Permeable membrane soil probe

A permeable membrane sensor probe has a housing with a gas permeable membrane. The membrane has an outer surface and an inner surface. The membrane outer surface is disposed adjacent an outer soil engaging surface of the housing. The membrane is adapted to allow the gas phase of at least one particular chemical compound found in the soil to permeate through the membrane from the membrane outer surface to the membrane inner surface while substantially preventing the transfer of liquids and solids found in the soil through the membrane. A gas detecting arrangement is disposed adjacent the membrane inner surface and inside of the housing. This detection system detects the presence of a particular compound at different soil levels.

This invention relates to a soil probe which is driven into the ground, 
and, more particularly, to a soil probe for sensing and/or measuring 
naturally occurring compounds and/or foreign contaminants in soil or 
ground water. 
In recent years, small diameter soil probing tools have been increasingly 
used for subsurface investigations. These tools are typically driven into 
the ground using percussion hammers and are primarily used for sampling 
soil vapor, soil cores, or ground water. With the increasing usage of 
these probing tools, improvements have been made in the tools and the 
driving mechanisms such that the depth of investigation at which the 
probing tools are used is gradually increased. One use of such probing 
tools has been the exploration of a site for naturally occurring compounds 
or foreign contaminants in the soil and ground water of the site. The 
investigation of such a site, however, involves a very labor intensive and 
time consuming operation. More specifically, to determine whether 
particular compounds or contaminants are present at a particular soil 
level, an actual soil or ground water sample is taken at that level. To 
perform such sampling, a special probe is positioned on the lower end of a 
probe rod string and driven into the ground to the particular level at 
which a sample is to be taken. The probe is actuated in some manner to 
allow collection of actual soil or ground water found at that level. The 
probe rod string is then removed from the ground to retrieve the probe and 
soil sample therein. Thereafter, the soil sample or ground water sample is 
removed from the probe and analyzed using various detection and 
quantifying instruments. As is apparent, if it is desirable to investigate 
a site to determine the presence and/or quantity of certain compounds or 
contaminants at a variety of different levels, the use of this direct 
soil/ground water sampling operation can involve a substantial amount of 
labor and time. More specifically, to detect compounds or contaminants at 
various levels the probe rod string must be driven to each particular 
level, a sample taken, and thereafter the sample removed from the ground 
by removing the probe rod string. Thus, numerous iterations of driving the 
probe rod string into the ground and then retracting the probe rod string 
from the ground are required to sample the various soil levels. 
Therefore, a probe capable of detecting and quantifying chemical compounds 
and contaminants at particular soil levels is needed which overcomes the 
disadvantages and shortcomings of the prior art probes discussed above. 
Furthermore, many of the chemical compounds and contaminants of interest 
in the subsurface are volatile compounds which either exist wholly in the 
gas phase at normal soil temperatures, or exhibit a substantial vapor 
pressure while existing in the soil in the dissolved, liquid, or solid 
phases. Many of these compounds, in addition to being volatile, will also 
rapidly diffuse through many solid materials. 
Accordingly, it is a primary object of the present invention to allow 
detection of certain compounds at a subsurface level without bringing a 
soil sample or ground water sample to the surface. 
A further important object of the present invention is to provide a probe 
that can detect contaminants in both saturated and unsaturated soil zones, 
the probe having a membrane that allows diffusion of chemical compounds in 
their gas phase into the probe. 
Another object of the present invention is to provide a probe construction 
with a permeable membrane that is resistant to tearing and collapsing as 
the probe is driven through the soil and the membrane is exposed to the 
soil. 
A still further object of the present invention is to provide a probe 
construction that conveys gas phase compounds which diffuse across the 
permeable membrane at certain subsurface soil levels to the surface for 
detection and evaluation.

A permeable membrane probe involving the principles of this invention is 
broadly designated in the drawings by the reference numeral 10. Probe 10 
has a cylindrical housing 12 which is adapted to be positioned on the 
lower end of a probe rod string 14 by a thread arrangement 16 as best 
shown in FIG. 1. Housing 12 has a lower conical drive point 13 for parting 
the soil as the probe is driven into the ground. Housing 12 has a central 
bore 18 which is aligned with a central bore 20 of probe rod string 14. 
