US2375949A - Geochemical prospecting - Google Patents

Description

R. T. SANDERSON GEOCHEMICAL PROSPECIING May 15, 1945.

2 Sheets-Sheet 1 Filed Feb. 15, 1941 May 15, W45. R. T. SANDERSON 2,375,949

GEOCHEMICAL PROSPECTING Filed Feb. 15, 1941 2 Sheets-Sheet 2 Patented May 15, 1945 GEOCHEMICAL PROSPECTING Robert Thomas Sanderson, Fishkill, N. Y., assignor to Stanolind Oil and Gas Company, Tulsa. Okla, a corporation of Delaware Application February 15, 1941, Serial No. 379,109

12 Claims.

This invention relates to geochemical prospecting and more particularly to a method and apparatus for geochemical prospecting which is particularly adapted to use in the field.

A set of procedures known generally as geochemical prospecting has been developed in recent years. In the main this prospecting technique has to do with the analysis of samples of soil, soil gas, or other geological components for minute traces of substances indicative of underlying mineral deposits. The samples are usually analyzed for minute traces of light hydrocarbons, principally the gaseous paraflin hydrocarbons heavier than methane, and in some instances the lighter normally liquid hydrocarbons, which are believed to seep upwards in gaseous or vapor form from underlying oil and gas deposits, and the presence of which in abnormally high quantities is indicative of the presence at greater depths of such oil and gas deposits. Other techniques using different indicator substances are known but are generally conceded to be inferior to those utilizing the lower paraflin hydrocarbons heavier than methane.

Geochemical prospecting has been conducted in the past by sampling either the soil or the gases present in the soil and taking these samples to the laboratory where they are analyzed for indicator substances by elaborate techniques unsuited to field use. The necessity for taking samples to the laboratory is particularly burdensome when soil gas or soil air samples, rather than samples of the soil itself, are used since the sample containers must necessarily be bulky and relatively expensive, particularly in view of the fact that any geochemical survey necessarily involves a large number of samples taken at spaced survey points. Largely for this reason many operators have gone to the use of soil samples rather than soil gas samples but this procedure is open to very serious question since the amount of hydrocarbons sorbed on the particles of soil is aifected by many factors and is not necessarily a true indication of the amount of hydrocarbons migrating upward through th soil from the underlying oil and gas deposit. To be sure the results obtained by the use of soil samples can be corrected to a considerable extent in various ways but the corrections add to the cost and it is never completely certain that entirely adequate corrections have been made.

An object of the present invention is to provide a method and apparatus for geochemical v prospecting utilizing soil gas samples but avoidchemical prospecting which is simple and rapid.

Other and more detailed objects, advantages and uses of my invention will become apparent as the description thereof proceeds.

In short I construct at each sampling station an enclosed hole in the soil and a suflicient amount of time is allowed to elapse to permit the air in this hole to come to substantial equilibrium with the soil gas. Apparatus of a type which will later be described is then put in communication with the hole, preferably with a purification train intervening, the apparatus is flushed out with gas withdrawn from the hole, and further gas is withdrawn from th hole and passed through cold traps which serve to condense significant hydrocarbons. These significant hydrocarbons are then evaporated by warming them and flushing them with further gas, preferably withdrawn from the hole, and they are then passed to a measuring device.

My method and apparatus will be more fully described with reference to the accompanying drawings which form a part of this specification and are to be read in conjunction therewith. In the drawings:

Figure 1 is a diagrammatic elevation of one form of apparatus in accordance with my invention; and

Figure 2 is a diagrammatic elevation of an alternative type of apparatus in accordance with my invention.

Turning now to Figure 1 in more detail, a pit or enlarged hole ill isv first formed in the soil and from the bottom of this pit a hole i l is drilled in the soil. Pit Hi can, for instance, be two or three feet long and at least a foot or two deep, while hole H can be from two to six inches in diameter, preferably about 4 inches in diameter, and from 5 to 50 feet deep; for instance 10 or 15 feet deep. Pits I U and holes ii are dug in the desired locations, for instance at sampling stations located one-tenth mile apart along a survey line or throughout a survey area, several days prior to sampling and are sealed at the top by inserting plug I2 carrying sampling tube l3 and packing pit it above plug i2 with moist earth, clay, or other sealing material It. The top of tube l3 can at the time the hole is originally drilled be sealed with a rubber tube having the upper end closed or by other means. Sampling tube l3 protrudes slightly above the surface of the ground and can be marked so that it can later be located by the analysis crew. This sampling tube preferably extends well downin hole so as to minimize further any possible contamination of the sample withdrawn therethrough with atmospheric air.

