Land Drainage Research Project (LanDraP) Final Report May 2014 Henry Webber BA Hons MPhil FACTS
Contents Acknowledgements... 2 1. Introduction... 3 1.1 The Project Aims... 3 1.2 Ground Penetrating Radar... 3 1.3 Site and Ground Conditions... 5 2. Methodology... 7 3. Results... 7 3.1 Test 1- Comparison between pre-rolled surface and a non-rolled surface... 7 3.2 Test 2 Drain detection... 9 3.3 Test 3 - Investigation into topsoil mapping... 15 4. Conclusions and Catchment Management... 16 Bibliography:... 19 Acknowledgements I must thank Teresa Meadows for partnering and supporting this project, also for helping to get funding from Essex & Suffolk Water. Secondly a thank you to Vincent Utsi for helping to teach me about GPR. 2
1. Introduction 1.1 The Project Aims The Land Drainage Research Project was formed because of mutual interests between the author and Teresa Meadows of the Chelmer & Blackwater Catchment Partnership, which is supported by Essex & Suffolk Water, Catchment Sensitive Farming and the Environment Agency. These interests involved the idea that certain ground based geophysical methods that are widely used in some precision farming companies, archaeological services and geological investigations could be applied to help investigate land drainage in a number of dimensions. Land drains have seen particular publicity in the past few years because of the more extreme weather patterns and a lack of maintenance over the past 30 years. Land drains are also a crucial pathway for water, catching and transporting that water and its contents from field to watercourse. They hence play a pivotal role in managing diffuse pollution, the Chelmer & Blackwater Catchment Partnership s main objective. To date there has been little published research on the usefulness and extent that Ground Penetrating Radar (GPR) has for detecting and mapping land drainage patterns. Much research has used magnetic methods such as Caesium Magnetometers and even low resolution magnetic susceptibility (Rogers et al 2005; pers. comm. Linford 2014). Thus this project aims to identify how effective, and more importantly how consistent, GPR data is for detecting land drains on two sites in Essex. The data will be evaluated to consider its ability to create land drainage maps, which could then be used to aid the management of diffuse pollution with farmers in the future. 1.2 Ground Penetrating Radar The general principles of GPR need to be briefly described before applying this technology to land drainage. The GPR antenna has a transmitter and a receiver. The transmitter sends out electromagnetic waves of a certain frequency and as such, a certain wavelength. A higher frequency will mean the wavelength is smaller and will not penetrate the soil as much as a low frequency, large wavelength antenna. The transmitter sends out signals at the 3 Picture 1
programmed rate, and the receivers collect reflected signals that reflect off of significant changes in the soil. The actual parameter within the soil that affects the travel of electromagnetic waves is called the dielectric permittivity. Certain soil physical, biological and chemical compositions can affect this dielectric permittivity. However certain features that will absorb this energy are things like water, and therefore soil that is too saturated with water will not yield good results. Equally soil types such as heavy clays will also attain more signal and thus the reflections will be less significant and fewer in number. However circular pipes, air voids, changes in physical composition (such as the difference between topsoil and subsoil) are all regular features when analysing GPR data (for more basic description of GPR methodology see Campana and Piro 2009, Gaffney and Gater 2003). One of the largest problems in acquiring and interpreting GPR data is the complicated nature of the reflections from certain anomalies, and the signal to noise ratio that can be achieved on a site. The interpretation of features is complicated in itself, however the signal to noise ratio is also critical for collecting clear data. GPR antennae can be used at a certain distance above the ground, or at the ground surface; however it is imperative to keep this distance consistently the same to create smooth data profiles. In picture 2 there is an example between a GPR survey of a medieval bridge with a 250 MHz antenna, in comparison with a survey with a 200MHz antenna of a stubble field. It is clear that the smooth surface of the bridge provides a much more consistent surface for the GPR to run along compared to a stubble field with undulations, wheat stubble and tractor wheelings. Hence it is much harder to see specific features and reflections is the noise level of the data is high (processing of this data may differ). Picture 2 Left = Medieval Bridge Right = Stubble Field The interpretation of GPR reflections is also not a simple task and can be very hard without great experience or test features to relate to. For this project our main concern is to identify land drains. These will generally consist of a trench dug into the ground, through top soil and sub soil. A pipe would be placed (plastic or more traditionally clay tile) and then back filled with pebble to a suitable depth that this pebble fill would not be disturbed by any agricultural operations. Thus above this pebble a relatively normal soil profile would be expected after a few years cultivation. The first feature that we would expect to see from a GPR reflection would be a hyperbola for any rounded objects, such as a round clay pipe. Secondly the pebble 4
