ground truthing magnetometer data using soil coring: initial results ...
INTRODUCTION - Earth Science India archival/download.php?file=pdf-… · Ground Magnetic Method...
Transcript of INTRODUCTION - Earth Science India archival/download.php?file=pdf-… · Ground Magnetic Method...
e-Journal Earth Science India www.earthscienceindia.info Popular Issue, January, 2011
1 | P a g e
Geophysical Prospecting as a Tool for Site CharacterizationGeophysical Prospecting as a Tool for Site CharacterizationGeophysical Prospecting as a Tool for Site CharacterizationGeophysical Prospecting as a Tool for Site Characterization forforforfor Mining Dump WasteMining Dump WasteMining Dump WasteMining Dump Waste Rolland Andrade and Karunakar Goud.B
Any mining activity results into generation of disintegrated waste materials “Tailings”.
Tailings are also known as slimes or leach residue which are the left over materials after
the process of separating the valuable fraction of radioactive as U3O8 in Uranium mining.
Mine tailings are usually produced from the mill in slurry form, which demands proper
disposable sites, as it generally consists of trace quantities of metals/radioactive elements.
Hence, surface waste disposal site has to be characterized for its sub-surface nature to
understand its role in environmental impact due to the loading of waste materials. The
objective of this article is to provide a description of the site investigation techniques and
approaches that can be used to characterize the flow of subsurface water, map the lithology
at near surface disposal facilities. Surface geophysical techniques like ERT (2D resistivity
tomography), ground magnetic and borehole seismic studies in integration with each other
can be an ideal tool in mapping the geophysical characteristics of sub-surface formations
in and around the disposal site.
INTRODUCTION
Mining generates a large quantum of tailings otherwise termed as slimes or leach residue,
basically a mixture of fine disintegrated mineral particles and fluid, which needs to be disposed
safely without causing any environmental hazard like leaching and erosion by wind or water.
The global legacy of mining and disposal of tailings had been for more than few centuries.
Tailings facilities consist of tailings ponds or lagoons, tailings dams and tailings transport
systems (generally pipelines). Usually a very large area is required to contain the tailings
which is man-made, and is the most critical elements of these facilities. The surface disposal
site is to be characterized for its sub-surface nature in order to understand its role in
environmental impact due to the loading of tailings. Geophysical exploration plays a pivotal
role in mapping the subsurface geological formation in terms of its geophysical parameters in
and around the waste disposal site.
Several near surface geophysical explorations methods like magnetic survey, resistivity
investigations, shallow seismic survey etc., can be adopted in integration with geological
mapping or litholog analysis to better understand the subsurface geological settings below the
waste disposal site. The thickness of weathered zone (overburden), presence of intrusive
bodies, dipping beds etc., are some of the geological inferences expected from geophysical
investigations. Geophysical investigation results are generally represented in terms of
resistivity contrast, density contrast, susceptibility contrast etc., for the subsurface lithology
(geological settings). These inferences are to be corroborated with hydrological and
geochemical results from the study area to concretize the geophysical findings.
e-Journal Earth Science India www.earthscienceindia.info Popular Issue, January, 2011
2 | P a g e
PONDING AREA
Tailings ponds are the areas of refused mine tailings where the water borne refuse material
is pumped into a pond to allow the sedimentation (meaning separation) of solid particles from
the water. The pond is generally impounded with a dam, and is known as tailings
impoundments or tailings dams. It was estimated in 2000 that there were about 3,500 active
tailings impoundments all over the world. There are different methods for dumping mining
waste which includes valley impoundments, ring dikes, in-pit impoundments, and specially-
dug pits. The most commonly adopted method is the valley pond, in which the natural
topographical depression of the ground is taken into utility. Large earthen dams are
constructed, which may be either upstream, downstream or centerline construction, which are
later filled with tailings. In all possible instances for dump site selection, due consideration
must be given to the probable possibility of groundwater contamination and other
environmental issues. The tailings (mining waste) are generally in a semi-liquid state with
considerable amount of water present in it. Dewatering treatment is an important part of pond
storage, as the tailings are added to the storage facility, the water is removed generally by
draining into decant tower structures. Dump site (Tailings Pond) for mining waste disposal are
generally selected over a considerable areal space with supportive geomorphological settings.
