Download presentation
Presentation is loading. Please wait.
1
Resistivity Practical The Crossley Heath School, Halifax
27/04/2019 Resistivity Practical Will Brown The Crossley Heath School, Halifax
2
Practical 27/04/2019 A construction firm is looking to build new houses in Silsden, Yorkshire. They want to know if sites near the River Aire are suitable or not so have called a geophysical contractor to survey possible sites (us!). There are no boreholes nearby so exact measurements are not available. They want to know: Where is the bedrock? Is the bedrock level? Where is the water table?
3
Practical We recon. the site before doing a proper survey:
27/04/2019 We recon. the site before doing a proper survey: The site is a grassy field sloping slightly to the river, it’s spring time and the surface is dry. We know the local geology is Millstone Grit (sandstone) the surface soil is river sediment. Looking at the nearby river bank we can see the water is high and can see about 1m of soil before the bank cuts into rock. We then do a desk study: What are the resistivity properties of the area likely to be? What would we expect to see with a resistivity survey? Is this type of survey suitable?
4
Practical 27/04/2019
5
Practical We decide to go ahead with a resistivity survey.
27/04/2019 We decide to go ahead with a resistivity survey. We want to see the depths of the subsurface layers and how they change horizontally – a pseudo-section. We take 8 soundings parallel to each other, using electrode spacings from m: Soundings River Aire Section - Centre line
6
Practical We decide to go ahead with a resistivity survey.
27/04/2019 We decide to go ahead with a resistivity survey. We want to see the depths of the subsurface layers and how they change horizontally – a pseudo-section. We take 8 soundings parallel to each other, using electrode spacings from m: Soundings River Aire Section - Centre line
7
Practical We decide to go ahead with a resistivity survey.
27/04/2019 We decide to go ahead with a resistivity survey. We want to see the depths of the subsurface layers and how they change horizontally – a pseudo-section. We take 8 soundings parallel to each other, using electrode spacings from m: Soundings River Aire Section - Centre line 20m
8
Practical We decide to go ahead with a resistivity survey.
27/04/2019 We decide to go ahead with a resistivity survey. We want to see the depths of the subsurface layers and how they change horizontally – a pseudo-section. We take 8 soundings parallel to each other, using electrode spacings from m: Soundings River Aire Section - Centre line 20m 128m
9
Practical Profile Sounding 128m Sounding 1 20m Sounding 8
27/04/2019 Soundings ρa spacing a (m) log(a/h) 1 2 3 4 5 6 7 8 log(ρa/ρ1) 0.25 1838 2351 2626 1752 2367 2343 2052 1698 0.5 1738 2538 3289 2098 2134 2469 2147 1588 1231 3170 1979 974 1947 2263 1223 1074 1139 1803 1080 563 900 1115 805 772 506 507 371 272 322 432 403 399 350 252 205 190 198 211 264 16 602 408 292 210 194 168 153 126 32 1096 643 434 303 247 163 111 119 64 2812 1123 690 485 402 277 181 90 128 5851 4195 2274 921 689 411 237 85 Profile Sounding Sounding 1 River Aire Section - Centre line 20m 128m Sounding 8
10
Practical 27/04/2019 A pseudo-section is an approximation of a depth section, not actual depth. Less than 1000Ωm Greater than 1000Ωm Soundings ρa spacing a (m) log(a/h) 1 2 3 4 5 6 7 8 log(ρa/ρ1) 0.25 1838 2351 2626 1752 2367 2343 2052 1698 0.5 1738 2538 3289 2098 2134 2469 2147 1588 1231 3170 1979 974 1947 2263 1223 1074 1139 1803 1080 563 900 1115 805 772 506 507 371 272 322 432 403 399 350 252 205 190 198 211 264 16 602 408 292 210 194 168 153 126 32 1096 643 434 303 247 163 111 119 64 2812 1123 690 485 402 277 181 90 128 5851 4195 2274 921 689 411 237 85 Dry soil Ωm Water saturated soil Ωm Sandstone 1000’s Ωm
11
What do these regions represent?
27/04/2019 What do these regions represent? Less than 1000Ωm Greater than 1000Ωm Soundings ρa spacing a (m) log(a/h) 1 2 3 4 5 6 7 8 log(ρa/ρ1) 0.25 1838 2351 2626 1752 2367 2343 2052 1698 0.5 1738 2538 3289 2098 2134 2469 2147 1588 1231 3170 1979 974 1947 2263 1223 1074 1139 1803 1080 563 900 1115 805 772 506 507 371 272 322 432 403 399 350 252 205 190 198 211 264 16 602 408 292 210 194 168 153 126 32 1096 643 434 303 247 163 111 119 64 2812 1123 690 485 402 277 181 90 128 5851 4195 2274 921 689 411 237 85
12
What do these regions represent?
