Apparatus and method for ground survey

Abstract

A rod (200) for ground surveying, comprising: an elongate shaft (201); and a head (202) formed by or attached to the first end of the shaft (201) so as to facilitate penetration of the rod (200) into the ground or to produce a specific effect when a force is applied thereto. The rod comprises: a plurality of ground current electrodes (203) positioned at intervals along at least a portion of the length of the shaft (201); and a ground current connection (204) between the ground current electrode (203) and a corresponding connection point (205). The connection point (205) is located within the rod (200) closer to the second end of the shaft (201) than any one of the plurality of ground current electrodes (203) or outside the second end of the shaft (201).

Description

Technical Field

[0001] The present invention relates generally to the technical field of ground surveying. In particular, the present invention relates to determining electrical conductivity and related quantities within a three-dimensional target volume of the ground. Background Art

[0002] As a general term, ground survey covers all methods and practices used to obtain qualitative and / or quantitative information about the subsurface conditions within a certain surface target volume. The most common example is a ground survey carried out in preparation for a new construction project in order to check what are the prerequisites for the characteristics of the ground set for the construction task. A non-exhaustive list of other applications of ground surveys includes, for example: checking the condition of earth dams; assessing the need for additional drainage around or under existing buildings; estimating the impact of spills from landfills and other potential sources of pollution; locating and assessing the condition of underground structures such as pipelines, cables and foundations; and so on.

[0003] A special case of ground surveying consists of the measurement of electrical conductivity. Two basic examples are shown in FIG1 . On the left, two electrodes 101 and 102 have been placed on the ground surface and a potential difference has been created between them. The potential difference causes the current to be distributed over various parts of the ground. If the conductivity of the ground is uniform, the equipotential surfaces will have a regular form shown by dashed lines, where lines 103 and 104 are shown as examples. The current passes vertically through the equipotential surfaces, as schematically shown by solid lines 105 and 106. On the right side of FIG1 , two electrodes 111 and 112 have been placed under the ground, for example in a borehole. Again, the potential difference between electrodes 111 and 112 causes the equipotential surfaces and the distribution of the current through the ground.

[0004] In an actual ground target volume, the conductivity is not uniform, but varies with various factors such as soil type and composition, moisture content, etc. By making measurements such as those in Figure 1 between multiple measurement points and using the measurement results as input to the inversion calculation, an estimate of the most likely three-dimensional distribution of conductivity within the ground volume being examined can be found that will cause such measurements to occur. Although pure conductivity itself may have limited value as information, it can be used to infer related quantities such as moisture content, soil type, and the presence of large rocks, animal burrows, underground building structures, etc.

[0005] The technical difficulty with measurements like those on the left side of Figure 1 is how to get accurate results at relatively shallow depths in the ground. Measurements like those on the right side of Figure 1 give more accurate information, but also involve deeper formations, and getting a sufficient number of electrodes deep enough into the ground is laborious and time-consuming. Summary of the invention

[0006] This summary is provided to introduce some concepts in a simplified form, which are further described in the detailed description below. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

[0007] The object of the present invention is to provide an apparatus and a method for generating more accurate information about ground properties within a target volume of ground, while observing constraints related to cost, complexity and applicability to various types of ground and various fields of application.

[0008] According to a first aspect, there is provided a rod for ground surveying. The rod comprises: an elongated shaft having a first end and a second end; and a head formed by or attached to the first end of the shaft, for facilitating penetration of the rod into the ground when a force is applied from the direction of the second end of the shaft, or for producing a particular effect when a force is applied to the rod after the rod has penetrated the ground. The rod comprises: a plurality of ground current electrodes positioned at intervals along at least a portion of the length of the shaft; and a ground current connection between the ground current electrodes and a respective connection point. The connection point is either located within the rod closer to the second end of the shaft than any of the plurality of ground current electrodes or is located outside the second end of the shaft.

[0009] According to one embodiment, the ground current electrode is firmly attached to the shaft. This involves at least the advantages of a simple and robust structure.

