Direct Push Logging

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Direct push (DP) technology can be used to advance a variety of logging tools into the subsurface to provide high-resolution information on vertical variations in soil and unconsolidated formation properties. Commonly used tools provide qualitative and/or quantitative indicators of soil strength (cone penetration test (CPT)), salinity and/or clay content (electrical conductivity (EC)), permeability (hydraulic profiling tool (HPT)), levels of volatile organic chemicals (membrane interface probe), and fuel related hydrocarbons as non-aqueous phase liquids (NAPL) (optical imaging profiler (OIP)).

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CONTRIBUTOR(S): Wesley McCall, M.S., P.G. and Tom Christy, P.E.


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Introduction

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Figure 1. General components of a DP logging system.

DP logs are run in unconsolidated materials, often to depths of 50 ft [15 m] and depths of 100+ ft [30+ m] may be achieved under amenable conditions. The first logging tool for DP application was the cone penetration tool, which was developed in the 1930s for geotechnical applications[2]. Several new DP logging tools have been developed since the 1980s focused primarily on geo-environmental investigation needs[1][3]. These tools are pushed or driven into virgin (no pre-existing borehole) formation to perform logging of various physical and chemical properties of the formation. A string pot or similar device, is used to track the depth of the probe. Most of these logging systems have several features in common (Fig. 1). The probe provides an interface between the measurement device and the formation being logged. The measurement device may consist of electrodes, a semi-permeable membrane, a screen, load cells or transducers for measurement of pressures, or other devices. Some probes have an optical window to allow for illumination of the formation with visible light for photos or video or illumination with ultra-violet (UV) light to monitor for fluorescence of some organic compounds. There is often a wiring cavity immediately above the probe where wiring and plumbing connections are made to the trunkline. The trunkline is a cable with plastic cladding that contains all of the electrical wiring and any tubing for fluid flows. For efficiency, the trunkline is pre-strung through all of the drive rods before any tooling is advanced into the subsurface. The top end of the trunkline is usually attached to an analog-to-digital instrument that converts the analog signals from the probe downhole to digital signal for output to software on a laptop computer for live-time viewing and saving data. When spaced across the site appropriately the DP log data may be used to develop high resolution site models for the parameter(s) investigated.

Cone Penetration Testing (CPT)

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Figure 2. A CPT cone (left) and piezocone (right) with porous ring.
Figure 3. A CPTu log displaying left to right: lithologic graph based on log interpretation, tip resistance, sleeve friction, pore pressure and friction ratio.

CPT tests are primarily used to assess soil strength or bearing capacity for foundations. More recent work has led to use of CPT data for soil classification[4][5] and also for environmental investigations[6][7], where it may be used to understand unconsolidated formation stratigraphy. The cone is advanced at approximately 2 cm/sec for logging. Often a large truck weighing 10 tons or more is used to advance the cone. However, smaller vehicles including DP track machines may use soil anchors to provide resistance for pushing the cone. The modern cone penetrometer is designed with a conical tip and a friction sleeve (Fig. 2). A load cell behind the conical tip measures the tip resistance (qc) required to push the cone through unconsolidated materials. A separate load cell behind the sleeve measures the frictional force exerted on the sleeve (fs) as the probe is advanced. Logs obtained using tip resistance and sleeve friction data are designated as CPT logs. A porous ring may be placed behind the point so that pore water pressure can be measured with a pressure transducer. This device is called a piezocone and logs obtained with this system are called CPTu logs. The pore pressure data helps with determining soil type and also is used to correct tip resistance. When advancing a piezocone the operator may stop probe advancement and perform a pressure dissipation test. The pressure dissipation test may be used to obtain estimates of local hydraulic conductivity[5]. CPTu logs typically display data plots of tip resistance (qc), sleeve friction (fs), and pore pressure (u). The friction ratio (Rf) is often used in classifying the soils and some CPT software programs will provide a soil classification log along with the digital data (Fig. 3).

Electrical Conductivity (EC)

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Figure 4. A Wenner array probe (top) and a dipole array built into an MIP probe.
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Figure 5. A typical EC log with rate of penetration (ROP) plotted on left and bulk formation EC on right. The decrease in EC and ROP at ~29 ft indicates transition to coarser grained materials (sands).