backfill will also affect the dielectric permittivity of the normal soil and thus may show as a slightly more square like rounded feature. The pebble back fill would not be more than 15-20cm and thus a similar size to the drainage pipe itself. In addition to the signal to noise ratio, there is also another difficulty with interpreting GPR data. This is the same for all prospective (non-invasive) data, and regards how you can determine the natural from the anthropogenic. In other words, how can you determine the specific features that the surveyor is looking for in contrast to other natural features? Above discussed how the pipe and the pebble back fill may be detected by the GPR, however this is completely dependent on the soil matrix that surrounds it. If the surrounding soil matrix has a high clay percentage, then the fired clay tile, in a material sense will be very similar to the clay soil surrounding it. It will then be less likely to reflect a clear boundary between the two materials. Secondly, the stone content of the soil matrix will also affect how clearly defined the boundary between the pebble fill of the drainage trench in comparison to the normal soil is. Thus the GPR has often been used most effectively in sandy and silty soils rather than heavy clays. With this in mind, however, it is suggested here that because of this timidity over clay soils, there has been very little testing to establish if there are other methods of applying GPR to these soils. Is it the soil type that presents the major problem with GPR on agricultural fields or is it in fact the practicality of ground surface consistency? 1.3 Site and Ground Conditions There were two main test sites that were used in this project, both in Essex in South East England. One site was at Skreens Park Farm (NGR 626 085) and the second site was near Chipping Ongar (NGR 548 033). Both of these sites have a range of soil types on them, ranging from alluvial clays to silty clays over gravel to heavy clay loams with large flint Picture 3 5
inclusions (London Boulder Clay). The ground conditions and agricultural situation of each test site was also different to give some insight into how different ground conditions can affect the GPR data. At the Skreens site, there were a number of different tests as shown in picture 4. In picture 5 the GPR transects are marked out in the relative areas of the same field. This field had no cultivations occur on it and had been left since harvest. Picture 4 Picture 5 6
2. Methodology The methodology of this project was to conduct a series of small tests, dividing larger questions up into smaller areas that meant that the control of variables was more effective. At the Skreens site, land drainage maps were available and so were scanned and geo-referenced into QGIS. This meant that here the GPR transects could be placed over known drains to test what sort of responses are common from the drains themselves. Each of the tests was completed by configuring the GPR antennae behind a quad bike which had a GPS attached for positioning. 1 st Test This first test involved the design and construction of a roller, using 56lb weights, a frame of steel and a set of car wheels, to compact the ground or vegetation prior to the GPR antennae. Picture 6 and Figures 1 and 2 show the results of the cart system. 2 nd Test This test concentrated on finding drains and testing the effectiveness of this technique. A number of GPR transects were undertaken and features were dug where possible to identify their origin. 3 rd Test Experimental transect over an old pond area at Ongar to look at the topsoil and subsoil relations. Throughout some of the earlier tests it became clear that there could be a potential use for GPR to determine topsoil depths and subsoil relations. 3. Results 3.1 Test 1- Comparison between pre-rolled surface and a non-rolled surface One of the initial problems with GPR data collection has been the ground contact consistency, and this is especially relevant when investigating specifically agricultural contexts. This prompted the testing of a heavy roller to flatten small soil clods and any vegetation to create a smoother surface. In addition to this, UTSI Electronics (the GPR Picture 6 7