[0010] According to one embodiment, each of the ground current electrodes comprises an exposed contact surface on the exterior of the shaft. This involves at least the advantage that a good and reliable conductive connection can be established between each of the ground current electrodes and the soil in its immediate surroundings.

[0011] According to one embodiment, each of the ground current electrodes comprises: an outer contact surface portion, the outer contact surface portion forming the exposed contact surface; an insulating layer, located between the outer contact surface portion and the exterior of the shaft, the insulating layer attaching the outer contact surface portion to the shaft; and an electrical connection, passing through the insulating layer and the wall of the shaft, for connecting the outer contact surface portion to a corresponding ground current connection inside the shaft. This involves at least the following advantages: the structure is relatively simple to manufacture, is robust and can be manufactured so that it does not have undesirable directivity.

[0012] According to one embodiment, the ground current electrode is releasably coupled to the shaft. This involves at least the advantage that the shaft can be retrieved while leaving the ground current electrode in place in the ground.

[0013] According to one embodiment, the shaft is tubular. In this way, at least some of the ground current electrodes may be located inside the tubular shaft, and the tubular shaft may be configured to slide off in its longitudinal direction to reveal the at least part of the ground current electrodes. This involves at least the advantage that the ground current electrodes are safe and not subject to mechanical wear or damage when the rod is driven into the ground, but are fully operational when the tubular shaft has been removed.

[0014] According to one embodiment, the rod comprises a releasable joint between the shaft and the head. This involves at least the advantage that the head can be left in the ground if necessary when retrieving the shaft.

[0015] According to one embodiment, the head is one of the ground current electrodes or accommodates at least one of the ground current electrodes. This involves at least the advantage that the lowest electrode gives information from a position at least as deep in the ground as reached by the head.

[0016] According to one embodiment, the head is a standardized head for ground survey rods or augers. This involves at least the advantage that the same rod can be used for mechanical ground surveys that comply with standards.

[0017] According to a second aspect, there is provided an apparatus for ground surveying. The apparatus comprises: at least two rods of the above-mentioned type; and a measuring device configured to feed a measuring current to the ground current electrodes and to measure the current received from the ground current electrodes.

[0018] According to a third aspect, a method for ground survey is provided. The method comprises:

[0019] - sinking at least two rods of the above-mentioned type into the ground within the target ground volume,

[0020] - feeding a measuring current to at least a first part of a ground current electrode comprised in said at least two rods,

[0021] - measuring the received current from at least a second portion of the ground current electrodes comprised in said at least two rods, and

[0022] - generating a three-dimensional model of ground characteristics in at least a portion of said target ground volume based on calculations from said measured currents and received currents. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In the attached picture:

[0024] FIG1 shows two examples of prior art measurements,

[0025] Figure 2 shows the rod,

[0026] Figure 3 A section of a rod is shown,

[0027] Figure 4 A section of a rod is shown,

[0028] Figure 5 The rod and some of its structural details are shown,

[0029] Figure 6 The structural details of the rod are shown.

[0030] Figure 7 The structural details of the rod are shown, and

[0031] Figure 8 The device is shown. DETAILED DESCRIPTION

[0032] In the following description, reference is made to the accompanying drawings, which form a part of the present disclosure and in which are shown by way of illustration specific aspects in which the present disclosure may be placed. It should be understood that other aspects may be utilized and structural or logical changes may be made without departing from the scope of the present disclosure. Therefore, the following detailed description should not be taken in a limiting sense, as the scope of the present disclosure is defined by the appended claims.

[0033] For example, it should be understood that the disclosure related to the described method may also be applicable to a corresponding device or system configured to perform the method, and vice versa. For example, if a specific method step is described, the corresponding device may include a unit for performing the described method step, even if such a unit is not explicitly described or shown in the figure. On the other hand, for example, if a specific device is described based on a functional unit, the corresponding method may include steps for performing the described function, even if such steps are not explicitly described or shown in the figure. In addition, it should be understood that the features of the various example aspects described herein may be combined with each other unless otherwise specifically noted.

