WO2006015463A2

WO2006015463A2 - Soil survey device

Info

Publication number: WO2006015463A2
Application number: PCT/BE2005/000129
Authority: WO
Inventors: Abdul Mounem Mouazen
Original assignee: K.U.Leuven Research And Development
Priority date: 2004-08-13
Filing date: 2005-08-11
Publication date: 2006-02-16
Also published as: EP1776577A2; WO2006015463A3; AU2005270677A1; GB0418108D0

Abstract

This optical soil survey devices comprise two parts, a soil excavation unit, which goes under the surface and excavates it and a sensing unit attached to the rear portion of the excavation u nit, which collects the data. The present invention provides a particular design of an optical soil survey device, whereby the sensing unit is mounted on the excavation unit such that the lower part of the sensing unit penetrates the soil below the lower part of the excavation unit. When using such a survey device the sensing unit penetrates the trench bottom that was opened by the excavation unit, assuring that the lower part of the sensing unit is continuously submerged in the soil. In a second object the invention provides a sensing unit specially designed for penetrating the trench bottom and for preparing the bottom of the obtained trench for optical surveying.

Description

SOIL SURVEY DEVICE

FIELD OF THE IMVEMTIOM

This invention concerns an optical soil survey device to survey soil properties based on the analysis of the optical characteristics of a given soil, a soil survey system and a vehicle comprising the soil survey device. More specifically, it concerns the devices mentioned above which create, at a desired depth, a space in order to survey the characteristics of the soil.

BACKGROUND OF THE INVENTION

In recent years the concept of precision agriculture (field management) has become increasingly popular as a means of preserving the environment and at the same time insuring a profit. The objective is to reduce the amount of input agricultural materials, namely, fertilizers, herbicides, and so on. The most crucial requirement for precision field management is an accurate understanding of the soil conditions which prevail in a given field. Soil, after all, is the most important element in agricultural production. Previously it was demonstrated that optical characteristics of the soil can be used to asses soil properties. Indeed, part of the light projected onto a soil surface is absorbed and the intensity of the absorption of light at a given wavelength varies with the soil components and their quantities. Accordingly, the reflected light displays characteristic optical spectra (i.e., reflected spectra), depending on the components of the soil. Heretofore, then, it was found to be possible to analyze the composition of soil by surveying and analyzing its optical spectra.

Examples of optical soil survey devices to survey the optical characteristics of a soil in order to investigate soil conditions in a given field are disclosed in U.S. Pat. No. 5,044,756, U.S. Pat. No. 6,608,672 and US Pub. No. US2003/0009286. These optical soil survey devices create, at a desired depth, a soil surface that can be exposed to light, allowing to analyse the composition of the soil by surveying and analyzing its optical spectra. The survey devices are mounted on a vehicle which allows a continuous analysis of the soil characteristics while the survey device is dragged through the soil. Essentially said optical soil survey devices comprise two parts, a soil excavation unit, which goes under the surface and excavates it and a sensing unit attached to the rear portion of the excavation unit, which collects the data. Typically, the sensing unit comprises a housing wherein means for illuminating the soil and means for capturing the reflectance are mounted. A spectrophotometer splits the light captured by the sensing unit into specific wavelengths.

The experience has shown that accurate optical survey of the soil requires that the sensing unit is not exposed to the pernicious effects of stray light and that the distance between the optical sensor and the surveyed soil surface remains virtually constant during the movement of the sensing unit over the soil surface. Variations in the distance between the soil surface and the optical sensor mainly result from undesired inclinations of the survey device when protracted through the soil. To avoid the problems associate with stray light or distance variations between sensor and soil surface, different designs for optical survey units have been proposed. Some of these designs (Pat. No. 6,608,672 and US Pub. No. US2003/0009286) are highly sophisticated and require the presence of additional sensors within the sensing unit in order to obtain optimal survey results, while other devices (U.S. Pat. No. 5,044,756) are more straightforward but have the disadvantage that they encounter high resistance of the soil during excavation. Some designs are also prone to clogging of the sensing unit with particles of the surrounding soil. When this happens, dirt gradually accumulates in the sensing unit until it becomes impossible to survey the soil characteristics.