Rod string 14 includes a plurality of probe rod segments which can be 
interconnected to one another as the probe is driven into the ground, as 
will be more fully described below. 
Housing 12 has a generally rectangular recess 22 formed on its outer 
peripheral surface 24 as best shown in FIGS. 2 and 3. A through aperture 
26 extends between and spatially connects bore 18 and recess 22. Recess 22 
and aperture 26 receive a sensing unit 28. 
Sensing unit 28 includes a permeable membrane 30, a gas distribution screen 
32, a holding member 34, a cylindrical reinforcing plug 36, and a plug 
retaining cap 38 as best shown in FIGS. 3 and 4. Holder 34 has a generally 
rectangular configuration to conform with recess 22. The outer surface of 
holder 34 has a centrally disposed flat portion 40 and generally curved 
side portions 42 as best shown in FIGS. 2 and 4. Curved portions 42 
generally follow the curvature of the peripheral surface 24 of housing 12 
to reduce the soil buildup around unit 28 as the probe is driven into the 
ground. Holder 34 further includes a threaded nipple 44, which partially 
forms a bore 46, extending from the distal end of the nipple all the way 
through holder 34 to front flat portion 40 of holder 34. Bore 46 has a 
reduced diameter portion 48 and an annular recess 50, which opens up to 
flat portion 40. Recess 50 has a larger overall diameter than bore 46, and 
is used to receive membrane 30 as best shown in FIGS. 3 and 4. 
Membrane 30 is formed of a circular disc of stainless steel screen which 
has been coated with a polymer. The polymer is applied in a manner well 
known in the art such that the openings of the screen are filled, thus 
making the screen impervious to the bulk flow of either gases, liquids or 
solids. However, the polymer itself is actually porous, and is permeable 
to the diffusion of certain compounds which may be present in the soil 
surrounding the probe. Many different types of polymers could be used for 
the screen coating, depending on the type of compound to be detected in 
the surrounding soil. More specifically, polymers vary greatly in their 
permeability to gaseous diffusion. Thus, a particular polymer can be 
chosen which will allow diffusion of a particular compound to be detected 
from the soil surrounding the probe. It has been found that a TFE polymer 
manufactured by E. I. Du Pont de Nemours & Co. of Wilmington, Del., is a 
preferable type of polymer to be used to coat the screen. This polymer is 
baked onto the screen in successive layers, and allows maximum diffusion 
and minimum sorption of contaminant compounds found in the soil. However, 
as indicated above, other polymers could be used which have different 
permeability attributes. For instance, a polymer could be chosen which 
will admit polar compounds to the exclusion of nonpolar compounds. 
Membrane 30 can be secured in annular recess 50 by resistance welding, 
brazing, or any other suitable means of attaching the membrane to holder 
34. 
The mass of a compound passing through the membrane can also be regulated 
or adjusted by varying the size and thickness of the membrane. More 
specifically, presently a membrane having an area of 0.058 square inches 
and a thickness of 0.030 inches is used. To increase the mass transfer of 
a compound across the membrane, the surface area of the membrane could be 
increased. 
Gas distribution screen 32 is positioned adjacent an inner surface 52 of 
membrane 30 and within reduced diameter portion 48 of bore 46 as shown in 
FIGS. 3 and 4. The interstices of screen 32 form a gas mixing area wherein 
gas that has permeated through membrane 30 from the surrounding soil can 
be picked up by a carrier gas circulated within the mixing area, as will 
be more fully described below. Further, screen 32 supports membrane 30 to 
prevent the inward collapsing of the membrane due to soil pressure exerted 
on the outer surface 54 of the membrane, also as will be more fully 
described below. 
Plug 36 is positioned within bore 46 and serves to prevent inward 
collapsing of membrane 30 due to soil pressure. Plug 36 has a generally 
cylindrical nose portion 56 which is partially disposed in portion 48 and 
abuts against the inner surface of screen 32 as shown in FIG. 3. Plug 36 
also has an annular shoulder 58 which engages an annular ridge 60 formed 
where bore 46 transitions to reduce diameter portion 48. The inner end of 
plug 36 also has a protruding portion 62 which extends into an aperture 64 
formed in the end of cap 38 as best shown in FIG. 3. An annular shoulder 
66 of plug 36 engages the inner surface of cap 38 adjacent aperture 64 to 
hold plug 36 within bore 46. Cap 38 is secured to nipple 44 through thread 
arrangement 68. 