[0034] One aspect related to the present invention is that a large number of ground surveys are, and will be, conducted using conventional mechanical methods that involve repeatedly driving a so-called ground survey rod to a certain depth in the ground at a large number of grid points covering the target ground volume. A non-limiting exemplary list of conventional mechanical methods includes cone penetration testing (CPT), standard penetration testing, dynamic probing, weight probing, and field blade testing. In many cases, this type of ground survey is mandatory under local regulations governing building and construction activities. Therefore, there are well-established practices and even standards related to the equipment used and the methods performed. An example of such a standard is a head standard that defines a standardized head for a ground survey rod (sometimes also called a ground survey auger). Such a standard is needed to ensure that conclusions drawn from the way a ground survey rod penetrates the soil are commensurable between different measurements.

[0035] An important discovery is that the apparatus provided for the purpose of this previously known mechanical ground survey method can be enhanced so as to also be used to measure the electrical conductivity between a matrix of measurement points covering the entire target area. Here, and in the continuation below, electrical conductivity refers in particular (but not necessarily exclusively) to complex electrical conductivity. Alternatively or additionally, electrical conductivity may take into account (complex) resistivity, which is essentially the inverse of electrical conductivity.

[0036] The integration of a measuring device for conductivity with an apparatus suitable for mechanical ground survey has at least three important advantageous results. First, it eliminates the need to manufacture and manipulate a separate mechanical device for the purpose of bringing such a measuring device deep enough into the ground: the measuring device can "hitch a ride" inside the apparatus suitable for mechanical ground survey. Second, it offers the possibility of carrying out conductivity measurements dynamically as a function of depth: the measurement of conductivity can also be performed while the rod is still in the process of penetrating into the ground. This makes it possible, if necessary, to carry out measurements of conductivity over a substantially ungraded depth range. Third, it allows combining the results from the conductivity measurements with those of the mechanical ground survey, allowing a two-way interrelated interpretation of the results.

[0037] In at least some cases, some further advantages can also be obtained. In mechanical ground surveys, it is not uncommon for the rod to hit a rock or other obstacle before it reaches the expected depth. Obviously, in this case, the mechanical method will not give any information about the situation deeper than this. However, the measurement of conductivity can give information about the greater depth at this location, because the current flows in three dimensions and also propagates through the ground deeper than the lowest measuring electrode, and its effects can then become tangible and conceivable in the processed measurement results. As an example, the measurement of conductivity can be used to find out that the underground obstacle in question is indeed solid bedrock, and not just some erratic boulder.

[0038] Another advantage is the added accuracy and potential for avoiding interpretation errors compared to a purely mechanical ground survey. The personnel performing a mechanical ground survey make their observations and interpretations as the rod continues to penetrate the ground. If something is not observed or is misinterpreted, such as some more subtle change in the ground properties, a mechanical ground survey is not usually able to step back and go back to check that property. In contrast, because the conductivity measurement is made in three dimensions, it is also possible to "look back" or follow the penetration head at a distance, giving information about the formations that the head has passed through.

[0039] Figure 2 A pole 200 for ground surveying is schematically shown. Figure 2 The schematic representation in does not take any position with respect to the detailed mechanical structure of the components of the rod; examples of such detailed mechanical structure will be given later in this document. The rod 200 includes an elongated shaft 201 having a first end and a second end. For illustrative purposes, the Figure 2 , the first end of the shaft 201 is its lower end, and the second end of the shaft 201 is its upper end.

[0040] The rod 200 includes a head 202 formed by or attached to a first end of the shaft 201. The purpose of the head 202 can be to extend from the second end ( Figure 2 The purpose of the head 202 may be to cause a specific effect when a force is applied to the rod 200 once it has penetrated the ground. An example of such a head is a blade head used in field blade testing.

[0041] The shape and material of the head 202 should be selected in consideration of the purpose. For example, the head 202 can be made of a hard and strong material such as an iron alloy. It can have a pointed tip and / or a threaded spiral pattern of grooves and ridges covering its sides. If the intended purpose is related to field blade testing, the head 202 can have a corresponding characteristic paddle wheel form. According to an advantageous embodiment, the head 202 is a standardized head for ground survey rods or auger.