After a sufficient time has elapsed to accomplish at least approximate equilibrium between the air in the hole and the soil gas, tube I3 is uncovered and the analytical apparatus is at tached by means of connector l5 as shown in Figure 1 or otherwise. The time required to accomplish equilibrium is usually at least five days and longer periods are preferable.

The apparatus shown in Figure 1 is particularly adapted to be transported by hand if necessary and can conveniently be mounted on one or more panels to facilitate this. Thus, geochemical propecting can be carried out in relatively inaccessible locations without the necessity for transporting bulky apparatus or bulky gas samples.

My apparatus, as shown in Figure 1, preferably includes a purification sorption train made up of bulbs I611, I61) and |6c interconnected by connectors Ila and I'll). This sorption train can suitably contain materials for the removal of basic substances, acidic substances and the bulk of the water vapor. Thus bulb |6a can contain sodium bisulfate or other acidic material adapted to remove ammonia and other basic substances, bulb |6b can contain Ascarite (potassium hydroxide supported on asbestos) or other alkaline material for removing acidic substances such as CO2, etc., and bulb |6c can contain a drying agent, for instance calcium chloride. Other types of purification trains can, of course, be used and the purification train can, moreover, be omitted in some instances.

Conduit H3 is attached to the purification train by connector I9 and leads to three-way stopcock which is adapted to connect conduit |8 with conduit 2| leading to the first trap 22a. of a trap system and alternatively to connect conduit 2| with conduit 23 leading to a gas detection and measurement apparatus to be described later.

Trap 22a, and also trap 22b, is preferably of the coil type and can suitably be Packed with glass wool or the like to make the trapping operation as complete as possible. The two traps are connected by conduit 24 and from this or some other convenient part of the system, conduit 25 leads to manometer 26. From trap 22b conduit 21 leads through three-way stopcock 28, conduit 29 and connector 30 to bulb 3|, the purpose of which will later be described. Conduit 32 containing three-way stopcock 33 connects conduits 8 and 21 as shown.

Bulb 3| contains activated charcoal or other highly efiicient sorbent which is adapted to sorb much greater quantities of gas at a low temperature, such as liquid air or liquid nitrogen temperature, than at higher temperatures such as atmospheric temperature.

When the apparatus has been assembled, as shown in Figure 1, three-way stopcock 20 is positioned to connect conduit 2| and conduit 23 leaving the former unconnected with conduit l8. Three-way stopcock 34 is positioned to connect one of bulbs 35a and 35b with conduit 23. Stopcocks 36a and 361) are opened, connecting gas analysis chambers 31a and 31b with bulbs 35a and 351), respectively. Stopcock 33 is closed and stopcock 28 is positioned to connect bulb 3| with conduit 21 and to close ofi vent conduit 38. with the apparatus in this condition a bath 39d of liquid air or, better, liquid nitrogen, the latter being at approximately 196 C., is placed around bulb 3|. The result is that the air present in the apparatus connected with bulb 3| is rapidly sorbed and the pressure quickly falls. If a good grade of activated charcoal is used in a quantity of about 50 to 100 grams it will sorb at 196 C. from 10 to 25 liters of air or soil air in excess of the amount sorbed by it at atmospheric temperature. In other words, bulb 3| acts as a pump. When the pressure has dropped to a low value, for instance less than 5 mm. of mercury, stopcock 34 is turned to connect the unevacuated bulb 35a or 3512 with conduit 23. When the pressure has again dropped to a low value the apparatus is evacuated and ready for use.

Stopcocks 28, 36a and 36b are now closed and stopcock 33 is turned to connect conduit l8 with conduit 21 through conduit 32. Soil gas now fiows through the purification train, conduit 32, the traps (at atmospheric temperature) and stopcocks 20 and 34 into one of bulbs 35a and 35b, say bulb 35a, which is thus filled with a sample of purified soil gas. Stopcocks 20 and 33, and preferably also stopcock 34, are now turned to the completely closed position.