The present invention provides a particular design of an optical soil survey device, which provides a solution to the above discussed problems by mounting the sensing unit such that the lower part of the sensing unit penetrates the soil below the lower part of the excavation unit.

SUMMARY OF THE INVENTION

The present invention provides a particular design of an optical soil survey device, whereby the sensing unit is mounted on the excavation unit such that the lower part of the sensing unit penetrates the soil below the lower part of the excavation unit. When using such a survey device the sensing unit penetrates the trench bottom that was opened by the excavation unit, assuring that the lower part of the sensing unit is continuously submerged in the soil. It was observed that the efficiency of an optical survey device according to the present invention was influenced by the design of the sensing unit and in particularly of the sensing unit housing. So in a second object the invention provides a sensing unit housing specifically designed for penetrating the trench bottom and for preparing the bottom of the obtained trench for optical surveying. DETAILED DESCRIPTION OF THE IMVEMTIOM List of figures

Figure 1: Illustration of the effects of the variations of the distance between the soil surface and the optical sensor due to undesired inclination (α) of the optical soil surveying device when the sensing unit is mounted such that (I) the lower part of the sensing unit is at the same level as the lower part of the excavation unit and (II) the lower part of the sensing unit is below the lower part of the excavation unit.

Figure 2: Representations of different designs of the sensing unit housing: I. basic design (a. view of the side of the housing, b. view of the bottom of the housing, c. view of the rear part of the housing); II. housing comprising a sharpened lower frontal part (1 ); III. housing comprising a U-shaped extension of the bottom surrounding void (5) and opening (3) and (4); IV. Housing comprising both a sharpened frontal part (1) and an U-shaped extension of the bottom (2).

Figure 3: Front and sideway view of the optical soil survey device comprising a specific subsoiler-optical unit set up

Figure 4: The sensing unit with the sensing unit housing mounted in a protective metal casing: I. side view, II. Bottom view.

Figure 5: Sensor optical parts

Figure 6: Presentation of the measured spectra over the distance surveyed. Figure 7: Comparison between laboratory and field measurement of soil raw spectra of a same moisture content of about 0.10 kg kg^"1

Figure 8: Comparison between laboratory and field measurement of soil raw spectra of a same moisture content of about 10 kgkg^"1 after maximum normalisation.

Description

The present invention relates to mobile soil surveying equipment and more particularly to devices which create, at a desired depth, a soil surface that can be exposed to light, preferably near infrared light, allowing to analyse the composition of the soil by surveying and analyzing its optical spectra. In general such devices are mounted on a vehicle and comprise a soil excavation unit, which goes under the soil surface and excavates it and a sensing unit, which collects the spectra reflected by the illuminated soil. Typically, the sensing unit comprises a housing wherein means for illuminating the soil and means for capturing the reflectance are mounted. Said devices further comprise a spectrophotometer, which splits the light captured by the sensing unit into specific wavelengths, a computing means, which allows the analysis of the output of the spectrophotometer and means to store the captured and/or analysed data. The present invention is based on the finding that the problems of the interference of ambient light and the variations of the distance between the optical sensor and the soil surface can be overcome by mounting the sensing unit such that the lower part of the sensing unit penetrates the soil below the lower part of the excavation unit. Therefore, a first object of the present invention relates to an optical soil survey device comprising an excavation unit and a sensing unit whereby the sensing unit is mounted to the rear part of the excavation unit such that the lower part of the sensing unit is below the lower part of the excavation unit. The sensing unit can either be attached directly to the excavation unit or can be mounted in a casing, which is attached to the rear part of the excavation unit. When using a soil survey device of the present invention the sensing unit penetrates the trench bottom (20) that was opened by the excavation unit, assuring that the lower part of the sensing unit is continuously submerged in the soil. This excludes the problems associated with stray light as the slit (21) resulting from the penetration of the sensing unit in the trench bottom (20) is readily filled by the soil flowing back into the trench. Submerging the lower part of the sensing unit within the soil body also has the important advantage that it allows to minimise the distance variations between the optical sensor and the soil surface due to inclinations of the survey device during operation. Fig. 1 , illustrates how distance (D) between the soil surface and the optical sensor increases considerably when a soil survey device comprising a sensing unit, which is mounted such that the lower part of the sensing unit is at the same level as the lower part of the excavation unit, is subjected to an inclination forcing the tip of the excavation deeper into the soil. On the other hand, when using a soil survey device according to the present invention the same inclination only results in a distance (d) between the soil surface and the optical sensor. As long as the inclination does not lift the entire sensing unit out of its slit in the trench bottom, distance (d) is limited to the sinus of angle α multiplied with the distance between the front of the sensing unit and the optical sensor (distance A in Fig. 2.I). So in a preferred embodiment of the present invention the sensing unit is mounted on the excavation unit such that the sensing unit is not lifted out of its slit as a result of inclinations of the survey device as can be expected during normal operation.