[0042] In summary, the materials, dimensions and other properties of the shaft 201 and head 202 have been selected so that the rod 200 itself can be used directly to perform mechanical ground surveys of the type described earlier in this document. Alternative names for the rod 200 include, but are not limited to, ground survey auger, drill rod, (ground survey) probe and sonic survey (sound).

[0043] Unlike conventional poles used for mechanical ground surveying, pole 200 includes a plurality of ground current electrodes positioned at intervals along at least a portion of the length of shaft 201. Figure 2 Electrode 203 is shown as an example in FIG. 1 , so reference numeral 203 may be used to refer to one or more of the plurality of ground current electrodes. The electrode referred to here is a point or area of ​​finite size provided for conducting connection. Examples of electrodes are given later in this article.

[0044] In addition, the rod 200 comprises a ground current connection 204 between the ground current electrode 203 and the corresponding connection point. Figure 2 205 is shown as an example in the figure, and thus the reference numeral 205 may be used to refer to one or more of the plurality of connection points. The connection point 205 and the ground current connection 204 are intended to facilitate conductive connection with the ground current electrode 203, even when the portion of the length of the pole 200 where the ground current electrode is located has been penetrated into the ground or is otherwise not directly accessible. To this end, the connection point 205 may be located within the pole 200, closer to the second end of the shaft 201 than any of the plurality of ground current electrodes. Additionally or alternatively, the connection point may be located outside the second end of the shaft 201, such as at the distal end of one or more cables extending from the pole 200.

[0045] According to an embodiment, the ground current electrodes 203 are firmly attached to the shaft 201. The outer surface of the shaft 201 may have holes or recesses to accommodate such firmly attached ground current electrodes. Alternatively, each ground current electrode 203 may include an exposed contact surface located on the exterior of the shaft 201.

[0046] Figure 3 and Figure 4 An example of the latter ground current electrode is shown. Figure 3 and Figure 4Both schematically show a cut-away portion of the shaft 201 along its length. Figure 3 The cut-away portion is shown in an axonometric view, and Figure 4 The cut-off portion is shown in schematic cross section. The ground current electrode according to this embodiment comprises an outer contact surface portion 301 forming an exposed contact surface. An insulating layer 302 is located between the outer contact surface portion 301 and the exterior of the shaft 201. The insulating layer 302 mechanically attaches the outer contact surface portion 301 to the shaft 201 and at the same time electrically insulates it from the shaft 201.

[0047] exist Figure 3 and Figure 4 In the embodiment of the present invention, both the insulating layer 302 and the outer contact surface portion 301 are annular and surround the cylindrical outer surface of the shaft 201. This is not necessary, as many other shapes can be used. However, it is advantageous to a certain extent if the shape of the outer contact surface portion is such that the conductivity is not limited in the radial direction. In other words, it may be advantageous to use such a shape of the outer contact surface portion 301 that does not make an electrical connection with the surrounding medium depending on the rotational position of the rod around its longitudinal axis.

[0048] An electrical connection 401 is provided passing through the insulating layer 302 and the wall of the shaft 201 for connecting the outer contact surface portion 301 to a corresponding ground current connection 303 inside the shaft 201. Basically, the ground current connection 303 may also be routed outside the shaft 201, either as a loose cable or as a wire or conductive strip, mechanically supported by the shaft 201 but electrically insulated from the shaft 201. However, considering that the pole will be driven into the ground, the ground current connection 303 is better protected from wear and damage if it extends inside the shaft 201.

[0049] Instead of being firmly attached to the shaft 201, the ground current electrodes 203 may be releasably coupled to the shaft 201. Such an arrangement may have at least two purposes, which are not mutually exclusive. First, if the rod 200 is intended to be used also for purely mechanical ground surveys when needed, it is desirable to remove the ground current electrodes 203 at those times so that they do not enter and, of course, do not become futilely worn or damaged when not needed. Second, the rod 200 may be driven into the ground with the ground current electrodes 203 in place, and then the entire rod 200 or at least the shaft 201 may be removed, leaving the ground current electrodes 203 buried in the ground so that they can (also) be used to measure conductivity later.

