SE515279C3 - Method and system for detecting water level variations - Google Patents

Description

DESCRIPTION OF THE INVENTION According to one embodiment of the present invention, there is provided a method having the features set forth in claim 1. Preferred embodiments have one or more of the features set forth in subclaims 2-4. .

The invention also comprises a system, which has the features stated in claim 5. Preferred embodiments have one or more of the features set forth in subclaims 6-8.

The inventive method and system has a number of advantages over prior art. Because it is easy to perform the measurements, the measuring point for measuring point can be performed on repeated occasions. In this way, the level care received is very reliable. In addition, it is possible with the inventive method and the system to place the measuring points at a selectable distance from each other, which distance can be very small; thus, the measurements can be performed with a high horizontal resolution. It is also possible to perform water depth measurements at the same time as the level nets.

DESCRIPTION OF THE FIGURES Fig. 1 shows an example of an aircraft equipped with a laser bathymetry system.

Fig. 2 schematically shows a system according to an embodiment of the present invention.

Fig. 3 shows a pulse response at a receiver in the laser bathymetry system in fig 1.

Fig. 4 illustrates an example of a method according to the present invention. 10 15 20 25 30 515 279 3 PREFERRED EMBODIMENTS / IER I fi g l indicates reference number l an eye plane with an aircraft-based laser bathymetry system 2 for water depth measurements in seas, lakes, rivers and other watercourses. In an alternative embodiment (not shown), the laser bathymetry system 2 is arranged in a helicopter.

Reference numeral 3 denotes the water surface and reference numeral 4 the bottom of the sea, lake or watercourse.

Figure 2 shows that the system 2 comprises a laser 5. Directing means (not shown) are placed in front of the laser 5 to direct the laser radiation towards the water surface at a selected angle.

The aiming means consist, for example, of at least one joint rotatable mirrors placed in the beam path from the laser. The laser emits pulsed radiation within two wavelength ranges simultaneously, partly within the infrared wavelength range and partly within the wavelength range for visible light, preferably green light. Most of the infrared radiation is reflected towards the water surface 3 while a significant part of the green light penetrates into the water and is reflected at the bottom 4. The system 2 comprises a receiver 6 arranged to register the intensity of the reflected radiation. Fig. 3 shows the thus recorded pulse response with two intensity peaks. The first peak represents the radiation re-reflected in the surface of the water and the second peak represents the radiation re-reflected in the bottom. A calculation unit 7 connected to the receiver 6 calculates the time difference between the two intensity peaks, the water depth being calculated as half the time difference multiplied by the speed of light. The laser 5, the receiver 6 and the calculation unit 7 are also used for determining the distance between the system 2 and the water surface 3, for example by measuring the time that passes between a pulse being emitted from the laser 5 and the infrared radiation reflected in the water surface being detected by receiver 6, divide the time by two and multiply the result by the speed of light.

The aircraft 1 has a receiver 8 for receiving position data from a satellite positioning system, for example GPS. The GPS receiver 8 receives position data for the planplane specified in a global coordinate system called World Geodetic System 1984 (W GS 84). WGS 84 provides a global frame of reference fixed in relation to the earth. The frame of reference is described by a reference ellipsoid 11, which is a mathematical surface depicting the general shape of the globe and which is used to indicate positions in space, i.e. latitude, longitude and altitude. The height is thus stated in relation to said reference ellipsoid ll. The reference ellipsoid used in WGS 84 is a global reference ellipsoid that seeks to approximate the entire surface of the earth as well as possible. There are other reference ellipsoids that try to approximate certain parts of the earth's surface as well as possible. In Sweden, for example, Bessel's reference ellipsoid is used, which is considered to approximate Sweden's earth surface best. It is possible to transform coordinates given in relation to a certain reference ellipsoid to coordinates given in relation to another reference ellipsoid and the invention shall not be considered limited to refer to coordinates given in WGS 84, although in this description only WGS 84 is used.