It was observed that a narrow cutting width of the excavation unit promoted the soil to flow directly back into the opened trench, providing an additional natural cover to the optical unit. Therefore, in a preferred embodiment the cutting width of the excavation unit is less than 10 cm, more preferably 6 cm or less. Preferably said excavation unit is a subsoiler comprising a chisel and a shank.

One of the variables affecting soil reflectance is the surface roughness. The energy reflected from a soil surface is decreased by increased surface roughness. Surface roughness tends to be more of a problem at closer ranges due to a smaller sampling area. The rough surface diffuses light over a larger scene than is normally viewed by the sensor. Thus, it is important to provide for some minimum amount of soil conditioning to produce a uniform, constant surface when attempting to continuously analyse the optical characteristics of the soil. This is insured in the current invention by the downwards forces acting on the excavation device (subsoiler) and the smooth surface of the sensing unit. The downwards forces are transferred into pressure exerted by the bottom of the excavation device and sensing unit on the trench (20) and slit (21 ) bottom, respectively, leading to a smooth surface of the bottom of the slit (21 ) opened by the sensing unit. So in a second object the invention provides a sensing unit specifically designed for penetrating the trench bottom and for preparing the bottom of the obtained slit (21 ) for optical surveying. Figure 2.I represents a basic design of said sensing unit. This unit comprises a, preferably metallic, housing which comprises two openings (3) and (4) wherein the reflectance capturing means and illumination means, respectively, are mounted. It is preferred that the entire or at least part of the lower frontal part of the housing, for instance the part that is submerged in the soil (1 ) (Fig. 2.Il & Fig. 2.IV)) is sharpened in order to facilitate the penetration of the soil of the excavated trench bottom. Preferably, the bottom parts of the housing, which slide over the soil surface and particularly the parts preceding the openings (3) and (4) are smoothened in order to obtain a smooth soil surface, suited for optical surveying. In a preferred embodiment the two openings (3) and (4) make contact above a void (5) within the lower part of the housing. Said void (5) is in direct connection with an opening (6) in the lower rear part of the housing. The presence of the void (5) in connection with the opening (6) prevents that soil particles clog the openings containing the illumination and reflectance capturing means. During the forward movement of the sensing unit, the soil particles can move freely in the void (5) and subsequently exit the sensing unit through the opening (6). Preferably the height of the opening (6) is lower than the depth of the slit in the trench bottom in order to avoid that stray light enters through said opening. In a particular embodiment, the housing further comprised a smoothened U-shaped extension of the bottom of the housing (2) surrounding void (5) and the openings (3) and (4) (Fig. 2.Il and Fig. 2.IV). In a more preferred embodiment the housing of the sensing unit comprises a sharpened lower frontal part (1 ) and U-shaped extension of the bottom.