[0050] One way in which the principle of releasably coupled ground current electrodes may be implemented is where the shaft is tubular, at least some of the ground current electrodes are located inside the tubular shaft, and the tubular shaft is configured to slide down in its longitudinal direction to expose the at least a portion of the ground current electrodes.

[0051] Figure 5 A rod 200 is shown according to an embodiment in which ground current electrodes are releasably coupled to a shaft 201. As shown in the upper partial enlargement, the shaft 201 is tubular, and at least some of the ground current electrodes are located inside the tubular shaft 201. In this embodiment, there is an independent mechanical support structure, namely an inner tube 501, which is concentrically located inside the shaft 201. Such an independent mechanical support structure need not be tubular or concentric with the shaft 201, as other alternatives are possible.

[0052] exist Figure 5 The upper partial enlargement of the figure shows that the ground current electrode has the same Figure 3 and Figure 4 The structure is substantially similar to the structure shown in FIG. 1 , except that it is supported only by the inner tube 501 and not by the shaft 201. In other words, Figure 5 The ground current electrode in the upper partial enlarged view in FIG. 5 comprises: an outer contact surface portion 502, which forms an exposed contact surface; and an insulating layer 503, which is located between the outer contact surface portion 503 and the outside of the inner tube 501. The insulating layer 503 attaches the outer contact surface portion 502 to the inner tube 501 and electrically insulates it from the inner tube. An electrical connector 504 is provided through the insulating layer 503 and the wall of the inner tube 501 for connecting the outer contact surface portion 502 to a corresponding ground current connector 505 inside the inner tube 501.

[0053] Figure 5 The lower partial enlargement in shows a detail that may also be used in other embodiments, such as in an embodiment where a ground current electrode is securely attached to the shaft along the length of the shaft. Regarding the graphical representation, Figure 5 The lower partial enlarged view in FIG. 5 is a partial cross-section, so that the shaft 201 is shown in cross-section, while the head 202 and the inner tube 501 are not shown in cross-section.

[0054] like Figure 5As shown in the lower partial enlarged view in FIG. 2 , the rod may include a releasable joint between the shaft 201 and the head 202. The exemplary embodiment shown here is a bayonet-type releasable joint. The head 202 includes an upwardly extending rod 506, the outer diameter of which matches the inner diameter of the tubular shaft 201 relatively closely. Therefore, the rod 506 of the head 202 can slide within the first end of the tubular shaft 201. The groove 507 in the rod 506 is configured to receive a pin 508 protruding from the inner surface of the shaft 201. Rotation of the shaft 201 in one direction causes each pin 508 to reach the blind end of the corresponding groove 507 and engage with it, so that continuing to rotate the shaft 201 in this direction causes the head 202 to rotate together. Rotating the shaft 201 in the opposite direction causes each pin 508 to come out of the corresponding groove 507, thereby releasing the shaft 201 from the head 202.

[0055] Figure 5 The mechanism shown in the lower partial enlargement in FIG is naturally only an example. A person skilled in the art of having releasable mechanical joints between solid pieces may give a variety of alternative ways of achieving a releasable connection between the shaft 201 and the head 202. Since one intended use of the releasable connection is to allow the shaft to be retrieved leaving the head buried in the ground, it is recommended that the releasable connection (if used) be constructed so that it can be released without direct access to the head.

[0056] According to an embodiment, the head 202 may be one of the ground current electrodes 203. Alternatively or additionally, the head 202 may accommodate at least one of the ground current electrodes 203. Figure 2 and Figure 5 This possibility is illustrated in FIG. 1 by schematically showing one of the ground current connections from the head 202 .

