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Geophysical survey for the risk management


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Geophysical investigations for the due diligence study of dismissed urban and industrial sites

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Geophysical survey for the risk management

  1. 1. Geophysical survey methods: How To Manage the Due Diligence Risk
  2. 2. One of the primary reasons for conducting a due diligence assessment on a property that will either be purchased or sold is to understand the potential hazards and liabilities associated with the past and present activities conducted on-site. In summary, the goal is to “manage risk”. The main issue is the clear understanding of what operations occurred and what the subsurface holds. MANAGING RISK AND UNCERTAINTY Georadar section on an area nearby a river with location of old bridge arcades 10 m 1 m
  3. 3. LOCATION OF THE POTENTIAL RISKS UXO (unexploded ordnance) location with Magnetometer – CIRPARK Prj – Turin, 2014 In many instances, a geophysical investigation can be part of the solution to the problem. These non- invasive exploratory methods can greatly reduce the risk of missing critical subsurface problems or encountering a buried hazard by providing a screening tool and strategy for proposed follow-up subsurface investigations. How to identify an unknown buried object (potential risk and hazard) on a property? Invasive investigations (borehole drilling, excavations, etc.) are: • Sometime not permitted by the current site owner or facility operations • Limited at a single point and without any certainty to locate buried object • location of buried utilities or other subsurface hazards may render invasive methods as unsafe or impossible
  4. 4. • Non-invasive collection of data • Cost-effective • Quick mobilization • Fast on-site set-up and breakdown • Identification of hazardous prior to subsurface work • Permitting generally not required • Wide and comprehensive area of coverage • No generation of impacted media BENEFITS OF GEOPHYSICAL ASSESSMENTS EM survey for location of waste disposal Sacmi Prj – Scandiano (Italy), 2014
  5. 5. There are several types of geophysical methods available, all of which can be used independently or in tandem with each other. Fr the due diligence risk assessment we can consider two different geophysical survey: LARGE SCALE SURVEY AND (IF NECESSARY) SMALL SCALE SURVEY Large-scale geophysical survey (surface mapping of the underground properties) – MAGNETIC ELECTROMAGNETIC MAPPING GEOPHYSICAL METHODS Location of the main targets (buried tanks, underground pipes, unknown waste disposals, any underground anomalies) satisfying detail NOYES small-scale geophysical survey (anomalies characterization) GROUND PROBING RADAR, ELECTRICAL TOMOGRAPHY Direct ground investigation/remediation FIRST PHASE SECOND PHASE
  6. 6. Frequency Domain Electro Magnetic induction (FDEM) involves generating an electromagnetic field (primary field) which induces current in the earth which in turn causes the subsurface to create a secondary magnetic field. By measuring this secondary magnetic field and the difference with the primary field, subsurface soil properties and the main features (buried objects) can be detected. This method measures the magnitude and phase of induced electromagnetic currents, which are related to the subsurface electrical conductivity. Electrical conductivity is different for soil types, ad it is a function of the soil and rock matrix, percentage of saturation, and the conductivity of the pore fluids. EM instruments provide two measurements simultaneously, the electrical conductivity data and the in-phase component, which responds to magnetic susceptibility and metal. FDEM has distinct advantages over many other techniques. Because no contact with the ground is required, FDEM can cover a large area quickly and therefore with cost saving. LARGE-SCALE GEOPHYSICAL SURVEY Common applications of FDEM include • the mapping of buried wastes, metal drums, UST tanks, and metal utilities • Detect archeological remnants • Locate UXO (unexploded Ordnance) FD EM Survey The survey area is divided in regular grid. The spacing between line is usually 1 or 2 m a Transmitter Receiver target Primary EM Field Secondary EM Field
  7. 7. Example 1 - EM SURVEY TO LOCATE ILLEGAL UNDERGROUND DUMPING FDEM Survey Electrical conductivity FDEM Survey Magnetic susceptibility Gas pipe Illegal landfill Natural soil Foundry waste Detection and location of an illegal waste disposal - Brescia (ITALY) The ground conductivity maps obtained with electromagnetic survey can easily locate buried waste or contaminated areas. The magnetic susceptivity is particularly suitable for the detection of metals and helps a lot in the detection of buried drums or, for example, to recognize different waste type (in the example below, foundry wastes from ferrous foundries) REMARKS: THE EM METHOD INVOLVE A GROUND THICKNESS OF 5-6 M
