Borehole drilling, Soil investigation, Geophysical, Environmental, Oil and Gas

Category: water borehole drilling (Page 2 of 2)

How to carry out Geophysical VES Survey for ground water investigation

what you need to know

The main purpose of vertical electrical sounding surveys is to identify groundwater yielding zones, their geometry, variation in quality (in terms of salinity), and direction of groundwater movement.  Groundwater containing various dissolved salts is conductive and enables electric currents to flow into the ground. Thus, the measurement of subsurface resistivity gives information on the presence of water as well as the lithology.

Vertical Electrical Sounding is a geophysical method in which an electrical current is passed into the ground through a pair of electrodes. The potentials developed due to the current within the ground are measured across another pair of electrodes on the ground. Most soils and non-ore bearing rocks are electrically resistive. Soil moisture and ground water are often electrically conductive because they contained dissolved minerals. Therefore the resistivity measured in the ground is predominantly control by the amount of moisture and water within the soil and rock (a function of the porosity and permeability) and the concentration of dissolved solids (salts) in that water. The principle of operation depends on the fact that any subsurface variation in conductivity alters the form of current flow within the Earth and this in turn affects the distribution of electric potential.

Igneous and metamorphic rocks typically have high resistivity values from about 1000 to 10 million Ω.M depending on whether it is wet or dry. The resistivity of these rocks is greatly dependent on the degree of fracturing, and the percentage of the fractures filled with ground water.  Sedimentary rocks, which are usually more porous and have higher water content, normally have resistivity values from 10 to 100Ω.M depending on the concentration of dissolved salts. Salt water usually has low resistivity due to the relatively high salt content. This make the resistivity method an ideal technique for mapping the saline and fresh water interface in coastal areas.

Data Acquisition

The common electrode arrays suitable for VES work are the Wenner and the Schlumberger arrays.

What is Wenner array?

The Wenner electrode array is the simplest of arrays; in it, the four electrodes—A, M, N, and B—are placed in line and spaced at equal distance from each other. The two outer electrodes, A and B, are current electrodes, and the two inner electrodes, M and N, are potential electrodes. Detection of horizontal changes of resistivity is achieved by moving the four electrodes across the surface while maintaining constant electrode separation. The Wenner array is commonly used in profiling for lateral exploration of the ground, like soil testing. The logistic advantage of using the Wenner array when profiling is you only have to move four electrodes for each new measurement along the line.

wenner array (after Hassan 2017)

What is the Schlumberger array?

The Schlumberger array is where four electrodes are placed in line around a common midpoint. The two outer electrodes, A and B, are current electrodes that are moved outward to a greater separation throughout the survey for each measurement.  

schlumberger array (after Hassan 2017)

In most interpretation methods, the curves are sampled at logarithmically spaced points.  The ratio between successive current electrode spacings can be obtained from the relation:

where

 n = number of points to be plotted in each logarithmic cycle.

For example, if six points are wanted for each cycle of the logarithmic plot, then each spacing a will be equal to 1.47 times the previous spacing.  The sequence starting at 10 m would then be 10, 14.7, 21.5, 31.6, 46.4, 68.2, which, for convenience in layout and plotting, could be rounded to 10, 15, 20, 30, 45, 70.  In the next cycle, the spacings would be 100, 150, 200, and so on.  Six points per cycle is the minimum recommended; 10, 12, or even more per cycle may be necessary in noisy areas. The potential electrodes M and N stay in the same position until the observed voltage becomes too small to measure. At this point, the potential electrodes M and N are moved outward to a new spacing. As a rule of the thumb, the reasonable distance between M and N should be equal or less than one-fifth of the distance between A and B at the beginning. This ratio goes about up to one-tenth or one-fifteenth depending on the signal strength.

The current is driven into the ground using the current electrodes A and B and the resulting potential difference is measured using the two inner electrodes M and N placed close together. The value of the resistance is measured by Terrameter and the apparent reisistivity calculated by multiplying the geometry factor K. Geometric factor is a parameter which is dependent on the potential and current electrode spacing which is calculated using the equation below.

Geometric factor formula

Where:

AB is the distance between the current electrodes

MN is the distance between potential electrodes

π is a constant= 3.142

Apparent resistivity is calculated using ohms law

ρ a = KR

Where, K = Geometric factor

R = Resistance

ρ a = Apparent resistivity.  

