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Integrated Geophysical Approach for Mining Applications


Integrated Geophysical Approach for Mining Applications

Dr SANJAY RANA,
Director, PARSAN Overseas Private Limited, New Delhi, INDIA
(e-mail of corresponding author: sanjay@parsan.biz)

ABSTRACT:

Geophysical methods have great potential to solve various mining related problems. Due to their non-destructive nature and quick application, using these methods results in huge savings in terms of time and money. Newer deposits are deeper and exploring those using conventional means of drilling is becoming increasing expensive and time consuming. Geophysical methods are also being routinely used as a reconnaissance tool to quickly determine potential of a new site before getting into lease agreements. 

Apart from typical application of mineral exploration, modern techniques can also be utilized to make operations of mines markedly safer; it is possible, through geo-physical studies of soil foundation, to provide the parameters that facilitate the installation of infrastructure (access roads, processing plants etc.) and allow blasting to be planned for maximum efficiency and output. Important aspects of slope stability and rippability assessment are also addressed through geophysical techniques.

The state-of-the-art subsurface geophysical investigations are helpful towards minimizing involvement of the conventional direct exploration methods, aiding in accelerated and economical development of the mining projects.

Integrated application of various geophysical techniques help determine various properties of subsurface like ore body extent, depth, shape etc. Present paper discussed integrated geophysical approach for mining applications with case studies.


INTRODUCTION:

Sub-surface imaging by means of geophysical survey is a powerful tool for site assessment and mapping which historically has been under-utilized world-over. Continuing improvements in survey equipment performance and automation have made large area surveys with a high data sample density possible. Advances in processing and imaging software have made it possible to detect, display, and interpret small geological features with great accuracy.

Some of the unique advantages of geophysical survey: 

Geophysical methods are quick to apply, saving in terms of time and money.
Light and portable equipment allows access to remotest of sites.
Provides information on critical geological features like faults/ fractures/ weak zones/ shear zones, not visible from surface information 
Large areas mapped quickly and inexpensively
Researchers can assess site conditions, and target specific locations for detailed investigations by drilling, while avoiding others.
Geophysical methods can quickly produce subsurface geology avoiding delays during execution due to meeting the unexpected. 
Shear wave profiles can be quickly obtained for ascertaining liquefaction potential and earthquake response.
Buried utilities, pipes and cables, can be detected before drilling/ excavation, avoiding damage to utilities and costly accidents.
Concrete structures can be quickly scanned to ascertain integrity and detect defects like voids, honeycombing etc. 

A detailed survey plan is worked out in consultation with the client, to address to critical issues. The right combination of various available tools should be chosen to resolve the problem in unique manner.

Surveys which are both successful and cost-effective must satisfy a number of basic requirements. They must be implemented using appropriate and properly configured survey equipment. The data sampling strategy and density must be matched to the spatial resolution and statistical requirements of the survey. Monitoring the quality of data while in the field is mandatory, and post survey data processing must be both appropriate and mathematically sound.
 
There exist various geophysical techniques to address to various problems in mining sector:

Seismic Refraction Survey & Shear Wave Surveys:
o For Rippability Assessment
o Determination of stratigraphy
o Slope stability studies
Seismic Reflection Surveys
o For coal surveys
Gravity & Magnetic Surveys
o For Mineral Exploration
Electrical Imaging
o For ore body mapping
o For water table mapping
o For environmental studies
Ground Penetrating Radar
o For environmental mapping
o For mapping shallow contacts
o For mapping pockets in soil/ overburden
o For Rippability studies

The state-of-the-art subsurface geophysical investigations are helpful towards minimizing involvement of the conventional direct exploration methods, aiding in accelerated and economical development of the mining projects.

It is an established fact that a single geophysical method cannot resolve all the problems associated with subsurface investigations. As an example, seismic refraction cannot  see low velocity zones under rock interface, which might be present under certain geological conditions. It is therefore of utmost importance to focus on integrated application of various geophysical techniques to determine various properties of subsurface.