Bath 39a, which can suitably be at a temperature in the neighborhood of C., is now placed around trap 22a. This bath can, for instance, be an ether-carbon dioxide bath or an acetone-carbon dioxide bath. Bath 39b containing liquid air or preferably liquid nitrogen, the latter being at approximately -196 C., is placed around trap 221). At this point three-way stopcock 20 is turned to connect the traps with the purification train or, in other words, to connect conduit l8 with conduit 2| and stopcock 28 is turned to connect pump 3| with conduit 21,

Soil gas from bore hole now enters through the sorption or purification train, being freed of basic substances, acidic substances and most of its water in the course of its progress through this train. The purified gases pass through trap 22a. at approximately -80 C; and trap 22b at approximately -196 C. and on into the sorption bulb or pump 3|. Other bath temperatures can be used.

As previously indicated, a very large volume of soil gas can be passed through the apparatus by means of sorption bulb 3 However, the total volume of gas so withdrawn from bore hole II should be considerably less than the total volume of that bore hole, for instance from 5% to 75% of its volume and preferably from 15% to 40% of its volume.

It is also important that the conditions be maintained entirely constant at the various survey stations so that comparable results can be obtained in all instances. Thus the amount of material passed through the traps should be standardized for all survey stations. This result can be secured at least approximately by using a standardized quantity of a standardized activated charcoal and standardizing the pressures in each step in the process as well as standardizing the dimensions of the hole from which the soil gas sample is withdrawn.

As the soil gases pass through trap 22a practically all substances non-volatile at -80 C. are condensed therein and, similarly, substances volatile at 80 C. and condensable at --l96 C. are removed by trap 22b. After five to ten minutes the sorption by the charcoal in pump 3| is practically complete. Three-way stopcock 20 is then moved to completely closed position and threeway stopcock 28 is positioned to connect charcoal bulb 38' with vent line 38. Bath 39d is removed from the charcoal bulb permitting its temperature to rise to atmospheric with resultant desorption which puts the charcoal in condition for re-use.

At this point bath 39b is removed and trap 22b is thereby warmed. Three-way stopcock 20 is positioned to interconnect bulb 35b with trap 22a and stopcock 33 is positioned to interconnect tubes l8 and El. This assumes that bulb35b is the evacuated bulb and that bulb 35:: contains a soil gas sample. Since bulb 35b is rather highly evacuated and the apparatus between three- way stopcocks 20 and 28 as well as the purification train and the bore hole H are at a pressure appreaching atmospheric, further soil gas is drawn in through conduit l3, the purification train and conduit 32, and passes through traps 22b and 22a in sequence. This soil gas or soil air" flushes hydrocarbons, principally an ethane-pentane fraction, originally condensed on the inner walls of trap 22b out therefrom, through trap 22a and into bulb 35b. Stopcock 3A is then closed.

The sample is retained in bulb 35b long enough to warm to approximately the same temperature as that of the sample in bulb 35a.

The use of at least two traps is definitely advantageous since trap 22a serves to remove residual water and other materials which tend to interfere with the determination of hydrocarbons. However, a single trap 221) operating at a very low temperature such as the boiling point of liquid air or, better, liquid nitrogen, can be used omitting trap 22a. When utilizing two traps any traces of materials non-volatile at --80 C. which may have been blown in mist or frozen spray form into trap 221; by the soil gas current original-1y passed in succession through traps 22a and 22b is recondensed in trap 22b. Then when soil gas from conduit 32 is passed in sequence through traps 22b and 22a. this material non-volatile at ,80 C. orwhatever the temperature of trap 2211. may be, does not reach bulb 35b and is not measured. Thus a determination is made of an ethane-pentane hydrocarbon fraction (or other desired hydrocarbon fraction, depending on the trap temperature chosen) uncontaminated with substances non-volatile at the temperature of trap 22a.

The gas detecting part of the apparatus includes two platinum coils 40a and dob, or coils having a surface containing at least one platinum group metal, adapted to promote surface combustion. These two coils are disposed in chambers 31a and 31b respectively. Along with resistances dla and Mb and associated electrical connections, they form a Wheatstone bridge which can be balanced by adjustment of the relative value of these resistances. A potential, for instance that supplied by battery 42, is applied to one diagonal of the bridge when switch 63 is closed. The other diagonal is connected through a galvanometer or milliammeter it. Battery d2 serves to heat coils Ma and tub.

