Lecture 5b - DC resistivity surveying

Flashcards based on a lecture about resistivity surveys, including electrode array types, sounding, profiling, and tomography.

What is the basic principle of resistivity surveying?

Injecting current into the surface with electrodes and measuring electric potential differences.

What are the main types of resistivity surveys and what do they measure?

Sounding surveys vertically, profiling surveys horizontally, and tomography generates 2D or 3D images.

What factors are needed to Calculate the apparent resistivity?

Current (I), voltage (ΔV), and a geometric factor based on electrode separations.

What are the most common electrode array types?

Wenner, Schlumberger, and dipole-dipole.

What is the configuration of electrodes in a Wenner array?

All electrodes are at equal separation, and potential electrodes are located within the current electrodes.

What is the defining characteristic of a Schlumberger array?

Potential electrodes are much closer together than they are to either of the current electrodes, and they are located within the current electrode pair.

How are electrodes arranged in a dipole-dipole array?

The potential electrodes are located adjacent to, rather than within, the current electrodes.

What is a key consideration when choosing different array styles?

Different arrays have different patterns of sensitivity in the subsurface.

How does electrode separation affect the depth of investigation?

Increasing electrode spacing increases array sensitivity at deeper depths.

What is the purpose of sounding?

Surveying for vertical variation in resistivity below a single point on the surface.

Which array type is best suited for sounding, and why?

A Schlumberger array, because only one pair of electrodes need to be moved for each independent measurement and it is less sensitive to lateral variations.

What do apparent resistivity measurements represent?

Apparent resistivity measurements do not give an absolute measurement of resistivity at any point, but represent an averaging over a broad volume of investigation.

How are sharp subsurface boundaries represented in sounding curves?

Sharp boundaries between subsurface layers are represented by gradual changes in apparent resistivity.

How does fault orientation affect resistivity measurements?

Measurements aligned along the fractures are likely to be lower resistivity compared to those aligned perpendicularly.

What is the purpose of profiling?

Looking for lateral variations by moving the array along a profile line with a constant separation between electrodes.

Which array types are commonly used in profiling?

The Wenner array, due to equal electrode separations, and the dipole-dipole array, due to good sensitivity to lateral variations.

What is tomography?

Combining elements of profiling and sounding to develop two or three dimensional images of the subsurface.

What is a pseudosection?

A visualization of apparent resistivity measurements, not a model of subsurface resistivity.

How are tomographic techniques applied from boreholes?

By placing electrode pairs in adjacent boreholes.

What is the capability of resistivity methods

Identifying subsurface variations related to hydrological parameters such as porosity and permeability.