27/04/2019 What do these regions represent? Less than 1000Ωm Greater than 1000Ωm Soundings ρa spacing a (m) log(a/h) 1 2 3 4 5 6 7 8 log(ρa/ρ1) 0.25 1838 2351 2626 1752 2367 2343 2052 1698 0.5 1738 2538 3289 2098 2134 2469 2147 1588 1231 3170 1979 974 1947 2263 1223 1074 1139 1803 1080 563 900 1115 805 772 506 507 371 272 322 432 403 399 350 252 205 190 198 211 264 16 602 408 292 210 194 168 153 126 32 1096 643 434 303 247 163 111 119 64 2812 1123 690 485 402 277 181 90 128 5851 4195 2274 921 689 411 237 85 Dry soil Saturated soil Bedrock?
13
Practical Calculate log(ρa/ρ1) and log(a/h) to plot on master curve:
27/04/2019 Calculate log(ρa/ρ1) and log(a/h) to plot on master curve: k = (ρ2- ρ1)/( ρ1+ ρ2) ρ1=1698Ωm a is the electrode spacing h is the depth of the first layer Assume h=1 (h) Assumed to be =1 from min depth of drift on river bank. Rho1 taken from shallowest sounding in line 8. Draw a table of: (a), log(a/h), (rho a for sounding 8), log(rho a/rho1).
14
Practical k = (ρ2- ρ1)/( ρ1+ ρ2) ρ1=1698Ωm a is the electrode spacing
27/04/2019 Log(a/h) -0.6 -0.3 0.3 0.6 0.9 1.2 1.51 1.81 2.11 Log(ρa/ ρ1) -0.03 -0.14 -0.31 -0.62 -0.81 -1.13 -1.15 -1.28 -1.3 k = (ρ2- ρ1)/( ρ1+ ρ2) ρ1=1698Ωm a is the electrode spacing h is the depth of the first layer Assume h=1 What do you expect to happen if h ≠1m?
15
Practical We can read off k: k = -0.9 We also know that:
27/04/2019 Log(a/h) -0.6 -0.3 0.3 0.6 0.9 1.2 1.51 1.81 2.11 Log(ρa/ ρ1) -0.03 -0.14 -0.31 -0.62 -0.81 -1.13 -1.15 -1.28 -1.3 We can read off k: k = -0.9 We also know that: k = (ρ2- ρ1)/( ρ1+ ρ2) What is the second layer resistivity? ρ2 = ?
16
Practical k = (ρ2- ρ1)/( ρ1+ ρ2) k = -0.9 ρ1 = 1698Ωm
27/04/2019 Log(a/h) -0.6 -0.3 0.3 0.6 0.9 1.2 1.51 1.81 2.11 Log(ρa/ ρ1) -0.03 -0.14 -0.31 -0.62 -0.81 -1.13 -1.15 -1.28 -1.3 k = (ρ2- ρ1)/( ρ1+ ρ2) k = -0.9 ρ1 = 1698Ωm ρ2 = (-ρ1-k*ρ1)/(k-1) ρ2 = 100Ωm
17
Practical Why might the curves not match?
27/04/2019 Log(a/h) -0.6 -0.3 0.3 0.6 0.9 1.2 1.51 1.81 2.11 Log(ρa/ ρ1) -0.03 -0.14 -0.31 -0.62 -0.81 -1.13 -1.15 -1.28 -1.3 Why might the curves not match? What is h, the depth of the first layer? Remember:
18
Practical h = 1m h = 2m k = (ρ2- ρ1)/( ρ1+ ρ2) k = -0.9 ρ1 = 1698Ωm
27/04/2019 Log(a/h) -0.6 -0.3 0.3 0.6 0.9 1.2 1.51 1.81 2.11 Log(ρa/ ρ1) -0.03 -0.14 -0.31 -0.62 -0.81 -1.13 -1.15 -1.28 -1.3 k = (ρ2- ρ1)/( ρ1+ ρ2) k = -0.9 ρ1 = 1698Ωm ρ2 = (-ρ1-k*ρ1)/(k-1) ρ2 = 100Ωm What is h, the depth of the first layer? Remember: h = 1m h = 2m
19
Practical Our clients wanted to know: Where is the bedrock?
27/04/2019 Our clients wanted to know: Where is the bedrock? 2. Is the bedrock level? 3. Where is the water table?
20
Did we succeed? (Are we getting paid!)
