UNIT 14 
                                      EXPLORATION METHODS 
             
           Structure______________________________________________ 
           14.1     Introduction                                  Self-Potential Method 
                    Expected Learning Outcomes                    Telluric Current Method 
           14.2       Exploration Methods                                Magnetotelluric Method 
           14.3     Surface Exploration                                  Resistivity Method 
                    Geological Exploration                               Induced Potential Method 
                    Photogeology, Remote Sensing                         Electromagnetic Method 
                    and Geographic Information                           Seismic Method 
                    System 
                    Geochemical Exploration                               Pitting, Trenching, Drilling and     
                                                                    Borehole Logging 
                     Geobotanical Exploration               14.5  Activity 
                     Biogeochemical Survey                 14.6  Summary 
           14.4     Subsurface Exploration                 14.7  Terminal Questions 
                    Methods 
                    Gravity Method                         14.8  References 
                    Magnetic Method                        14.9   Further/Suggested Readings 
                    Electrical Method                       14.10  Answers
            
            
           14.1   INTRODUCTION 
           In the previous unit, you have studied about basic principles and stages of mineral exploration. 
           Mineral exploration has its own characteristic and is dominated by “High Risk-High Reward” 
           proposition as the occurrence of mineral deposit is not uniform worldwide. In this unit, you will learn 
           about different mineral exploration methods that involve various activities to identify/ quantify and 
           extract economically hidden deposits to the surface. The physicochemical properties of the minerals 
           like specific gravity, electrical conductivity, magnetic susceptibility, radioactivity, propagation of 
           seismic waves and mode of occurrence of minerals enables in locating ore deposits of economic 
           interest.  
       Block 4                             Mineral Exploration and Mineral Economics 
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                      Expected Learning Outcomes_______________________ 
                      After reading this unit you should be able to:  
                      ❖ outline different exploration methods for mineral exploration; 
                      ❖ learn principles and practices of exploration methods;  
                      ❖ discuss their importance in various stages of mineral exploration; and  
                      ❖ explain limitations of each exploration method. 
                      14.2   EXPLORATION METHODS 
                      In the previous Unit 13, you have been introduced to prospecting and 
                      exploration. Now, let us discuss about exploration methods. Different types of 
                      exploration methods are essential to find out and estimate volume of a 
                      commercially viable mineral concentration as it involves high capital-intensive 
                      operations. The exploration methods are divided into two main groups, namely: 
                      A)  Surface exploration; and  
                      B)  Subsurface exploration.  
                      Involvement of different surface and subsurface exploration methods during 
                      various stages of mineral exploration projects is shown in Figure 14.1. Surface 
                      exploration methods includes: geological, geochemical, geobotanical, 
                      photogeology & remote sensing. Subsurface exploration methods include: 
                      gravity, magnetic, electrical, self-potential, telluric current, magnetotelluric, 
                      resistivity, induced potential, electromagnetic, seismic. We will discuss the 
                      surface and subsurface exploration methods in details in the following sections. 
                                                                          
                                Fig. 14.1: Types of mineral exploration methods. 
                      14.3   Surface Exploration 
                      The surface exploration method includes preparation of geological maps based 
                      on data collected through field survey (including elevation data). Incorporation 
                      of aerial photos, satellite imagery, geochemical and vegetation data into these 
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             Unit 14                                                                                         Exploration Methods 
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            maps provide useful details regarding rock outcrops and their boundaries, 
            vegetation, geomorphology, lithology and lithological variations of the area 
            under investigation. Various surface exploration methods viz. geological, 
            geochemical, geobotanical, photogeology and remote sensing are carried out 
            during preliminary & detailed survey.  
            Let us discuss them in the following sections.  
            14.3.1   Geological Exploration 
            The occurrence of ore bodies are mainly controlled by geologic setting with 
            specific structural and tectonic setting and their spatial distribution over a wide 
            area. 
            Selection of most prospective area for exploration requires a sound geological 
            knowledge which includes structure of the area, age of the rocks, rock types 
            and basin modelling etc. Good acquaintance is required to make perfect 
            predictions and draw parallels between an established ore deposit and the 
            unknown potential mineralisation sought after exploration. The broad activities 
            involved in geological exploration are:  
            •    to prepare a field database from different sources; and  
            •    plot in geological base map.  
            The detailed activities include: 
            i)  Review of existing data and field work: The exploration begins with the 
                regional appraisal of large area through available geological information, 
                review of literature, earlier published maps, survey and reports from different 
                sources. After preliminary findings from the review, the exploration team 
                visits the field to gather information related to geological, mineralisation, host 
                rock and hunting for the surface guides to incorporate and prepare the 
                geological base map for next phase of exploration. 
            ii)  Geological / Surface Guides: During geological traverse, the exploration 
                team explores the existence of various types of surface signatures of mineral 
                deposits called geological or surface guides. You have read about 
                geological / surface guides in the previous Unit 13.  
            iii) Study of Topographic Maps: A topographical map (toposheets or 
                topomaps) commonly shows quantitative and precise representation of 
                physical and cultural features of the Earth’s surface. They represent a 
                symbolic picture of the natural and man-made features of small areas which 
                are drawn at relatively large scale (Fig.14.2). As a result, these maps serve 
                as indispensable tool for city and regional planners, natural resources 
                managers, and for engineering, environmental and geological studies. Earlier 
                a topographic survey is conducted with tape and compass. Now a days, it is 
                being done using electronic total stations to capture 3D observation data on 
                the field to prepare digital terrain model that is transformed into a high quality 
                geological map. 
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       Block 4                             Mineral Exploration and Mineral Economics 
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                      Fig. 14.2: Topographic maps (a) 1:250000; (b) 1: 50000 and (c) 1: 25000 scales. 
                            (Source: www.surveyofindia.gov.in) 
                      The activities carried out during geological exploration can be subdivided into 
                      three main categories namely regional scale, belt scale and deposit scale. 
                       •  Regional Scale: Surface features, like broad geological contacts, 
                         existence of shear zones and weathering features are regionally plotted in 
                         the base map. Study of aerial photographs, satellite images, airborne 
                         geophysical data acquisition, wide spaced soil sample, grab and chip 
                         sampling of surface exposures for broad geochemical surveys and 
                         weathering profiles are collected and recorded. 
                       •  Mineral Belt Scale: Geological base map survey is scaled up to 1:25000 
                         or 1:10000 scales using theodolite and GPS survey instruments. Detailed 
                         study of geological structural setting such as stratigraphy and 
                         identification of host rock, surface shows of mineralisation, existence of 
                         ancient mining and smelting signatures are recorded and plotted in the 
                         base map. Additionally, ground geophysics, geochemistry, pitting, 
                         trenching data are acquired for data integration.  
                       •  Mineral Deposit Scale: After integrated study of the area, maximum 
                         information of the mineral deposit is acquired by triangulation station 
                         survey. Thereafter, base map in larger scale of 1:5000 or 1:1000 is 
                         prepared. Additionally, closely spaced surface drilling, trenches, pits and 
                         boreholes is carried out along and across the mineral body to delineate 
                         and extension of the mineral deposit (Fig. 14.3). 
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