company) also separated the transmitting and receiving antennae. In most circumstances they are combined in the same plastic box, however over rough areas there are some advantages in having these separated so that undulations do not affect the data as much. In all surveys, both a 250MHZ split antenna and a 400MHz split antenna were used. The results from the 400MHz however did not produce the best results. The higher frequency meant that more background noise was picked up from the topsoil and thus it was not easy to pick out features in the subsoil. All results presented here are from the 250MHz antenna specifically. Figure 1 Drain feature (Rolled) The results between Figure 1 and Figure 2 are negligible and there is very little difference between the two sets of data, showing that the roller did not affect the data in anyway. This is likely due to the amount of pressure needed to actually flatten the ground surface on a grass as well as on a drilled field. From this it was clear that significantly greater ground pressure was needed and particularly more direct pressure onto the antennae themselves (which was not done within this project because of the time constraints). Figure 2 Drain feature (Non-Rolled) 8
3.2 Test 2 Drain detection Once some data had been gathered and compared for background noise over different types of ground and with the roller design described in Test 1, it was then of most importance to concentrate on surveying more areas to see if land drains could be found in these soil types and with this GPR array. Picture 7 At Skreens there were around 3 transects undertaken that were placed over known (mapped) land drains. Out of those 3 transects, only 1 drain was found consistently. This drain was found along transect F4 on the map above. The drain can be seen running E-W and the GPR transect crossed it multiple times. It was found reliably every time the GPR crossed its path. Interestingly at the transect B1, which also crossed the drains path but at a different angle, the drain was not visible. This is quite an important result, as it seems that drain features are not as clear when running at an angle to the line of the GPR. This is because the feature becomes elongated and more complicated to determine rather than the clear hyperbola seen in Figure 3. Thus with the noise and ground disturbance that creates the natural background noise, it becomes much harder to identify where the features are. In this case it was easier to determine because of the geo-referenced drainage map, and use of GPS to limit the area down to a number of meters. If surveying without the aid of a drainage map, which would be most cases that this type of technology might be deployed, it would be much more difficult to find. 9
Once found, this feature was excavated to find the drain and take measurements of depths and construction of the drains and their back fill to confirm the GPR interpretations. This also enabled less drains to be excavated (time consuming process) because any features at the correct depth and roughly the same shape could be confidently interpreted as drains. Drain back fill started just below the topsoil at 35cm deep, stone pebbles filled from 35 until 65cm where the drain began. The drain was not removed because of the effect it would have had on the drainage system working after the trench was filled back in and the project finished. Figure 3 Drain feature Picture 8 Excavated drain 10
There were a number of other GPR anomalies found while surveying at Ongar which will be discussed further. At the Ongar site there were a number of attempts to find the cause of these anomalies however no more drains were excavated fully like the one at Skreens. The features picked out on the GPR data are most certainly significant features in the soil profile. However the main reason for not being able to excavate them arose from the GPS signals recorded while surveying. The GPS, a standard GPS with an average accuracy of +/- 1 or 2m, must have had fewer satellites available in Ongar as can be seen from Figure 4 and 5. Figure 4 is an example of the GPS lines from Skreens (smooth) and Figure 5 is from Ongar (not smooth). This has meant that when locating positions of anomalies and taking the GPS coordinates from the data to then excavate them, the co-ordinates were inaccurate. Figure 4 Example GPS Figure 5 Example GPS 11
Figure 6 GPR Anomaly Despite this loss of accurate GPS signal, there have still been a number of features that have produced anomalies similar to what would be expected from a drain. In addition there were also a number of other features that have not been clarified in the ground. Figure 6 shows an anomaly in Ongar, it is a feature that is around 1-2m wide and irregular in shape. The red line added shows the interpreted topsoil/subsoil interface, marked by the change of the thickness in the black and white layers which represent the time (in ns) of two way travel. There is a significant change in the normal layers that can be seen at the start and end of the radar gram suggesting that the definition or depth of the topsoil-subsoil change is not as clear or visible as would be normally. Equally the feature is not one single feature, but has another different material next to it. The width and shape of this feature is not concurrent with any drain like feature and so this anomaly is most likely representing some archaeological feature or early 20 th century agricultural disturbance. The second feature to discuss is shown in Figure 7. This anomaly was on the same GPR transect in Ongar that surveyed the length of low lying silty clay