Cap 38, plug 36 and screen 32 all serve the advantageous function of 
preventing inward collapsing of membrane 30 due to pressure exerted on its 
outer surface 54 by the soil through which the probe passes. More 
specifically, plug 36 is securely positioned within bore 46 by cap 38 and 
supports the inner surface of screen 32. Screen 32 in turn supports the 
inner surface 52 of membrane 30 to prevent inward movement. Additionally, 
the interstices of screen 32 allow gas that has permeated through the 
membrane to be picked up by the carrier gas circulated through the 
interstices and then returned to the surface for analyzing, as will be 
described. Screen 32 is preferably a rigid stainless steel mesh screen. 
However, screen 32 can be comprised of other materials, such as porous 
sintered stainless steels, so long as the material used for the screen can 
transfer support force from plug 36 to membrane 30 and allow for free flow 
of carrier gas between the membrane and the plug. 
A carrier gas is used to convey the gas which has permeated through the 
membrane upwardly to the surface and into a suitable analyzing detecting 
instrument as generally shown in FIG. 1. As discussed above, the carrier 
gas mixes with the permeated gas within a gas mixing area formed by the 
interstices of screen 32. Carrier gas is conveyed to this gas collection 
area via a flexible inlet tube 70 as shown in FIGS. 1 and 3. Carrier gas 
and the mixed permeated gas are returned to the surface via a flexible 
return tube 72. Tubes 70 and 72 extend from unit 28 through the bore 18 of 
housing 12, upwardly through the connected bores 20 of the probe rod 
string 14, and out the upper end of the probe rod string. Inlet tube 70 is 
connected at its upper end to a supply of carrier gas as shown in FIG. 1. 
The carrier gas is preferably an inert gas, such as nitrogen or helium. 
The lower end of inlet tube 70 extends through an inlet bore 74 formed in 
plug 36. The extreme lower open end 76 of tube 70 is positioned adjacent 
screen 32 to supply carrier gas to the mixing area formed by screen 32 as 
best shown in FIGS. 3 and 4. The lower end of return tube 72 is positioned 
in a return bore 78, also formed in plug 36. Tube 72 has an open end 77 
disposed adjacent screen 32 to collect carrier gas mixed with the 
permeated gas and return this mixture to the surface as best shown in 
FIGS. 3 and 4. The end of tube 72 and the end of bore 78 adjacent screen 
32 are formed in a generally half-moon shape to allow collection of the 
mixture of carrier gas and permeated gas. The upper end of tube 72 is 
connected to a suitable gas analyzer, which can determine the presence 
and/or quantity of particular types of gases. 
Sensing unit 28 is removably secured to housing 12 via fastening screws 80 
positioned in counter-sunk holes 82 of holder 34 and holes 84 of housing 
12 as best shown in FIGS. 3 and 4. 
In operation, probe 10, with sensing unit 28 attached thereto, is 
positioned on the lower end of a probe rod string and driven into the 
ground using a hydraulically driven percussion hammer (not shown). As the 
probe is driven into the ground, clean carrier gas is supplied from a 
carrier gas supply source to sensing unit 28 via tube 70. The carrier gas 
continuously flows within the interstices of screen 32 adjacent inner 
surface 52 of membrane 30. Certain volatile compounds found within the 
soil the probe is passing through permeate through membrane 30 and into 
the mixing area formed by the interstices of screen 32. In this mixing 
area, carrier gas is mixed with the permeated gas and flows upwardly and 
into return tube 72. The carrier gas and permeated gas are conveyed 
upwardly through tube 72 to a detector device located on the surface of 
the ground. 
Various different analyzing devices can be utilized for this return flow of 
carrier gas and permeated gas to detect and/or quantify the permeated gas. 
For instance, the return flow can be directed to a flame ionization 
detector which is commonly used for sensing hydrocarbons. Further, other 
detection devices may also be used in parallel with the flame ionization 
detector. Such devices could include carbon dioxide, oxygen, or humidity 
sensors. The returning carrier gas could also be directed to a mass 
spectrometry detector or, indeed, any device for the measurement of 
compounds in the gas phase. A typical data output from use of the 
arrangement shown in FIG. 1 will show increased detector response at 
depths where increased levels of a particular compound are encountered in 
the subsurface. More specifically, detections of particular compounds can 
be measured with respect to time as the probe is driven into the ground. 