Once the tailings are added, over time the intrinsic properties of the pre-existing soil and
shallow subsurface geological parameters are prone to change due to several mechanism like
adsorption, diffusion, mechanical dispersion etc. Also loading of tailings over a period, builds
the pressure on the ground surface, which is transferred to the deeper levels causing widening
of preexisting fractures or weak zones. These may behave as conduits for the tailings to
migrate to deeper levels causing groundwater contamination over time. Hence before
commissioning any waste dump site, mapping the site area through near surface geophysical
techniques would give an understanding of the sub-surface geological setting, over which a
considerable thickness of tailings is going to sit permanently.
GEOPHYSICAL INVESTIGATIONS – An overview
Exploration geophysics is the practical application of principles of physical methods (such
as seismic, gravity, magnetic, electrical and electromagnetic) to remotely sense the subsurface
anomalies. Analysis of these measurements reveals the variation in the physical properties of
the earth’s interior. Integrating the knowledge of geology, these measurements can be useful in
understanding and interpreting the anomalies with meaningful geological inference. With the
advancement in instrumentation the application of geophysical prospecting is extended in
monitoring environmental studies, investigating buried archaeological and civil engineering
sites and interplanetary imaging etc. There are different geophysical techniques as enlisted
below:
� Seismic methods, such as reflection seismology, seismic refraction and seismic
tomography.
� Magnetotellurics.
� Transient electromagnetic (EM).
� Radio frequency electromagnetic propagation (e.g., ground penetrating radar).
� Electrical techniques, including Electrical resistivity tomography and induced polarization.
� Magnetic techniques, including aeromagnetic surveys and ground magnetic.
� Gravity and Gravity Gradiometry.
e-Journal Earth Science India www.earthscienceindia.info Popular Issue, January, 2011
3 | P a g e
� Geodesy.
� Remote Sensing.
The use of various near surface geophysical methods which can be adopted for tracking
the movement of hazardous waste and groundwater pollution sites is often a rapid, cost-
effective means of preliminary evaluation. The information gained from a surface geophysical
survey can be used to choose optimal locations for the placement of boreholes, monitoring
wells etc., and also to correlate geology between existing boreholes. Combination of two or
more exploration techniques used in integration for interpreting the results can reduce the
degree of ambiguity. A comprehensive knowledge of the local geology and site conditions is
necessary in order to select an effective geophysical exploration technique in order to plan a
survey, and to interpret the acquired data.
NEAR SURFACE GEOPHYSICAL EXPLORATION
� Ground Magnetic Method (Total Intensity Mapping):
Ground magnetic investigations are carried out using magnetometer, an instrument which
measures magnetic field strength in units of gammas or nanotesla (1 gammas = 1 nanotesla =
0.00001 gauss). The earth’s magnetic field strength gets distorted from its regional equilibrium
state due to localized subsurface features, like buried bodies, lithological contacts, presence of
dyke or sills etc., causing anomalous signatures. The basic objective of conducting a magnetic
survey at a mining dump site or groundwater pollution site is to map these localized anomalies
and delineate the area of existence or burial of the source for these anomalies. Magnetic survey
is generally practiced perpendicular to the strike direction of host country rock or exposed /
buried features like dykes, and is carried out along transverse profile lines, aligned in a grid
format to map a2D or 3D perception of the subsurface geologic features as mentioned earlier.
There are several types of magnetometers with different working principles used for
specific problem oriented investigations, of which the total-field proton-precession
magnetometer (PPM) is most commonly used. It generally measures the total magnetic field
strength at a point of measurement, hence a scalar quantity consisting of the sum of the earth’s
field, the anomaly caused by the magnetic source if any, and the variations of the field caused
by diurnal drift, magnetic storms and micro pulsations. Of the two configuration (Transverse
profiling and Grid), traverse magnetic profiles are usually aligned in a northerly orientation in
order to define the asymmetric anomaly usually associated with buried ferromagnetic material.
Grid or traverse coordinates must be surveyed from a known land mark with GPS
(preferentially ‘DGPS’ Digital Global Positioning System) locations, such as Bench mark
points, Road milestones etc., which can be recovered at a future date. Looping magnetic survey
method is generally adopted, since the magnetic readings are repeated at the same location, it is
expected to be consistent under normal circumstances over a short period of time.
