Geophysical Study of Isfahan University of

Technology’s Geophysical Site, Isfahan, Iran

Pezhman Rasekh

April 9, 2015

Contents

1 INTRODUCTION 21.1 Purpose and Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 21.2 Geophysical Profile Lines and their Settings . . . . . . . . . . . . 31.3 Magnetic Declination of the Study Area . . . . . . . . . . . . . . 31.4 Studied Geophysical Methods . . . . . . . . . . . . . . . . . . . . 4

1.4.1 Field Studies . . . . . . . . . . . . . . . . . . . . . . . . . 41.4.2 Analytical and Sofware Requirements of the Study . . . . 4

2 Raw Geophysical Data 92.1 Electromagnetic Data . . . . . . . . . . . . . . . . . . . . . . . . 92.2 VLF Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.3 IP/RS Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3 Data Processing 183.1 IP/RS Data Processing and Eliminate the Outlier Data . . . . . 183.2 Electromagnetic and VLF Data Processing and Eliminate the

Outlier Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183.3 Choosing the Smoothest Total Field/Location VLF Diagram . . 193.4 Importing IP/RS Data to RES2DINV Software and Creating the

Subsequent Geophysical Plots . . . . . . . . . . . . . . . . . . . . 193.5 Importing Electromagnetic Data to Potent Software . . . . . . . 203.6 Importing VLF Data to VLF Filtering Software and Creating the

Consequent Resulted Plots . . . . . . . . . . . . . . . . . . . . . 21

4 Exported Plans of the Potent Software 29

5 Conclusion 355.1 Revealing the Probable Geophysical Objects . . . . . . . . . . . . 35

6 References 36

7 Aknowledgements 37

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Chapter 1

INTRODUCTION

1.1 Purpose and Scope

In general, geophysical surveying is the study of the Earth by the quantitativeobservation of its physical properties, in particular by seismic, electromagnetic,radioactivity, Induced Polarization (IP/RS), Resistivity (RS) and potential field(such as gravity and magnetism) methods. Exploration geophysics can be usedto directly detect the target style of mineralization, via direct measurement ofits physical properties.

The first objective of this geophysical surveying is to delineate the potentialof some buried conductive objects by electromagnetic, VLF and IP/RS geophys-ical surveying in Isfahan University of Technology’s geophysical site which islocated just next to faculty of mining engineering. We are supposed to interpretwhole the geophysical aspects of this site, process the obtained geophysical dataand finally, propose the probable locations and shapes of the buried anomalies.A schematic map of the study area is clearly visible in Fig. 1.1.

Electromagnetic surveys have been one of the geophysical methods of sur-veying for many years. EM conductivity surveys measure ground conductivityby the process of electromagnetic induction. In this area, the electromagneticsurveying was done with Scintrex Proton Magnetometer which was be able tomeasure both VLF and electromagnetic data simultaneously. The resulted datain this procedure has been processed by VLF filtering and Potent softwares andthe exported plots will be shown in the following chapters. A picture of theScintrex Proton Magnetometer is shown in Fig. 1.1.

In IP/RS surveying, electrical properties of the ground can be calculatedfrom comparing the transmitted signal to the received signal. ground resis-tivity the ability to conduct an electrical current affects the strength of thereceived signal. A change in IP the ability of ground material to polarize atinterfaces affects the shape or timing of the received waveform. When plotted,these properties provide information about faults, fractures, geologic structures,

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CHAPTER 1. INTRODUCTION 3

Figure 1.1: Google Map of the Study Area-the yellow rectangle indicates thegeophysical site of Isfahan University of Technology.

mineralization, and groundwater porosity. A picture of a di pole-di pole arreyconfiguration is visible in Fig.1.3. In this method the geophysicist tries to lo-cate portions of the earth where resistivity decreases as the frequency of appliedcurrent is increased. The induced electrical polarization method is widely usedin exploration for ore bodies, principally of disseminated sulfides. In order toIn this area, the IP/RS surveying was done with a digital Scintrex time domainIP/RS receiver and the resulted data has been processed by the RES2DINV soft-ware. A picture of the IP/RS receiver is shown in Fig. 1.4 and the procedureof transmiting signal is visible in Fig. 1.5.