Equipment used is Terrameter, Electrode (current and potentials), Rechargeable battery, Measuring Tape, Cables, Hammer, Global positioning system (GPS) and recording sheet. 

Example of VES data acquisition table

Analysis and Interpretation

Vertical sounding curves can be interpreted:

Qualitatively by Study of types of the sounding curves obtained and notation of the areal distribution of these types on a map of the survey area. Each map is prepared by plotting the apparent resistivity value, as registered on the sounding curve, at a given electrode spacing (common to all soundings) and contouring the results.

Quantitative with computer modeling.  The first step in the interpretation of a resistivity sounding survey is to plot on log-log sheet a graph of apparent resistivity against the current electrode spacing (AB/2) with the best fit synthetic model curve using the computer software IPI2WIN which is developed for the purpose of data processing, analysis and interpretation. The observed apparent resistivity curves are classified into types with respect to synthetic model curve. This classification is based on the basis of the shapes of the curves, but at the same time it is related to the geological situation in the subsurface (Rock type, grain size, degree of void spaces and amount of water present, degree of weathering etc).The shape of the VES curve depends on the number of layers with corresponding resistivity values in the subsurface and thickness of each layer.

The interpretation should be guided by the information from geologic studies of drill holes, road cuts in the survey area.

Example of Geoelectric layer, Field and theoretical curves (after Meindinyo et al 2017)

In the above example five geo-electric layers were identified. Using computer assisted interpretation (IPI2win), the geo-sounding synthetic curve will be of the QQH-type (ρa1 > ρa2 > ρa3 > ρa4 < ρa5 ). The topsoil which is the first layer has a resistivity of 141.8Ωm and a thickness of 1.9m. The high resistivity indicates the presences of small amount of water and sand, so the possible lithology is wet sandy clay. The second layer has a resistivity of 53.4Ωm and a thickness of 3.1m, with the amount of the resistivity measured, it shows that the layer is conductive which indicates the presences of a high amount of clay. The third layer has a resistivity of 14.1Ωm and a thickness of 11.9m. The lithology here can also be said to be that of a clayey sand. The fourth layer has a resistivity of 9.3Ωm and a thickness of 21.3m. Due to the low resistivity in this layer the lithology here is clay. The fifth layer having a resistivity of 29.2Ωm and a thickness of 20.4m will have a formation of clay mainly. From the table above one can noticed that there was a decrease in resistivity with increase in depth, though later increases in the fifth layer.

Talk to us for your upcoming project in VES survey

Geodata Evaluation & Drilling LTD. offers Vertical Electric Sounding (VES) to determine actual depth to good ground water zone. Let us handle the project for you. contact us at www.geodatadrilling.com Phone: +2348037055441

What you need to know about water borehole drilling

When you have the knowledge of borehole drilling you will stand a better chance of understanding certain tricks being played by quack borehole contractors in other to cheat clients out of their money and the essence of this write up is for you to avoid such tricks in order to cut down borehole drilling cost and ensure good quality borehole. The quacks will always tell you such things like, “Sir water is very far down from here, we need to drill up to 120 ft before we can get good water. The truth is that they want to play tricks with you and at the same time ready to compromise project materials quality to give you a substandard borehole job.

It is important for the client to be aware that there are two types of water borehole, depending on your water needs and affordability. We have Private shallow borehole and community or industrial deep borehole. Each one of these borehole types is designed to serve a particular purpose of usage. Private borehole is for few individuals or domestic use. While community or industrial borehole are for large residential areas, hotels and high rise buildings, production factories, bottle or pure water processing factories, community water schemes etc. This borehole type is the most reliable in terms of production capacity and resistance to contaminants infiltration. The aquifer is confined and deeply located. The deep borehole is the most expensive of all the borehole types, because of the depth and the nature of materials involved in the drilling operations.

Before I go on, I advice every prospective client to entrust a geologist experienced in water borehole drilling with his water project. In carrying out a drilling project, you need to have the basic knowledge of Geology or underground earth drilling location.