After incorporating appropriate corrections followed by processing, the analytical data is
represented either as an X-Y plot or is contoured to give an image for the gridded study area.
In practice if the terrain over which the magnetic survey measurements are carried out is highly
undulating with elevation difference of more than 3-5mts within short interval, then the
elevation correction is also incorporated. Contour image represents the total magnetic field
strength over an area of investigation, clearly delineating the anomalous zones with contrasting
field intensity change as shown in Figure.1. Further in order to quantitatively analyze the
e-Journal Earth Science India www.earthscienceindia.info Popular Issue, January, 2011
4 | P a g e
depth, dimension and orientation of any anomalous body, individual grid profiles are analyzed.
Usually conductive bodies like dykes etc., are mapped, which are considered to be natural
hydrological barriers. Hence an appropriate site recommendation based on magnetic
investigation results can be useful in retarding the surface water seepage into the groundwater
and further migration of pollution plume.
Fig.1: A contour image showing various anomalous zones (dyke intrusions) mapped through
magnetic survey in and around a proposed waste dump site– case study from
sedimentary terrain of Cuddapah basin.
� Electrical Resistivity Method (Vertical Electrical Sounding):
Electrical resistivity exploration method maps the subsurface electrical resistivity structure,
which is interpreted to determine appropriate geologic structure and/or physical properties of
the subsurface. The electrical resistivity of a geologic unit or target is measured in ohmmeters,
and is a function of porosity, permeability, water saturation and the concentration of dissolved
solids in pore fluids within the subsurface. It measures the bulk resistivity of the subsurface
which is regarded as apparent resistivity. In this survey method the current and potential
electrodes can be placed in various configurations. However, there are three commonly used
configurations called Wenner, Schlumberger and Dipole for carrying out resistivity soundings.
Selection of suitable configuration is made based on the type of investigation and area of study.
The depth of investigation and depth of resolution are two independent properties
associated with each individual configuration. Selection of appropriate configuration for
investigation requires supplementary information like well inventory report, local geology,
hydrological conditions etc., in order to have appropriate site selection. The site for carrying
out a resistivity sounding should be in an area with more or less flat terrain and also without
any visible lateral surface inhomogeneities like dykes, veins etc. Presence of appreciable
thickness of soil cover along the sounding profile line will facilitate good galvanic contact
between the ground and various electrodes. Using VES (Vertical Electrical Sounding) it is
possible to infer the depth, thickness and the nature of various layers in the sub surface at a
particular place of investigation.
e-Journal Earth Science India www.earthscienceindia.info Popular Issue, January, 2011
5 | P a g e
Depending upon the total area of investigation, number of VES is done either randomly or
along a profile, with particular orientation (normally perpendicular to the strike of the host
formation) and spread length. Further, the interpreted results are reduced to the mean sea level,
by incorporating elevation for each electrical sounding location. Bore well site
recommendations are made based on the VES data, which is used in conjunction with
resistivity interpreted results to prepare a 2D or 3D lithological section for the entire
investigated site as shown in Figure.2. This kind of attempt can be a useful guide for an
engineer or planner to assess a site for construction of dam axis or storage tanks for decant
pond etc.
Fig.2. Geological section prepared based on VES (inset) and bore / dug well litholog analysis-
case study from granite terrain of Nalgonda district.
� Electrical Resistivity Tomography (Two Dimension):
The new advancement and development in recent years is the use of 2-D electrical
imaging/tomography surveys to map the study line as a whole with variable depth. Hence a
more realistic model accommodating the resistivity changes in the vertical direction, as well as
in the horizontal direction along the survey line is obtained. However, it is assumed that the
resistivity change in the direction that is perpendicular to the survey line on either side is less
influential. 2-D resistivity surveys are the most practical economic compromise for obtaining
accurate results and keeping the survey costs down.