                                                                                                       


GRAVITY SURVEYS:

Gravimetry is a potential field technique which measures variations in the Earth's gravitational field.  These variations are caused by density contrasts in the near surface rock and sediment.  Gravimetric surveys are carried out using extremely sensitive instruments capable of measuring tiny variations in the gravitational field.  These surveys are always carried out in conjunction with a precise topographic survey, to enable terrain corrections to be carried out. Typical applications are:

Regional geological mapping;
Oil and gas exploration;
Mineral exploration;
Sediment thickness studies;
Archaeological surveys;
Void detection.

The technique is extensively used for mineral exploration, especially for ore bodies having a high difference in density compared to the host rock. 

 
 
Scintrex CG-5 gravimeter




                                                              
                                                                                                           Example Gravity Contour Map 


Gravity measurements define anomalous density within the Earth; in most cases, ground-based gravimeters are used to precisely measure variations in the gravity field at different points. Gravity anomalies are computed by subtracting a regional field from the measured field, which result in gravitational anomalies that correlate with source body density variations. Positive gravity anomalies are associated with shallow high density bodies, whereas gravity lows are associated with shallow low density bodies. Thus, deposits of high-density chromite, hematite, and barite yield gravity highs, whereas deposits of low-density halite, weathered kimberlite, and diatomaceous earth yield gravity lows. The gravity method also enables a prediction of the total anomalous mass (ore tonnage) responsible for an anomaly. Gravity and magnetic (discussed below) methods detect only lateral contrasts in density or magnetization, respectively. In contrast, electrical and seismic methods can detect vertical, as well as lateral, contrasts of resistivity and velocity or reflectivity. 


MAGNETIC SURVEYS:

The magnetic method exploits small variations in magnetic mineralogy (magnetic iron and iron-titanium oxide minerals, including magnetite, titanomagnetite, titanomaghemite, and titanohematite, and some iron sulfide minerals, including pyrrhotite and greigite) among rocks. Measurements are made using fluxgate, proton-precession, Overhauser, and optical absorption magnetometers. In most cases, total-magnetic field data are acquired; vector measurements are made in some instances. Magnetic rocks contain various combinations of induced and remnant magnetization that perturb the Earth's primary field (Reynolds and others, 1990). The magnitudes of both induced and remnant magnetization depend on the quantity, composition, and size of magnetic-mineral grains. Magnetic anomalies may be related to primary igneous or sedimentary processes that establish the magnetic mineralogy, or they may be related to secondary alteration that either introduces or removes magnetic minerals. In mineral exploration and its geo-environmental considerations, the secondary effects in rocks that host ore deposits associated with hydrothermal systems are important (Hanna, 1969; Criss and Champion, 1984) and magnetic surveys may outline zones of fossil hydrothermal activity. Because rock alteration can effect a change in bulk density as well as magnetization, magnetic anomalies, when corrected for magnetization direction, sometimes coincide with gravity anomalies. Magnetic exploration may directly detect some iron ore deposits (magnetite or banded iron formation), and magnetic methods often are an useful for deducing subsurface lithology and structure that may indirectly aid identification of mineralized rock, patterns of effluent flow, and extent of permissive terraces and (or) favorable tracts for deposits beneath surface cover. Geo-environmental applications may also include identification of magnetic minerals associated with ore or waste rock from which hazardous materials may be released. Such associations permit the indirect identification of hazardous materials such as those present in many nickel-copper or serpentine hosted asbestos deposits.

Following are examples of magnetic survey maps obtained during investigations for iron ore:

    
 

RESISTIVITY IMAGING:

2D Resistivity Imaging uses an array of electrodes (typically 64) connected by multicore cable to provide a linear depth profile, or pseudosection, of the variation in resistivity both along the survey line and with depth. Switching of the current and potential electrode pairs is done automatically using a laptop computer and relay box. The computer initially keeps the spacing between the electrodes fixed and moves the pairs along the line until the last electrode is reached. The spacing is then increased and the process repeated in order to provide an increased depth of investigation.
The technique is extremely useful for investigations of important sites to get information on weak zones or buried channels, under the rock interface, which goes undetected in seismic refraction, which terminated at rock interface. Resistivity imaging can also be effectively used to determine rock profile along dam axis across high current shallow rivers where deployment of hydrophones is not possible restricting use of seismic refraction. For deep penetration seismic refraction techniques requires use of explosives, which are not always feasible to deploy especially in sensitive areas. In such cases resistivity imaging can be effectively used to get detailed information of deeper layers.