The dimension of the sensing unit, and more particularly of the said housing, are mainly determined by the dimensions of the reflectance capturing means and illumination means used. In a preferred embodiment the illumination and reflectance capturing means are lenses, more preferably converging lenses, connected to optical fibres, which guide the light from a light source to the sensing unit and from the sensing unit to the detector of the spectrophotometer, respectively. In a particular embodiment, the accumulation lenses having a width of about 1 cm were mounted in a sensing unit housing having a width of 3 cm. In order to limit the resistance of the soil on the sensing unit, it is preferred that the sensing unit is not larger than the excavation unit, more preferably the sensing unit is narrower than the excavation unit. In a preferred embodiment the width of the sensing unit is half or less than half of that of the excavation unit. In an even more preferred embodiment the length of the sensing unit is a quarter or less than a quarter of that of the excavation unit. It is clear that the resistance of the soil on the sensing unit is determined by the width of the sensing unit on the one hand and the depth of penetration of the sensing unit on the other hand. Therefore, it can be understood that a narrower sensing unit allows a deeper penetration into the soil before the resistance of the soil attains unacceptable limits than a larger sensing unit. For instance, when using a sensing unit set to penetrate the soil 5 mm below the lower part of the excavation unit it is preferred that the sensing unit housing has a width below 50 mm.

In a preferred embodiment the sensing unit housing is made in an abrasion resistant material, for example abrasion resistant steel, such as Roc 400 (CASEO, Belgium). The abrasion resistance of the housing can also be enhanced by applying an abrasion resistant coating on at least the parts of the housing submerged in the soil. However, the person skilled in the art will understand that, regardless of the nature of the material of the housing or the coating applied thereon, the sensing unit housing has to be replaced after a given period of use. Therefore, it is preferred that the sensing unit housing can be easily attached to and removed from the excavation unit and that the illuminating and reflectance capturing means can be easily mounted in and removed from the sensing unit housing.

A third object of the present invention relates to a vehicle, preferably a tractor, or a device that can be protracted by a vehicle, for instance a seeding, tillage or other protracted machine, on which the surveying device of the present invention is mounted such that the sensing unit can be dragged through the soil at the desired depth. This can be achieved by attaching the shank of the excavation unit to a frame on the vehicle or protracted device. In a preferred embodiment the frame further comprises means to measure the depth of the sensing unit. In an even more preferred embodiment, the frame further comprises a ballast in order to minimise undesired movements of the survey device. In a particular embodiment a soil surveying device of the present invention is attached to the vehicle or the protracted device by the three point hitches of the vehicle.

In many cases it is important to relate the data obtained with the optical survey equipment to the geographical location of said measurements. Therefore, it is preferred that the vehicle or the protracted device is equipped with a system allowing to determine the position of the vehicle in the field, such as a differential global positioning system (DGPS). During the operation of the survey device of the present invention it was observed that in given circumstances, for example when measurements are conducted on slopes, lateral inclinations of the survey device occur. Such lateral inclinations result in a decrease of the measured reflectance. To overcome this shortcoming, a proper post-processing of the spectrum is performed, in a preferred embodiment the post processing is a spectral normalisation. So a fourth object of the present invention relates to the use post¬ processing algorithms to compensate for the decreased reflectance due to lateral inclinations of the optical soil survey device.

Additional features and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of a preferred embodiment.

A visible near infrared (VISNIR) spectrophotometer soil surveying device according to the present invention was developed and its performance was evaluated in the field. Said device comprised mechanical, optical and electronic parts: 1. Mechanical parts 1.1. An excavation unit:

The excavation unit is a medium-deep subsoiler (STEENO, Belgium), which is used in the practice to depths not exceeding 0.5 m. The subsoiler used comprised two parts; the chisel (8) of 0.06 m width, and the shank (19) of 0.03 m width, as shown in Fig. 3. The subsoiler opens a narrow trench in the soil to an assigned depth between 0.1 and 0.5 m. The tip of the subsoiler penetrates deeper than the backside of the chisel, aiming to reduce friction between the trench bottom and the subsoiler bottom side. 1.2. An optical sensing unit:

The optical sensing unit was attached to the subsoiler backside, as shown in Fig. 3. A housing, comprising illumination and reflectance capturing means, was fitted within a protective iron casing (15) attached to the subsoiler (Fig. 3 & 4). The light illumination (11) and reflectance fibres (10) were collected together at a 45° angle position in the housing, as shown in Fig. 4.I. The housing was tightened within the iron casing by means of several escrows (22). The top of the housing was attached to a rectangular prism piece of metal by means of a strong escrow (23). The lower part of the sensing unit housing (18) (Fig. 3) was mounted outside of the protective casing such that it was set to penetrate 5 mm deeper in the soil than the subsoiler tip. In consequence the lower part of the housing performed a direct penetration of the trench bottom that was opened by the preceding subsoiler during operation of the survey device. To obtain a smooth soil surface, the housing's bottom side was carefully smoothened. This allows a continuous sliding of the housing within the soil body and hence eliminating of soil-to-optical unit distance variation. Furthermore, this design provides a continuous cover of the optical unit by the soil flowing back into the opened trench, insuring that the targeted soil spot on the bottom of the trench received the light of the spectrophotometer halogen lamp only. This eliminated the need of a special cover for the optical unit to protect from the surrounding light.

1.3. Secondary parts:

The secondary mechanical parts include a jointing mechanism, by which the subsoiler shank was attached to the frame. A commercially available frame (TYPE, STEENO, BELGIUM) was used to attach the sensor to the three-point hitch of the tractor, and to be a platform for electronic device and laptops. A combination of a metal wheel (type,, STEENO BELGIUM) and a linear variable differential transducer (LVDT) (SOLARTRON, DIMED ELECTRONIC ENGINEERING, BELGIUM (USA)) is used to measure the variation in the depth of the soil penetrating optical unit. This allows to measure the depth at which the optical measurements are done at different measurement points in the field.

2. Sensor optical parts:

A Corona fibre visible near infrared (VISNIR) spectrophotometer (12) is used. It is fast, precise and robust, without moving parts, which make it suitable to be permanently aligned on mobile machines. In addition to the InGaAs diode-array for measurement in the NIR region (944.5 - 1710.9 nm), a Si-array is available for the measurement in the VIS and short infrared wavelength region (306.5 - 1135.5 nm). The light source is a 20 Watt tungsten halogen lamp (13) illuminating the targeted soil surface. The light is transferred to the soil surface by means of 1 m fibre (11) that is attached to the spectrophotometer at one and is attached to a converging lens (17) at the other end (Fig. 5). The lens fits within the sensing unit housing, which is mounted within the protective casing (15) (Fig. 3). The reflected light from the soil surface was collected by another converging lens (16) positioned perpendicularly to the soil surface. Light was transferred from the lens to the detector of the spectrophotometer (14) by another fibre (10).

3. Sensor electronics: The electrical system, besides the spectrophotometer, consisted of several modules: a basic power supply, travel speed sensor, global positioning system, signal conditioning system, amplifier and data acquisition system. The travel speed is measured using a doppler radar that is mounted pointing backwards to avoid the effects of stubble or grass movement after the measurement frame passed. The accuracy of the sensor was tested in previous experiments and all errors were smaller than 2.5%. Position, latitude and longitude, are determined with a Trimble AgGPSI 32 differential global positioning system (DGPS). A laptop is used to acquire the different signals.

The performance of the mentioned system during operation is described below. The 20 Watt tungsten halogen lamp illumines light, which is transferred by means of the illumination fibre (11) (Fig. 3) to the converging lens (17) situated within the sensing unit housing (18) protected by the iron case (15). The lens transferring light (17) encloses a 45° angle with the soil surface as well as with the light collecting lens (16). When the light reaches the smooth soil spot through an opening (5) in the bottom of the housing, part of the light is absorbed by the soil, whereas the unabsorbed light is reflected from the soil spot. When light is illuminated towards the soil surface, the radiant energy is distributed through three different processes: reflection, absorbance and transmission. As transmission in soils equals zero, the balance between reflection and absorbance is governed by the influence of the soil physical and chemical properties. These properties determine the colour and roughness of the soil surface, influencing the amount of light reflection and/or absorbance. Bowers and Hanks (1964) found that the intensity of reflected light from the soil surface decreased with increasing moisture content, particle size and organic matter.

The reflected light from the soil surface is collected by means of light collecting lens (16), and transferred back to the spectrophotometer detector by means of the detecting fibre, shown in Fig. 5.

