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Induced Polarization How-To Guide

Getting Started with Induced PolarizationThe Induced Polarization How-To Guides walk you through tasks you perform in the Induced PolarizationTM (IP) system. The procedures are divided into common procedures that you use every time you work with the program. The IP topics include:

Getting Started with IP

Importing IP Data

Import and Display 3D IP Data

Quality Control for your IP Data

Processing your IP Data

Plotting Your IP Results

Exporting IP Data

Geosoft provides sample data for you to use when working through these guides. These data files can be found in your "../Program Files (x86)/Geosoft/resourcefiles/data/ip" folder.

To Begin...This Getting Started with IP guide introduces you to the IP extension, which provides the ability to import, process, visualize, present and export IP data.

This guide explains the IP system, including:

1. What you need to know to use the IP System 2. IP Resistivity Theory and Survey Techniques 3. Create a Project 4. Load the IP menu 5. Set the IP Configuration

System Capabilities and ConceptsIP and apparent resistivity data play a key role in mineral exploration. When applying data to exploration problems, the IP and resistivity interpreter must employ various strategies to deal with the following:

Different types of data

Different instruments and survey configurations

Data reduction from instrument formats to standard IP parameters

Manipulation and visualization of complex multi-point data (time domain windows and multi-frequency samples)

Duplicate samples

Topographic variations that can distort estimated depth and other interpretation parameters

Presentation of data in standard formats (pseudo-sections and stacked pseudo-sections).

The Oasis montaj IP extension is designed to assist IP users, including contractors and in-house geophysicists, in addressing the above challenges. Specifically, the system enables you to import, process, visualize, and present IP data.

The IP system imports data from a variety of instrument dump files, IP files, and array configurations, including:

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IP File Types Instrument Dump Files Array Configurations

Zonge FLD Format

Zonge AVG Format

Scintrex Geophysical Data Format

Interpex 12X

Geosoft IPDATA

Geosoft IPRED

Generic 3D Time Domain IP

GDD

Iris Elrec-2

Iris Elrec-6

Iris Elrec-10

Iris Elrec-Pro

Iris SYSCAL-R2

Scintrex IPR11

Scintrex IPR12

Phoenix V2

Phoenix V4-V5

Dipole-Dipole

Pole-Dipole

Pole-Pole

Gradient

Raw IP data are simplified by extracting standard types of information – such as IP average, apparent resistivity, chargeability, self potential, voltage potential, “N” value, and metal factor – from the instrument data. The IP system extracts this information automatically as it imports raw instrument data into a database.

The series of values obtained at the same measurement point (time windows or frequency responses) are displayed as a curve (array) in a single cell of the database. This way, the system handles data channels containing both single values and value arrays.

Data series for duplicate samples can be edited interactively for each line and station in the database. The Quality Control tool in the IP system enables you to visually inspect the samples and eliminate data series that appear suspicious or incorrect. This process does not alter the integrity of the initial data, but rather tags it through the use of a mask channel.

IP data is commonly collected on a local idealized coordinate system, however, at some point you may want to introduce georeferencing or import topography. Both are built into the workflow.

The IP system enables you to produce pseudo-section and stacked pseudo-section plots from your IP data, as well as represent the data in a 3D view.

What Does the IP System Do?The Oasis montaj IP system is designed for importing, performing quality control, processing, and outputting data derived from IP surveys.

The general processing steps for time domain and frequency domain data are the same.

The system capabilities include:

Processing 2D and 3D time-domain and frequency-domain data from dipole-dipole, pole-dipole, pole-pole, and gradient survey configurations

Handling different instruments and arrays

Reducing raw IP data

Performing interactive quality control

Adding georeferencing and topographic information



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Plotting values in pseudo-section format

Plotting stacked pseudo-sections

Creating 3D views of stacked-section maps

Additional System SpecificationsReduces and filters up to 2048 readings at a time.

Data entry can be performed in any order; data can be appended to the previously entered data.

One of the five pant-leg or triangular filters can be applied to the data, and the weighting of the filter can be varied as a function of the plotted depth.

Up to eight pseudo-sections can be plotted in a single map, displaying sections of IP, Metal Factor, Self-Potential, and any selected slice from the IP decay curve. Also, up to six data profile windows can be plotted in a map.

Optional contours may be drawn on industry standard logarithmic base at levels 1, 1.5, 2, 3, 5, 7.5, 10, 15, etc., or linearly at a user-specified interval.

What You Need to Know to Use the Induced Polarization SystemThis section introduces you to the elements of the spreadsheet and profile panes, and describes the essential concepts that are important for understanding the IP system.

