Membrane Interface Probe
- Logs VOC contaminants and soil EC with depth
- Shows distribution and relative magnitude of VOC contaminants
- Real time mapping of petroleum and chlorinated VOC plumes
- Used worldwide for characterization of subsurface VOC contaminants
- Only tool capable of logging VOCs in situ in real time
The MIP (membrane interface probe) is a direct push tool used to log the relative concentration of volatile organic compounds (VOCs) with depth in soil.
Fig. 1 : An MIP log showing an EC soil log (left) with MIP-XSD contaminant log (right).
The MIP was developed by Geoprobe Systems® (U.S. Patent No. 5,639,956) and is presently manufactured and sold exclusively by Geoprobe Systems® and its distribution agents. The MIP has been used extensively in the U.S. and Europe for mapping the extent of VOC contamination in the subsurface.
Principle of Operation:
The Membrane Interface Probe (MIP) is a screening tool with semi-quantitative capabilities acting as an interface between volatile contaminates at depth in the soil and gas phase detectors at the surface (Fig. 2). The MIP membrane is semi-permeable and is comprised of a thin film polymer impregnated into a stainless steel screen for support. The membrane is approximately 6.35mm in diameter and can be easily replaced. The membrane is placed in a heated block attached to the probe. This block is heated to approximately 100-120 degrees C as the probe is advanced into the soil. Heating the block accelerates membrane diffusion while at the same time minimizing membrane absorption.
Fig. 2: The MIP Principle of Operation. The downhole, permeable membrane serves as an interface to a detector at the surface. Volatiles in the subsurface diffuse across the membrane and partition into a stream of carrier gas where they can be swept to the detector. The membrane is heated so that travel by VOCs across this thin film is almost instantaneous. MIP acquisition software logs detector signal with depth.
Diffusion across the membrane is driven by the concentration gradient between the contaminated soil and the clean carrier gas behind the membrane. A constant gas flow of 35-45 mL/min sweeps behind the membrane and carries the contaminants to the gas phase detectors at the surface. Travel time from the membrane interface to the detector(s) is approximately 30-45sec (depending on the length of trunkline and flow rate).
Sweep gas from the MIP membrane is directed to detectors that are part of the MIP instrument system at ground surface (Fig. 3). The standard detectors installed by Geoprobe Systems® include the photoionization detector (PID), the flame ionization detector (FID), and the halogen specific detector (XSD). These detectors are employed in series with each detector providing sensitivity to a particular group or type of contaminant. The XSD is highly specific to halogenated compounds and is the best detector when the MIP is used for logging chlorinated solvent plumes or source areas (TCE, PCE, Carbon Tet, etc.). The PID provides sensitivity to aromatic compounds (BTEX) as well as confirmation of chlorinated ethylene compounds detected by the XSD. The FID is a general detector useful for hydrocarbon detection as well as confirmation of high concentration of all compounds seen on the other two detectors.
Le MIP Log :
In practice, multiple MIP logs are used on a site to find a source area of contaminant, to track the movement of a contaminant plume, to define the mass of contaminant for remediation, or to investigate the efficacy of a remediation treatment. Whatever the purpose of the MIP logging, there are always multiple logs used at any one site.
The MIP log shows the response of the MIP detectors with depth in the soil. The MIP data acquisition system makes this log by taking into account the travel time required for the membrane sweep gas to reach the MIP detectors at ground surface, thus converting the detector time data into MIP response with depth data. An example series of MIP logs from a contaminated site are shown in Fig. 4. The logs in this series, or cross section, show the response of the XSD detector in the MIP system with depth. The contaminant at this site is a mixture of Tetrachloroethylene (PCE) and Trichloroethylene (TCE). This figure shows the main utility of MIP logs in that the position of the contaminant, which varies from location to location, is described by the MIP log. The concentration of the contaminant also varies across the site, which is described by the wide range of responses shown on the MIP logs.