soil that runs into alluvial clays closer to the river. This feature is very clear and must be fairly significant in the soil profile. Once again the 3 thicker layers at the top of the profile are most likely representing the topsoil layers, with the feature just beneath these layers. This feature appears to be V shaped, much like a ditch feature with a defined but feint difference between the soil filling this ditch, in comparison to the subsoil. However there also appear to be 3 features within this ditch. These 3 features are all relatively strong reflections and are not hyperbolic, but square type features. These features and the ditch itself is a number of metres wide and must be something fairly substantial. Again it is not likely that this is a drain, and from discussions with the landowner, is doubtful to be anything modern. However this is a substantial feature that does at first glances, look similar to a large ditch that may have been used for water and electrical pipes and cabling. However this is unconfirmed and is certainly not a singular land drain. 12
Figure 7 GPR Anomaly Figures 8 and 9 are both also on the same GPR transect at Ongar, and both could potentially be land drains. Both of these were excavated but the excavation could not find either of them. As explained above, this is due to the GPS accuracy rather than the features themselves and if more time was allowed for the excavation before the crop was drilled, then a longer trench would have been more beneficial. Both features are relatively clear, Figure 8 being slightly more ephemeral than Figure 9. Both of these features do not have the same particular hyperbolic shape to them in comparison to the drain detected at Skreens. However this is most likely because of the angle of traverse over the drain, both of these transects must have crossed the drain at an angle different from 90 degrees, thus appearing to give a slanted anomaly. They are both at approximately the correct depth, just below the main topsoil/subsoil boundary. They are also consistent with the landowner s opinion of where drains run in the field even though no map was available. 13
Figure 8 GPR Anomaly Figure 9 GPR Anomaly 14
3.3 Test 3 - Investigation into topsoil mapping The final test was not on the original plan of investigations, however came about through some of the other tests that were conducted. It involved looking at the stratigraphy of the soil profile in the GPR data to learn more about how the layers change and undulate. It has become fairly clear that while looking for anomalies from the soil profile to delineate drains, there could also be great use in being able to delineate the layers in the soil profile. The difference between the topsoil and the subsoil in this area is fairly substantial and is often quite significant. On both test sites the subsoil contains much higher clay content than the topsoil. Hence, Figure 10 is an example of a very obvious stratigraphic change that had been surveyed as an initial test for topsoil and subsoil relations. This test was placed over an old pond that has been filled in since the 1970s. In the data, although the data is bunched together more than all of the other Figures in the report (at least 50m long), there is a disappearance of a whole layer. This set of black and white responses disappears, showing that this response must be caused by the boundary change between the topsoil and the subsoil. Thus in the centre of this GPR profile, there is no detectable subsoil for a length of around 20m where the old pond was situated. This data has not been validated and migrated (a process by which the data better reflects the actual soil profile dimensions) as there are no accurate measurements to compare this depth information with. It is suggested here that this would be a piece of further work. An auger survey would be required along the same line as the GPR transect to determine how accurate and easy it would be to predict the changes in topsoil depth from the GPR data alone. Figure 10 Soil Stratigraphy 15
The potential use of topsoil mapping is becoming quite relevant in modern farming. It has been noticed that Precision Farming could potentially use this information to better understand crop growth in certain areas. An accurate topsoil map could also be used to inform variable rate cultivation, relating depth of cultivation to the topsoil so as not to disturb subsoil layers and use excessive amounts of fuel to do so when it is unnecessary to do so. Precision Farming companies such as SOYL, IPF and Agrii could thus use this data in addition to GPR being used for land drainage only. Below is an example of some raw data that was collected and also has the list of sequenced processes that were completed on each GPR transect. These are all fairly standard processes to make the GPR data more clear and easily interpreted. Figure 11 Raw data and Processing Sequence 4. Conclusions and Catchment Management Conclusions from the GPR testing that this project has completed: GPR has been used successfully to locate land drains in a clay dominant topsoil and subsoil. Ground surface consistency is still problematic but not a complete hindrance. GPR is not 100% effective and survey method needs to reflect this if to be used on a field scale. GPR has demonstrated usefulness in identifying topsoil and subsoil relations and heavy compaction. 16