In order to have an appropriate on-line spectroscopy measurement, the distance between the soil spot receiving light and the detecting lens should be kept constant, while having a smooth soil surface. The later is perfectly satisfied by the sensor proper mechanical design described above. The smooth bottom of the sensing unit housing, in addition to the light scrape of the trench bottom, done by the steel piece results in a reasonably smooth θ surface. A smooth soil surface increases the amount of reflected light to the soil to the detector. The distance variations between the soil spot receiving light and the detecting lens are kept minimal by allowing the housing to penetrate deeper than the subsoiler tip, keeping the housings bottom side in a continues touch and sliding over the smooth soil surface. However, this is enhanced by adding additional weight to the frame, providing a sort of damping against vibration encountered during on-line measurement. The optical measurement by the designed spectrophotometer is performed in the field with tractor driving in straight lines with a speed to be selected. Therefore, spectra are taken along with straight lines, whose length differ according to the tractor speed, number of measured spectra per time and integration time, the time needed to measure one spectrum. In the field, we found that the best measurement is done by considering 5 spectra with 475 ms integration time.

The optical measurement is done on-line during tractor driving, during which the position, speed and depth are measured by different sensors above-mentioned. This is needed in order to determine the horizontal and vertical position of each sample, which is needed for map development or variable rate application of fertilisation, manure, etc,.... Figure 6 presents respective spectra collected over a trajectory of 25 meter. Each spectrum is an average of the observations over a distance of 1.2 meter. This figure clearly illustrates that the optical survey device of the present invention allows a consistent surveying of the optical soil characteristics while it is dragged through the soil.

Spectral correction of deviations due to lateral inclinations of the sensing unit housing during measurements on slopes

In particular situations, for instance when on-line measurement is conducted on slopes, the survey device is inclined laterally, which results in the sensing unit housing making an angle with the bottom of the trench. This angle that extends along with the travel direction induces less reflectance due to less light collection by the detecting fibre. To overcome this shortcoming, a proper post processing of the spectrum is needed. For instance during the on-line measurement of soil moisture content, it was found that two spectra of a same moisture content of 10 kg kg^"1 could had different measured reflectance properties due to said lateral inclinations, as shown in Fig. 7. This was solved by carrying out spectral normalisation, which resulted in two spectra within approximately the same range, as shown in Fig. 8. Compared to the devices of the prior art the above described illustrative embodiment of the present invention has following advantages:

1. Simple and easily manufacture.

2. Relatively cheap; the expenses include the spectrophotometer, two fibres, two lenses, subsoiler and sensing unit housing, frame, two metal wheel, laptop, metal wheel and LVDT to measure depth (optional), DGPS, radar and electrical instruments.

3. Most mechanical parts are commercially available, including the subsoiler, frame, depth wheel and jointing mechanism between the frame and subsoiler and local manufactures can produce them. All the electrical and optical units are also available commercially. The sensing unit with the sensing unit housing are the only unit that should be specifically produced.

4. The different sensors with the frame can be installed onto any commercially available tractor. 5. Since the subsoiler and sensing unit are relatively narrow in addition to the continuous submerging of the sensing unit housing within the soil, no special cover is needed for the optical unit to prevent the ambient light to reach the soil spot under measurement. In fact, the narrow cutting width of the subsoiler (0.06 m) allows the soil to flow back directly to the opened trench, providing a natural cover to the optical unit.

6. A smooth soil surface is produced by means of the sensing unit housing bottom side designed for a light scrape of soil from the bottom of trench opened by the preceded subsoiler. The subsoiler downward force assists obtaining the smooth surface by pressing the bottom trench by the subsoiler tip downwards. 7. The distance variation between the soil surface at the bottom of the trench and the detecting lens is minimised by means of the proper design, setting the sensing unit housing deeper than the subsoiler tip. As long as the subsoiler tip touches the trench bottom opened, the bottom of the housing continuously touches the trench bottom. Additional weight put on the frame are very helpful to reduce the vibration influence on the distance variation, by acting as a damper.