1.2 Geophysical Profile Lines and their Settings

The study area consists of 20 separate profiles but we were assigned to studyonly five of them. (Lines: 20,25, 30, 35 and 40). A schematic map pf the profilelines is clearly visible in Fig. 1.6.

1.3 Magnetic Declination of the Study Area

Magnetic declination or variation is the angle on the horizontal plane betweenmagnetic north (the direction the north end of a compass needle points, cor-responding to the direction of the Earth’s magnetic field lines) and true north(the direction along a meridian towards the geographic North Pole). Accordingto national geophysical data center (NGDC), magnetic delineation of Isfahan

CHAPTER 1. INTRODUCTION 4

Figure 1.2: Scintrex Proton Magnetometer

University of Technology is changing is changing by 11 degrees E per year. Fig.(1.7).

1.4 Studied Geophysical Methods

1.4.1 Field Studies

Field studies were done in 3 days during the November of 2014 and includedesigning and conducting the electromagnetic, VLF and dipole-dipole IP/RSsurveys, Fig. 1.3. Consequently, numeric results were collected as separate datasets from different profiles.

1.4.2 Analytical and Sofware Requirements of the Study

Laboratory work mainly consisted of detailed data interpretation and geophys-ical modelling by the relevant softwares such as: Potent, RES2DINV and VLFFiltering Softwares.

CHAPTER 1. INTRODUCTION 5

Figure 1.3: dipole-dipole array configuration: The dipole-dipole electrode arrayconsists of two sets of electrodes, the current (source) and potential (receiver)electrodes. A dipole is a paired electrode set with the electrodes located rela-tively close to one another; if the electrode pair is widely spaced it is referredto as a bipole. The convention for a dipole-dipole electrode array is to maintainan equal distance for both the current and the potential electrodes (spacing =a), with the distance between the current and potential electrodes as an integermultiple of a. The electrodes do not need to be located along a common surveyline.

CHAPTER 1. INTRODUCTION 6

Figure 1.4: Digital Time Domain IP/RS Receiver

Figure 1.5: Transmite and Recieve Singnals in IP/RS Surveying

CHAPTER 1. INTRODUCTION 7

Figure 1.6: Geophysical Profile Lines and their Settings. As it is clearly obviousin the picture, the five profiles of 20, 25, 30, 35 and 40 have been studied by theparticipations of this geophysical surveying.

CHAPTER 1. INTRODUCTION 8

Figure 1.7: Magnetic Declination of Isfahan University of Technology

Chapter 2

Raw Geophysical Data

2.1 Electromagnetic Data

The five raw magnet data sets of different profiles are presented as diverse plotsin the following pages.

2.2 VLF Data

The five raw VLF data sets of different profiles are presented as diverse plots inthe following pages.

2.3 IP/RS Data

The data sets of IP/RS survey which is resulted from the 20th profile is visiblein the following pages.

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CHAPTER 2. RAW GEOPHYSICAL DATA 10

