Introduction:

Hydrogeologists all over the world use the data of geological, hydrogeological and geophysical surveys to identify and delineate potential aquifers. In most cases geophysical surveys was used for ground water targeting was done for industries, residential apartments and agricultural landscapes. Over the last decades, there has been great advancement in the application of geophysical tools in groundwater exploration.

Magneto Telluric survey technique is the latest State-of-the-art available to locate potential borewell locations in areas where there are limited lateral space. In hard rock areas groundwater potential lies in weathered rock and fractured rock presence. Historically and even nowadays Electrical Resistivity surveys are used. The greatest limitation of 1D resistivity sounding method is that it requires lateral space. For 1 D Electrical Resistivity surveys lateral space twice the depth of the investigation is required. For conducting a Typical 1-D resistivity sounding to 100m depth, a lateral distance of 200m is required.

Electric Field Geophysical Prospecting Instrument, referred to as the 2D Audio magneto telluric water detector (ADMT-800S) in this article, and was used for electromagnetic propagation. ADMT 800 S is a portable equipment and works by measuring the electric field component of the earth’s magnetic field in different and can easily demarcate the potential points of aquifers. ADMT 800 S is an effective tool to provide a geolectric overview of the subsurface sequence of a particular area of interest In many terrains lateral space is not available for conducting Electrical Resistivity Surveys. In such terrains. Magneto Telluric Survey is very useful to scan and identify potential groundwater locations especially in hard rock terrain. Few research articles have documented the effectiveness of using Magneto Telluric Survey as an effective tool to determine Aquifer potential.

Geophysical investigation of groundwater potential in Iwo, Osun State, Southwestern Nigeria using audiomagnetotelluric method Etido Nsukhoridem Bassey a,*, Olumide Oyewale Ajani a, AbdulGaniyu Isah b, Adetunji Ayokunnu Adeniji a a Department of Physics, Bowen University, PMB 284, Iwo, Osun State, Nigeria b Department of Geological Technology, Federal Polytechnic Ede, PMB 231, Ede, Osun State, Nigeria, 2023 The above research outlines the significance of groundwater for human activities and the challenges of prospecting in complex basement terrain. The study suggests using the Audio magnetotelluric (AMT) method to explore the subsurface conductivity structure as an indicator of potential groundwater resources. The research demonstrates the effectiveness of AMT in mapping groundwater and characterizing geological materials up to a depth of 200 m. The analysis identifies distinct layers with varying resistivity values, primarily composed of Pegmatite and Migmatite gneiss. Pegmatite, with its lower resistivity, acts as a water-bearing zone when weathered, while Migmatite gneiss’s higher resistivity suggests reliance on fracture zones within the bedrock for groundwater exploration. Shallow potential groundwater zones are identified at 10–30 m depth in certain profiles, while deeper zones are detected at 40 m and beyond across the study area. The prevailing aquifer systems are influenced by fractures and weathered zones. The saprock region, located between fresh bedrock and regolith at 40 m depth and above, is deemed most promising for drilling purposes.. The research concludes that combining the AMT method with ADMT software provides valuable tools for groundwater prospecting in complex basement terrain, offering detailed data on the subsurface resistivity structure and facilitating the identification of potential groundwater resources.

The above research article documents successful delineation of aquifers in granitic terrain. An Integrated geophysical approach i.e magnetic impedance(lps1200) and Audio magneto telluric(ADMT 300s) survey was carried out in hard rock area of Pathalapattai village, Thiruverambur Block in Tiruchirappalli District of Tamil Nadu State, India, for delineation of groundwaterbearing zones that would be feasible for drilling deep bore wells for tapping good quantum of water. The entire area (1 acre of land) was surveyed and only two places was the favorable where the magnetic impedance difference was encouraging. Those anomalous areas were scanned by 2D Audio magneto telluric scanner up to the depth of 300m. The technique has provided information on the subsurface lithology & structures up to 300 m depth. The study shows that the groundwater occurrence in the area is confined to fractured and weathered zones. One of the location was recommended to drill a bore well. Comparing and correlating these results with the drilling data, it is found to be good. It can be concluded that the established interpretation of 2D Audio Magneto Telluric data and LPS 1200 data pretty well correlates with the actual fracture depth and lithological conditions. Thus, the method not only aids in locating water aquifers but also helps to avoid bore hole failures.