Fig. 4 : A series of MIP logs from a site contaminated with PCE and TCE. The logs shown above are MIP-XSD (halogen specific detector) logs. These logs illustrate the use of MIP in defining the depth at which contaminants are encountered. The concentration represented in each of these logs is also vastly different. The first log has a scale of 105 microvolts (μV) response, the second 106 μV, and the third 107 μV. All three logs are from close proximity on the same site.
In practice, an MIP detector log is seldom used apart from a simultaneous log indicating lithology. This accompanying log can be of electrical conductivity (EC), hydraulic profile (HPT), or CPT parameters.
Figure 5 shows an MIP-FID log along with the EC at this location. This example log shows the true power of this combination of logging tools. The contaminated zone (indicated by the MIP log) is located in a fine grain unit (indicated by the elevated response on the EC log).
Fig. 5 : A typical suite of MIP logs including an EC log (left), and MIP-FID (right). This log was obtained on a hydrocarbon contaminated site. The MIP-FID indicates the majority of contaminant mass is at a depth of 6 to 8 meters. The EC log indicates a fine grain soil unit at this depth.
MIP equipement : Geoprobe Systems® manufactures all of the equipment needed for MIP logging. This equipment can be divided into two basic categories: surface instrumentation (MIP controller, data acquisition, and detectors), and downhole probes (including probes, trunklines, connectors, etc.).
Fig. 6 : MIP Instrumentation
A basic set of MIP instruments is shown in Figure 6. These instruments include the following as labeled in the Figure
- FI6003: Data acquisition instrument, acquires data from the MIP system’s detectors and sensors and relays it to the computer via a USB connection. The FI6003 is the general data acquisition instrument used in all Geoprobe® DI logging systems (EC and HPT).
- MP6507 Series MIP Controller: This instrument regulates and measures gas pressure and flow to the MIP probe and controls heating of the probe. Data from this controller is sent to the FI6003 via a data cable.
- Detection system: This is the platform of detectors used in the MIP System. Standard detectors offered in this system include the PID, FID, and XSD. Analog data outputs from this instrument are directed to theFI6003.
Basic downhole MIP equipment is shown in Figure 7. There are many variations and combinations of this tooling, depending on the size of the rod string being driven into the ground and the lithology or permeability sensors that are to be used in combination with the MIP. The standard, most commonly deployed components are shown in Figure 7. These components include:
- MP6520 MIP probe: 120 VAC heated probe with removable membrane and dipole electrical conductivity array.
- MIP Trunkline (P/N 14929): 150 ft. (46m) with PEEK return line.
- Connection section and drive head. Gas line and electrical connections are carried in this section.
- Probe rods. Geoprobe®5 inch (38mm) rods are the most commonly used for MIP logging. Successive sections of these rods are added to push or percussion drive the probe to depth.
MIP Tool Configurations :
There are several standard configurations of the MIP probe. Tool string diagrams showing part numbers for these various configurations are found on this website http://geoprobe.com/mip-tool-string-diagrams.
Standard probe configurations are shown in the following figures:
Fig. 10 : MIP-CPT. The standard Geoprobe® MIP sub (P/N 20131) with Geotech AB – NOVA CPT probe. This configuration combines both EC and CPT measurements with the MIP. This tool combination features easy connection of the CPT cone. CPT deployment of MIP typically yields much longer life for the MIP probe compared to percussion driven models.
Purchasing a MIP System
The MIP system is available exclusively from Geoprobe Systems® and its authorized distributors. Please contact Geoprobe® Environmental Technologies for a quotation for this equipment. Companies who already operate Geoprobe® EC or HPT systems may already have some of the instrumentation required to operate an MIP system (such as the FI6003 and various probe pushing hardware). All MIP operators must receive a proper training (usually 3 days). This training is designed to teach proper set-up and operation of the MIP system, practice of log Quality Assurance (QA) and Quality Control (QC) procedures, field trouble shooting, and log interpretation. Training is typically designed to include field exercises in MIP logging.