Electrical conductivity (EC) and it’s inverse, resistivity, have been used for petroleum and mineral resources investigations[8] and groundwater resource assessments[9] for many years. Conversely, soil scientists have typically used EC for determining the salinity of soils[10]. EC logging tools for DP application were developed more recently[1][11] and are sometimes pushed statically with a heavy CPT truck or advanced dynamically with smaller percussion probing machines. Often EC probes are designed with four evenly spaced electrodes (Wenner array) while many other DP logging tools will include a simple dipole array (Fig. 4). The volume of formation measured is proportional to the length of the EC array. While a Wenner array averages over a larger volume of formation, a dipole array can detect smaller features, such as thin clay layers within a large sandy aquifer. In fresh water formations, the bulk electrical conductivity of the formation is most often controlled by clay content. Clays typically have a higher electrical conductance than silts and sands so that higher EC values usually correlate with increased clay content and lower permeability[1][12]. However, some clays do not exhibit high EC. Also, when ionic contaminants or ionic remediation fluids are present, they may cause a very high bulk formation EC, masking variations in natural formation EC readings. EC logs have been obtained to depths exceeding 100 ft (30 m) with DP methods in many unconsolidated formations. Direct push EC logs have been found to be useful for many geo-environmental investigations[1][11][12][13][14][15][16][17]. The logs provide data on bulk formation EC at the inch scale versus depth (Fig. 5). They usually include a speed graph which can help interpret formation changes. EC logs may be used to locate brine plumes or seawater intrusion and help to define the distribution of ionic injectates (e.g. sodium persulfate) used for contaminant remediation. An EC array is often run in tandem with other DP logging sensors to provide additional information on subsurface lithology.

Hydraulic Profiling Tool (HPT)

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Figure 6. Schematic of an HPT probe.
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Figure 7. An HPT log with EC on left, injection pressure in middle, and flow rate on right. Note good agreement between the EC and HPT pressure logs. Mostly coarse grained below 29 ft with 2 clay layers defined. See also Fig. 5.

Formation permeability is important in controlling subsurface contaminant migration. Permeable zones that are continuous form migration pathways for contaminants to reach receptors. Recent studies have found that clay-rich, low permeability zones may behave as sinks for many organic contaminants which are later released by back diffusion[18][19] greatly extending the life of the contaminant plume. The HPT probe[20][21][22] and similar devices[23][24][25][26] were developed to better understand formation permeability and the distribution of permeable and low permeability zones in unconsolidated formations. While advancing the HPT probe at approximately 2 cm/sec, water is injected at a rate of about 300 ml/min through a small screened port on the side of the probe (Fig. 6). A pressure transducer located just above the probe measures the pressure required to inject water into the formation. The injection pressure and flow rate are plotted versus depth to provide a log of relative formation permeability (Fig. 7). Interpretation of the HPT log is relatively simple with higher HPT pressure indicating lower permeability and vice versa, at a constant flow rate. HPT probes typically include an EC array. Under most field conditions in fresh water formations, the EC log and HPT pressure log will display a similar trend. Sometimes the EC and HPT pressure logs do not correlate. This is especially true when high concentrations of ionic contaminants are present in the groundwater. This behavior makes the HPT-EC probe useful for tracking brine plumes, seawater intrusion, and the distribution of ionic remediation injectates. Some of these logging devices can be used to collect groundwater samples at multiple depths during logging[25][27]. Pneumatic slug testing[28][29] of DP groundwater samplers may be used to confirm HPT permeability data and obtain quantitative data for hydraulic conductivity.

Membrane Interface Probe (MIP)

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Figure 8. Schematic of MIP depicting principles of operation.
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Figure 9. An MIP log left to right: EC log, XSD, PID and FID detector responses versus depth identifying presence and levels of VOC contamination.