Stopcocks 36a and 38b are now opened and gases from bulbs 35a and 35b enter chambers 31a and 31b and impinge on heated coils 40a and 4102). The surface oxidation of the hydrocarbons contained in the gases entering chambers 31a and 3'lb causes the temperatures of coils 40a and 40b to increase, with resultant increase in their electrical resistances. Indicating device M gives a reading which is a measure of the relative temperature increases of coils 40a and 40b and thus a measure of the relative amounts of soil hydrocarbons contained in the samples of soil gas stored in bulbs 35:: and 3512. Bulb 35a contains the original soil gas while that contained in bulb 35b contains a similar sample enriched by the heavier soil hydrocarbons trapped from a much larger volume of soil gas. Thus coil 40a measures a blank and balance this out against a sample of the soil gas containing the trapped heavier hydrocarbons. Methane and other low boiling combustibles contained in the soil gasare not measured by instrument 46 which only measures the significant volatile hydrocarbons heavier than methane.

This measure can by suitable experiments with known samples be converted to give approximate values of hydrocarbon content in terms of parts per million or parts per billion or in such other terms as may be desired but this is not essential since in geochemical prospecting all that is needed is relative values for the various survey stations and the readings of indicating instrument 44 at various survey stations can thus be directly compared without any compelling need for converting them into hydrocarbon concentration figures.

Instead of using a sample of soil gas in bulb 35a and using soil gas to flush the trapped hydrocarbons into bulb 35b, atmospheric air can be used for these purposes. Thus instead of filling bulb 3511 with soil gas stopcock 33 can be turned to connect this bulb with the atmosphere via conduit 45 and air from this same conduit can be used to flush the trapped hydrocarbons into bulb 35b.

The volume of bulb 35b (the same as that of bulb 35a) should preferably be in excess of the volume of traps 22a and 22b and the remainder of the apparatus disposed between three- way stopcocks 20 and 28 so that a considerable volume of gas is drawn through the two traps, forcing at least the bulk of the hydrocarbons initially trapped in trap 221) into bulb 35b and ultimately into contact with coil 40b.

It will be understood, of course, that the methods and apparatus described are subject to considerable variation in detail and I have illustrated certain modifications which are suitable in many instances in Figure 2 in which parts corresponding to Figure l carrying corresponding reference numbers. Various features of Figure 2 can be used in conjunction with the apparatus of Figure 1 and vice versa as will be apparent to those skilled in the art.

In Figure 2 a board lZa and screen l2b are used to replace plug l2 of Figure 1, and serve the purpose of keeping soil It or other sealing material out of bore hole ll.

Figure 2 also illustrates a purification train using liquid purification agents instead 'of solid purification agents as in Figure 1. Thus conduit 46 leads from connnector l5 to tightly stoppered bottles Ito and lfib, which can suitably contain concentrated sodium hydroxide solution and concentrated sulfuric acid respectively and serve to replace purification bulbs "5a, 661) and 460 of .Figure 1. v.

The trap system of Figure 2 is similar to that of Figure 1 except that a third trap 22c and bath 390 are added for purposes which will hereinafter be described.

In Figure 2 a different means for evacuating the system is provided and this means is particularly suitable where the survey stations are accessible to a small truck while the set-up of Figure 1 is particularly suitable where hand carrying is necessary. Three-way stopcock 28 of Figure 2 is connected to a vacuum chamber 41 to which manometer 48 is connected. This vacuum chamber is connected through stopcock 49 with a motor driven vacuum pump 3|, dia grammatically indicated.

Vacuum chamber 41 is used to standardize the volume of soil gas withdrawn from bore hole H. When the system including vacuum chamber 41, traps 22a, 22b and 220 and the gas detection device has been evacuated by means of motor driven vacuum pump 3| to a predetermined low pressure level as indicated by manometer 48, stopcock 46 isclosed and the pump is turned off. From this point on the analysis is similar to that using the apparatus of Figure 1 and consists in connecting the trap system to vacuum chamber 47 by means of three-way stopcock 28 and to the purification system by means of three-way stopcock 20. Thus soil gas is drawn through the purification system and the trap system into the vacuum chamber and the amount of this gas thus drawn through the system can be standardized for each survey station by simply evacuating the system in the first instance to the same pressure so that with a constant volume vacuum chamber 41 and standardized initial vacuum in this chamber the volume of gas withdrawn before the pressure increases to atmospheric or some other predetermined level (as indicted by manometer 26 or 48) is necessarily a constant.