Figure 2.1: Electromagnetic Data of Profile Number: 20

CHAPTER 2. RAW GEOPHYSICAL DATA 11

Figure 2.2: Electromagnetic Data of Profile Number: 30

CHAPTER 2. RAW GEOPHYSICAL DATA 12

Figure 2.3: Electromagnetic Data of Profile Number: 35

CHAPTER 2. RAW GEOPHYSICAL DATA 13

Figure 2.4: Electromagnetic Data of Profile Number: 40

CHAPTER 2. RAW GEOPHYSICAL DATA 14

Figure 2.5: VLF Data of Profile Number: 20

Figure 2.6: VLF Data of Profile Number: 25

CHAPTER 2. RAW GEOPHYSICAL DATA 15

Figure 2.7: VLF Data of Profile Number: 30

Figure 2.8: VLF Data of Profile Number: 35

CHAPTER 2. RAW GEOPHYSICAL DATA 16

Figure 2.9: VLF Data of Profile Number: 40

CHAPTER 2. RAW GEOPHYSICAL DATA 17

Figure 2.10: IP/RS Data of Profile Number: 20

Chapter 3

Data Processing

3.1 IP/RS Data Processing and Eliminate theOutlier Data

Removing outliers can cause the data to become more normal and it wouldcause the consequent reliable results. According to the variation of the resulteddata from IP/RS surveying and the recorded amounts of geophysical noises,we can conclude that whole the data are reliable and there is not any need toeliminate any of the data. Considering the resulted amounts of voltage andIntensit de Courant, we can calculate the specific resistance of the study areain Dipole-Dipole configuration, by the following equations:

K = (PI ∗ Spacing ∗ n ∗ (n + 1) ∗ (n + 2)) (3.1)

Thus, the Specific Resistivity will have the following form of:

Resistivity = (K ∗ V/I) (3.2)

The resulted resistivity data from this procedure was inserted to an Excelfile to be processed in RES2DINV software.

3.2 Electromagnetic and VLF Data Processingand Eliminate the Outlier Data

Assuming the variation of data and recorded measurment noises, some of theresulted data was eliminated. In order to have more accurate data, we haveto do some corrections on the resulted data. Such as: Day/Night correctionand IGRF correction. In terms of Day/Night correction, the recorded Tie Pointdata must be processed through the statistical procedure which is enclosed inthe attached Excel files. To do IGRF correction, we have two choices! Thefirst one is to calculate the amount of changing field by softwares as Geomagics

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CHAPTER 3. DATA PROCESSING 19

and the second one is to calculate it online. In our study, the online resultedamount of filed is: 46960 nt which is visible in Fig.(3.1). By deducing thisamount from the total field, IGRF correction is supposed to be done. Since theKarous and Hjelt filter is characterized by a large degree of accuracy in depthestimation, The synthetic VLF data is filtered by the Karous and Hjelt filter at5m of interval distance between measuring points.

Figure 3.1: IGRF Correction of the Study Area.

3.3 Choosing the Smoothest Total Field/LocationVLF Diagram

In the VLF surveying, the measurments were done by 3 different amounts offrecuency (23.4, 19.2 and 24 Hz). For each amount of frecuency, the resultedplots of filed amounts per location have been presented in the attached Excel file.In order to have the most accurate data, it’s a good idea to choose the smoothestone. In this study, the smoothest plot was resulted from the fecuency amountof 23.4 Hz. Thus, data which resulted from this frecuency is the best one to beprocessed in the VLF filtering software. Corresponding plots for each profile isgoing to be shown in the following pages.

3.4 Importing IP/RS Data to RES2DINV Soft-ware and Creating the Subsequent Geophys-ical Plots

The processed and corrected data (Fig. 2.10) are ready to be imported inRES2DINV software. Thus, the following plots will result from RES2DINVsoftware and these plots are going to be interpreted in the following chapters.

CHAPTER 3. DATA PROCESSING 20

Figure 3.2: Total Field/Location VLF Diagram (Frq: 23.4 Hz)

3.5 Importing Electromagnetic Data to PotentSoftware

Potent can import observed data from ASCII files (such as Geosoft XYZ formatfiles) and from Geosoft or ERMapper grids. Typical data sources include:

• Airborne surveys

• Regional gravity surveys

• Detailed ground surveys

• Down-hole surveys

• Any combination of the above

All datasets can be multi-component. Different types of data can be combinedin the one workspace, allowing true multi-component, multi-dataset forwardand inversion modelling. In this study, the processed and corrected OBS elec-tromagnetic file is ready to be imported to Potent software.(Fig. 3.10) Thus,the following plots will result from Potent software and these plots are going tobe interpreted in the following chapters.

