Electrical Conductivity & Hydraulic Profiling The Combining of Two Subsurface Investigation Methods (With More to Come)
How has field work been progressing? Split Spoon and Standard Penetration Testing (SPT) Thin walled tube samples Soil Resistivity Testing Upgrades to SPT with the auto-hammer Cone Penetration Testing (CPT) Upgrades to CPT to include seismic testing Electrical Conductivity (soil resistivity going down-hole) Hydraulic Profiling Tool (HPT) Membrane Interface Probe (MIP)
What does EC provide? Electrical Conductivity (EC) measures the flow of electrical current through soil over a three inch interval. Based on the soil conductivity measurement, software estimates the soil type. Small particles with a larger surface to surface contact area transmit current better than larger particles that have less surface to surface contact.
The EC Probe The EC probe has four electrodes that send and measure electrical current. Measurement is similar to soil resistivity using the Wenner 4- probe system.
What Does Hydraulic Profiling Provide? HPT is used in conjunction with EC logging to assist in soil classification, identify permeable and impermeable zones in a soil profile, and determine groundwater depths.
EC & HPT Platforms EC & HPT are designed to work from direct push platforms The instrumentation is hydraulically pushed or advanced by hammering Logging depths are measured by attaching a string pot from the direct push unit to the advancing tool string.
Geoprobe 6620DT with EC/HPT
How does HPT work? The Hydraulic Profile Tool (HPT) measures the volume of flow and pressure required to inject water into the soil. Volume is used to determine the soil type Higher flow occurs in sand and gravel, or in loose materials Lower flow occurs in clays, or in dense materials Pressure is also used to determine soil and groundwater properties. Higher pressure can indicate clays or dense soil conditions Pressure that increases with increase depth can indicate depth below water Low pressure can indicate sand, gravel, or loose material
The HPT Probe The HPT portion of the probe has a screened discharge port to allow the water to be pumped into the soil formation. Both flow rate and pressure are measured. New models can also be reversed to collect water samples from an interval.
What can EC & HPT provide together? Using the EC & HPT data together we can better define: Soil types Location of the water table Soil density (relative measure) Estimated Soil permeability
Applications Water Well Siting Environmental Remedial Design Geotechnical Borrow Sources Directional Drilling
Some pros of EC & HPT Relatively simple to use Productivity 300 to 400 linear feet per day Low soil disturbance 1.5-inch diameter hole Produces no soil cuttings Data available on-site as the work progresses Direct push platform no tripping in and out of rods
Some cons of EC & HPT No physical soil samples are collected No direct soil strength data Use of the HPT can be limited in freezing conditions Exploration depth limited to the available electrical and fluid line Limited to soils that can be readily penetrated by hydraulic pressure and hammer action. Trouble shooting electrical problems may require some talent
Equipment for EC EC logging can be performed independently of HPT. EC requires the instrumented probe EC/HPT Rods typically 1.5 inch diameter The EC cable The Direct Imaging Unit A power source
The Probe The EC portion of the probe is near the base of the cone. It has four pins that produce or measure current. The HPT portion of the probe is located above the EC pins and provides a port for fluid to be introduced into the formation.
Setting up the equipment The equipment stack from top to bottom: EC/HPT Direct Image Unit (records the data) HPT Controller (controls pump pressure and flow rate) HPT Metering Pump (the pump unit)
The connections Water and electrical inputs on the back of the units. Everything is labeled and connections are made to fit only where they belong.
Setting Up The software walks you through the calibration and test set up
EC Calibration The electrical conductivity system is tested using a test conductivity block attached to the probe sensors. The EC software runs a self-test and will not allow data collection if anything is out of tolerance.
HPT Calibration The hydraulic profiling tool is calibrated by filling a test cylinder and measuring the pressure.
HPT Calibration Test Tough to see the pressure increasing as the calibration test cylinder fills.
EC & HPT Field Set Up
Trunk Line Length To provide ease of access when handling the rods the trunk line should be at least 25 feet and preferably 50 feet longer than the exploration depth.
Water Source The HPT flow rate is around 250 ml per minute (less than 0.07 gallons per minute) or about 4 gallons an hour. The two 5-gallon buckets provide the water source and a return.
