Four (4) main sources of Geophysical Noise in DC resistivity Data

There are many different thoughts among persons when it comes to the understanding of the term Noise. However the precise meaning of this term varies depending on the field laid behind it. For Example in environmental field, Noise can  be regarded as the form of pollution that is related to significant increase of sound level above the average level. In geophysics in layman terms the Geophysical Noise can be regarded as any unwanted signals that are either related or unrelated to the source of interest (target). I will give you various simple examples so that you can grasp the basic concept easily. 

Okay! Our first example is when we are interested in taking physical measurements regarding resistivity variation caused by  local geological faults, it means that any other variation of resistivity caused by regional fault structures may be regarded as Noise. Another example is that if our objective is to measure the amplitudes of reflected seismic waves from subsurface reflector that is generated by hammering a metal plate, then any other amplitudes that are not related to such source may be regarded as Noise. Another example is that if our objective is to measure the magnetic effect caused by a buried metal conductor, then any other magnetic effects that are caused by other sources may be regarded as Noise. 

Hope now you have a clear and simple clarification on the term geophysical noise. As the rule of thumb that any geophysical noise should be reduced from data to enhance Data quality. There are so many sources of geophysical noises, However this post is tailored to sources of geophysical noise in Direct Current (DC) resistivity data. So after knowing these sources you can be alerted when either carrying out or interpreting resistivity data.

The following are the four (4) main sources of Geophysical noise in DC resistivity data.

Terrulic currents: These are natural electric currents within Earth's crust that are produced by time changes in the geomagnetic field and ionospheric flow of charged particles such as lightning. Self-Potential (SP) signals originate naturally in the ground as a result of  various electrochemical processes such as electrical filtration, diffusion, adsorption and others.

Geologic topography: As due to uneven nature of the ground surface, high topography such as hills tends to disperse currents while low topography such as valleys tend to focus the currents. This situation will result in what is known as perturbation in measured potentials. Also heterogeneity (non-uniform) due to uneven distribution of rocks and structures may result in formation of cusps.

Cultural features; These features include man-made structures such as metal pipes, fences, power lines, pumps. For Example power lines have the tendency of generating Electromagnetic (EM) signals that may interfere with our results. Also some fences are induced with  electric charges to provide protection against thieves. So when we are carrying out a DC resistivity survey near these features there will be a possibility of incorporating these signals in resistivity data.

Instruments noise. This is when your instrument that was used in measuring resistance became itself the source of geophysical noise. This may be due to many factors such as signal processing components of the instrument, for example when a transient time noise results when a certain input variable is converted into output variable. Also when the instrument is exposed to heat, some electrons may vibrate and produce sound noise. It is related to instrument manufacturers. If so that is not our concern but theirs. 

There are many resistivity meters that have been designed to give best and quality results in resistivity surveys. So the selection of the proper resistivity meter is highly recommended when you make a purchase or renting your device so as to maximize the chance of having quality data throughout your resistivity project. Of Course you have to know these selection criteria for you to choose the best resistivity instrument but don't lose hope. I would like to talk clearly  about this  in the upcoming posts. 

All in all geophysical Noise is a wide and general term and may be regarded as different in various geophysical techniques. To have proper knowledge of sources of geophysical noises of particular geophysical techniques is highly recommended, so that you can apply proper noise reduction strategies and win ongoing resistivity projects.

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