Telluric Noise Research Articles

Delineation of Groundwater Contamination Using Self Potential Method in Effurun, Delta State, Nigeria

Spontaneous Potential profiling has been carried out around a dumpsite in Effurun near Warri, Delta State, Nigeria. This is with a bid to determine qualitatively the presence of contaminant in the groundwater around the area due to electrochemical and electrokinetic processes arising from arbitrary disposal of both domestic and industrial waste. The study was carried out by engaging two different electrode configurations, which include the gradient and the fixed base arrays. This is to reduce or eliminate cumulative errors and electro - telluric noise connected with variations in the earth's magnetic field and other cultural sources. The data acquisition was carried out along the two orthogonal traverse lines. Electrode spacing of 5 m and 10 m in both techniques were adopted. Also, the data was acquired during the raining and dry season, to determine the effect of climatic changes on leachate generation and migration. The profiles generated show high positive potential of up to 70 mV outside the dumpsite area to as low as -60 mV around the dumpsite area using the gradient array, and 120 mV to -30 mV using the fixed base array in both the raining and dry seasons. This low and negative potential readings around the dumpsite area could be the confirmation of the redox activities taking place at depth, and contaminating the aquifer and groundwater.Keywords: electrokinetic, electrochemical, contaminant, self potential, electrodes

Induced Polarization and Resistivity of Second Potential Differences (SPD) with Focused Sources Applied to Environmental Problems

A novel technique is proposed to improve shallow induced polarization (IP) and resistivity survey results. We propose the apparent resistivity and apparent chargeability of second potential differences (SPD), employing two focused sources (FS) mathematically manipulated by superposition. To test the idea, a synthetic model is developed with two bodies. The first body is a small shallow heterogeneity which is above the second and larger body. This synthetic model illustrates the shape and response of the apparent resistivity and chargeability for FS under the random noise and masking effect. These processes reduce electromagnetic coupling, telluric noise, contamination in channel links, and small heterogeneous responses. A field test of the SPD for FS was carried out in an agricultural site irrigated with wastewater, where contaminated water laden with metals has been accumulating for years in the soil. Soil samples were collected and analyzed throughout the geophysical survey to correlate the resistivity-IP results. Soil laboratory analysis included metal content, moisture content and texture. The parameters computed after applying the SPD for FS depicted a better lateral resolution where vertical and horizontal boundaries of the anomaly zones were well defined. It was possible to determine the low permeability horizontal layer made of clay-soil (called tepetate), which is a barrier for water and metals.

2D Model Study of CO2Plumes in Saline Reservoirs by Borehole Resistivity Tomography

The performance of electrical resistivity tomography (ERT) in boreholes is studied numerically regarding changes induced by CO2sequestration in deep saline reservoirs. The new optimization approach is applied to generate an optimized data set of only 4% of the comprehensive set but of almost similar best possible resolution. Diverse electrode configurations (mainly tripotentialαandβ) are investigated with current flows and potential measurements in different directions. An extensive 2.5D modeling (>100,000 models) is conducted systematically as a function of multiparameters related to hydrogeology, CO2plume, data acquisition and methodology. ERT techniques generally are capable to resolve storage targets (CO2plume, saline host reservoir, and impermeable cap rock), however with the common smearing effects and artefacts. Reconstructed tomograms show that the optimized and multiply oriented configurations have a better-spatial resolution than the lateral arrays with splitting of potential and current electrode pairs between boreholes. The later arrays are also more susceptible to telluric noise but have a lower level of measurement errors. The resolution advance of optimized and multiply oriented configurations is confirmed by lower values for ROI (region of index) and residual (relative model difference). The technique acceptably resolves targets with an aspect ratio down to 0.5.

Cole-Cole Inversion of Telluric Cancelled IP Data

SUMMARY Induced Polarisation, like all geophysical methods, suffers from the presence of noise. Of the many sources of noise, tellurics can often be one of the most problematic to remove. An effective method of removing telluric noise has been trialed on dipole-dipole data acquired by MIM Exploration's proprietary MIMDAS system, during routine surveying in NE South Australia. The method utilises impedances determined from previously acquired MT data to estimate the natural field component of the measured signal. This paper presents results obtained thus far, displaying significant improvement in data quality when compared to the uncorrected data. The benefits of increased signal to noise and higher confidence in Cole-Cole parameter estimation are outlined.