Typical 1-D resistivity sounding surveys usually involve about 10 to 20 readings, while 2-
D imaging surveys involve about 100 to 1000s of measurements, hence denser the data
acquisition, better is the resolution. In many geological situations, 2-D electrical imaging
survey can give more useful information that are complementary to the information obtained
by other geophysical methods. Two-dimensional electrical surveys should be used in
conjunction with seismic or GPR surveys as they provide complementary information support
about the subsurface. The result of a 2D ERT (Electrical Resistivity Tomography) is a pseudo-
depth representation of the area investigated in terms of its resistivity variation as shown in the
Figure.3 (Inset).
e-Journal Earth Science India www.earthscienceindia.info Popular Issue, January, 2011
6 | P a g e
Fig.3: 2D resistivity image showing different subsurface geology (inset; image-1 and image-2)
and a 2D Geological section prepared based resistivity results and borehole lithologs-
case study from sedimentary terrain of Cuddapah basin.
In areas of investigations, similar to the VES sounding, the orientation of profile is kept
perpendicular to any anomalous buried / exposed body to map its dimensions. Also the depth
of required information depends on suitable protocol (schlumberger, wenner, dipole etc.,) and
length of profile is selected. As discussed earlier, the data acquisition is basically a
combination of VES and profiling, hence it is possible to deduce true resistivity variation with
depth pertaining to a given electrode position. If a number of 2D resistivity investigations are
executed in a grid format, resistivity data pertaining to desired electrode position and depth can
be integrated to form a 2D or 3D section of the area of study as shown in Figure.3. This kind of
2D perception helps in deciding appropriate sites for structure making like reservoir dams,
check dams etc.
� Shear wave mapping (MASW):
Surface seismic techniques used in groundwater studies are largely restricted to either
seismic refraction or seismic reflection method. These methods usually measures the travel
time of acoustic waves propagating through the subsurface and its attenuation pertaining to the
subsurface geology is reflected. In the refraction method, the travel time of waves refracted
along an acoustic interface is measured, were as in the reflection method the travel-time of a
wave which reflects off an interface is measured. These waves are detected at the surface by
small receivers or sensors (geophones), which transform mechanical energy (ground vibration)
into electrical voltages. The voltages are relayed along cables to the seismograph, which
records the voltage output versus time, similar to an oscilloscope.
There are many types of seismic sources used to impart sound into the earth. The most
common type of source in shallow seismic investigations is a sledgehammer and strike plate.
The types of sources used are dependent on the signal versus noise ratio in the survey area.
Noise can come from vehicular traffic, people or animals walking near the geophones,
electrical current in the ground (electromagnetic interference which affects the geophone
e-Journal Earth Science India www.earthscienceindia.info Popular Issue, January, 2011
7 | P a g e
cables), low-flying aircraft, or any sound source. Generally, the noise can be overcome by
using a larger source, which effectively increases the signal or by proper filtering technique.
Survey design is site dependent and must be planned so that the geometry of the geophone
spread will allow the target to be resolved. The spacing of the geophone stations within the
spread can vary from several feet to tens of feet, depending on the depth of the geologic layer
and required resolution. A closer spacing of geophones within the spread is chosen when a
higher resolution of a shallow target is the objective. Shot points should extend along the entire
traverse length and show a redundant sampling of the resolved interfaces. Small differences in
horizontal displacements can cause a considerable change in the interpretation. The geophone
stations should lie along as straight a line as possible (for profile data). Deviations from a
straight path will result in ray path projection inaccuracies. Shot point and geophone elevations
are necessary in case were there is very high undulation or rugged terrain. The interpreted
results are presented either as a 2D cross section or as 3D block model as shown in Figure.4,
when a grid of data is collected within a given study area.
Fig.4: Representation of Shear wave velocity map in sedimentary and hard rock terrain and its
presentation in 2D cross section and 3D block image.
CONCLUSION
In this paper the potentiality of geophysics as an exploratory tool for monitoring the
conditions, and site evaluation for mine tailings dumping is briefly elaborated. Evaluation of a
selected site area for dumping mining waste can be done prior to its commissioning through
near surface geophysical investigations in integration with drilling program, hydrological
investigations and geological mapping studies. Presence of natural barriers like dykes, conduits
like faults etc., can be mapped and also its dimensions can be quantified. This helps in better
understanding the site and to take respective precautionary measures to avoid groundwater /
environmental pollution. It is always necessary to have periodic monitoring of tailings pond
and dam, once it is in active state and also assess the transient variation in the geophysical
signature/ parameters with natural inputs like rainwater, tailings etc. Deeper investigations
through near surface geophysical tools over the tailings can be carried out to monitor in case
e-Journal Earth Science India www.earthscienceindia.info Popular Issue, January, 2011
8 | P a g e
there is any leakage to the groundwater system. Such an integrated approach will be a
beneficial tool for planners and engineers to make suitable designs as per recommendations.
