FIELD OF THE INVENTION

The present disclosure relates to a system and method for monitoring subsurface vertical dynamics. The system and method can for instance be used as extensometer for monitoring changes in strata over time.

BACKGROUND OF THE INVENTION

There is an ever growing interest in monitoring movement and thicknesses of layers of soil. Historically, mining for hydrocarbons may have caused manmade changes to formation layers. More recently, there is a growing interest in peat lands and similar wetlands, where human influence on ground water levels may cause the top surface layer or layers of the wetlands to dry and subsequently oxidize. The oxidization process is irreversible, and the process releases carbon dioxide and methane into the atmosphere, greenhouse gases (GHG) contributing to global warming. To limit global warming, and to live up to various international treaties intended to limit global warming, governments are ever more interested in not only monitoring direct CO2 emissions, but also carbon released from the ground as described above.

State of the art to monitor formation compaction includes the use of extensometers. Extensometers however are relatively complex devices and as a result are relatively expensive to produce, to place, and to use. A side-effect of the relative expense is that a system using extensometers typically only uses a limited number thereof, resulting in low definition and accuracy.

An alternative is monitoring using satellites. InSAR (Interferometric Synthetic Aperture Radar) is a technique for mapping ground deformation using radar images of the Earth's surface that are collected from orbiting satellites. Unlike visible or infrared light, radar waves penetrate most weather clouds and are equally effective in darkness. So with InSAR it is possible to track ground deformation even in bad weather and at night. Two radar images of the same area that were collected at different times from similar vantage points in space can be compared against each other. Any movement of the ground surface toward or away from the satellite can be measured and portrayed as a "picture" - not of the surface itself but of how much the surface moved (deformed) during the time between images. Imagery is provided by space agencies in Italy, Germany, Canada, Japan, Korea, Europe, and the U.S. InSAR can indicate ground uplift as well as compaction patterns, but cannot be used to infer at which depth the compaction occurred. Although InSAR has proven to provide accurate data with respect to solid structures, such as buildings or roads, the data relating to agricultural land is a lot less accurate, or generally unsuitable. In other words, the InSAR system provides accurate data for cities and roads, but is typically unable to provide useful data for agricultural land. The latter is due to, for instance, the changing reflectivity of agricultural land due to growth of crop or grass, plowing, or other activities which are relatively common in agricultural areas. Yet, as indicated above, measuring ground compaction in agricultural areas is of particular concern if GHG release from soil is to be monitored.

AU-2017421071-B2 discloses a monitoring method integrating three aspects of data relating to strata control, comprising: downhole pressure observation, in which a pressure monitoring device arranged in a goaf and a pressure sensor installed at a hydraulic support are used to perform a collection operation; overburden strata displacement observation, in which monitoring of data relating to a mining-induced fracture movement of overburden strata is performed on the basis of a location of a key stratum of the overburden strata by using a multipoint extensometer provided to measure internal strata CO displacement and disposed in a surface borehole in combination with a borehole TV system; and surface subsidence observation, in which measurement is performed by means of a GPS to obtain subsidence data.

US20050247136A1 discloses a wireline extensometer that uses a tensioned cable extending between a supply spool and an anchor (e.g. of a type connected to a slope mass). The extensometer measures movement of the cable to reflect movement of the anchor (e.g. due to movement of the slope mass), and includes a magnetic brake configured to provide a predetermined constant non-frictional braking force on the supply spool. The wireline extensometer is designed to (i) minimize the risk of the extensometer reporting data that suggest slope movement where the slope mass has not moved to an undesirable extent, (ii) measure slope movement at relatively slow rates as well as rapid rates, and to (iii) measure slope mass movements continuously, and in a manner that can be efficiently and effectively communicated to responsible personnel. Because of the environment, the cable is flexible, multi-stranded, and stainless steel to resist corrosion. The wireline extensometer of US20050247136A1 can only measure movement of the anchor away from the supply spool. Its use is therefore relatively limited, and is specifically designed for detecting movement of slopes.

CN-110836654-A discloses a device and method to measure differential motion of subsurface layers. The device is composed of different vertical sections consisting of nodes and connecting pipes. For each section, the pipe connecting the two nodes can observe extension or compression. So the length of the pipe is L, and this can change over time. The nodes are able to measure rotations and angles (alpha and beta). If you have this for every section, one can derive how the subsurface nodes move relatively to each other.