  8. 8. Example 2 - EM SURVEY TO LOCATE ILLEGAL UNDERGROUND DUMPING 0 50 100 150 200 Distanza [m] 0 50 100 150 200 -120 -90 -60 -30 0 Distanza[m] 143500 145500 147500 149500 151500 Illegal disposal 1 Tyre waste illegal disposal – Old trench excavation filled with tyre wire waste (HIGH CONDUCTIVITY) Illegal disposal 2 Old gravel pit excavation filled with rubber waste (LOW CONDUCTIVITY) Mapping of an illegal waste disposal - Cuneo (ITALY)
  9. 9. SMALL-SCALE GEOPHYSICAL SURVEY Small-scale geophysical survey has the main objective of better defining the ground characteristics (soil profiling, waste disposal thickess and geometry, shape of the buried structure, etc) There are two main methodologies: • ELECTRICAL RESISTIVITY TOMOGRAPHY (ERT) • GROUND PROBING RADAR 0m 50m 100m 150m 200m -150 -100 -50 0 Resistivity Map Resistivity section Mapping of an illegal waste disposal - Cuneo (ITALY) Thickness of the waste disposal = 10-11 m
  10. 10. The electrical resistivity tomography (ERT) is used to map the subsurface distribution of electrical resistivity by means of injection of DC current in the ground and by measuring the voltage on the ground surface or inside boreholes. The characteristics of electric resistivity are tightly correlated to the chemical-physics characteristics of the ground materials and, for this reason, ERT provide a very precise vision of the subsurface Electrical resistivity survey are applied to evaluate: 1. Presence of groundwater 2. Depth of bedrock 3. Mapping of contaminant plumes 4. Location of faults and rock fractures 5. Detection of buried landfill cells 6. Location of clayey zones that may form aquitards 7. Zones of buried constructions debris 8. Void spaces, such as large culverts, pipelines, caves, or abandoned mine adits Electrical resistivity values are digitally recorded on a multi- electrodes georesistivimeter and processed using inversion software (RES2DINV) to create 2D sections or a three-dimensional model of the underground. Depth of penetration: 1/3 of the length of the line ELECTRICAL RESISTIVITY TOMOGRAPHY
  11. 11. 0 5 10 15 20 25 30 35 40 45 Distance [m] -7 -6 -5 -4 -3 -2 -1 0 Profondità[m] 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 The plume of contamination usually show a strong contrast with the natural soil or groundwater. The oil (and all hydrocarbons) have a high electrical resistivity, and can be easily recognized with the geoelectrical survey LNAPL MW3 Linea ERT7 GWL DNAPL Underground pipe Floating oil Polluted water 3 3.2 3.4 3.6 3.8 4 4.2 4.4 <3 (log-resistivity) Natural soil Oil tak Source of the contamination LNAPL Example 3 – ERT SECTION TO LOCATE THE CONTAMINATION PLUME OF PLASTIFICANT OIL IN THE GROUNDWATER TABLE Mapping of the pollution plume due to a leakage from an oil storage tank (project not citable)
  12. 12. GPR (GROUND PROBING RADAR) SURVEY METHOD The ground penetrating radar (GPR - Ground Probing Radar) is a geophysical method used to investigate the near surface underground. Thanks to its high degree of resolution, the GPR is the most effective method for locating cavities, underground tanks or utilities, and buried artifacts, archaeological remains and structures in general. In addition, the GPR can be used to identify geologic contacts, substrates and surface geological features of various kinds (fractures, groundwater levels, etc..). The GPR method can detect underground storage tanks and pipes, underground utilities of any type (metal, plastic and concrete). A typical GPR survey requires the acquisition of a series of lines arranged in a regular grid, in order to investigate the entire site. When a radar profile crosses a cylindrical shape (pipe or tank) across its axis, the electromagnetic signal undergoes a reflection effect that results in a typical hyperbolic shape .
  13. 13. For any kind of work involving excavation of the ground, the exact knowledge of the location of underground structures is a key element for proper execution of the work. In this perspective, the use of GPR is an excellent tool to obtain all the information needed to perform the actions provided for in an effective way, safe and without damage to the infrastructure. Georadar section 3D reconstruction of the buried structures UST Underground tank utilities B A UST GPR SURVEY METHOD
  15. 15. 3 4 GPR section 3 Sketch map GPR section 4 Site: restoration of a warehouse (Novara, 2010) The purpose of the survey: The survey was conducted to assess the characteristics of the pavement of an old industrial building to converted in a storage warehouse. More in detail, we detected the potential weakness zones, which may cause differential settlement or structural collapses due to concentrated loads . Design Survey: The survey methodology was conducted with GPR (400 MHz antenna) on the whole area of the building (2000 m2): The operation required the use of two technicians for about half a day of site acquisition. Results: The investigation showed the presence of several underground old pipes (concrete and fiber-concrete), 15 hidden metal plates (basement of industrial machines) and the mapping of the reinforced concrete pavement. Example 4 –CONCRETE FLOOR ASSESSMENT OF A CONVERTED INDUSTRIAL BUILDING pipesReinforced concrete pavement Small tunnel Small tunnel Concrete rebars Reinforced concrete