Acknowledgement: The authors are highly thankful to Dr.Y.J.Bhaskar Rao, Director of National Geophysical
Research Institute for his encouragement and support towards the submission of this article for publication. The
authors also like to extend their gratitude towards Dr.R.Rangarajan, for his kind support and encouragement.
BIBILIOGRAPHY
Abraheem AM, Hamburger MW, Bayless ER, Krothe NC (1990) A study of acid mine drainage using earth
resistivity measurements. Groundwater 28:361–368.
Benckert A (2003) Tailings dam safety in Sweden. Proc. International Symposium on Major Challenges in
Tailings Dams 71st Annual Meeting of the International Commission on Large Dams (ICOLD), 15 June
2003, Montreal, Canada: 35–45.
Buero für Geophysik Lorenz: Wiederholungsmessungen mit der Methode der Induzierten Polarisation zur
Erkundung von Bergbau- und Schlackehalden. Report to the Federal Institute for Geosciences and Natural
Resources (BGR), Hannover, 1999 (unpublished).
Campbell, D.L., and Fitterman, D.V.: Geoelectrical methods for investigating mine dumps, in: Proceedings from
the Fifth International Conference on Acid Rock Drainage (ICARD2000), Denver, Colorado, May21-24,
2000. Society for Mining, Metallurgy, and Exploration, Inc., v. II, p.1513-1523.
ICOLD International Committee of Large Dams: Tailings Dams Risk of Dangerous Occurrences. Lessons Learnt
from Practical Experiences. Bulletin Nr. 121, 144 p., Paris, 2001.
Loke, M.H., 2000, Electrical imaging surveys for environmental and engineering studies: A practical guide to 2-D
and 3-D surveys, 61 pp.
Meggyes, T.: Reducing the risks for tailing facilities. Proceedings of Consoil 2003, in press.
Niederleithinger, E., Grissemann, Ch., & Rammlmair, D.: SIP Geophysical Measurements on Slag Heaps: aNew
Way to Get Information about Subsurface Structures and Petrophysical Parameters. Proceedings ofICAM
(International Conference on Applied Mineralogy), Göttingen, 2000.
Niederleithinger, E.: Spectral Induced Polarization - a tool for nondestructive testing of soils and building
materials. Proceedings of NDT-CE 2003 (this volume), Berlin, 2003.
Palacky, G.J., 1988, Resistivity characteristics of geologic targets: in Electromagnetic Methods in Applied
Geophysics, Vol. 1: Theory, Investigation in Geophysics, No. 3, Nabighian, M.N. (ed.), Society of
Exploration Geophysicists, 53–129.
“TAILSAFE: Investigation and Improvement of Tailings Facilities”, E. Niederleithinger et. Al., International
Symposium (NDT-CE 2003).
Ward, S.H., 1988, “The resistivity and induced polarization methods: in Proceedings of the Symposium on the
Application of Geophysics to Engineering and Environmental Problems”, 109–250.
About the authors
Dr. Rolland Andrade joined National Geophysical Research Institute, Hyderabad, in 2002 as a research
fellow under the RGNDWM Project. Since then he was associated in successful completion of several
projects both In-house and sponsored. His main area of specialization is water management and exploration.
Currently he is working on Geohydrological characterization of nuclear waste dumping sites.
E-mail: [email protected]
Mr. Karunakar Goud joined National Geophysical Research Institute, Hyderabad as a research fellow in
groundwater division under the sponsored project entitled “Integrated approach for hydro geological
management at Tummalapalle uranium project area in Cuddapah district, Andhra Pradesh”. He was also
associated in network project “Sustainable development and management of water resources in different
problematic terrain” in Ananthapur District, Andhra Pradesh
E-mail: [email protected]