Detection of crustal inhomogeneity in the Nojima Fault zone using earth current noise

Abstract A resistivity survey method using artificial telluric noise was examined and applied to a field of a fault zone. The electric earth current was measured at 50 sites in the Nojima Fault zone, which is in the northwestern part of Awaji Island, southwestern Japan. The dominant component of the observed electric field is supposed to be leakage currents from DC electric railways running outside the island. Amplitude and polarization of the stray current were systematically investigated and were revealed to represent the subsurface electrical structure of the study area. Some features on the fault zone’s electrical structure have been pointed out, including: (i) an electrical boundary that corresponds to a geological one between granite (resistive) and sediments (conductive); and (ii) a low resistivity spot on the surface rupture of the earthquake fault. The structure estimated in the present study is both qualitatively and quantitatively consistent with previous resistivity surveys done using other methods pursued in the same area. It shows the validity of the ‘stray current method’ as one that is easy and uses low‐cost resistivity exploration tools in a region where the effect of artificial noise caused mainly by leakage currents from electrical railways cannot be ignored.

Groundwater contamination monitoring with multichannel electrical and electromagnetic methods

Electrical and ground-based electromagnetic surveys have been carried out at the Ranger minesite in the Northern Territory, Australia to investigate their use in detecting any seepage from structures used to store ore processing tailings. The main aim of this work has been to develop a clearer understanding of any seepage problems at this minesite using a combination of self-potential (SP), direct current (DC) resistivity, induced polarisation (IP), and transient electromagnetic (TEM) methods, with the results being interpreted in conjunction with hydrogeological data. Ultimately, it is aimed to apply an optimal combination of the methods to long-term monitoring of potential seepage. A 64-channel system developed at the Cooperative Research Centre for Australian Mineral Exploration Technologies (CRC AMET) has been used to record simultaneously the response at a number of electrodes with the SP, DC resistivity and IP methods. A 2D array of electrodes was set up to monitor the SP response over an area measuring 200×300 m. The simultaneous measurements enable time-varying telluric noise associated with SP responses to be minimised. In-line array DC resistivity and IP measurements were made efficiently at 10-m station intervals with the multichannel system, e.g. complete Schlumberger soundings centred at 30 separate stations were made in approximately half a day. Data collected in October and December 1998 north of the Ranger minesite tailings dam are the first of a long-term project, in which changes of responses will be monitored. A trend of increasing chargeability is observed towards a fault that is the main path of any seepage from the tailings dam. At this stage, maps of the SP response measured with the 2D array at different times indicate that the results are reproducible, and there is an association of an SP anomaly with the presence of seepage in an intersection of two faults. It appears that the IP and SP methods offer the best possibility for the direct detection of seepage in this case. Furthermore, the DC resistivity and TEM methods help to resolve geological structure relevant to hydrogeological processes that could affect the behaviour of seepage over time.

Regions of Anomalously Large Telluric Interference in IP: The Cobar Syndrome

Regional differences are observed in the severity of telluric noise interference in induced polarization surveys. In some areas, of which Cobar (New South Wales) is an extreme example, surveys must often be postponed or confined to particular times of day. Using simple resistivity and magnetotelluric models, and typical magnetic micropulsation spectra, we show how this arises. The situation is worst when a very resistive basement is overlain by a thick conductive cover. In that case the IP signal is small on account of the conductive cover, while telluric voltages are large because of the resistive basement. For typical IP frequencies, and for the values of overburden resistivity found at Cobar, noise to signal ratio is shown to be greatest for overburden thicknesses of 200?300 m. This compares with the 80?150 m thicknesses actually observed. Noise to signal ratio is relatively insensitive to dipole length for a given separation of dipole centres.

Computer simulation of low‐frequency electromagnetic data acquisition

Computer simulation of low‐frequency electromagnetic (EM) digital data acquisition in the presence of natural field noise demonstrates several important limitations and considerations. Without a remote reference noise removal scheme, it is difficult to obtain an adequate ratio of signal to noise below 0.1 Hz for frequency‐domain processing and below 0.3 Hz base frequency for time‐domain processing for a typical source‐receiver configuration. A digital high‐pass filter substantially facilitates rejection of natural field noise above these frequencies; however, at lower frequencies where much longer stacking times are required, it becomes ineffective. Use of a remote reference to subtract natural field noise extends these low‐frequency limits by one decade, but the remote reference technique is limited by the resolution and dynamic range of the instrumentation. Gathering data in short segments so that natural field drift can be offset for each segment allows a higher gain setting to minimize dynamic range problems. The analysis is also applicable to the induced polarization technique in which similar problems arise at low frequencies in the presence of telluric noise.