The three traps of Figure 2 permit a more eificient fractionation of the hydrocarbons than do the two traps of Figure l. The procedure with the three trap system is the same as with the two trap systems until stopcock 33 has :been positioned to connect conduit 27 with conduit 18 and the pressure in the trap system has increased to about /2 atmosphere as shown by manometer 26. Stopcock 33 is then closed and bath 39a at -80 C., or other temperature of that order, is removed from about trap 22a and bath 390, which can be at this same temperature, is placed about trap 22c. As a matter of fact,'baths 39a and 390 can be the same since they are not used simultaneously in the method under description. When trap 22a has warmed to approximately atmospheric temperature, stopcock 33 is again positioned to connect conduits l8 and 2'! until the system comes to atmospheric pressure or some predetermined pressure level approaching atmospheric. The purpose of this redistillation procedure is to free any traces of ethane-pentane hydrocarbons which may have been trapped in trap 22a.

The gas detection or measurement apparatus of Figure 2 difiers somewhat from that of Figure 1. While that of Figure l is preferable so far as accuracy is concerned, that of Figure 2 simplifies the procedure somewhat. In the first place, an enlarged bulb 50 connected with the gas detection apparatus 'by conduit can be evacuated initially along with the rest of the system and when gas is admitted to the gas measurement apparatus the bulb increases the volume of gas drawn through the system and makes certain that the hydrocarbons reach the combustion coils. The volume of the evacuated gas measurement apparatus including. in this instance; bulb 50 should exceed that of the trap system.

Furthermore, the gas detection system of Figure 2 includes four coils 40c, 4011, 40c and 40f in place of the two coils 40a and 40b of Figure l. Coils c and 40d are disposed in a first bulb or com-- bustion chamber 310 and coils 40c and 40, in a second and symmetrical bulb 31b. The two bulbs are connected by a conduit 52 surrounded by a metal tube or water jacket 53, or other cooling element, so that the heat formed by combustion on coils 40c and 40d does not affect coils 40c and 40 the purpose of which is to balance out atmospheric effects, etc. Coils 40c, 40d, 40c and 40) form the four arms or a Wheatstone bridge which can be balanced by means of variable resistance 56. Indicating or recording device 44, for instance a galvanometer or milliammeter protected by shunt resistance under the control of switch 58 is connected across one diagonal of the Wheatstone bridge while battery 42 under the control of switch 43 is connected across the other diagonal. Thus the reading of indicating or recording device 44 reflects the temperature of cells 460 and 40d and provides an indication of the amount of hydrocarbons, particularly ethanepentane hydrocarbons, present in the soil gas samples. The hydrocarbons passing through conduit 52 and cooled by tube or jacket 53 impinge on coils 40c and 40} which are thus subjected to the same conditions as coils 40c and 40d g lcgept for the removal of combustibles in bulb While I have described by invention in connection with certain preferred embodiments thereof, it is to be understood that these are by way of illustration and not by way of limitation and I do not mean to be restricted thereto but only to the scope of the appended claims in which I have endeavored to describe the novelty inherent in my invention.

I claim:

1. A method of geochemical prospecting comprising forming an enlarged soil gas collecting zone in the soil, withdrawing soil gas from said soil gas collecting zone through at least one low temperature trapping zone whereby soil hydrocarbons are condensed in said trapping zone, connecting a hydrocarbon measurement zone to said trapping zone, raising the temperature of said trapping zone to promote the vaporization of said condensed soil gas hydrocarbons, passing an oxygen-containin gas through said trapping zone and into said hydrocarbon measurement zone to carry said vaporized hydrocarbons into said hydrocarbon measurement zone, and measuring the comparative effects of a sample of said gas carrying said vaporized hydrocarbons and a sample of comparable gas free from said vaporized hydrocarbons on the resistance of hot wires having surfaces adapted to promote the surface oxidation of hydrocarbons, whereby the amount of said vaporized hydrocarbons is measured.