Spreadsheet and Profile PanesThe following image identifies the elements of the IP spreadsheet and profile panes for a database containing time-domain survey data on an N-S line.

The spreadsheet displays a database containing all the survey data. The current survey line number is located in the top left cell of the spreadsheet. For IP data, each spreadsheet row contains a fiducial number. The fiducial number is incremented for each survey station where a reading was taken.

The second and third columns contain location information. For IP data, these columns contain line and station numbers.


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An IP spreadsheet also contains array channels. Array channels appear as a profile curve, with multiple columns of single channel data. At each reading in an IP survey, several different time windows or frequencies are recorded in separate channels. An array channel represents all these values or value windows by a curve.

Profiles are displayed in the Profile pane for the channel selected to be profiled. In the sample image above, the Profile pane displays the IP array channel in red. The selected data point or cell in the spreadsheet is indicated by a square in the Profile pane. The numbers at the left indicate the range of data values. By changing these numbers, you can modify the scale of the profile pane and alter the appearance of the profile lines.

Array ChannelsArray channels enable you to store multiple time-windowed or frequency measurements in a single cell of the database. It is essential to understand how array channels work.

The first thing you will notice about an array channel is that each cell in the spreadsheet column contains a curve instead of a number. The reason for this is that an array channel contains more than one channel or column of data. For example, in an IP time domain survey, a curve in a spreadsheet cell of an array channel would represent a decay curve for a single survey location.

By representing data in an array channel, all the readings for a single location can be put into one column of the spreadsheet instead of having several channels for the multiple readings at each survey location. The array channels do not display the data numerically, but represent the data as a curve. The numerical data stored in an array channel may be displayed in sub-channels. For example, if the decay curve contains 256 time slices, you have the option of looking at any slice of that curve in its own sub-channel.

Important! As with all cells in the database, when a cell in an array channel is selected (highlighted), pressing a key overwrites the data.

Array Channel MaskingYou can create a mask channel for any single array channel or a pair thereof. This enables you to turn “off” individual data window values in the Average duplicate samples dialog. The dialog creates a “mirror” array channel of type “byte”, with the same number of columns as the selected channel. By default, all these values are “1”, which indicates that all the values are selected to be used.

The IP QC Tool recognizes that an array channel has been assigned a mask channel, and checks the Mask check box. Two channels can be selected to use the same mask channel. This is useful, for instance, for frequency domain systems where channels typically occur in pairs for In-Phase and Quadrature, or Amplitude and Phase. Normally, if one of these values is bad, the corresponding value in the other channel is also bad, and both will be disabled at the same time.

Database ChannelsThe table below provides a list of the IP database channels you will find in the columns of the Spreadsheet pane. The channels in this list may vary depending on the instrument used to collect the data. You may not see some of these channels, or may have imported other channels not listed here.



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Channel Description

X Station This example assumes an N-S line direction. For a database with an E-W line direction, these channels are swapped.

Y Line number

Z Estimated depth. The intercept point in the vertical plane joining the transmitter and receiver at a 45° dip angle

IP In time domain, after the injected Voltage Vc is cutoff, the transient voltage V(t) is recorded at specific time intervals. For each physical Tx-Rx pair, a response array over a predetermined time window is recorded in units of V(t)/Vc x 10e+3 . Note that the scale of the curve in each array channel cell is unique.

IP_Avg IP readings are averaged upon import and saved in the database. Some systems calculate this average at the instrumentation level.

N Receivers are placed at a regular interval from the transmitter. N denotes the number of intervals separating a Tx-Rx pair. N is undefined for pole-pole and gradient arrays. N is calculated by the IP system during import.

I Transmitted Current.

MF The IP response varies with effective resistivity of the host rock, temperature, electrolytes, porosity. The Metal Factor corrects to some extent for these variations. It is calculated upon import.

QC & QC_RES

Flags generated upon import. Using the IP Quality Control tool these fields can be set to exclude specific readings from the subsequent calculations. The flag values are:

1 Accept: The value is used in calculations and plotted.

2 Flag: The value is not used in calculations, exports, and averaging, but is plotted with square brackets on pseudo-sections if no “better” values exist for the same station.

* Reject: The value is not used for calculations, export, or plotting.

R1X; R2X Receiver positions.


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For N-S line directions, these channels are labelled as R1Y, R2Y. Calculated upon import.

T1X; T2X Transmitter positions.

For N-S line directions, these channels are labelled as T1Y, T2Y. Calculated upon import.

ResCalc The apparent resistivity is calculated during import, taking into account the distant electrodes. This calculation uses all 4 potential and transmitting electrodes.

Sp The Self Potential is a measured entity.

Stn Station position along the survey line.