The device and method of CN-110836654-A are designed to detect landslides, where layers at different depths move with different directions or velocities. The part of the device extending above the surface contains a mechanism to connect either a prism (which is used to observe it, and determine its position, with a total station using distance and angle measurement), or to temporarily screw a GNSS antenna on the mechanism, to determine its position.

The device of CN-110836654-A requires multiple relatively high-tech, expensive, and failure-prone sub-parts, i.e. the nodes, to be installed underground. The subsurface nodes consume power and require a power connection. The device has additional drawbacks, for instance it will be a challenge to operate the device below the ground water table. Although the device may be able to measure compaction or extension of an intermediate layer, the rod L with its two nodes is, in practice, a relatively expensive device. Finally, it is relatively complex to install the multiple subsurface devices of CN- 110836654-A at depth.

CN-108 548 524-A discloses a construction comprising a concrete block, sunken to the bottom of a river. A mast is arranged on the concrete block, which sticks out above the water table. On top of that mast is a GNSS antenna. CN-108 548 524-A describes how, in the case when a tunnel is dug under the respective river, the river bottom may subside, which can be measured since the block, the mast and therefore the GNSS antenna will subside as well.

The device of CN-108 548 524-A relates to rivers, i.e. to measurement of sub-aquatic soil movement. Due to the characteristics of a block of concrete, the device is unsuitable to install at depth and measure sub-surface movement. The block of concrete is designed for a river bed. In soft soil, the heavy concrete blocks would sink. The concrete blocks would show autonomous motion (‘sinking’) if it would be applied in soft soils, as of particular interest to the measurement of release of carbon. Due to its particular design, the device is unsuitable and is purposed for being an extensometer.

SUMMARY OF THE INVENTION

The present disclosure aims to provide a simpler, faster, and/or more cost effective system and method for monitoring strata compaction. The present disclosure aims to overcome one or more drawbacks of devices of the prior art.

Aspects of the present invention are set out in the accompanying claims. The disclosure provides an extensometer system for monitoring movement of at least one layer of a formation, the system comprising one or more extensometer units, each extensometer unit comprising: an anchor adapted to be arranged in the formation at a predetermined depth; an antenna fixedly connected to the anchor, the antenna being adapted to receive Global Navigation Satellite System (GNSS) signals; and a processing unit adapted to process the GNSS signals to provide changes of depth of the anchor with respect to a reference depth over time.

In an embodiment, each extensometer unit comprises a rod having a first end adapted to be connected to the anchor, and a second end adapted to extend above the surface of the formation, the antenna of the respective unit being adapted to be connected to the second end.

In an embodiment, at least one of the extensometer units comprises a tube enclosing the rod.

In an embodiment, the system comprising at least two extensometer units adapted to function in conjunction.

In an embodiment, each extensometer unit comprises a radio link connection for remote analysis.

In an embodiment, each extensometer unit comprises solar panels for power provision and/or rechargeable batteries.

According to another aspect, the disclosure provides a method for monitoring movement of at least one layer of a formation, the method comprising the steps of: providing one or more extensometer units, each unit comprising an anchor, an antenna, and a processing unit; arranging the anchor of the at least one extensometer unit in the formation at a predetermined depth; fixedly connecting the respective antenna to the anchor, using the antenna to receive Global Navigation Satellite Systems (GNSS) signals; and processing the GNSS signals using the respective processing unit to at least provide changes of depth of the respective anchor with respect to a reference level relative to time.

In an embodiment, the step of fixedly connecting the respective antenna to the anchor comprises: fixing a first end of a rod to the respective anchor, the rod extending from the anchor to the surface of the formation, and attaching the antenna of the respective extensometer unit to a second end of the rod, the second end extending above the surface .

In an embodiment, the method comprises a step of arranging a tube in the formation extending from substantially the predetermined depth to the surface of the formation, and arranging the rod in the tube, the rod being moveable in longitudinal direction with respect to the tube.

In an embodiment, the method comprises the steps of: providing at least two extensometer units; arranging a first anchor of a first extensometer unit at a first depth, the first depth being regarded as reference; and arranging a second anchor of a second extensometer unit at a second depth; and monitoring relative depth of the second anchor with respect to the first anchor.