2. A method of geochemical prospecting comprising forming an enlarged soil gas collecting zone in the soil, withdrawing soil gas from said soil gas collecting zone through at least one low temperature trapping zone whereby soil hydrocarbons are condensed in said trapping zone, discarding the remainder of said soil gases, connecting a sample storage zone to said trapping zone, raising the temperature of said trapping zone to promote the vaporization of said condensed soil gas hydrocarbons, passing an oxygen-containing gas through said trapping zone and into said sample storage zone to carry said vaporized hydroca'rbons into said sample storage zone, transferring gases from said sample storage zone to a hydrocarbon measurement zone, and measuring the comparative effects of a sample of said gas carrying said vaporized hydrocarbons and a sample of comparable gas free from said vaporized hydrocarbons on the resistances of hot wires having surfaces adapted to promote the surface oxidation of hydrocarbons, whereby the amount of said vaporized hydrocarbons is measured.

A method of geochemical prospecting comprising removing soil gases from soil, trapping condensable hydrocarbons therefrom, conveying said trapped hydrocarbons to a hydrocarbon measurement zone in a sample of oxygen-containing gas, and comparing the amounts of hydrocarbons in said sample of gas and in a second sample of gas from the .same source but free of said trapped hydrocarbons.

4. A method of geochemical prospecting comprising removing soil gases from soil, trapping condensable hydrocarbons therefrom, vaporizing said trapped hydrocarbons, conveying said vaporized hydrocarbons to a hydrocarbon measurement zone in a sample of gas, collecting a second sample of gas from the same source as said firstmentioned sample of gas in a second hydrocarbon measurement zone, and measuring in said hydrocarbon measurement zones the difierential hydrocarbon contents of the gas samples contained therein.

5. A method of geochemical prospecting comprising removing soil gases from soil, trapping condensable hydrocarbons therefrom, vaporizing said trapped hydrocarbons, conveying said vaporized hydrocarbons to a hydrocarbon measurement zone in a sample of oxygen-containing gas, conveying a second sample of gas from the same source as said first-mentioned sample oi gas to a second hydrocarbon measurement zone, each of said hydrocarbon measurement zones containing a hot wire having a surface containing at least one platinum group metal, and measuring in said hydrocarbon measurement zones the differential efiects of the hydrocarbon contents of said two samples of gas on the resistances of said hot wires.

6. A method of geochemical prospecting comprising forming an enlargedsoil gas collecting zone in the soil,' connecting at least one low temperature trapping zone to said soil gas collecting zone, withdrawing soil gas from said soil gas collecting zone through said low temperature trapping zone whereby soil hydrocarbons are condensed in said trapping zone, discarding the remainder of said soil gases, connecting a hydrocarbon measurement zone to said trapping zone, raising the temperature of said trapping zone to promote the vaporization of said condensed soil gas hydrocarbons, passing gas from a source of imiform oxygen-containing gas through said trapping zone and into said hydrocarbon measurement zone, passing gas from said source into a second hydrocarbon measurement zone, each of said hydrocarbon measurement zones containing a hot wire having a surface containing at least one platinum group metal, and measuring in said hydrocarbon measurement zones the difierential efiects of the hydrocarbon contents of said two streams of gas on the resistances of said hot wires.

7. A method of geochemical prospecting comprising forming an enlarged soil gas sample collecting zone in the soil, sealing said sample collecting zone from atmospheric air, Withdrawing soil gas from said sample collecting zone through a trapping zone held at a low temperature and thence through a second trapping zone maintained at a lower temperature, said first-mentioned trapping zone being adapted to cause the precipitation of relatively high boiling impurities present in said soil gas and said second-mentioned trapping zone being adapted to cause the precipitation of light hydrocarbons heavier than methane, increasing the temperature of said secand-mentioned trapping zone while maintaining said first-mentioned trapping zone at said low temperature, passing further soil gas from said soil gas collecting zone first through said secondmentioned trapping zone while at said increased temperature and thence through said first-mentoined trapping zone while at said first-mentioned low temperature and thence to a hydrocarbon measurement zone, and measuring in said hydrocarbon measurement zone a function of the hydrocarbon content of the gases entering said hydrocarbon measurement zone.

