Today’s Lecture  Field Geophysics

Announcements  Lecture  today: field geophysics  Thursday: guest lecture (Alice Conovitz, Integral)  Lab  Lab 1: Phase I ESA  re-write grading in progress  Lab 3: Soil sampling  grading in progress  Lab 4: Well drilling  grading in progress  Lab 5: Slug tests  lab report due Wed Nov 11

Announcements  Lab  Lab 6 – geophysics; resistivity and GPR  tentative  Sampling and Analysis Plan (SAP)  description of sites  SAP resources (submission)  SAP outline (submission)  SAP report (submission)

Sampling and Analysis Plan  Site 1: Boulder Town Gas Plant The City of Boulder purchased the property in 1975 and became aware of subsurface contamination in The City decided to address the contamination with a voluntary cleanup. The site was characterized in and the voluntary cleanup plan (VCUP) was submitted in October Prepare a sampling and analysis plan for the site characterization that was conducted before submission of the VCUP.

Sampling and Analysis Plan  Site 2: Burlington Mine The Burlington Mine underwent a voluntary cleanup in to prevent interaction between surface waters and waste rock, but acid mine drainage still discharges from a collapsed mine opening into Little James Creek. The EPA has considered the site for Superfund designation. Prepare a sampling and analysis plan to characterize the site in its current condition for risks relative to Superfund designation.

Sampling and Analysis Plan  Site 3: Valmont Butte Mill The City of Boulder acquired the Valmont Butte property in Over the next few years, various uses were proposed, but concerns surfaced about the effect of contamination on these uses. A site investigation was conducted in 2009 to provide information for a voluntary cleanup plan (VCUP), which was submitted in Prepare a sampling and analysis plan for the site characterization conducted in 2009 to provide information for the VCUP.

Sampling and Analysis Plan  Site 4: Captain Jack Mill The EPA added the site to the National Priorities List (the “Superfund list”) in The Remedial Investigation, Feasibility Study, and Proposed Plan were completed in 2008 and the remediation was mostly completed by The site is currently owned by a mixture of federal agencies, Boulder County, and private parties. Boulder County has an interest in purchasing the site to extend the Switzerland Trail and create a mountain bike park. Prepare a sampling and analysis plan to characterize the site for suitability for this proposed use.

Sampling and Analysis Plan  Site 5: Boulder Cleaners Contamination was initially discovered at the site in 2000 and monitoring was continued over the next decade. In 2013, environmental consultants proposed to reduce contamination by injecting zero-valent iron into the groundwater. The reaction of compounds like PCE with iron should result in degradation by reductive dechlorination. Prepare a sampling and analysis plan to monitor the effectiveness of the zero-valent iron injection for reducing groundwater contamination.

Sampling and Analysis Plan  Site 6: Centerline Circuits Disposal of chlorinated solvents to the plant’s septic system in the 1960s led to groundwater contamination that affected the drinking water supplies of over a dozen North Boulder residents in the late 1980s. The residents were eventually transferred to City of Boulder drinking water, but the groundwater contamination was never addressed. Prepare a sampling and analysis plan to assess the spread of the groundwater contamination in North Boulder for the design of a pump-and-treat remediation plan.

Field Geophysics  Methods  seismic – refraction, reflection  gravity – density contrasts and anomalies  geomagnetic – induced rock magnetism  electromagnetic – ground-penetrating radar  electrical – resistivity  borehole – well-logging  radiometric – radioactive decay and isotope dating  geothermal – heat flow and conduction

Field Geophysics  Methods  seismic – refraction, reflection  gravity – density contrasts and anomalies  geomagnetic – induced rock magnetism  electromagnetic – ground-penetrating radar  electrical – resistivity  borehole – well-logging  radiometric – radioactive decay and isotope dating  geothermal – heat flow and conduction

Field Geophysics  Passive  naturally-present sources and fields to create image of subsurface  gravity field  magnetic field  earthquakes  heat  Active  man-made sources to create image of subsurface  hammer, dynamite, air guns, …  electromagnetic waves  electrical currents

Field Geophysics  Choice of methods  what relevant physical properties?  porosity, permeability, seismic velocity, density, …  what spatial scales?  meters, tens of meters, hundreds of meters, kilometers, …  what field conditions?  e.g., urban, wetland, off-shore, …  any useful prior information?  e.g., previous geophysical surveys  any cheaper alternatives?  e.g., water level measurements

Field Geophysics  Some references  Burger H.R., Sheehan A.F., and Jones C.H., Introduction to Applied Geophysics: Exploring the Shallow Subsurface. W.W. Norton, 550 pp.  Reynolds J.M., An Introduction to Applied and Environmental Geophysics, 2 nd Ed., Wiley, 710 pp.

Field Geophysics  References  Burger H.R., Sheehan A.F., and Jones C.H., Introduction to Applied Geophysics: Exploring the Shallow Subsurface. W.W. Norton, 550 pp.  Reynolds J.M., An Introduction to Applied and Environmental Geophysics, 2 nd Ed., Wiley, 710 pp.

