing the distance on ground. The data were interpreted by comparing geology of the area with electrical resistivity of earth materials. The color bars indicates the range of electrical resistivity values in ohm-meters, the color legend scale is logarithmic and consistent with contour intervals. Cool colors (i.e., blue) represent areas of low resistivity values, warm colors (i.e., red) represent areas of high resistivity values (Rungroj, et al, 2020). The green, blue and purple color reveals lower resistivity zones which are not conductive and most of its resistivity values varies from one profile to the other, light green to yellow color shows slightly weathered geologic materials and red colors shows fresh basement (Fig. 1-11) The result of magneto-telluric survey, indicated by colored legend with blue to green color shows areas with lower resistivity values from 0.08- 0.34 Ωm for Fig 1, 2 and 3, 0.11- 0.34 Ωm for Fig 7, 8 and 9, 0.18- 0.43 Ωm for Fig 11 and 12, 0.16-0.31 Ωm for Fig 13, 14 and 15, 0.20-0.32 Ωm for Fig 16 and17, and 0.23-0.35 Ωm for Fig 19. The depths to potential points for drilling varies between 140 m to 200 m. The light green to yellow colour indicates slightly milledfracturing in basement and red colors represents fresh basement (Loke, 2004). The resistivity of potential area in Fig. 9; Profile13 range from 0.20-0.40 Ωm, this profile shows an exceptional case of very thick overburden weathered granite, it has a thickness of more than 160 m.In basement terrain, weathered basement are known to enhance groundwater recharge in boreholes. Inview of the above ground water exploration can be carried out in those points with black rectangular vertical colums on the figures. The results of all the profiles shows clearly the advantage of low frequency equipment over current based in delineating deep groundwater sources in basement terrain. With these method groundwater exploration in basement terrain is made easy. The depth of investigation using this equipment is subject to choice of investitigation depth but could not exceed 600 m.

Conclusion

Magneto-telluric equipment (ADMT 600 SX equipment) are relatively new in Nigeria but are becoming more common due to low cost and ease of field application and its precision. Groundwater exploration using Advance Magneto-telluric method has proved to be the most recent and useful tool in delineating deep groundwater depending the on the depth of investigating settings in basement terrain areas. The results of the surveys are positive for groundwater exploration, more especially in rural areas where there are no or less interferences. This equipment is prone to interferences if used close to power lines and communication mask. Caution must be taken to achieve desired result.

Acknowledgment

The author is grateful to Adamawa State University Mubi for funding the survey.

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