HPT/EC Rod Carrier
Stringing the trunk line through the rods
Evaluating data on-site
Lessons Learned Where there is steam there is a problem. Encountering a hard strata that required extensive hammering heated the drive cap and melted the EC/HPT cable.
What it takes to run
The Problem Water bills are increasing, the available water is decreasing and the public wants answers.
Real Project Application Water Well Water well siting in Medicine Lodge, Kansas New municipal well location and depth is determined by locating the largest permeable zones with high flow capacity. A hydrogeologist analyzed the EC & HPT data to prepare crosssections of the area They located the largest sand seams using EC data, confirmed flow using HPT flow data and assessed the depth of water using HPT pressure data. From the data they sited the well and are preparing for a pump test.
What the client had available The client had a quarter section (160 acre) project site to investigate Subsurface data was limited to the Kansas Geologic Society on-line logs sporadic wells not consistently logged. A limited budget that must include: site investigation, test wells, pump testing, and production wells.
Using EC/HPT versus Auger Borings EC/HPT 14 locations varying from 60 to 80 feet in 2.5 days Continuous soil profile Continuous hydraulic flow data No spoil material to handle Client estimated cost savings of 60% Auger Boring with SPT 16 locations in 6 days Sampled at specific intervals No permeability data Problems with augers locking in the sand
Verifying the Conditions Based on the EC/HPT data, two auger borings were advanced at possible test well locations to verify the conditions. The EC/HPT corresponded very well with the auger samples.
Is it accurate? An EC/HPT log has been overlaid with the results of three split spoon samples taken at specific intervals. The results were remarkable.
What the final results look like
Installing the Test Well Once the subsurface conditions were verified with auger borings and split spoon sampling, a large diameter test well was installed.
Additional Data Based on the EC data, the clients hydrogeologist identified a possible issue with chlorides in the soil profile. An additional boring will be performed to collect water samples at specific intervals to determine if zones in the profile contain high chlorides.
Pump Testing Once the well was installed, a short term pump test was performed. The production was over 85 gallons per minute with a 24-inch draw down at the well. A long term well test will be performed this month.
An Environmental Project Use The project consisted of a spill that covered tens of acres and was believed to have migrated across a stream onto adjacent properties. The consultant needed to determine the lateral extent of the spill migration and needed to determine how deep the spill penetrated. Once the extent of the spill was determined, the consultant needed to design a remediation program.
What the Consultant Did The investigation program was on private properties with picky landowners Membrane interface probes were used on track mounted CPT rigs to determine the extent of the spill both laterally and in depth. Electrical conductivity logging was performed to depths of more than 125 feet to profile the soil and groundwater conditions. The consultant evaluated the soil, groundwater and spill data to determine a remedial plan.
The Plan The consultant remobilized to the site and used EC/HPT logging to define the soil types and permeability for a combination of injection and pump and treat remediation.
Remediation The consultant is working on a remediation plan and pilot test for the use of injection and/or pump and treat system.
Why did they use MIP, EC and HPT? The MIP provided the client with environmental data on-site in real time allowing them to adjust the testing locations. MIP, EC and HPT limited the disturbance to the properties and produced no soil cuttings. The schedule was expedited using these techniques over auger sampling that would require sending samples to an off-site laboratory for analysis.
Wrapping It Up the Pro s EC/HPT is a great tool for classifying soils and estimating permeability relatively quickly with little site disturbance. The combination of EC/HPT/MIP can provide consultants with data on the soil type, permeability and potential contamination in a single boring and in real time while on site saving time and money. In the right subsurface conditions investigation depths can exceed 125 feet. Its relatively easy to learn and use.
Wrapping It Up - the Limitations EC/HPT/MIP can only be used where advancement using hydraulic pressure and hydraulic hammering are possible. No physical samples are obtained using these methods. Separate borings are required to obtain samples. No strength data is obtained using these methods.
Wrapping it up Where are we going? In the last 10 years technology and the people using it have improved. Formerly considered Black Magic by engineers and geologists, technology like the CPT are becoming more readily accepted and used. The benefits of EC/HPT and MIP will become recognized and more readily accepted and used. Use of any of these technologies is dependent on competent field and office personnel. None of these can replace auger drilling, but will likely be used in conjunction with drilling.
Drillers Out Standing in Their Field