Following is an example resistivity imaging carried out to delineate at manganese body. 

                                        
                                                    

SEISMIC REFRACTION SURVEYS

Seismic technique is one of the most developed geophysical techniques, providing vital information on subsurface, crucial for most of the engineering projects. Seismic refraction consists of recording the length of time taken for an artificially provoked surface vibration to propagate through the earth. By processing the data recorded at various sensors absolute velocities, velocity contrasts and the depths of the underlying layers are determined. These results give information about the nature and thickness of overburden (alluvium deposits), surface of bedrock, the depth of weathering zones in the rock mass, location of geological boundaries and identifies faults or weak zones, scale and width, etc.

Applications:

Bedrock profile, rock quality and depth. 
Thickness of overburden 
Fractures and weak zones 
Topography of ground water 
Rippability assessment in mines 
Slope stability studies 
Pipeline route studies 

Following is an examples from seismic refraction:

                                                                

Based on the knowledge of seismic velocities, rippability can be quickly determined using standard charts available with equipment manufacturers. Rippability is the ease with which soil or rock can be mechanically excavated. According to Bieniawski, rippability of rock is assessed by numerous parameters including uniaxial strength, degree of weathering, abrasiveness, and spacing of discontinuities.  Nevertheless, seismic refraction has historically been the geophysical method utilized to indirectly predetermine the degree of rippability. Ripping is typically performed by tractor-mounted equipment.  The size of the tractor (dozer) is determined by the ripping assessment of the rock.  The hardness and competency of each individual material will determine the ease of rippability.  Rock that is too hard to be ripped is fragmented with explosives. Rocks can be classified into three categories: igneous, sedimentary, and metamorphic.  Igneous rocks, formed by cooling of molten masses originating within the earth, are the most difficult to rip.  This is partly because they lack lines of weakness such as stratification or cleavage planes.  Metamorphic rocks are generally defined as any rocks derived from pre-existing rocks by mineralogical, chemical, and/or structural changes, in response to marked changes in temperature, pressure, shearing stress, and chemical environment. Common metamorphic rocks are gneiss, quartzite, schist, and slate.  These rocks vary in rippability, depending on their degree of stratification or foliation.  Sedimentary rocks consist of material derived from the destruction of preexisting rocks.  Water action is responsible for the largest percentage of sedimentary rocks, although some are formed by wind or glacial ice. Sedimentary rocks are generally the most rippable. Few or no problems are found with hardpan, clays, shales, or sandstones.  Likewise, any highly stratified or laminated rocks, and rocks with extensive fracturing are usually rippable. 
The above rippability criteria are presented by The Caterpillar Company in a book titled Handbook of Ripping.

            
                                             

A well conducted seismic refraction provides a quick assessment of rippability.

OTHER GEOPHYSICAL METHODS :

There are various other geophysical methods available for mining applications, which are not detailed in this paper to keep it shot and concise. Following is a brief listing of these methods:

Self-Potential (SP) :  for sulphides, graphite, formation contacts, detection of land slides, seepage etc.

Electro-Magnetic (EM) :  for sulphides, some oxides, mapping of conductive formations & geological structures, ground water.

Induced Polarization (IP) :  Sulphides, some oxides, particularly disseminated ores like porphyry copper deposits, ground water & environmental problems

Seismic Reflection: Mapping of coal beds.

Radiometric :  Radioactive minerals, mapping of geological structures and formations


CONCLUSIONS

A well designed integrated geophysical investigation program can remove most of ambiguities generally known to be associated with geophysical investigation campaigns for mining applications. The state-of-the-art subsurface geophysical investigations are helpful towards minimizing involvement of the conventional direct exploration methods, aiding in accelerated and economical development of the mining projects. The survey methodology and tools to be used should be decided in consultation with the client and depending on site conditions and objectives of survey.