CHAPTER 3. DATA PROCESSING 21

Figure 3.3: Total Field/Location VLF Diagram (Frq: 23.4 Hz)

3.6 Importing VLF Data to VLF Filtering Soft-ware and Creating the Consequent ResultedPlots

The processed and corrected data which are corresponding to the smoothestplot are ready to be imported in VLF filtering software. Hence, the followingplots will result from VLF filtering software and these plots are going to beinterpreted in the following chapters. The Karous and Hjelt filter has beenlong time used as a qualitative interpretation of VLF-EM data. It is deriveddirectly from the concept of magnetic fields associated with the current flow inthe subsurface and resulted in a 2-D cross section showing the current densitydistribution at different depths. Practically, as the distance between measuringpoints increases, the total depth of the 2-D current density distribution sectionincreases. Theoretically, the common guide to estimate the depth of penetrationof an electromagnetic wave is the skin depth, which depends on the frequency ofthe electromagnetic wave and the conductivity of the host geological material,regardless of the distance interval between measuring points.

CHAPTER 3. DATA PROCESSING 22

Figure 3.4: Total Field/Location VLF Diagram (Frq: 23.4 Hz). According tothe resulted signal, we can conclude that: As we go through the more distances,there is a gradual increase in the amounts of total field, specially between thedistances 30 and 40. Hence, the probable coordinate of the burried object wouldbe between 30 and 40 meters from the base station.

Figure 3.5: Total Field/Location VLF Diagram (Frq: 23.4 Hz)

CHAPTER 3. DATA PROCESSING 23

Figure 3.6: Total Field/Location VLF Diagram (Frq: 23.4 Hz)

Figure 3.7: Resistivity/Depth Plots of Profile 20.According to the above 3 maps,it is obvious that there is an anomalous object with low resistivity in easternside of the 20th profile. Thus, a high chargeability would be expectable forthe eastern parts of the mentioned profile.On the other hand, high amounts ofresistivity in central parts of the profile would indicate a reluctance host rock.

CHAPTER 3. DATA PROCESSING 24

Figure 3.8: Chargeability/Depth Plots of Profile 20

Figure 3.9: Chargeability and Resistivity Plots of Profile 20. Assuming theresistivity responses of Fig. 3.7, high amounts of chargeability in the easternparts of this map, confirms the anomalous conductive buried object.

CHAPTER 3. DATA PROCESSING 25

Figure 3.10: Importing the Corrected Data to Potent Software

Figure 3.11: Imported Data to Potent Software

CHAPTER 3. DATA PROCESSING 26

Figure 3.12: Karous-Hjelt VLF filtering Plot of Profile 20

Figure 3.13: Karous-Hjelt VLF filtering Plot of Profile 25

CHAPTER 3. DATA PROCESSING 27

Figure 3.14: Karous-Hjelt VLF filtering Plot of Profile 30: According to themap, it is obvious that there is an anomalous object between the distances 30and 40. The probable depth of the object would be 08-10 meters. In the follow-ing chapters, data integration will be done due to reveal the exact properties ofthe buried object.

Figure 3.15: Karous-Hjelt VLF filtering Plot of Profile 35

CHAPTER 3. DATA PROCESSING 28

Figure 3.16: Karous-Hjelt VLF filtering Plot of Profile 40

Chapter 4

Exported Plans of thePotent Software

Inversion modelling is based on the non-linear least squares method describedby Jupp and Vozoff (Geophysical Journal of the Royal Astronomical Society,vol 42, 1975). The implementation is versatile and, in its simplest form, veryeasy to apply. There are two steps:

• Create and display one or more subsets that include the observations thatyou want to include in the inversion. Minimise or close all other subsetwindows.

• In the dialog boxes for each body, check the INVERT checkboxes thatcorrespond to the parameters that you want to be varied during the in-version. In this study the inversion will optimise the X coordinate, Z (thedepth), dip, shape and physical properties. Similarly, other bodies of themodel might also be optimised during the same inversion.