[0057] The use of the head 202 as one of the ground current electrodes (and / or the use of the ground current electrode housed in the head) involves certain advantages. The head 202 can be made of a material such as an iron alloy, which is inherently a relatively good electrical conductor, which means that (except for the corresponding ground current connection) no other structure or component may be required to use it as a ground current electrode. In addition, due to the positioning of the head 202 at the lower end of the rod 200, the head will penetrate the deepest into the ground, establishing a good measurement point when it is considered that the purpose is to bring the measurement point into the ground anyway and not just on the ground surface (or very close to the ground surface) as in many measurement schemes of previously known types. In those embodiments involving a releasable connection between the shaft and the head, the advantage is that it can be continued for a long time after the initial ground survey, because the head can remain in place and buried in the ground for a long time. Some standardized ground survey methods even require a new head each time, so making the head a disposable item for the above-mentioned purpose does not even significantly increase the consumption of material resources.

[0058] Figure 6 and Figure 7 Some possibilities for realizing connection points are schematically shown, from which there are ground current connections to the corresponding ground current electrodes. Figure 6 In the illustrated embodiment, the second end of the shaft 201 includes one or more electrical connectors 601 from which there are a plurality of ground current connections 602 connected to corresponding ground current electrodes ( Figure 6 not shown). Figure 7 An embodiment is shown where there is an inner tube 501 inside the tubular shaft 201 and one or more connectors 701 at the end of the ground current connection 702 are temporarily stored inside the inner tube 501. In this embodiment, once the shaft 201 has been released and removed, the connectors 701 can be taken out of the inner tube 501 for use. Figure 6 and Figure 7 The principle schematically shown in can be mixed in many ways: for example, even if there are no other structural components besides the shaft, it is possible to Figure 7 The connector may be temporarily located inside the shaft as in the embodiment of the present invention, so that once the rod is driven into the ground as required, the connector can be removed from the inside of the shaft. In this case, the second end of the shaft may include an openable hatch or just an opening. As another example, there may be a similar Figure 6 The fixed connector in the embodiment of the present invention is not fixed to an additional support structure in the (releasable and removable) shaft; for example, fixed to a Figure 7 The upper end of the inner tube 501 in.

[0059] As a further possibility, the connection point may be located outside the second end of the shaft from the outset. An example of this is an embodiment in which the ground current connection comes out of an opening at or near the second end of the shaft and continues from there for a considerable distance before terminating in a connector or the like, or simply in a bare cable head to which a suitable connector can be attached if required.

[0060] Figure 8 A device for ground surveying according to an embodiment is schematically shown. Since the aim is to measure the conductivity in a three-dimensional matrix between a plurality of measuring points, the device comprises at least two rods of the above-described type. Figure 8 In the embodiment shown, there are eight rods, all of which have been driven deep enough into the ground so that even the topmost ground current electrode (which is closest to the second end of the shaft) is below the ground surface. The device includes a measuring device 801 configured to feed a measuring current to the ground current electrodes in the rods and to measure the current received from the ground current electrodes.

[0061] Although not required, it is also possible to have ground surface electrodes in the device. Figure 8 In FIG, a measuring cable on the ground is schematically shown with five ground surface electrodes at known positions along its length. Ground surface electrode 802 is shown as an example. In this example, most of the ground surface electrodes are located within the surface area generally defined by the outermost rods. Additionally or alternatively, at least some of the ground surface electrodes can be placed outside the surface area. This involves the additional advantage that information can be collected from a larger ground volume that extends further than the outermost rods.

[0062] In this embodiment, the measuring device 801 is located locally, and the ground current connection may be extended to a connector in the measuring device 801 via an extension cable. Other embodiments are possible, for example such that there is a current transceiver integrated with each rod, for feeding the measurement current to the ground current electrode in the rod, and for measuring the current received from the ground current electrode in the rod. Such a current transceiver may be further connected to a central processing unit in a wired and / or wireless manner, which may collect the measurement results and perform calculations that ultimately give a three-dimensional conductivity model of the target ground volume. In yet another alternative embodiment, the current transceiver may be common to some or all of the rods used within a particular target ground volume, but such a common current transceiver may then send the measurement data it collects to an independent central processing unit.

[0063] Method embodiments of the present invention may be characterized as including certain method steps, such as:

[0064] - sinking at least two rods of the above-mentioned type into the ground within the target ground volume,

[0065] - feeding a measuring current to at least a first portion of a ground current electrode comprised in said rod,

[0066] - measuring the received current from at least a second portion of a ground current electrode comprised in said pole, and

[0067] - generating a three-dimensional model of ground characteristics in at least a portion of said target ground volume based on calculations from said measured currents and received currents.