BASIC GEOLOGY:

Geology is the study of the earth. It describes the origins and formation of the rock types under the surface of the earth. The original material or “basement rock” of the earth are the hard rocks such as granite and volcanic formations, formed when molten material cooled beneath or at the surface of the earth. These are known as the igneous rocks (“made by fire”). It is from these rocks that sedimentary layers have been formed.

Sedimentary layers are formed by the weathering, transport (by wind or rivers) and deposition (sediment) of particles broken down from rocks. Those particles can range in size from extremely fine (clay particles) through silt-sized to the larger sand and gravel particles. Sedimentary layers may be unconsolidated (loose such as clay and sand) or consolidated (cemented together) to form harder rocks such as sandstone and lime stones. For example the clay particles of the clay layer that you have encountered during drilling may have arrived from somewhere else. The clay particles were formed by the weathering  of rocks. Then they may have been eroded and transported to your drilling location by a river or the sea. Finally the clay particles were deposited (sediment, settled down) in still water, for example a lake.

In the same way a sand or gravel layer could have been deposited. The sand and gravel particles may have been transported by a river and were deposited along the river bed. Although now there may not be a river or a lake present, the deposition of particles could have happened thousands or even millions of years ago. Another way to transport particles is by wind. Particles can be blown to another location by the wind.

When a mixture of sand and fine particles has been compacted by pressure, created by the weight of layers on top of it and cemented by minerals present in the mixture, sandstone is created. Sandstone is hard and may look like solid stone, but is in fact consolidated sediment and may be difficult to drill through.

Other factors to be taken into consideration when drilling water borehole are underground water contaminants, depth of the fresh water aquifer, productivity of the well, septic contamination, salinity of underground water etc. At this point, a geophysical survey is highly recommended. But if the ground water condition is well known, Geophysical survey should be avoided.

Geophysical survey is a scientific method of acquiring geological and hydro-geological information of the earth. The interpreted data assist both the client and the Geologist in good decision making in the course of drilling the water borehole.

Borehole Schematic

HOW DEEP SHOULD BE YOUR WATER BOREHOLE?

“How deep will the well be?” is a common question before drilling a well. If the driller has drilled several wells in the nearby area, he may be able to estimate the approximate depth where water will be encountered. However, the depth needed to find the required water yield can be determined accurately prior to drilling by Geophysical VES survey. A well is hole in the ground through which ground water can be brought to the surface. Drilling Rig can drill to great depths, deeper wells usually cost more than a shallow well to construct in the short-run. However, not drilling deep enough can result in later problems that will be much more expensive to fix.

LISTED BELOW ARE SOME OF THE FACTORS THAT MAY INFLUENCE DECISIONS ABOUT THE DEPTH OF A WATER WELL

SEASONAL RISE AND FALL OF THE WATER TABLE

During the year, the water table will fluctuate in the well in response to seasonal precipitation in the area and local ground water use. The well must therefore be drilled deeper than the lowest expected elevation of the water table. Water level fluctuations may occur over several years if there have been drought conditions. Knowing the lower limit of the range of water levels over several years therefore can be helpful.

SURFACE CONTAMINATION RISKS

Deeper wells that are properly constructed (including grout, gravel pack, casing and well head) usually provide guaranteed protection from bacterial contamination sources originating at the surface. Increasing the well depth and the length of well casing will result in a longer flow path of water from recharge at the surface to pumping from the well. The longer the length of time water is in the subsurface, the more opportunity there is for bacteria to die-off or be trapped by soil and rock.

• POOR QUALITY WATER ZONES

In some areas of the country with multiple aquifers, there may be zones of poor water that should be avoided or “cased off” so this lower quality water does not adversely impact the well.

LOW YIELDING ROCK FORMATIONS

In low yielding rock formations the well may have to be drilled deep enough to serve as a storage cavity for ground water. Once a well is drilled, the total depth, depth to the top of the ground water table (static level) and diameter of the well will determine the quantity of water stored within the well cavity. The deeper the well and larger the well diameter the more water will be stored for a given well depth and water table elevation.

Talk to us for your upcoming project in Water Borehole Drilling

Geodata Evaluation & Drilling LTD. offers water borehole drilling services. Let us handle the project for you. contact us at www.geodatadrilling.com Phone: +234 8037055441

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