Further, the depth of the probe with respect to time can also be measured 
using means well known in the art. Thereafter, the two sets of data can be 
correlated so that a reading of detection of compounds with respect to 
depth can be obtained. Further, it may be possible to measure the 
detection signal with respect to depth directly using a depth-measuring 
system that is well known in the art, for instance, a string pot system. 
As is apparent, the preferred depiction of the data gathered by this 
device is in a graph format wherein time or depth is marked on one axis 
and detection levels are marked on the other axis. 
As is further apparent, there is a time lag in detecting compounds in the 
soil when using the detection system of this invention. More specifically, 
this time lag arises from the time required for a compound to pass through 
the membrane and then be conveyed to the ground surface in the carrier gas 
stream. The time lag is primarily determined by the diffusion coefficient 
of the compound encountered, the thickness and gas permeability of the 
membrane, the temperature of the soil, the rate of carrier gas flow, and 
the length of the gas tubes. All these factors are taken into account when 
analyzing the time and depth data from the various instruments. 
A major advantage of the probe of the present invention over equipment 
presently used in subsurface investigation is that levels or areas of 
contamination can be found without actually bringing a sample of either 
soil or ground water to the surface. Because probe 10 does not allow for 
the bulk flow of either water, solids or gas across membrane 30, probe 10 
can be used both in unsaturated zones of soil where the voids of the soil 
are filled with gas or in saturated zones of soil where void spaces are 
filled with ground water. Compounds which exist in the soil atmosphere in 
phases other than the gas phase (e.g., liquid or solid phases) can be 
detected by probe 10. Such compounds will partition into the gas phase 
adjacent the outer membrane surface and diffuse through the membrane into 
the carrier gas. Various compounds could potentially be detected using 
probe 10 depending upon the type of polymer used in membrane 30. These 
compounds typically will exhibit a vapor pressure at ambient temperature. 
The types of compounds that may be detected include aliphatic hydrocarbons 
such as butane, propane, ethane or pentane; aromatic hydrocarbons such as 
benzene or toluene; chlorinated hydrocarbons such as trichloroethylene, 
chloroform, tetrachloroethylene, or 1,1,1-trichlorethane; permanent gases 
such as carbon dioxide or oxygen; and water. 
Another advantageous feature of probe 10 is the supporting of membrane 30 
by plug 36 and screen 32. More specifically, as the probe is driven into 
the ground, soil pressure is exerted on outer surface 54 of membrane 30. 
This soil pressure will attempt to deform membrane 30 inwardly. The 
positioning of support plug 36 within bore 46 and the holding of the plug 
therein by cap 38 serves to provide a firm support surface to prevent 
inner movement of membrane 30. Further, rigid screen 32 acts to transfer 
the support of plug 36 to inner surface 52 of the membrane while at the 
same time allowing the mixing of permeated gas and carrier gas. Further, 
it has been found to be advantageous to have outer surface 54 of membrane 
30 generally flush with peripheral surface 24 of housing 12, as best shown 
in FIG. 3. This positioning allows the membrane surface to be 
self-cleaning. In other words, soil encountered at one depth is not 
carried with the probe to the next depth, but rather is sheared off and 
replaced with new soil from the next depth increment. Therefore, it is 
ensured that the compounds sampled by the probe are those found in the 
soil at the level the probe is at as opposed to soil that has been carried 
along with the probe during driving. 
As an alternative to circulating carrier gas along the inner surface of the 
membrane, a chemical sensor can be positioned directly within the probe 
adjacent the inner surface of the membrane. The sensor can sense directly 
the presence of a particular type of compound that has permeated through 
the membrane and relays such information to the surface electrically 
through wires or photometrically through optical fibers disposed in the 
central bores of the probe rod string and housing. Such a sensor could 
possibly take the place and be configured to be the same shape as plug 36. 
The advantage of placing the chemical sensor in the probe is that the time 
lag of conveying a carrier gas to the surface for analysis is eliminated, 
as is the need for a carrier gas supply system.