The research article documents the Magento Telluric Technique can be effectively used for mapping potential groundwater locations, ADMT-600S-X was used and set to transmit signal into ground for the depth of 300 m for each and a horizontal distance on the ground to 100 m. In this regard electric current is sent into the ground to investigate the subsurface. The result of each profile are presented in 2D inversions which makes it easy to interpret in terms of electrical distribution and variation within the subsurface. Total of 20 profiles of Magneto – telluric data were investigated in Adamawa State University Tsahuda Road Campus and environs. This geophysical method is basically used in basement terrain which makes it easy and quickly delineate areas for groundwater exploration. The results obtained shows potential profiles for Borehole drilling. This means that the study area has good groundwater potentials since majority of the profile gives a positive results. However, the potentials points shows that the depths to be drilled varies from 140 m to 200 m depending on the profile.

The article documents delineation of the borehole aquifer and the potential of the aquifer with the Audio Magnetotelluric (ADMT)method. The research was conducted on six measurement trajectories. The result obtained is that in all six trajectories there is a layer of rocks. These rocks are thought to come from the eruption of Mount Batur Purba. The study conludes that in the research site, the potential and distribution of water is more in the western area, while in the eastern area there is less.

Magneto Telluric survey can solve many hydrogeological problems, including (1) detection of potential ground water zones or determination of conductive weathered rock and hard rock zones); The objective is to map and understand the resistivity of formations at shallow and deeper depths belonging to Hard rock formations and interpret presence of aquifer for locating borewell.

Study Area:

The application of Magneto Telluric survey was conducted at Annalur Area near Irinjalakuda in Trichur district . The study area is a one acre and is a dairy farm. The farm had tried few poor yielding borewells and was not able to meet its daily requirements. Earlier ground water surveys were not successful in the area. The study location is shown in Fig. 1

Regional Geology and Hydrology:

Thrissur district is principally made up of hard rocks and sedimentary formations. Hardrock is made of greanites and gneisses. The geology of Thrissur District is such that it can be divided into two parts; the western coastal region comprising of loose unconsolidated sediments that have been deposited on the shore by rivers and ocean currents and the eastern region, which is underlain by hard, crystalline basement rocks such as gneiss and charnockite. Except for a thin coastal strip of 10-15 km, the major area of Thrissur district is composed of ancient crystalline rocks. The charnockites and gneisses have a characteristic ‘foliated structure’ imparted by the dark coloured tabular and flaky minerals. Cutting across these older rocks are linear bodies called dykes, which are doleritic and gabbroic in nature. The hard rocks terrains such as granites are usually devoid of primary porosity. Secondary porosity was developed due to weathering and fracturing of the hard rock and forms a good aquifer zone for groundwater occurrence and movement. In Annalur the geology consists of granite and gneissic rock.

Geology Map of the Area is shown in Fig 2

Methodology:

Magneto telluric (MT) is an electromagnetic geophysical method for inferring the earth's subsurface electrical conductivity from measurements of natural geomagnetic and geo-electric field variation. ADMT equipment was used for the survey.

Magnetic fields in the frequency range of 1 Hz to approximately 20 kHz are part of the audio magneto telluric (AMT) range.

Depth of investigation depends on the transmitted frequency and resistivity of the subsurface. Lower the frequency, greater the depth of penetration. The prospecting technique is based on natural electrical field which is influenced by resistivity contrast of underground minerals and ground water. Without heavy power supply, it uses low frequency signals & natural electric field as signal source. Equipment is automatically controlled by micro PC with a high resolution of measuring accuracy of 0.001 mill volt. The output will be in the form of 2 D profile with anomalous zones & in conjunction with geological & hydro geological conditions, recommendations were provided. Currently this is one of the most updated and effective detecting method in the world. It can be widely used for detail and general survey of underground water detecting, geothermal and other energy prospecting work. It improves the successful rate of drilling water well.

For Magneto Telluric method the survey data outputs were generated in the form of contour sections. The low resistivity zones and high resistivity zones were demarcated using contrasting thematic colour. The water bearing zones were shown in blue color and light violet colour. Traverse with more fractures zone are ideal locations for targeting ground water. The fractured zones are shown in blue color and location having more blue zoning are selected for drilling borewell. Magneto Telluric Survey Method was conducted at 4 locations and is shown in Fig. 3.

The magneto Telluric instrument has two electrodes(MN) which are grounded in the target area.

The Magneto Telluric instrument with M and N electrodes into ground the survey is conducted along straight lines. When the point sampling finished, move the M and N in a certain distance and the same direction to measure the second point. By this analogy, work until completely measuring the profile. ADMT- 800S penetration depth is around 800 m; when the resistivity fixed, the lower the frequency, the deeper the penetration depth.

Readings were carried out at 5m interval starting from Station 0 to Station 40m. In site the second station 20m was chosen to exactly pass through the investigation location. The station details are shown in Fig. 4

Interpretation of Survey Data :

Magneto Telluric survey data is computed through a software and sections are produced for each traverse which are intrepreted.