In relation to the catchment management initiatives, what could this technology be used for and how could it aid the programs objectives? It has become evident that there is more work needed to develop and prove GPR if the main aim is to survey large numbers of fields to map every land drain that the field contains. To prove this it would be useful to concentrate on one field, surveying it at regular intervals in a mix of directions to detect as many land drains as possible and create a map from such data. Since existing research has had success in certain areas with geophysical methods that measure part of the magnetic response from objects, perhaps a combination of GPR and magnetic methods might have potential. It would also be useful to auger survey the field to also look at the good potential that this technology has for mapping topsoil depths. In this project it has been proved that drains can be seen on the GPR data, however it is the direction in which you travel that affects much of the information that you can get from the data. Thus approaching an unknown field, the survey would have to be undertaken in a direction that would be most likely to pick up any land drains that may exist. In addition to this, it has been learnt that the GPS locating also has to be more accurate to save time. If such a survey could not only pick out the major drainage pattern, but also identify a potential risk for diffuse pollution, due to topography, soil type, soil depth (from GPR) and land drainage density (from GPR), it could be of great use to the Chelmer & Blackwater Catchment Partnership and other catchment management initiatives. This may not be true on large areas of land such as whole catchments, but could be very useful if concentrated where there are known issues with diffuse pollution from agricultural fields. In addition to this, as discussed earlier, the detailed topsoil/subsoil mapping that could be used for precision farming purposes could also open up ideas to connect diffuse pollution areas with precisely farmed management zones. So areas that are of particular risk to diffuse pollution could be digitally mapped into the precision farming methodology, allowing a certain area to be spread with different rates of fertiliser, slug pellets and crop sprays than the rest of the field to prevent diffuse pollution. Thus GPR could lead to a connection between understanding land drainage patterns (whether existing or planned), topsoil depths and site specific management of fertilisers and sprays in particular areas through precision farming technologies that exist today on many farms. Disregarding the field scale approach to these ideas, GPR could still also present use for CSF in investigating small problematic areas of pollution through the identification of drainage and other subsurface features that could be aiding water movement and thus nutrient and chemical movement off field. GPR has the advantage of also being able to detect drains and pipes through concrete, with many farms being encouraged to look at diffuse pollution from the farm yard itself, perhaps this is another area that GPR could be of use. Overall, there is evidence that shows how GPR could be useful and effective at finding land drains. At this point there is however still more experimentation needed to look at one field in detail to confirm drainage, and confirm any other interpretations drawn from the data, 17
especially with regards to the direction and interval of the survey. The potential of GPR has been displayed in this report but more work will help identify the extent and accuracy of this technology in relation to finding land drains and mapping topsoil depths. 18
Bibliography: Campana et al, 2009, Seeing the Unseen - Geophysics and Landscape Archaeology, Taylor and Francis Group, London. Gaffney and Gater, 2003, Revealing the Buried Past: Geophysics for Archaeologists, Tempus Publishing, Stroud. Jonard et al, 2013, Characterization of tillage effects on the spatial variation of soil properties using ground-penetrating radar and electromagnetic induction, Geoderma 207-208, p310-322. Natural England, Campaign for the Farmed Environment; 2010, Farming for cleaner water and healthier soil (NE230), Natural England. Novo et al, 2012, The STREAM X Multichannel GPR System: First Test at Vieil-Evreux (France)and Comparison with Other Geophysical Data., Archaeological Prospection 19 179-189. Picture 2 = P. Arias, J. Armesto, D. Di-Capua, R. González-Drigo, H. Lorenzo, V. Pérez- Gracia, Digital photogrammetry, GPR and computational analysis of structural damages in a mediaeval bridge, Engineering Failure Analysis, Volume 14, Issue 8, December 2007, Pages 1444-1457. Rogers M. Cassidy J. and Dragila M. (2005) Ground-based magnetic surveys as a new technique to locate subsurface drainage pipes: a case study, Applied Engineering in Agriculture, Volume 21, no. 3, pages 421-426. 19