In an embodiment, the first depth exceeds the second depth.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will be made to the figures on the accompanying drawings. The figures are schematic in nature and may not necessarily be drawn to scale. Similar reference numerals denote similar parts. On the attached drawings:

Figure 1 shows a schematic cross-sectional view of an embodiment of a system according to the present disclosure; and

Figure 2 shows a schematic perspective view of an embodiment of an extensometer unit of the system.

DETAILED DESCRIPTION

The following phrases may be used herein.

"Strata" may indicate a layer or a series of layers of rock or soil in the ground. Strata, or sedimentary strata (singular: stratum) are the horizontal layers, or beds, present in most sedimentary rocks. During or immediately after the accumulation of sediments, physical, biological, and chemical processes produce sedimentary structures. Strata are typically seen as bands of different colored or differently structured material exposed in cliffs, road cuts, quarries, and river banks. Individual bands may vary in thickness from a few millimeters to several meters or more.

The invention will be further elucidated in exemplary embodiments thereof as described below.

The present disclosure provides a system suitable to estimate subsurface vertical dynamics from depths to be determined, referenced to a global or local vertical datum. The system can be used as extensometer, to determine thickness changes of strata. The system uses ground anchors arranged at a respective predetermined depth in the ground, and antennas fixedly attached to the anchors. The antennas may extend above the ground. A global navigation satellite system (GNSS) can be used to monitor changes in the level of the antenna with respect to a reference level, directly indicating changes in the depth of the respective anchor with respect to the reference level.

Please note that the reference level may be provided by, for instance, a unit arranged at surface, a measurement of a satellite of the surface (for instance of a solid reference point at or near the location of the underground anchor), or an anchor unit of another system.

The principles of the system are described in connection with an extensometer designed to measure compaction of ground strata over time. However, the principles of the disclosure can also be used in various types of ground monitoring environment, indicating for instance both downward as well as upward movement (translation) of strata.

Figure 1 illustrates a system 1 of the disclosure. Figure 1 shows a cross section of an exemplary landscape, including ground 2 having multiple layers 4, 6 and 8 extending horizontally at different depth. The surface 10 of the ground 2 may be covered with any combination of, for instance, buildings 12, roads 14, and agricultural land 16.

The system 1 of the disclosure comprises one or more extensometer units 20. Each extensometer unit 20 comprises an anchor 22 adapted to be arranged in the ground 2 at a predetermined depth. An antenna 24 is fixedly connected to the respective anchor. For instance, the antenna is adapted to receive Global Navigation Satellite Systems (GNSS) signals 26. The GNSS signals typically originate from a satellite 30. Each extensometer unit comprises a processing unit 32 adapted to process the GNSS signals 26, received by the antenna 24. The processing unit 32 calculates and stores changes of depth of the respective anchor 22 with respect to the reference level over time.

To fixedly connect the antenna to the anchor, the extensometer unit 20 may comprise a rod 34 having a first end 36 adapted to be connected to the anchor 22, and a second end 38 adapted to extend above the surface 10 of the ground. The antenna 24 of the respective unit may be connected to the second end 38. Preferably, the antennas are positioned such that a direct, unobstructed line of sight can be established between the antenna 24 and the satellite 30. Satellites 30 may, in turn, communicate with a control center 40 via one or more ground based transceiver stations 42. The rod 34 may be relatively stiff, allowing direct translation of movement of the respective anchor 22 to corresponding movement of the related antenna 24. Thus, movement of the antenna provides a direct indication of movement of the anchor. Movement herein mainly relates to changes in height of the antenna, and to changes in depth of the anchor. The rod may be cylindrical. The rod may be tubular or have a solid core.

In a practical embodiment, the rod is comprised of a material having stiffness in both compression and extension. The rod may allow some lateral movement. Suitable materials for the rod may include nickel-iron controlled expansion alloys, such as FeNi36 (Invar®) and InVar® Steel, stainless steel, and/or composite materials comprising layers of glass. With a composition of 36% Nickel (nominal) and the balance Iron (Fe), Invar Alloy has the lowest thermal expansion of any known alloy. Another key factor is that it exhibits a very low expansivity below its Curie Temperature. This anomaly has been termed the “Invar Effect”. For this reason, Invar Alloy is particularly beneficial in applications where minimum thermal expansion and high dimensional stability is required.

