Spectrum geophysics

Services: Landfill Delineation

Several different methods can be used to delineate landfill extent.  The method chosen depends on several factors, such as whether lateral or vertical delineation is required, the dimensions of the area of investigation, anticipated depth of landfill materials, lithology of background materials, anticipated type of landfill materials, and cultural features/sources of interference in the area of investigation.  Spectrum commonly utilizes the following methods to delineate landfill boundaries:



   
• Frequency Domain Electromagnetics Methods

  
  Frequency domain electromagnetics methods measure the lateral variation of conductivity in subsurface soils.  As there is generally a measurable contrast in conductivity between background soils and landfill materials, frequency domain methods are useful for the lateral delineation of landfills.  Frequency domain methods are especially useful where there is a large area to investigate, as an extremely large number of measurements can be made quickly and cost effectively.  Spectrum commonly uses walking frequency domain terrain conductivity meters such as the Geonics EM-31 or the Geonics EM-34 for the delineation of landfill boundaries.

  These instruments are composed of a transmitter and a receiver separated by a fixed distance, which determines the depth of investigation.  The transmitter sends out a primary electromagnetic field that induces eddy currents in subsurface materials by Faraday’s Law of Induction.  The eddy currents induce a secondary magnetic field which is measured at the receiver and compared with the primary field; this secondary field has a strength and a phase shift that depends on the conductivity of subsurface materials.   The data are acquired on a rectangular grid and, for a given transmitter-receiver separation, represent measurement within a particular depth range.  The data are then contoured to create plan view maps representing lateral variation of conductivity within that depth range.  These plan view maps are then color-enhanced and used to distinguish the boundaries between materials exhibiting contrasts in conductivity such as landfill materials vs. background soils.
  
  
• Magnetics

  
  The magnetics method measures lateral variation in the intensity of the earth’s magnetic field.  Anomalies in the earth’s magnetic field are often caused by the induction of a secondary magnetic field in a ferromagnetic material (such as steel pipelines, well casings, tanks, drums and pits) by the earth’s magnetic field.  The magnitude of the induced field (and hence the anomaly) is proportional to the intensity of the earth’s magnetic field and the magnetic susceptibility of the underlying material.   As there are usually ferromagnetic materials in landfills, the magnetic method is useful for the lateral delineation of landfills.  Like frequency domain electromagnetics, the magnetic method is particularly useful where there is a large area to investigate, as an extremely large number of measurements can be made quickly and cost effectively.  Spectrum commonly uses the Geometrics G-858 cesium vapor walking magnetometer or the Geometrics G-858G cesium vapor magnetic gradiometer for the delineation of landfill boundaries.

  
  The operating principle of these instruments is optical pumping, where the energy state of electrons in the cesium vapor is changed and used to measure the Larmor frequency, which is related to the earth’s magnetic field.  The magnetics data are acquired on a rectangular grid and represent measurements of the shallow subsurface.  The data are then contoured to create plan view maps representing lateral variation of magnetic field in the shallow subsurface.  These plan view maps are then color-enhanced and used to distinguish the boundaries between areas with magnetic anomalies (landfill materials) and background soils.  Comparing and contrasting conductivity maps with maps of total field or vertical magnetic gradient for the same area allows discrimination between buried ferromagnetic objects (drums, metallic pits) and chemical and/or non-ferromagnetic disposal materials, allowing further characterization of the materials within a landfill.
  
  

• Electrical Resistivity  

  
  The DC electrical resistivity method measures both the lateral and vertical variation in electrical resistivity of subsurface materials.  In this method a DC circuit is established in the ground via cables and electrodes and the ground acts as the resistor to complete the circuit.  The electrical resistivity method is useful for identifying both the lateral boundaries and depth of landfills at sites where there is a measureable electrical resistivity contrast between landfill materials and background soils.  In Spectrum’s experience most landfills do exhibit a characteristic resistivity that is measurably different from background (although the actual values vary from site to site), and the DC resistivity method is extremely useful for imaging both the depth and lateral boundaries of landfills.   In addition, this method is useful for the detection of leaks in landfills.  Spectrum uses the SuperSting R8/IP system (SuperSting) manufactured by Advanced Geosciences, Inc. to acquire electrical resistivity data.
  
  During an electrical resistivity survey, Spectrum establishes a linear array of equally-spaced electrodes (usually 56, but more can be supplied if necessary) in the ground.  Once the electrodes are established, the circuit is connected and a known current is transmitted into the ground via a pair of electrodes.  This current travels through the ground and the voltage between another pair of electrodes some distance away is measured.  The electrical resistivity of the ground between the pairs of electrodes is then computed using a geometric factor and Ohm’s Law which states V=IR.  The distance between pairs of current and potential electrodes determines the depth of investigation of a particular measurement.  The SuperSting system steps through suites of readings all the way down the line such that a two-dimensional apparent resistivity vs. depth pseudosection is generated.  The data are then processed to generate a two-dimensional color-contoured model resistivity section that represents the subsurface materials along the line.  Once the model section has been generated it is used to identify areas of anomalous resistivity that may be associated with landfill boundaries. 

   

METHODS IN ACTION


Landfill Delineation – WESTLEY TIRE FIRE SITE, WESTLEY, CALIFORNIA

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    A geophysical investigation was conducted at the Westley Tire Fire site near Westley, California to delineate the lateral and vertical extent of buried tires within a 15-acre area where portions of a “tire landfill” had burned for 30 days. To obtain the lateral extent of buried tires Spectrum collected EM-31 terrain conductivity and in-phase data using 5-foot station spacing along parallel lines spaced 10 feet apart within the area of investigation. A detailed map of surface features, such as roads, utilities and piles of tires, was made within the entire area of investigation so that anomalies from these sources could be identified in the EM-31 data.
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      Once several high-conductivity EM-31 anomalies were identified, the depth of these anomalies was investigated along four DC resistivity transects using AGI’s Sting/Swift electrical resistivity system, four-meter cable and arrays of between 28 and 56 electrodes. The electrical resistivity data revealed several shallow extremely low resistivity anomalies which were further investigated by the Client in order to correlate resistivity values with subsurface features.
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    Line 2 was approximately 669 feet long. This section represents the best-fit electrical resistivity model interpreted from the data collected along Line 2. The extremely low resistivity (deep blue) anomaly evident between Stations 76 and 192 meters beginning at a depth of 3 meters and extending to 8 to 17 meters below ground surface was interpreted to be associated with buried burned tires at the Site. An excavation made by the Client at Station 84 revealed fills material from 0 to 10 feet, underlain by buried burned tires from 10 to 17 feet.