[0068] The method may involve repeated rounds of the steps of feeding measurement currents, measuring the received currents and generating a three-dimensional model. Such repeated rounds may even occur over very long time periods, such as several years, if the aim is, for example, to monitor the long-term development of ground features of interest.

[0069] An example of a measuring current that can be used in the method is a low-frequency alternating current. Low frequency here means a frequency below 5 Hz, and preferably below 1 Hz. For example, the device can feed a current of constant absolute amplitude between two measuring electrodes and switch the polarity of the current at intervals of about two seconds. This will generate a potential field in the measurement volume.

[0070] After each switch of the input polarity, the measured current first increases rapidly, then forms a circular inflection point and finally saturates at a certain constant level. If a constant level is reached, an indication of the DC conductivity (or resistivity) over the measuring distance is given. The frequency-dependent component of the (complex) conductivity (or complex resistivity) gives the form of the circular inflection point in the measured current. If the current pulse is short, the measured current does not have time to reach a constant (DC) level, in which case only the frequency-dependent components are observed and used in the calculations. Such short current pulses can be used as measuring currents at higher frequencies (typically up to 100 Hz).

[0071] Measurements and calculations can be aimed at detecting the full-wave form of the potential field formed, which essentially indicates resistance that varies with frequency. Inverse calculations from the measurements can then reveal the desired three-dimensional complex resistance (or complex conductivity) distribution or various responses to so-called induced polarization.

[0072] The inversion calculation usually involves simulating the ground volume under examination with an element model, wherein the continuous mass of ground is represented by a three-dimensional matrix of elementary volume cells. These elementary volume cells may have, for example, the form of tetrahedrons, each tetrahedron being defined by a selected set of four ground current electrodes. The techniques of how to feed the measuring current through such a selected set of ground current electrodes and how to exploit the relationship between the fed current and the measured potential difference in the inversion calculation are known per se to the person skilled in the art.

[0073] The inversion calculations may utilize prior information of the inspected ground volume to build an initial model, from which calculations are then iteratively advanced toward a computational model that provides the best obtainable match to the measurements. For example, if a mechanical ground survey (where ground current electrodes "hitch a ride" into the ground on their rods) shows the presence of a hard surface of bedrock at a certain depth, the bedrock surface may be set as a constant boundary condition for the inversion calculations. The same applies to all kinds of known features within (or near) the inspected ground volume. In an advantageous embodiment, there may be multiple known, precisely located features on the inspected ground volume, such that the inversion calculations need only fill in the spaces of previously unknown conductivity between the known points.

[0074] As a general method for inversion calculation, for example, the iterative Newton-Raphson method can be used. The measurement results are represented by a large system of equations, in which the conductivity values ​​in the basic volume unit constitute the unknowns. The best matching solution (in the root mean square sense) of the system of equations is basically found by a series of iterative matrix inversion operations, which can be computationally quite intensive because the matrices involved are large and the amount of random access storage required is therefore quite extensive.

[0075] Most advantageously, the above-mentioned type of measurements are associated with soil borings in order to collect information about the stratification of the ground and obtain geotechnical parameters or direct input for design methods. Field tests can give results about the following information: geology / stratification of the ground; type of structure, possible foundation and expected work during construction; type of geotechnical parameters required; design method to be adopted in the planned construction, etc.

[0076] The method may involve using the computationally generated three-dimensional model of ground properties as feedback to further actions in an associated mechanical ground survey. For example, a decision may be made based on the knowledge gained so far as to where to sink another rod into the ground to obtain further results either mechanically or electrically or both.

[0077] Any range or device value given herein may be expanded or altered without losing the effect sought. In addition, any embodiment may be combined with another embodiment unless explicitly not permitted.

[0078] Although the subject matter has been described in language specific to structural features and / or acts, it should be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Instead, the specific features and acts described above are disclosed as examples of implementing the claims, and other equivalent features and acts are intended to fall within the scope of the claims.

