Resistivity pseudosections

Summary

This tutorial explains how to calculate and plot resistivity data obtained from field surveys using a dipole-dipole array. The process involves measuring voltage and current to derive resistance, converting it into resistivity using the geometric factor, and plotting the results to create a pseudosection. The tutorial covers how to handle data from various electrode spacings, calculate resistivity at different points, and visualize the results in Excel. The goal is to help users identify conductivity anomalies in the ground by interpreting contoured pseudosections in their reports.

Takeaways

• 😀 Understanding resistivity vs resistance: In the field, the key measurement is resistance, but resistivity, which is independent of length, is the actual property being measured.
• 😀 Measuring resistivity requires a conversion factor: The geometric factor (k) is used to convert resistance into resistivity in dipole surveys.
• 😀 The formula for k: In a dipole survey, the formula for the geometric factor is pi × n × (n+1) × (n+2) × a, where n is the spacing factor and a is the electrode spacing.
• 😀 Spreadsheet calculation steps: The resistivity is calculated by multiplying resistance by the geometric factor (k). Additional columns in a spreadsheet can assist with this process.
• 😀 Importance of electrode spacing: The resistivity data is sensitive to the distance between electrodes. Different spacings (n = 1, 2, 3, etc.) give different measurements of the ground's resistivity.
• 😀 Pseudosection construction: By shifting the electrode positions and calculating new resistivity values, a pseudosection is created to visualize the resistivity profile of the subsurface.
• 😀 Midpoint calculation: For each set of electrode positions, the midpoint is calculated as the average distance between the inner electrodes. This midpoint is important for visualizing the resistivity at specific depths.
• 😀 Visualizing data with contour plots: Once resistivity data is processed, it can be represented visually through contour plots, which show variations in resistivity at different points.
• 😀 Data interpolation in Excel: When there are gaps in the resistivity data, an average of surrounding values can be used in Excel to fill in missing data for smoother plotting.
• 😀 Pseudosection interpretation: The pseudosection provides a simplified view of subsurface resistivity, approximating the depth and structure of the ground. It helps identify conductivity anomalies and other features of interest in the soil or rock.

Q & A

What is the difference between resistance and resistivity?

Resistance is the opposition to current flow, measured as the ratio of voltage to current. Resistivity, on the other hand, is a material property that quantifies how much a material opposes the flow of current, and it’s independent of the electrode spacing or survey length.

Why is it important to convert resistance to resistivity in a field survey?

Converting resistance to resistivity allows for measurements that are independent of the electrode spacing, providing a more accurate representation of the soil or rock's properties. This is essential for consistent comparisons between different surveys and conditions.

What is the geometric factor (k) and why is it used in resistivity calculations?

The geometric factor (k) is a constant used to convert the measured resistance to resistivity. It depends on the electrode spacing and arrangement, and its calculation accounts for the geometry of the measurement setup, ensuring the resistivity value is consistent regardless of the survey setup.

How do you calculate the geometric factor (k) for a dipole-dipole survey?

For a dipole-dipole survey, the geometric factor (k) is calculated using the formula k = π * n * (n+1) * (n+2) * a, where n is the number of electrode spacings and a is the electrode spacing distance.

What does the 'n' represent in the dipole-dipole survey?

'n' represents the multiple of the electrode spacing between the potential electrodes and the current electrodes. It indicates how far apart the electrodes are and is used to calculate the geometric factor and resistivity.

How do you plot a resistivity pseudosection using field data?

To plot a pseudosection, you first calculate the midpoint between the potential and current electrodes. Then, you record the resistivity at different intervals, ensuring each point corresponds to a measurement taken at varying electrode spacings. These values are then arranged in a grid, and a contour plot can be used to visualize the resistivity variations.

Why is the midpoint important in the resistivity pseudosection?

The midpoint represents the average position along the path of the current flow where resistivity is being measured. This helps in approximating where the measurement took place in the ground, crucial for creating a meaningful pseudosection that reflects the subsurface conditions.

What is the significance of moving the current electrodes further apart during the survey?

By increasing the distance between the current electrodes, you are effectively measuring resistivity at deeper layers of the ground. This allows for capturing resistivity data at varying depths and constructing a more accurate subsurface profile.

How can you fill gaps in resistivity data when using Excel?

To fill gaps in the data in Excel, you can average the resistivity values on either side of the missing data point. This simple method provides a reasonable estimate for missing values, allowing for a complete dataset that can be plotted effectively.

What steps are involved in creating a contour plot for resistivity data in Excel?

First, organize your resistivity data into columns with midpoints and electrode spacings. Then, use Excel's surface plot options to create a contour plot. You may need to adjust axis formatting and invert the vertical axis to ensure the plot is oriented correctly.