The Crossley Heath School, Halifax 17/01/2019 Electromagnetic Methods: Resistivity 17/04/2013 Will Brown The Crossley Heath School, Halifax

Overview Resistivity: EM Methods: Making measurements 17/01/2019 Resistivity: EM Methods: Making measurements Electromagnetic (EM) “Apparent” resistivity Equipment Ground Penetrating Radar (GPR) Looking deeper Looking further Spontaneous Potential (SP) Sections (ERT) Examples Induced Polarisation (IP) Impacts Practical

EM Electromagnetic methods rely on two main principles: 17/01/2019 Electromagnetic methods rely on two main principles: Measuring directly the electrical properties of the Earth Measuring the response of different materials to an EM field or waves Geophysical methods use a wide range of this spectrum above and below visible light. I’m mainly talking about geoelectrics today but we’ll get onto more general EM methods later. Active or passive methods.

Resistivity l A Ohm’s Law: V = Voltage I = Current R = Resistance 17/01/2019 Ohm’s Law: V = Voltage I = Current R = Resistance ρ = Resistivity l = Length A = Cross-section area A l Resistance is a fundamental material property, resistivity is an object-dependent property. Resistance is a constant for a material, resistivity gives you much more detail about porosity, fluid type, etc...

Conductivity l A Ohm’s Law: V = Voltage I = Current G = Conductance 17/01/2019 Ohm’s Law: V = Voltage I = Current G = Conductance σ = Conductivity l = Length A = Cross-section area A l Conductivity is the inverse of resistivity.

Units 17/01/2019

Properties 17/01/2019 All materials have a resistivity, some have a specific value, but many have characteristic range. This can be quite large.

Measurements l A In the earth, things are a bit more complicated.... 17/01/2019 A l V In the lab: we input a current we measure the voltage we calculate the resistance we measure the geometry we calculate the resistivity In the earth, things are a bit more complicated.... Very complex (heterogeneous) Strange geometries Very high contact resistance

Measurements In the earth, things are a bit more complicated.... 17/01/2019 Homogeneous – uniform, evenly distributed, the same everywhere. Heterogeneous – unevenly distributed, properties vary in different places. In the earth, things are a bit more complicated.... Very complex (heterogeneous) Strange geometries Very high contact resistance

Measurements An earth resistivity measurement: 17/01/2019 An earth resistivity measurement: We input a current just like we would in the lab, but measure the voltage somewhere else. (because of high contact resistance) In the lab:

Measurements An earth resistivity measurement: 17/01/2019 An earth resistivity measurement: We input a current just like we would in the lab, but measure the voltage somewhere else. (because of high contact resistance) We can approximate the geometry In the lab: In the field: Geometrical factor

Geometrical factor: Why 2πa? 17/01/2019 Geometrical factor: Why 2πa? Current lines Equi-potentials

Geometrical factor: Other arrays. 17/01/2019 Geometrical factor: Other arrays.

What do we actually measure? “Apparent ρ” 17/01/2019 What do we actually measure?

What do we actually measure? “Apparent ρ” 17/01/2019 What do we actually measure?

What do we actually measure? “Apparent ρ” 17/01/2019 What do we actually measure? We measure an “apparent” resistivity

Equipment Why are they all different distances apart? Electrodes 17/01/2019 Why are they all different distances apart? Electrodes

Seeing vertically – a sounding Soundings 17/01/2019 Seeing vertically – a sounding The wider our spacing, the deeper we measure. Spacing Is a combination of and Tends towards layer resistivities at ends. Currents concentrated around a depth equal to electrode spacing (a). So the wider the spacing, the deeper we see. If you want to see deep you need long cables...know you know why we use different arrays.

Seeing vertically – a sounding Soundings 17/01/2019 Seeing vertically – a sounding The wider our spacing, the deeper we measure. Spacing Is a combination of and Tends towards layer resistivities at ends. Currents concentrated around a depth equal to electrode spacing (a). So the wider the spacing, the deeper we see. If you want to see deep you need long cables...know you know why we use different arrays.

Seeing vertically – a sounding Soundings 17/01/2019 Seeing vertically – a sounding The wider our spacing, the deeper we measure. Spacing Is a combination of and Tends towards layer resistivities at ends. Currents concentrated around a depth equal to electrode spacing (a). So the wider the spacing, the deeper we see. If you want to see deep you need long cables...know you know why we use different arrays.

Geometrical factor: Other arrays. 17/01/2019 Geometrical factor: Other arrays. If A and B have to be connected together and M and N, what is the main difference between these arrays? Remember width of array implies depth of survey.

Soundings 17/01/2019 Theoretical curves give resistivity of second layer not independently measured. Old versions are log-log scaled, harder to plot on but easier read off rho2 and h with overlay. Plotting K allows rho2 calculation and linear plotting. (a/h) is used since a layer is sampled when the spacing is approximately equal to the depth. Curves based on ratio of layer resistivities. Positive K is resistivity increase with depth, negative K is decrease with depth.

There’s a whole book of master curves for 3 layers! Soundings 17/01/2019 There’s a whole book of master curves for 3 layers! The more layers there are, the more complex to curve and the more permutations of resistivity contrasts.

Seeing laterally – a profile Profiles 17/01/2019 Seeing laterally – a profile Distance along profile

Combining a profile and sounding: a section Sections 17/01/2019 Combining a profile and sounding: a section

Measurements We see CONTRASTS 17/01/2019 We see CONTRASTS This means that resistivity surveys can be used for: Archaeology Finding fresh water Locating chemical contaminations Determining subsurface structure/geology Plus many more...

Examples 17/01/2019 Archaeology ?

Examples Contaminant Plumes Lower resistivity plume of contaminants 17/01/2019 Contaminant Plumes Lower resistivity plume of contaminants

Examples Landfills Monitoring old landfill sites High resistivity 17/01/2019 Landfills Monitoring old landfill sites High resistivity Low resistivity

What do all these examples have in common? Impacts 17/01/2019 What do all these examples have in common? i.e. Really good for environmental purposes, does no damage and leaves no trace.

What do all these examples have in common? Impacts 17/01/2019 What do all these examples have in common? Buried materials with contrasting properties. Unknown locations. Large areas. Impractical to measure directly – can’t dig it all up! i.e. Really good for environmental purposes, does no damage and leaves no trace.

What do all these examples have in common? Impacts 17/01/2019 What do all these examples have in common? Buried materials with contrasting properties. Unknown locations. Large areas. Impractical to measure directly – can’t dig it all up! Resistivity is non-invasive, silent, quick and cheap but requires contrasts in properties, especially good for fluids i.e. Really good for environmental purposes, does no damage and leaves no trace.