As exemplified in Figure 2, the rod 34 may be embedded in a wider cylinder or tube, which may be referred to as outer cylinder 50. The cylinder 50 avoids, or at least minimizes, friction of the rod 34 within the ground layer between the depth of the respective anchor 22 and the antenna 24. The cylinder 50 may have a bottom end arranged or adapted to be arranged at or near the position of the respective anchor. At or near herein may indicate a position between 0 and 5 m with respect to the anchor. In a practical embodiment, the bottom end of the cylinder may be positioned at, for instance, a vertical distance of about 10 cm, for instance about 20 cm, 30 cm, 50 cm, 1 m, 1.5 m, 2 m, 3 m, 4 m, 5 m with respect to the position of the respective anchor. The cylinder may have a top end extending above the surface. Thus, the cylinder 50 allows substantially friction-free movement of the rod 34. The cylinder may be made of a material suitable to withstand lateral compression of strata. The tube 50 may be made of, for instance, a relatively strong steel. Alternatively the cylinder may be made of the same material as the rod 34. The latter may be particularly advantageous for longer term operation, for instance to avoid galvanic corrosion. Longer term herein relates to operations lasting more than, for instance, 1 year, 2 years, 5 years, or 10 years.

The rod may be arranged in the cylinder at surface, providing an assembly of a rod 34, an anchor 22 attached to a lower end of the rod 34, and a cylinder 50 enclosing the rod. The latter may be done in a workshop. The assembly may be assembled at a location of interest, for instance at the location of intended use. In an alternative embodiment, the tube 50 may be arranged in the formation first, allowing to introduce the rod, potentially with the anchor attached in a folded position, through the cylinder. The anchor may be collapsible between a folded position, allowing to fit within the tube 50, and an expanded position, wherein the anchor can engage the surrounding formation.

In operation, one or more extensometer units 20 may be arranged in the ground at a location of interest. The anchor of each extensometer unit 20 is arranged at a predetermined depth. The depth of each anchor may differ. Also, anchors may be positioned in different layers 4, 6, 8 of the formation, allowing to monitor in which of the layers a change, such as compaction, originated.

As an example, to place the ground anchors 22, holes of the required size may be drilled with the help of, for instance, an auger or a drill rig, at the locations selected for the installation of the anchors. The target depth of the holes will be the desired depth of the respective anchor. Holes can be drilled with or without casing pipes, either vertically or at an inclination as required. Typically, the holes will be drilled vertically. In a second step, the ground anchor can be inserted in the hole and lowered to the target depth. Once at target, the anchor can be activated to be locked in place. The anchor may be attached to the rod at surface, allowing lowering the anchor in the hole while being attached to the rod. Alternatively, the anchor can be lowered while being attached to a wireline. Once activated and in position, the wireline can be released and the rod can be inserted in the borehole and engage with the respective anchor using a suitable coupling mechanism.

In an embodiment, a grout, such as cement or bentonite, can be pumped into the space left between the wireline and the hole at predetermined grouting pressure. If a casing pipe is used in drilling the hole, it can be withdrawn during grouting. The grout is finally allowed to cure, in fact forming the rod extending from the anchor to the surface.

In another embodiment, the anchor comprises one or more helical blades of a screw like structure. The blades can be turned either clockwise or counter-clockwise, depending on the orientation of the blades. Turning and application of a suitable torque will make the blades move into the ground. Turning is continued until the blades are at target depth. The rod herein may be sufficiently strong to withstand torque applied at surface - by a suitable drilling rig or similar apparatus - and transfer said torque to the blades. Alternatively, once in position the rod or pipe used for drilling can be replaced with a more modest structure, such as a relatively thin steel or glass fiber rod.

At least two extensometer units 20 may be used. To increase accuracy, discriminatory power, and representativity, the system 1 of the disclosure may include many extensometers 20. In a practical embodiment, extensometers 20 may be arranged in a grid at mutual horizontal distances of, for instance, 0.2 m up to 1 km. Mutual horizontal distance between adjacent anchors may be, for instance, 0.4 m, 0.5 m, 0.75 m, 1 m, 1.5 m, 2 m, 2.5 m, 5 m, 10 m, 15 m, 20 m, 25 m, 50 m, 75 m, 100 m.