8. A method of geochemical prospecting comprising forming an enlarged soil gas sample collecting zone in the soil, sealing said collecting zone from atmospheric air, leaving said sample collecting zone in sealed condition for a prolonged period of time sufhcient to achieve substantial equilibrium between the gases present in said collecting zone and the gases present in the soi1 surrounding said collecting zone, attaching directly to said collecting zone a purification zone and at least two trapping zones, one of said trapping zones being maintained at a low temperature in the vicinity of --80 C. and the second of said trapping zones being maintained at a still lower temperature in the vicinity of -196 0., withdrawing soil gas from said collecting zone and passing said soil gas so withdrawn first through said purification zone and thence in sequence through said first-mentioned trapping zone and said second trapping zone, said trapping zones being maintained at the temperatures aforementioned, increasing the temperature of said second trapping zone to vaporize hydrocarbons condensed therein, passing further soil gas from said collecting zone in sequence through said second trapping zone and said first-mentioned trapping zone while said first-mentioned trapping zone is at said low temperature and said second trapping zone is at said increased temperature, passing said last-mentioned soil gas carrying hydrocarbons originally condensed in said second trapping zone from said first-mentioned trapping zone to a hydrocarbon measurement zone, and measuring in said last-mentioned zone a function of the hydrocarbon content of the gases entering said last-mentioned zone.

9. In apparatus for soil gas analysis adapted foruse in the field, a first low temperature trap, a second low temperature trap, a conduit connecting said traps, a pair of conduits leading from a source of soil gas one to each of said traps, valve means disposed in each one of said pair of conduits, valved vent means communicating with the conduit connecting said second trap with said source of soil gas, means connected to the system for pumping soil gas through said traps and conduits, whereby soil gas may be passed in sequence through said first tra then through said second trap and then vented, and whereby additional soil gas may be passed first through said second trap and then through said first trap, and measuring means in valved communication with said first trap for measuring the amount of hydrocarbons contained in said last-mentioned gas.

10. In apparatus for soilgas analysis adapted for use in the field, a first low temperature trap,

a second low temperature trap, a first conduit leading from a source oi soil gas to said first trap, a second conduit connecting said first and second traps, a third conduit leading from said second trap to the source of soil gas, valves disposed in said first and third conduits, valved vent means in communication with said third conduit, means connected to the system for pumping soil gas through said traps and conduits, the operation oi said valves permitting soil gas to be passed in sequence through said first trap, then through said second trap, and then vented and permitting additional soil gas to be passed through said second trap and then through said first trap and measuring means communicating with said first conduit for measuring the amount of hydrocarbons contained in said last-mentioned gas.

11. Apparatus for geochemical prospecting by analyzing soil gas withdrawn from a soil gas collecting chamber in the soil comprising a trapping system, a first conduit means leading from said collecting chamber to said trapping system, said trapping system including at least one low temperature trap for condensing soil gas hydrocar= bons heavier than methane, pumping means connected to said trapping system for withdrawing soil gas from said collecting chamber and for passing said soil gas through said low temperature trap, a gas measurement chamber containing a wire having an oxidative surface, a second conduit means connecting said gas measurement chamber with said first conduit means, a second gas measurement chamber containing a second wire having an oxidative surface, a third conduit means connecting said second gas measurement chamber with said, second conduit means, twoway valves disposed at the junction of said first and second conduit means and at the junction of said second and third conduit means, whereby the hydrocarbons heavier than methane may be passed in admixture with a sample of soil gas from said low temperature trap to said first gas measurement chamber, and whereby a second sample 01 fresh soil gas may be passed from said collecting chamber to said second gas measurement chamber, means for heating said wires and means for measuring the relative resistances of said wires.

12. Apparatus for geochemical prospecting comprising conduit means communicating at one end with the soil for removing gas therefrom, extracting means communicating with the other end of said conduit means for extracting hydrocarbons from said soil gas, a first gas measurement chamber communicating with said extracting means and containing a wire having an oxidative surface whereby said extracted hydrocarbons in admixture with a sample of soil gas may be passed from said extracting means to said first gas meas urement chamber, a second gas measurement chamber containing a second wire having an oxidative surface and communicating with said conduit means whereby a second sample of fresh soil gas may be introduced into said second gas measurement chamber, means for heating said wires and means for measuring the relative resistances of said wires.

R. THOMAS SANDERSON.

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