Field Geophysics

VMD (EM31): vertical magnetic dipole; electromagnetic ground conductivity HMD (EM31): horizontal magnetic dipole Magnetometer

Field Geophysics

 Applications forensic geophysics hydrocarbon exploration (coal, gas, oil) regional geologic studies (100s km 2 ) exploration for mineral deposits engineering site investigations hydrogeological investigations detection of subsurface cavities mapping of contaminant plumes location of buried metallic objects archaeogeophysics

Field Geophysics  Applications forensic geophysics hydrocarbon exploration (coal, gas, oil) regional geologic studies (100s km 2 ) exploration for mineral deposits engineering site investigations hydrogeological investigations detection of subsurface cavities mapping of contaminant plumes location of buried metallic objects archaeogeophysics

Field Geophysics  Applications forensic geophysics hydrocarbon exploration (coal, gas, oil) regional geologic studies (100s km 2 ) exploration for mineral deposits engineering site investigations hydrogeological investigations detection of subsurface cavities mapping of contaminant plumes location of buried metallic objects archaeogeophysics

Field Geophysics  Applications forensic geophysics hydrocarbon exploration (coal, gas, oil) regional geologic studies (100s km 2 ) exploration for mineral deposits engineering site investigations hydrogeological investigations detection of subsurface cavities mapping of contaminant plumes location of buried metallic objects archaeogeophysics

Field Geophysics  Interferences

Field Geophysics  Influence of spatial sampling resolution

Field Geophysics  Influence of spatial sampling resolution

Field Geophysics  Influence of spatial sampling resolution

Field Geophysics  Influence of spatial sampling resolution  inverse problem often non-unique  reinforce interpretation with  multiple geophysical methods  geologic knowledge

Field Geophysics  Property contrasts  electrical resistivity

Electrical Resistivity  Electric current through ground  survey of variations in electrical resistance  Ohm’s Law  resistivity (ohm m;  m)  apply known potential difference (V) to circuit through resistive material of known area and length

Electrical Resistivity  Electric current through ground  circuit through multiple units (layers)  series or parallel

Electrical Resistivity  Resistivity of geologic materials  metallic ores  decrease resistivity  temperature  higher T with depth decreases resistivity  water saturation  higher water, lower resistivity  higher salinity, lower resistivity  anthropogenic features  cavities increase resistivity

Electrical Resistivity  Porous water-saturated rocks  ionic conductors  Archie’s Law (empirical)  t resistivity (ohm m) a  0.5 – 2.5  w density  porosity m  1.3 (Tertiary) – 2.0 (Paleozoic) for cementation s w water saturation n  2 if sw  0.3

Electrical Resistivity  Rock and mineral resistivity

Electrical Resistivity  Subsurface current paths  70% of current within depth equal to electrode separation  electrode spacing typically 2  target depth

Electrical Resistivity  Typical setup  number of electrodes: usually four  applied voltage: about 100 V  applied current: milliamps or less (shock possible)  current flow reversed a few times per second

Electrical Resistivity  Vertical electrical sounding  identification of layers of different resistivity  depth profiling by expansion of electrode spacing

Electrical Resistivity  Vertical electrical sounding  current refraction  variation in resistivity with depth

Electrical Resistivity  Current flow path geometry changes  greater resistance measured over longer flow path  apparent resistivity  accounts for geometric factor    = a/l for rod shape

Electrical Resistivity  Wenner array of electrode spacing  equal spacing between four electrodes  apparent resistivity  spacing increased progressively  spacing increased until 2  desired depth  modern systems include many electrodes to minimize re-arrange- ment time

 Wenner survey  results plotted as log  a versus log a  inverse problem  master curves calculated for values of resistivity and layer thickness  computer codes Electrical Resistivity shallower depths deeper depths

 Wenner survey  two layers, or more than two layers Electrical Resistivity

 Lateral changes in resistivity Electrical Resistivity

Ground-Penetrating Radar  GPR  Radio Detection and Ranging (RADAR)  initial interest in detecting ice depth (U.S. Air Force)  first commercial systems 1974  high resolution (cm scale)  contrast in electrical and magnetic properties detected with antenna

Ground-Penetrating Radar

 GPR contrast for layers  dielectric permittivity   electrical conductivity   must be greater than 50 ohm m for GPR  magnetic permeability   attenuation of radar wave

GPR  Example of field application

Field Geophysics  Seismic

Field Geophysics  Seismic Material V p (m s -1 )

Field Geophysics  Gravity

Field Geophysics  Gravity density range average density material (Mg m -3 ) (Mg m -3 )

Field Geophysics  Geomagnetics

Field Geophysics  Geomagnetics magnetic mineral or rock susceptibility

Next Lecture  guest lecture Thursday (Alice Conovitz, Integral)  guest lecture Tuesday (Ned Turner, Indian Peaks)