Type Type of reading: 0 indicates a single reading, 1 is an averaged reading. When plotting, the averaged readings take priority over single readings.

Vp Primary voltage.

IP QC Tool recognizes the following hot keys:

A = Accept

B or Left Arrow = Previous Sample

D = Delete (same as reject)

F or S = Flag

H or Up Arrow = PreviousTx/Stn

N or Down Arrow = Next Tx/Stn

M or Right Arrow = Next Sample

R = Reject (same as delete)

IP/Resistivity Theory and Survey TechniquesThe Oasis montaj IP system assumes you are familiar with ground electrical methods, the supporting theory, and interpretation techniques. This chapter provides a brief overview of the electrical theory and methods. For a comprehensive description of electrical methods you are referred to Luo and Zhang1 1997, Sumner2 1976, Van Blaricon3 1992.

IP and Resistivity TheoryElectrical methods intend to provide a quantitative measure of the resistivity of underlain rocks. Resistivity is the resistance that the rock manifests to the passage of electricity. When a current I is injected into the surface of a half space of uniform resistivity, the resulting voltage V at a point of distance r from the current electrodes provides a measure of the apparent resistivity ρ of the half space through which the current has travelled. This relationship is defined as:

ρ π= 2 *V

I

The resistivity of the half space can be determined by four electrodes. Two current electrodes, through which the current is injected, and two Voltage electrodes at which the resulting Voltage is measured. The standard coplanar configurations are illustrated below:



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The horizontal spread identifies lateral variations, while the depth sounding indicates vertical changes in resistivity.

Recently a 3-Dimensional configuration has gained momentum. In 3D-IP, a set of parallel Receiver electrodes are placed on the ground and the transmitters are moved in between receiver lines. This configuration is illustrated in plan view below:


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Generally rock minerals are insulators and do not effectively conduct electricity. Porous rocks on the other hand fill with water. Water conducts electricity and the more saline it is, the better conductor it becomes. The resistivity of the half space is directly related to its porosity. Porosity varies with fracturing and alteration and sedimentary grain size. There are a few uncommon rock types that are good to excellent conductors. Metallic rocks - most sulfides as well as graphite - are excellent conductors.

The ground electric methods are used in exploration to identify units of different resistivity. Faults, shear zones, alteration zones and overburden layers will respond differently to the current.

The apparent resistivity is the resistivity of the equivalent uniform half space between the transmitter and receiver electrodes. The apparent resisitvities are then interpreted relative to the electrode placement, distance between electrodes and injected current intensity.

Self-PotentialSelf-potential is a measurement of the natural direct currents flowing through the half space. The most common cause of self -potential is the chemical reaction of the oxidization process.

The self-potential is inspected along profiles or as a plan map. Oxidization zones are generally indicated by self-potential lows.



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IP EffectThe current flow is maintained by charged ions in the fluid filling the pores. The IP effect is created when this ionic current flow is converted to electronic current flow at the contact between the fluid and metallic minerals. In this case, when the current flow is interrupted, the induced polarization does not instantaneously return to its normal state. The transient voltage of charges essentially decays to 0 after a lapsed time.

In Frequency domain IP, the geophysicist seeks to locate portions of the Earth where the resistivity of the rock decreases as the frequency of the applied current increases. The time domain (transient) IP method identifies areas in the ground where the voltage decay takes a noticeable period of time to dissipate. This technique measures the bulk or average polarization of a volume of rock. Polarization is the ability of the ground to store electrical energy.

The frequency domain (or variable frequency) method uses the magnitude and phase shift of the frequency to calculate the apparent resistivity, which is the bulk or average resistivity of a volume of rock. The resistivity value of a rock mainly depends on the porosity and salinity (that are translated to resistivity) of the solution filling the pore spaces. The method is used to search for areas where the resistivity of the rocks decreases with the increase of the frequency of the applied current.

The relationship between the frequency domain and the time domain methods is defined by the following equation (the LaPlace Transform theory):

≅m fe=fe

I fe+ ,

Where the Chargeability parameter (m ) measured in the time domain method is exactly equivalent to the frequency effect parameter (fe ) used in the frequency domain method, and the current is represented by I .

IP and Resistivity Survey TechniquesIP surveys require two pairs of electrodes to introduce electric current into the ground and then measure the response voltage. The distance between the potential electrodes is generally represented by “a ”. As this distance increases, the survey depth also increases, meaning that a greater volume of rock is sampled. The “n ” value is an integer that refers to the distance and order of the potential electrodes. The n value increases with the distance of the potential electrodes from the current or transmitting electrode. In general the greater the n value, the deeper the penetration and the larger the sample location .