The aircraft fl, for example, travels at 120-190 knots at an altitude of about 1000 meters above the surface. Lasem then sweeps an area across the flight direction of 500 meters. Of course, it works just as well to perform the measurements with the help of a helicopter. For each sweep, the height of the eye plane above the water surface and the water depth is measured at a number of measuring points by means of the laser bathymetry system, while at the measuring point position the position of the eye plane is registered by means of the GPS receiver 8 indicated by latitude, longitude and height. The values thus recorded for each measuring point are stored in a unit 9. kmz / h. With a surface coverage of, for example, 200 krnz / h, an increase of about 200 hours would be required to cover the entire exploration area. If the exploration area is such that it is affected by wind, currents or other external environmental factors, all or parts of the area can be remeasured at a later appropriate time to compensate for the effect of these factors' impact on the water surface level.

After landing, the memory unit 9 is taken out of the eyepiece 1 and loaded into a unit 10 for processing the registered data. In an alternative embodiment, the memory unit 9 is not physically transferred to the processing unit 10, but the data is refueled according to some conventional method. The processing unit 10 preferably consists of a computer equipped with memories and peripherals as well as software for causing the computer to perform operations on the data in the memory unit 9. 10 15 20 25 30,. uw f. wf:: '_ ["_:',. '.,.. I:. u._ u na oa af ....“ - xn 0.- c n. nn .uu - II qunn. In Fig. 4, a first operation 12 comprises that the level of the measuring surface of the water surface is calculated in relation to the reference ellipsoid 11. In this operation, data is described for each measuring point describing the distance between the receiver 8 and the reference ellipsoid. the ellipsoid 11 to the data describing the distance between the laser bathymetry system 2 and the water surface 3, providing the level of the water surface 3 relative to the reference ellipsoid 11 provided that the GPS receiver 8 is located in direct connection with the laser bathymetry system 2. Should the receiver be located on a distance from the system 2, the level values instead indicate the level of the surface in relation to the reference ellipsoid in a constant, but this constant is the same for all level data and therefore irrelevant.The operation 12 is preferably performed in a calculation unit 10a in the processing unit 10.

The level values obtained constitute, in addition to representing the level of the water surface, an estimate of the height of the so-called geoid at the measuring points. The geoid describes the shape of the mean level of the sea surface and its elongation below the continents and, like the sea surface itself, an irregular, undulating surface affected, for example, by gravity.

Gravity is not the same all over the earth, but varies with latitude, but also more locally with the composition of the earth's crust. In areas where the geoid is curved towards the seabed, gravity is weaker, which may be due to the bottom having a lower density than the surrounding bottom and upward curves in the geoid means that the gravitational force is stronger, which may mean that the bottom has a higher density than surrounding bottom. Local variations in sea level and thus the geoid can thus result from density variations in the bottom due to the bottom containing natural resources such as metal, gas, oil, a freshwater reservoir, etc. Thus, it is possible to use sea level when exploring to determine where test wells should be. performed.

However, the sea level does not coincide with the geoid at all times; this is because the sea is affected by a number of conditions that change with time, such as tides, air pressure, wind, ocean currents, water temperature, salinity, etc. under different conditions; each series of measurements is stored in the memory unit 9. In the calculation unit 10a, in an operation 13 for each measuring point, a corrected level value is calculated to try to remove the influence from the various prevailing conditions. In a simple embodiment, an average level value is calculated for each measuring point based on the created level values for the measuring point. In an alternative embodiment, the level values are measured measurement series by measurement series in order to try to filter out the influence from the prevailing conditions. Thereafter, the average values are formed from the measurement series measuring point by measuring point.

The geoid and thus the height of the geoid are also affected by the water depth. When searching for density variations in the bottom, the effect of the water depth on the sea level is uninteresting and can also constitute a source of error when utilizing the sea level during exploration. In order to try to remove this source of error, the depth calculated in the calculation unit 7 is used in an operation 14 to calculate for each measuring point the effect of the water depth on the height of the geoid and thus the water level. In this calculation, preferably performed in the calculation unit 10a, an area is created around each measuring point, from which a water column extends downwards to the bottom so that the height of the water column corresponds to the depth. To determine the position of the water column on the geoid, each column is replaced by a column of the same volume, but whose density is approximately the same as that of the upper crust of the bottom. Then, using well-known methods for each measuring point, the contribution to the geopotential at the water surface is calculated due to the replacement of the water column with a mass having the density of the bottom. Then, in an operation, the level data measuring point is corrected for measuring point based on the calculated effect. The level data thus describes a modified estimated geoid to be used in exploration.