Practical 27/04/2019 Our clients wanted to know: Where is the bedrock? We saw it at the first few soundings, probably at least >64m deep. 2. Is the bedrock level? No, it appears to dip toward the river but we can’t be certain how much or how far. 3. Where is the water table? Within the soil layer, >2m below surface. Did we succeed? (Are we getting paid!)
21
Geophysics for A-level Physicists
27/04/2019 Geophysics for A-level Physicists Will Brown
22
What is geophysics? Literally: “geo” – Earth, physics.
27/04/2019 What is geophysics? Literally: “geo” – Earth, physics. Study of the physics behind natural processes of the Earth. Application of physics to study the Earth.
23
What does your A-level have do with geophysics?
27/04/2019 What does your A-level have do with geophysics? Seismology Satellite, GPS Material properties and stress-strain graphs Gravitational field strength Hooke’s law and Young’s Modulus Properties of light Gravitational methods Elastic strain and strain energy Newton’s laws Waves, polarisation, reflection, refraction and diffraction Radar Vector analysis of motion Volcanology, Atmospheric Electromagnetic Methods Fluid flow, laws of motion Electric circuits and resistivity Stoke’s law, density and viscosity Magnetism and electromagnetism
24
Electromagnetic Surveying
EM 27/04/2019 Electromagnetic Surveying Like the common “metal detector” Uses an ultra-low-frequency radio transmitter (e.g. 10kHz) Transmit a known “primary” signal, measure the response. The difference between what was transmitted and what is measured depends on the properties of the subsurface e.g. conductivity.
25
EM 27/04/2019 Landfills
26
Building foundations and contaminants
EM 27/04/2019 Building foundations and contaminants
27
EM 27/04/2019 Can be used for detecting aquifers, contaminants, landfills, archaeology, oil and gas, ore bodies. Work horse of EM surveying, can see deep and is very quick and cheap. Can be mounted to people, cars, trains, planes, ships...
28
Ground penetrating radar
GPR 27/04/2019 Ground penetrating radar Emit pulses of radio waves and measure the time taken for them to reflect off the subsurface and return to the sensor. Uses frequencies of 25MHz – 1GHz. The speed that radio waves travel in a material is governed by the dielectric permittivity – similar to conductivity. Like EM can mount equipment to pretty much anything...
29
GPR “Recent” technique developed since 1960s.
27/04/2019 “Recent” technique developed since 1960s. Good for glaciology, UXO detection, forensic investigations, construction, utility surveying, archaeology. Depth of survey decreases as frequency increases – attenuation. Resolution increases with frequency however.
30
Radar Not GPR but Radar - SRTM Shuttle Radar Topography Mission
27/04/2019 Not GPR but Radar - SRTM Shuttle Radar Topography Mission Extendable boom built onto space shuttle with radar transmitter on the end. Send out radio pulses which reflect off Earth’s surface to map topography. Best maps of remote regions available still.
31
InSAR Like other satellite radar uses lower frequencies than GPR.
27/04/2019 Like other satellite radar uses lower frequencies than GPR. Take radar measurements repeatedly over the same area, calculate the difference between each pass of the satellite. Used to monitor deformation of the Earth by volcanoes and earthquakes. An interferogram - the different colour bands represent 3cm changes in the distance between the satellite and the ground.
32
Spontaneous Potential
27/04/2019 Spontaneous Potential Oldest and simplest form of geophysical surveying. Place one fixed electrode and move another, measure the voltage but input no current. Naturally currents flow through the Earth – generated by magnetic field of Earth. Think circuits – current takes path of least resistance so is concentrated in conductive bodies and is not constant everywhere. Small differences but visible on large scales e.g. ore bodies. Can measure flow direction underground. -ve +ve
33
IP Induced Polarisation
27/04/2019 Induced Polarisation Between resistivity and SP surveying – induce a current into the ground then switch off and measure response. Time it takes for charge to build and dissipate depends on the conductivity of subsurface materials. Very simple fieldwork, 4 electrodes like resistivity but no moving. Particularly effective for saline fluids and disseminated metal ores. Voltage Time Earth acts like a capacitor
34
Summary 27/04/2019 Generally: Sensitive to EM properties, therefore fluids and metals common targets. Non-invasive – do not disturb ground and leave no trace, some don’t even touch ground. Quick, cheap and quiet. Best for relatively shallow surveys. Mobile, can be carried out from ground, air, sea or space. High conductivity/low resistivity reduces penetration. Therefore some are useless over water or waterlogged ground or volcanic deposits...but not ice. What methods might you use to... Find a plastic or a metal utility pipe? Find disturbed ground at a crime scene? Find an aquifer? Find a metal ore deposit?
Similar presentations