In most array configurations, the IP and resistivity results are plotted as profiles along a measurement line. For each survey line, a mathematical model is used to match each reading to a theoretical position below the surface. The data itself represents the true path of the current projected onto the surface. The model projects these data to a probable path and assigns a new location for each data point. The result is a two-dimensional vertical slice of data for each survey line, referred to as a “pseudo-section”. The vertical scale increases downwards, and represents the “n ” value – a rough measure of depth.

The resulting data points describe the relative positioning of anomalies inherent in the data. A sense of depth to individual anomalies is conferred by the vertical axis. This, however, is not a true measure of depth. Data inversion is necessary to obtain a more realistic depth measurement.

IP field measurements, voltage and resistivity, are directly proportional to polarization, the ability of the rock to hold a charge. A geometrical factor must be applied to the raw data to convert it to resistivity.

The IP system supports four array types for both time and frequency domain surveys:

Dipole-Dipole

Pole-Dipole

Pole-Pole


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Gradient

References:John S. Milsom, John Wiley & Sons, 2003, Metal factor. Field Geophysics, Third Edition, p.122.

Yanzhang Luo, Guiqing Zhang, 1997, Theory and Application of Spectral Induced Polarization. Society of Exploration, 171 pp.

J.S. Sumner, 1976, Principles of Induced Polarization for Geophysical Exploration. Developments in Economic Geology, 5, Elsevier, 278 pp.

Starting an IP ProjectThe IP system enables you to access files anywhere but it is a good strategy to carefully organize your data (project information and files) before carrying out any processing.

You need to perform the procedures in this How-To Guide before you can process your IP data. Once you have created a project, loaded the IP menu and set your survey parameters you can try subsequent tutorials including, Importing, Processing, Quality Control, Plotting and Exporting IP data.

To Start a Project 1. Start Oasis montaj.

2. From the File menu, select Project and then select New. The New Project dialog appears.



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Oasis montaj assumes that your data is in the directory containing this project.

3. Specify a project name and folder for the project.

4. Click the Save button.

The system saves the project and indicates that it is open by adding menus to the menu bar, adding buttons to the toolbar, and by displaying the Project Explorer pane. These are visual clues indicating that you are ready to start working with the system.

Loading the IP MenuBefore you can start working with the IP system, you have to load the IP menu in your project.

If you require more detailed information on setting menus, refer to the Oasis montaj Online Help System.

To Load the IP Menu 1. From the GX menu, select Load Menu.

The Load Menu dialog appears.

2. From the list of files, select ip.omn and click the Open button. The system displays the IP menu on the main Oasis


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montaj menu bar.

Setting the IP Configuration

Reasonable defaults have already been set and you can proceed without modifying the settings. However, if you need to modify a setting detail, it is important to understand the tool.

You can specify many of the default processing and mapping parameters used in the IP system by selecting the IP Configuration option from the IP menu. This menu option displays a series of dialogs that enable you to customize the global settings for your project. Most of these settings can also be modified from other IP dialogs. All such modifications take effect at the project level. The IP Defaults menu option is provided so that you can customize the project settings at the onset of the project and in one place.

For the majority of survey data, you do not have to change the IP default settings. If you encounter a problem with importing or processing your data, it is most probably a setting issue and you will need to go through the defaults option.

To Specify Survey Parameters 1. From the IP menu, select Set Configuration.

The Survey Parameters dialog appears.



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2. Specify the Measurement Domain, Array Type (configuration), Line Direction, Line Sense, Station number multiplier, Line number multiplier, and Distance Units parameters, as they pertain to your survey configuration.

The X & Y fields are populated on import. X is the East-West direction and Y is the North-South direction. For example, if you selected survey line 1000N, and specified an E-W line direction, all the Y values would be 1000.

3. Click Next. The Survey Type dialog appears.

4. The Array Type is for reference only. You can leave the Dipole Separation blank so that it is extracted from your data file.

5. Click Next. The Derived Channel Calculations dialog appears.


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6. You can leave the Windows for Averaged Total IP blank to include all the IP time series in the averaging. Individual array elements can be specified separated by commas, or a range can be specified using a dash.

7. You can also leave the Filter weights blank to assign the same weight to all elements of the array. Similarly to above, individual weights can be specified either comma separated or dash separated.

8. Using the dropdown list, select if you want to Normalise Average IP by window widths?

9. Accept the default settings for the remainder parameters: Resistively Units, Metal factor formula, Metal factor multiplier, Minimum N spacing, Maximum N spacing, N spacing increment, Pseudo-section filter, Filter weights and Adjust app. res. for topography? .

10. Click Next. The Map Annotations dialog appears.

11. Create a “boilerplate” text that is plotted on all maps you produce using the IP system.



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