Once this is done you can start the inversion. This study shows a 2D modelbefore and after a 50 iteration inversion. The position (including orientation)and shape of each body was allowed to vary. Images of the observed, calculatedand residual field are all shown with the same colour stretch in the followingpages:

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CHAPTER 4. EXPORTED PLANS OF THE POTENT SOFTWARE 30

Figure 4.1: Exported Plan of profile 20 from Potent Software

Figure 4.2: Exported Signal of profile 20 from Potent Software. As it is clearlyvisible in the plan, an isometric conductive body (object.3) is implanted in theeastern part of surveying line with the average depth of 4m and relative distanceof 50m from the base station. The two other objects are implanted to neutralizethe electromagnetic effects of the covering metal walls of the study area.

CHAPTER 4. EXPORTED PLANS OF THE POTENT SOFTWARE 31

Figure 4.3: Exported Plan of profile 25 from Potent Software

Figure 4.4: Exported Signal of profile 25 from Potent Software. As it is clearlyvisible in the plan, an isometric conductive body (object.3) might be implantedin the eastern part of surveying line with the average depth of 8m and relativedistance of 50m from the base station. But assuming the known information ofthe study area, 8 meters are too deep for this area and not reasonable, so wepropose this object as magnetic effects of the sidelong profiles. The two otherobjects are implanted to neutralize the electromagnetic effects of the coveringmetal walls of the study area.

CHAPTER 4. EXPORTED PLANS OF THE POTENT SOFTWARE 32

Figure 4.5: Exported Plan of profile 30 from Potent Software

Figure 4.6: Exported Signal of profile 30 from Potent Software. As it is clearlyvisible in the plan, a conductive rectangular prism (object.3) is implanted inthe western part of surveying line with the average depth of 2-3m and relativedistance of 20m from the base station. The two other objects are implanted toneutralize the electromagnetic effects of the covering metal walls of the studyarea.

CHAPTER 4. EXPORTED PLANS OF THE POTENT SOFTWARE 33

Figure 4.7: Exported Plan of profile 35 from Potent Software

Figure 4.8: Exported Signal of profile 35 from Potent Software. As it is clearlyvisible in the plan, two isometric conductive bodies (object.3 and object.4) mightbe implanted in the surveying line with the average depths of 2m and 8m,respectively. Assuming the known information of the study area, 8 meters aretoo deep for this area and not reasonable. Thus, we propose both the 4th and3th objects as electromagnetic effects of the sidelong profiles. The two otherobjects (object.1 and object.2) are implanted to neutralize the electromagneticeffects of the covering metal walls of the study area

CHAPTER 4. EXPORTED PLANS OF THE POTENT SOFTWARE 34

Figure 4.9: Exported Plan of profile 40 from Potent Software

Figure 4.10: Exported Signal of profile 40 from Potent Software. As it is clearlyvisible in the plan, an isometric conductive body (object.4) is implanted in theeastern part of surveying line with the average depth of 5m and relative distanceof 45m from the base station. The two other objects are implanted to neutralizethe electromagnetic effects of the covering metal walls of the study area.

Chapter 5

Conclusion

We proposed some conductive anomaly in a definite dimensions and locations.(Fig .5.1). The response of these conductive bodies is calculated by integrat-ing results of VLF, electromagnetic and also IP/RS surveyings. The presentstudy showed that geopysical data integration can increase the probability ofgeophysical exploration.

5.1 Revealing the Probable Geophysical Objects

According to te previous mentioned explanation, we can propose the probabledimentions and locations of the main buried objects in the whole of the studyarea as follows:

Figure 5.1: Schematic Map of the Burried Objects

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Chapter 6

References

• Kearey, P, Brooks M. and Hill, L. (2002) An introduction to geophysicalexploration, Blackwell Science Ltd.

• National Geophysical Data Center’s Official Website: (http://www.ngdc.noaa.gov)

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Chapter 7

Aknowledgements

We would like to express our special thanks of gratitude to Dr. N. Fathianpouras well as our principal Eng. Tavakoli, who helped us alot during this geophysicalsurveying.

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