The MIP was developed for investigation of volatile organic compounds (VOC) and light petroleum fuel contaminated sites in unconsolidated formations[30][31][32]. The MIP probe has a semi-permeable membrane on one side (Fig. 8) that allows VOCs to pass through, while minimizing the movement of water across the membrane. A heater block on the probe face enhances movement of VOCs across the membrane and into a nitrogen carrier gas stream. The carrier gas and contained VOCs travel up the trunkline to gas phase detectors at the surface. Detectors commonly used with the MIP include a photo-ionization detector (PID), flame ionization detector (FID) and a halogen specific detector (XSD). All three detectors can run simultaneously[32]. The PID is particularly sensitive to aromatic hydrocarbons like benzene and toluene, but will also detect the double bonds present in many chlorinated hydrocarbons like trichloroethylene (TCE) and perchloroethylene (PCE)[33][34]. The FID is effective at detecting fuel hydrocarbons (e.g. hexane and octane) and other alkanes and aromatic hydrocarbons[35][36]. The XSD is designed to detect chlorinated VOCs like dichloroethene (DCE), TCE and PCE[37]. The MIP does not provide for separation of analytes by chromatography for specificity but provides for total VOC detection[30][31]. The probe is advanced in one foot (~30 cm) increments across zones of contamination, at each depth increment the probe advancement is halted for about one minute[31][32]. This allows time for VOCs to cross the membrane and be swept up the trunkline to the detectors at the surface. This results in peaks in the detector response when contaminants are present (Fig. 9). The scale of the detector response is proportional to the level of contamination, with higher responses indicating more contamination. Several factors influence the MIP response such as formation grain size, organic carbon content, moisture content, and the analytes affinity to partition between the solid, liquid and gas phases. Thus, MIP logs provide a semi-quantitative indication of concentration[30][31]. A dipole electrical conductivity array is included near the MIP probe end to give information about formation lithology. The MIP and hydraulic profiling tool (HPT) also have been combined into one probe. This combination allows investigators to look at relative formation permeability along with VOC contaminant distribution[38]. Studies have been done to evaluate MIP operation and detection of DNAPLs[39][40][41][42][43][44].

Optical Imaging Profiler (OIP)

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Figure 10. Schematic of the OIP probe.
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Figure 11. OIP log with EC plotted on left, % area of fluorescence plotted on the right graph (shaded). Example of a captured image displaying fluorescence (mid-right) and analysis of the image used to determine % area of fluorescence below. Fuel product is focused in the area of lower EC (higher permeability) above the water level at this site. Visible light image (top center) provides information on texture and color at the selected depth.

The OIP is designed to detect UV (ultraviolet) light induced fluorescence of light nonaqueous phase liquids (LNAPL), primarily fuels and related hydrocarbons, in unconsolidated materials. Chlorinated VOCs (TCE, PCE, etc.) do not fluoresce under UV light. The OIP probe is constructed with a sapphire window and light emitting diodes (LED) illuminates the soil (Fig. 10). A digital camera is mounted inside the probe, behind the window, to capture images. Logging is performed with the UV LED and images are analyzed at 30 frames per second with one image also captured every 15 mm of depth and saved in the log file. Each image covers an area of 9.5 x 7 mm. Images are displayed onscreen as the probe is advanced at 2 cm/sec. The images are analyzed in real time in the acquisition software for percent area displaying fluorescence. The operator may halt advancement of the probe at any time and capture a visible light image. The visible light images are saved with the file and markers on the log indicate depths where these images were acquired. Visible light and UV images saved with the log file can be reviewed at any time with the viewing software (Fig. 11) and used in reports or presentations. Visible light images enable the investigator to observe soil texture and color at selected depths. The probe also includes an EC dipole to provide information on soil type/lithology. Several logs can be plotted in the viewing software to provide simple 2D cross sections of lithology and product distribution. The log data may be imported into graphical software packages to create detailed 2D or 3D plots. The 2D and 3D plots may be used to develop high resolution site models to guide sampling and remediation efforts.

Summary

Several direct push (DP) logging tools have become primary devices used to conduct high-resolution site characterization (HRSC) in the environmental industry. Probes such as the CPT, EC, and HPT help to define soil types, lithology, and hydrostratigraphy. These devices are often used to define migration pathways or hydrogeologic barriers. Other logging tools, such as the MIP and OIP, help to define the presence and extent of volatile organic contaminants or fuels, respectively. Many site investigators use 2D and 3D plots of DP logging data to generate detailed conceptual site models of subsurface soils and lithology and/or contaminant distribution. Some logging tools have been combined, such as the MIP and HPT into MiHpt, to provide information on lithologic controls of contaminant distribution in one log[38]. Targeted sampling and analyses of soils and groundwater are recommended to confirm log results.

References

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