1.0 INTRODUCTION Land subsidence is defined as an environmental geological phenomenon that causes the slow lowering of ground surface elevation. It is often a result of the natural compaction of sediments and extraction of ground water, geothermal fluids, oil, gas, coal and other solids through mining.
Land subsidence tends to change the topographic gradients, and thus causes infrastructure damage, ruptures in the land surface, aggravates flooding, causes inundation of land and reduces the capacity of aquifers to store water; ultimately posing a risk for society and the economy. The occurrence of land subsidence has been studied in many places around the world, including Tokyo, Japan, Mexico, Saudi Arabia, Texas, USA, Jakarta, Indonesia, Ravenna, Italy, Bangkok, Thailand, Taiwan, and China. The driving force behind land subsidence is mainly a combination of a primary factor and an immediate factor; the primary factor being the existence of unconsolidated sediment deposits that comprise the aquifer system, and the immediate factor being the diminishing groundwater level. An area is potentially prone to land subsidence if a thick sediment deposit prone to consolidation exists in the subsoil, along with water which is susceptible to being pumped. Lowering of the water table due to groundwater harvesting is the triggering factor of subsidence. Nonetheless, even if the water table is reduced, land subsidence will not occur if the aquifer system lacks the presence of unconsolidated sediments.
It has been found that excessive groundwater exploitation can result in a slow, but eventually significant, land subsidence. A close relationship between the amount of groundwater withdrawal for industrial activities and advancement of land subsidence was recognized early in Japan during observations made between 1954 and 1960. Additionally, geology also plays a vital role in the acceleration of land subsidence. Large amounts of groundwater extraction from certain types of underlying sediments, such as fine-grained sediments, result in compaction of these sediments, because the groundwater is partly responsible for the subsurface support. This ultimately triggers land subsidence.
Air-bone Laser Scanner (ALS) is an advanced remote sensing tool that has the ability to map displacements over vast areas at a very high spatial resolution, at a lower cost than other conventional techniques, such as GPS, topographic measure and extensometers. ALS tools mapping produces high-resolution topographic maps of very high accuracy. The unique capabilities of this technique yield more comprehensive and precise topographic information than traditional methods.
It can be used to accurately measure the topography of the ground, even where overlying vegetation is quite dense. The data can also be used to determine the height and density of the overlying vegetation, and to characterize the location, shape, and height of buildings and other man-made structures. Repeat observations provide the information necessary to derive estimates of subsidence. 2.0 METHODOLOGY Air-bone Laser Scanner (ALS) method relies on measuring the distance from an aircraft to the Earth’s surface by measuring precisely the round-trip travel time of a pulse of laser light. The travel-time is converted into distance from the aircraft to the ground by multiplying by the speed of light. Typical laser scanner transmitters used produce a near-infrared laser pulse that is invisible to the human eye.
Advanced ALS systems can send out up to 150,000 laser pulses per second. By scanning the laser pulses across the ground using a rotating mirror, a dense set of distances to the surface is measured Figure 1 along a narrow swath under the ground track of the aircraft. Figure 1 : Illustration of an Air-bone Laser Scanner system in operationThe distance measurements are converted to map coordinates and elevations for each laser pulse by combining the distance data with information on the position of the aircraft at the time the laser pulse was shot and the direction in which the pulse was shot. The aircraft position along its entire flight path is determined from GPS observations using a differential kinematic positioning technique. The direction of the laser pulse is established using an Inertial Navigation System (INS) that measures the 3-dimensional (3D) attitude of the airplane, and measurements of the orientation of the ALS scan mirror. An area is mapped by flying many parallel lines, guided by GPS, so that the narrow swaths of data overlap along their edges.
Perpendicular flight lines may also be flown to produce a grid of observations that can be used to determine and calibrate systematic errors based on comparing measurements at the grid intersections or “crossover” points. The combined set of geo-referenced data points that results from an ALS survey is often referred to as a “point cloud”. Early versions of laser altimeters measured the distance to the first feature reflecting the laser pulse.
More recent laser altimeter systems measure multiple returns for each laser pulse. Typical modern ALS systems produce a laser pulse that has a ground spot size of about 0.5 m in diameter. If the laser pulse reflects off of more than one feature within a ground spot, the one pulse can measure distances to the multiple features.. This capability is very important when trying to map ground topography beneath vegetation. The ‘last returns’ for each pulse are those from the lowest features and thus are more ikely to be reflections from the ground. Post-processing algorithms are used to identify the last laser returns that are from the ground.
Once the ground returns are identified, they are used to produce what is known as a digital elevation model (DEM) that describes the ground topography using a regularly spaced grid of elevation values. Post-processed ALS point clouds are produced in a global reference frame determined by GPS positioning with typical spatial densities more than 1 observation per square metre and data point accuracies of up to 3-10 cm in vertical position and 20-50 cm in horizontal position. The point cloud observation density recovered depends on the type of ALS instrument used and the altitude at which the ALS survey is flown. The resulting data products have important applications in many areas. ALS campaigns have yielded digital representations of topography at resolutions and accuracies sufficient to make measurements of surface deformation associated with earthquakes, landslides, and subsidence associated with ground water depletion. To examine motion of the ground surface due to subsidence using ALS, geo-referenced DEM’s computed using ALS point clouds collected at different epochs can be differenced. The resulting elevation-difference map shows apparent absolute vertical motion of the ground surface in the time interval between the surveys at each DEM grid point. Since the DEM’s can contain a relatively high levels of random noise, often a smoothing filter with a radius window of 1-5 metres is used before the differencing step.
This technique of measuring subsidence results in maps of vertical motion with very high spatial resolutions of more than 1 sample per square metre and absolute vertical accuracies of about 10 cm. 3.0 CONCLUSION In conclusion, we can say that Air-bone Laser Scanner had a high-density maps of deformation which is more than 1 sample per square metre where the survey can be scheduled and flown any time within the weather limitations. Hence it can make measurements in all terrain and vegetation conditions. However, there are certain weaknesses where the accuracy is only 5-10 cm in the vertical component.
It needs specialized processing of data is required to obtain the highest accuracy results which may not available from commercial operators. Surveys can only be flown in good weather.