U.S. Patent No. 5,615,114 discloses additional aspects of processing water level data to seek to refine the data so that substantially only variations remain that result from variations in bottom density.

The level data thus modified is searched in an operation 16 to detect variations in the level data resulting from density variations in the bottom which are due to natural resources in the bottom. The scan is performed by a scan unit 10b included in the treatment unit. It in turn comprises a filter designed, for example, to let through variations whose extent in the plane is less than 200 km times 200 km and greater than 50 m times 50 m. Characteristic of the relevant variations is also that the amplitude is less than one meter. The detected variations are characterized in an operation 17 on a map image of the exploration area stored in the processing unit 10.

The above-described operations 12-17 'in the described example were realized in software. It is to be regarded as professional measures to implement the functions required for operations 12-17 in the software.

When in this description the term GPS receiver is used, it is assumed that the GPS receiver is provided with a second receiver to receive a correction signal, a so-called DGPS. In order to further improve the accuracy of the measurements, a reference point with known coordinates is also used in an example, whereby it is possible to obtain an accuracy in the measurements which is of the order of centimeters.

However, it may be considered as professional measures to improve the accuracy of the position indication from a GPS receiver in this way.

Claims (1)

1. 0 15 20 25 515 279 PATENT REQUIREMENT Method for detecting variations in the level of the water surface (3) within an area of a sea or lake caused by variations in the density of the underlying bottom (4), characterized in that an eyelet (1) flies over the area and at a number of measuring points: a) measures its height above the water surface, at which altitude measurement - laser radiation is emitted from the eyepiece at a first wavelength for reflection in the water surface, - the time elapsed until the reflected radiation is received at the eyepiece and fl the height of the device above the water surface is determined from the time elapsed and b) determines its position in relation to a reference ellipsoid using a satellite positioning system, such as GPS, to calculate (12) for each measuring point the water surface level relative to the reference ellipsoid and that the level data is scanned (16) to find variations which are due to the density variations. Method according to claim 1, characterized in that the altitude measurements and position control rings are repeated under different conditions such as different air pressures, winds, sea currents and / or water temperatures and that the level data for each measuring point is corrected (13) to substantially remove effects from the prevailing conditions. Method according to claim 1 or 2, characterized in that at each measuring point the device also emits laser radiation at a second wavelength mainly for reflection at the bottom and that for the measuring point a water depth is determined based on the time difference between receiving the first laser and the second wave. A method according to claim 3, characterized in that the effect of the water depth on the water level is calculated (14) and that the level data is corrected (15) on the basis of the calculated effect. System for detecting variations in the water surface level (3) caused by density variations in the underlying bottom (4) within an ocean area on a sea or lake, characterized in that the system comprises: an aircraft (1) equipped with a laser bathymetry system (2) arranged to measure the height of the aircraft above the water surface, a position determining unit (8), such as a GPS receiver, arranged to determine the position of the vehicle in relation to a reference ellipsoid and storage means (9) arranged to store the height d and position data for fl era measuring points and a treatment unit (10) comprising calculating means (10a) arranged to calculate from the height and position data in the storage means (9) for each measuring point the level of the water surface in relation to the reference ellipsoid and means (10b) arranged to scan level data to find variations in the level of the water surface caused by the density variations. System according to claim 5, characterized in that the storage means (9) are arranged to store altitude and position data from repeated measurements at respective measuring points and that the calculation means (10a) are arranged to correct the level data for each measuring point in order to substantially remove impact from environmental factors low / high air pressure, winds, sea currents and / or water temperatures. System according to claim 5 or 6, characterized in that the laser bathymetry system (2) is also arranged at each measuring point to measure a water depth. System according to claim 7, characterized in that the calculation means (10a) are further arranged to calculate the effect of the water depth on the water level and to correct the level data on the basis of the calculated effect.