Claims

1. An optical soil surveying device, which can be protracted through the soil and which can create at a desired depth a soil surface, wherein said soil surveying device comprises (i) an excavation unit and (ii) a sensing unit mounted to the rear part of the excavation unit, said sensing unit comprising means to illuminate said soil surface and means to capture the light spectra reflected from said soil surface, said soil survey device being characterised in that the sensing unit is mounted to the excavation unit such that at least part of the sensing unit is set to penetrate deeper into the soil than the excavation unit.

2. The optical soil surveying device according to claim 1 wherein the sensing unit is mounted to the excavation unit such that at least part of the lower frontal part of the sensing unit remains submerged in the soil upon occurrence of inclinations of the soil surveying device, which are associated with the normal operation of the device.

3. The optical soil surveying device according to claims 1 or 2 wherein the sensing unit is mounted to the excavation unit such that the lower part of the sensing unit is set to penetrate the soil 2 to 30 mm deeper than the lowest part of the excavation device.

4. The optical soil surveying device according to claims 1 or 3 wherein the sensing unit is mounted to the excavation unit such that the lower part of the sensing unit is set to penetrate the soil 4 to 10 mm deeper than the lowest part of the excavation device.

5. The optical soil surveying device according to claims 1 to 4 wherein the width of the excavation unit is less than 100 mm.

6. The optical soil surveying device according to claims 1 to 5 wherein the width of the excavation unit is less than 60 mm.

7. The optical soil surveying device according to claims 1 to 6 wherein the excavation unit is a subsoiler comprising a chisel and a shank.

8. The optical soil surveying device according to claims 1 to 4 wherein the sensing unit has the same width or is narrower than the excavation unit.

9. The optical soil surveying device according to claims 1 to 5 wherein the width of the sensing unit is half or less than half of the width of the excavation unit.

10. The optical soil surveying device according to any of the claims 1 to 9 wherein the means to illuminate said soil surface comprises one or more lenses connected to an optical fibre which can guide the light from a light source to the sensing unit.

11. The optical soil surveying device according to any of the claims 1 to 10 wherein the means Io capture the light reflected from said soil surface comprises one or more lenses connected to an optical fibre which can guide the reflected light from the sensing unit to the detector of a spectrophotometer.

12. The optical soil surveying device according to any of the claims 10 to 11 wherein said lenses are accumulation lenses.

13. The optical soil surveying device according to any of the claims 1 to 12 wherein the sensing unit is mounted into a casing said casing being attached to the rear part of the excavation device.

14. A sensing unit housing, which comprises means to mount the sensing unit to the rear part of the excavation unit or to a casing attached to the rear part of the excavation unit, said sensing unit housing comprising two openings wherein the illumination and reflectance capturing means can be mounted, said openings connecting above a void, which connects to an opening in the lower rear part of said sensing unit housing whereby at least part of the lower frontal part of the housing is sharpened to facilitate the penetration of the soil.

15. The sensing unit housing according to claim 14 wherein the part of the frontal part of the sensing unit housing that is submerged in the soil is sharpened.

16. The sensing unit housing according to claim 14 wherein the entire frontal part of the sensing housing is sharpened.

17. The sensing unit housing according to claims 14 to 16 wherein the sensing unit housing comprises a U-shaped extension, surrounding the said void below the openings wherein the illumination and reflectance capturing means can be mounted.

18. The sensing unit housing according to any of the claims 14 to 17 wherein at least part of the sensing unit housing is made in an abrasive resistant material, such as abrasive resistant steel.

19. The sensing unit housing according to any of the claims 14 to 18 wherein an abrasive resistant coating is applied on at least part of the sensing unit housing.

20. A sensing unit for optical soil surveying comprising a sensing unit housing according to any of the claims 14 to 19.

21. An optical soil surveying device comprising a sensing unit according to claim 20.

22. A mobile soil survey system comprising a vehicle, a spectrophotometer and the optical soil survey device of any of the claims 1 to 13 or of claim 21 wherein said soil survey device is mounted on the vehicle such that the sensing unit can be dragged through the soil at a desired depth.

23. A mobile soil survey system according to claims 22 wherein said system comprises means to determine the geographical position of the system and means to relate such geographical information to the data of the survey.