To allow comparisons between movement, compaction or extension in different layers 4-8, a first anchor may be arranged at a first depth. The depth of the first anchor may be regarded as reference. Other anchors can be arranged at a second depth, being shallower than the first depth. See figure 1. Positioning the first anchor in a ground layer which is known or expected to be relatively stable allows to monitor movement in other ground layers which are expected to, or may at some point in time start to cause, changes in structure and related movement in vertical direction. In addition to absolute depth (with respect to the surface), this setup allows to monitor depth of the second anchor relative to the first anchor.

In a practical embodiment, the depth of the respective anchors, i.e. including the first depth and the second depth of the second anchor, may be any suitable depth of interest. The depth of the respective anchors may be, for instance, anywhere between 1 m and 10 km. Vertical distance between adjacent anchors relative to each other may be, for instance, in the order of 0.1 m, 0.2 m, 0.3 m, 0.4 m, 0.5 m, 0.75 m, 1 m, 1.5 m, 2 m, 2.5 m, 5 m, 10 m, 15 m, 20 m, 25 m, 50 m, 75 m, 100 m, or more.

In use, Global Navigation Satellite Systems (GNSS) signals 26 are transmitted by a web of GNSS satellites 30 orbiting earth. The signals 26 are processed using the respective processing units 32 of the extensometer units 20. Processing herein may include comparing a depth of the respective anchor 22 with respect to the predetermined depth. I.e. current depth of the anchor is compared to the original depth or target depth. Monitoring depth over times allows, for instance, to monitor changes in depth of the respective anchor with respect to the predetermined depth over time.

The processing units may include a power source, such as a battery, and a data storage unit such as a hard disc, SSD and/or memory unit. Data may be gathered at intervals. Power usage per unit can be relatively low, allowing relatively long intervals of operation. Also, the capital expenditure can be relatively low, i.e. each unit can be relatively cost effective. Combined, this allows the system 1 to include a lot of extensometers 20, covering a large area with relatively high accuracy and definition. The system 1 may comprise, for instance, in the order of 10, for instance 100, for instance 500, for instance 1000, for instance 2000, for instance 5000, for instance 10,000 extensometer units or more. The system of extensometer units may cover an area in the range of, for instance, 100 m ² , 500 m ² , 1 km ² , 5 km ² , 10 km ² , 15 km ² , 20 km ² , 50 km ² , 100 km ² , or more.

The system of the disclosure provides a relatively simple and cost effective manner to monitor subsurface movement. The system has relatively cheap mechanical parts to be arranged in the subsurface, and only a high-tech device (a relatively low-cost GNSS antenna) above the ground. The system obviates arrangement of high-tech, expensive, and failure-prone sub-parts in the subsurface. Also, the system obviates subsurface parts that consume power. This simple system can operate at virtually any depth, including below the ground water table (it is only a metal rod and anchor).

The system is intended for observing the vertical motion of the anchor depth. Yet, an integral part of the system is that it is possible to use two of the devices (with different anchor depths) in concert. The difference between the two devices represents the compaction or extension of the intermediate layer. To monitor changes in subsurface layers, the system obviates expensive devices arranged subsurface. In contrast, the system of the disclosure removes the explicit extensometer altogether, and reaches the same goal as a conventional extensometer arranged subsurface albeit significantly cheaper. Due to the cost benefit, the system presents an attractive solution to monitor relatively large areas.

Finally, the installation of the system is relatively easy. The system obviates the installation of complex devices (such as extensometer-rods and tiltmeters) at depth. The method simply creates a hole, allowing to push the anchor and rod in, attach the GNSS antenna to the end of the rod extending above the surface, and the system is ready for use. Thus, the system enables easy, cheap, and fast installation. The latter is suitable for, for instance, monitoring of relatively soft soil such as ubiquitous in the Netherlands. Also, the market has expressed a clear need to have a cheap, fast, robust, and easy-to-install device as alternative to monitor soft soil compaction at various different locations. Having such a device off-the-shelve would be a very useful asset.

The scope of the present disclosure is not limited to the embodiments described above. Many modifications therein are conceivable without deviating from the scope of the present invention as defined by the appended claims. In particular, combinations of features of respective embodiments or aspects of the disclosure can be made. An aspect of the invention may be further advantageously enhanced by adding a feature that was described in relation to another aspect of the invention. While the present invention has been illustrated and described in detail with reference to the figures, such illustration and description are illustrative or exemplary only.

In the claims, the word “comprising” does not exclude other steps or elements, and “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference numerals in the claims should not be construed as limiting the scope of the present invention.