Comparison of Induced Polarisation Measurements Over the Elura Orebody

Resistivity and induced polarisation measurements have been performed at identical locations over the Elura orebody using 100 m dipole?dipole arrays by operators of three advanced IP systems. While the three resistivity data sets correlate extremely well, the three sets of induced polarisation measurements correlate poorly. The polarisation response of the Elura deposit is most clearly shown by chargeability values measured using a HUNTEC Mark IV receiver. Phase measurements at 0.125 Hz using a ZERO GDP-12 receiver support the character of the chargeability pattern. Low frequency phase measurements acquired using the Phoenix Multispectral IP system were adversely affected by telluric noise, for which (owing to survey time limitations) inadequate signal stacking was employed. By contrast, the accompanying high frequency phase data contain useful information concerning the geoelectrical section and the electromagnetic response of the orebody. This case history may assist users of advanced IP systems in making the appropriate compromise between survey production and data quality.

TURAM EM Testing Over the Elura Orebody

Two Turam surveys with a frequency range between 35 and 660 Hz were conducted over the Elura orebody in 1977 and 1979. The first survey using a 700 m ´ 300 m loop centred on line 20100 E, produced a definable anomaly over the orebody but the data were generally noisy. Data from the second survey using an 800 m ´ 400 m loop centred on line 50800 N were less noisy. Anomalies were recorded on two lines over the orebody. No significant anomalies were recorded over two other lines, one on the edge and the other off the orebody. Over the limited extent that data could be collected owing to poor signal strengths (some 600 m from the leading edge of the large loops), it is possible to define orebody anomalies in a frequency range up to 105 Hz and possibly to 220 Hz. The problem with this exploration technique is that loop positioning is critical and that both geological noise (variations in the overburden depth and resistivity) and telluric noise degrade the data, making the Turam method at these frequencies unsuitable for exploration in the Cobar type environment. Analogue and digital modelling results are shown in an attempt to clarify and define the Turam anomaly expected over the Elura orebody.

The Induced Polarisation Technique Over the Elura Orebody

Following the original geophysical discovery work on the Elura orebody which included the INDUCED POLARISATION technique, various efforts have been made to improve the quality of the I.P. data so that this technique could be used confidently as an exploration tool in the area. Since the discovery, I.P., together with magnetics, has been the most widely used geophysical tool in the Cobar belt. However, major problems were encountered, some of which are probably unique to the Cobar region. Considerable time and e.ffort had to be applied in an attempt to overcome the severe telluric noise common to this area and ultimately to generate anomalies with an acceptable response over the background chargeabilities.The INDUCED POLARISATION technique has now been shown to be an effective geophysical tool for detecting the Elura orebody. A number of parameters must be chosen carefully so that the technique can be used in reconnaissance surveys for other bodies in the area. These include: – a) Taking measurements in the time-domain;b) Using the dipole-dipole configuration with an appropriate dipole length;c) Investigating the optimum transmitter "ON" time;d) Increasing the integration time of the received signal;e) Using equipment that can minimise the effects of the severe telluric noise in the area.

Spectral analysis of voltage measurements for the interpretation of dipole geoelectrical soundings in low resistivity geothermal areas

This paper deals with the application of the Maximum Likelihood Method (MLM) for power spectral analysis of voltage recordings at the receiving dipole, obtained during the execution of dipole vertical soundings in the geothermal area of Travale (Tuscany). After a brief recall of the dipole sounding method with particular emphasis given to a simple and quick field technique, we consider the problem encountered in the evaluation of the useful signal in presence of low resistivity formations and low signal-to-noise ratios. We then give a detailed description of the MLM, which permits the individuation of the useful signal at the frequency of the energizing current, by suppressing contemporarily the random stationary telluric noise without any signal distortion. Finally, we show and discuss some field recordings as proof of the validity of the field technique employed and of the spectral analysis method for apparent resistivity data acquisition.