This equipment is capable of running self-checks for connectivity of electrodes and generates warnings on bad contacts. Bad contacts were resolved by pouring water on the ground and then electrode where driven subsequently to reduce grounding resistance,. Normally a constant spacing between adjacent electrodes is used.

The Magneto Telluric Instrument is shown In Fig. 5

Reference :

Application of magnetotelluric (MT) study for the identification of shallow and deep aquifers in Dholera geothermal region Kriti Yadav a, Manan Shah b, Anirbid Sircar c Groundwater for Sustainable Development Volume 11, October 2020, 100472.

Results and Discussions :

• Magneto Telluric Survey Results.
• Location P1 642155E , 1136659 N.
• At Location 1 following Groundwater accumulations were inferred.
• Three minor zones with moderate resistivity were interpreted to contain minor moisture and fractures.
• Zone 1 10 to 20m depth Minor zone.
• Zone 2 150 to 160m depth Minor zone.
• Zone 3 200 to 210m depth Minor zone.
• Very low resistivity is not observed at this location.
• Location not selected as poor yield is expected.
• Magneto Telluric output of P1 is shown in Figure 6.

• Magneto Telluric Survey Results.
• Location P2 642171 E, 1136656 N.
• At Location 2 following Groundwater accumulations were inferred Low resistivity zones with 3 minor and 1 potential fracture zones were interpreted at this location.
• Zone 1 10 to 20m depth Minor zone.
• Zone 2 120 to 130m depth Minor zone.
• Zone 3 220 to 230m depth Minor zone.
• Zone 4 250 to 260m depth Major Zone.

Location 2 exhibits potential low resistivity zones in the range of 0.13 to 0.15 ohm-m and is selected for drilling as priority 2, Magneto Telluric output of P2 is shown in Figure 7.

Magneto Telluric Survey Results at Location 3.
Location 3 642174 E, 1136691 N.

   At Location 3 Very low resistivity values and maximum groundwater layer accumulations were inferred Five saturated zones were interpreted to contain moisture and fractures. Location 3 exhibited lowest resistivity values, maximum groundwater accumulation and also had better groundwater recharge conditions.

• Zone 1 10 to 20m depth Minor zone.
• Zone 2 50 to 60m depth Minor zone.
• Zone 3 80 to 90m depth Minor zone.
• Zone 4 100 to 120m depth Major zone.
• Zone 5 190 to 200m depth Minor Zone.

Lowest resistivity is observed at this location ranging between 0.09 to 0.13 ohm-m and indicates weathered fractured rock presence with saturation.
Location 3 Recommended for Drilling as priority 1

Magneto Telluric Survey Results at Location 4.
Location 4 642157 E, 1136675 N.
At Location 4 Moderate resistivity and very poor ground water accumulations were inferred.
There were no indications of any ground water accumulation or low resistive zones and fractures.

Conclusions:

The site falls under hard granitic gneiss terrain and red Lateritic soil masks the site. Four points were studied using Magneto Telluric Survey. Based on the survey it is interpreted that the northern portion exhibits better ground water potential and accumulations than the southern portion of the site.

     Of the Four points validated by Magneto Telluric method Point 3 exhibits promising saturated fractures at depths and was recommended for drilling boreholes to 200m.

Drilling Results :

Drilling was conducted immediately at Location 3 gave a high yield of around 12000 litres per hour. Good yield was observed between 100 to 120 m depth as inferred from Magneto Telluric results. The letter from the owner of the Farm is shown below

Conflict of Interest :

The authors declare no conflict of interest.

Declaration on Funding:

The authors declare that no funding was received for conducting the study and preparation of the research article.

Acknowledgments:

The authors acknowledge the Department of Geology, Presidency College, Chennai – 600 008, India for providing facilities and support. Incisive comments from the anonymous reviewers to upgrade the quality of the paper are thankfully acknowledged.

Data Availability Statement :

The section contour outputs are provided in the article.

References:

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* Groundwater exploration using Advance Magneto Telluric (ADMT-600S-X) in Tsahuda Road Campus Adamawa state University Mubi, Adamawa State S. Kasidi Adamawa State University, Department of Geology Kasidisimon2002@yahoo.co.uk (Received in March 2022; Accepted in May 2022) Revealing Aquifer Potential With Audio Magnetotelluric(ADMT)Method: Case Study At Bangli Bali Narcotics Correctional Centre I Nengah SimpenEarth Physics Laboratory, Physics Department, Matematics and Science Faculty, Udayana University Jl. Raya Kampus UNUD, Bukit Jimbaran, Kuta Selatan, Badung, Bali, Indonesia, 8036, 2023.
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