[0079] It should be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments. The embodiments are not limited to those embodiments that solve any or all of the problems described or those embodiments that have any or all of the benefits and advantages described. It will be further understood that reference to "an" item may refer to one or more of those items.

[0080] The steps of the methods described herein may be performed in any suitable order, or simultaneously where appropriate. In addition, individual step boxes may be deleted from any method without departing from the spirit and scope of the subject matter described herein. Aspects of any of the above embodiments may be combined with aspects of any other embodiment described to form further embodiments without losing the effects sought.

[0081] The term "comprising" is used herein to mean including the identified methods, step blocks or elements, but such step blocks or elements do not comprise an exclusive list and the method or apparatus may contain additional step blocks or elements.

[0082] It should be understood that the above description is given by way of example only, and that various modifications may be made by those skilled in the art. The above description, examples, and data provide a complete description of the structure and use of the exemplary embodiments. Although various embodiments have been described above with a certain degree of particularity or with reference to one or more separate embodiments, many changes may be made to the disclosed embodiments by those skilled in the art without departing from the spirit or scope of this specification.

Claims

1. A rod (200) for ground surveying, comprising: - an elongated shaft (201) having a first end and a second end; as well as a head (202) formed by or attached to the first end of the shaft (201) for facilitating the penetration of the rod (200) into the ground when a force is applied from the direction of the second end of the shaft (201) or for producing a specific effect when a force is applied to the rod after the rod has penetrated the ground, Characterized in that the rod comprises: - a plurality of ground current electrodes (203) positioned at intervals along at least a portion of the length of the shaft (201), and - a ground current connection member (204) located between the ground current electrode (203) and the corresponding connection point (205); Wherein, the connection point (205) is positioned within the rod (200) closer to the second end of the shaft (201) than any of the plurality of ground current electrodes (203) or is located outside the second end of the shaft (201).

2. The rod according to claim 1, wherein The ground current electrode (203) is firmly attached to the shaft (201).

3. The rod according to claim 2, wherein Each of the ground current electrodes (203) comprises an exposed contact surface on the exterior of the shaft (201).

4. The rod according to claim 3, wherein Each of the ground current electrodes (203) comprises: - an outer contact surface portion (301), which forms the exposed contact surface, - an insulating layer (302) located between the outer contact surface portion (301) and the exterior of the shaft (201), the insulating layer (302) attaching the outer contact surface portion (301) to the shaft (201), and - an electrical connection (401) passing through the insulating layer (302) and the wall of the shaft (201) for connecting the outer contact surface portion (301) to a corresponding ground current connection (303) inside the shaft (201).

5. The rod according to claim 1, wherein The ground current electrode (203) is releasably coupled to the shaft (201).

6. The rod according to claim 5, wherein: - the shaft (201) is a tubular shaft, - at least some of the ground current electrodes (203) are located inside the tubular shaft (201), and - the tubular shaft (201) is configured to slide down in its longitudinal direction to expose the at least a portion of the ground current electrode (203).

7. Rod according to any one of claims 5 or 6, comprising a releasable joint (506, 507, 508) between the shaft (201) and the head (202).

8. A rod according to any one of the preceding claims, wherein The head (202) is one of the ground current electrodes or receives at least one of the ground current electrodes.

9. A rod according to any one of the preceding claims, wherein The head (202) is a standardized head for ground survey rods or augers.

10. A device for ground surveying, comprising: - at least two rods (200) according to any one of the preceding claims, and - a measuring device (801) configured to feed a measuring current to the ground current electrode (203) and to measure the current received from the ground current electrode.

11. A method for ground surveying, comprising: - sinking at least two rods (200) according to any one of claims 1 to 9 into a target ground volume in the ground, - feeding a measuring current to at least a first portion of said ground current electrodes (203) comprised in at least two of said rods (200), - measuring the received current from at least a second portion of the ground current electrodes (203) comprised in at least two of said rods (200), and - generating a three-dimensional model of ground characteristics in at least a portion of said target ground volume based on calculations from said measured currents and received currents.

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