Delving Into Landfill Depths With Geophysical Exploration Methods

Ellen Allen, Rene Rodriguez and Hans Naumann

To define the subsurface structure of an existing landfill, land surveyors have traditionally plotted cross-sections based on surveys that must be conducted regularly throughout a landfill's life.

When survey records of subsurface areas are lost, fill construction progress records are unclear to the engineer or regulatory agencies request confirming data, the landfill operator and engineer face the difficult task of collecting the information without damaging the existing landfill structure or risking the environmental controls that are already in place.

Additional information is frequently requested on the:

* Original surface contour or shape;

* Shape and thickness of the waste layers relative to covering materials;

* Location and topography of the liner material;

* Thickness, make-up and contour of the underlying original soils;

* Location and approximate condition of the weathered zone of the bedrock member;

* Identification of distinct stratigraphic units in the bedrock member; and

* Proximity of any aquifers.

Conventional exploratory drilling is the most direct way to gather the required data. To reduce time and costs while minimizing environmental risks, geophysical exploration can be used in conjunction with drilling. These methods allow investigators to minimize or eliminate new drilling while collecting enough information to generate a comprehensive picture of the subsurface environment.

Geophysical, or indirect, exploration methods rely on scientists' or engineers' ability to read the seismic and electric characteristics of the landfill components in order to create a picture of the subsurface environment. Their interpretations are confirmed by frequently indexing information collected from direct methods such as drilling. Although geophysical methods are indirect, they can collect enormous numbers of data points over a short period of time; they are non-invasive; and the data interpretation yields great detail.

Seismic Exploration Surface-based seismic exploration methods, which are well-known and time-proven, form the basis of most oil and gas exploration programs and can penetrate several thousand feet into the earth's crust. Similarly, many geologists and geotechnical engineers use seismic exploration methods for shallow subsurface investigations involved in tunneling, mining and foundation installation.

The process generates seismic waves on or near the surface (usually with a small explosive blast). Vibration-sensitive geophones planted in the earth's surface at known intervals and distances from the seismic source record the waves, which carry the signature of the media through which they have traveled. Points can be plotted on a planar or vertical perspective and can provide insight into the structure of the earth underneath the geophones.

Unfortunately, applying traditional seismic methods to an existing landfill can be fruitless. Placing seismic sources (small explosive charges) into the landfill surface can be frustrating. A good coupling between the seismic source and the transmitting medium can be difficult to achieve, and the loosely compacted waste immediately below the surface in which the source is placed can interfere with the ability to propagate seismic waves into the interior of the landfill. This loose layer, in effect, attenuates the seismic energy and prevents the recovery of a seismic record from any depth. Methods, equipment or both must be modified so that seismic survey methods can be applied to landfill exploration assignments.

Selecting and installing geophones on the landfill surface so that the seismic record of the internal structure can be collected is a challenge. The coupling problem faced with standard geophones can be resolved by using standard geophones modified with multiple, long-legged ground anchors rather than the traditional spike.

Although additional legs provide contact with the waste material, more than one contact in the ground at each geophone will produce a phase shift between incoming waves to each leg, which generates a separate high-frequency resonance in the legs. The seismic waves through the waste material, however, are characteristically at the low-frequency spectrum.

Normal-frequency geophones (centered at 14 Hz or less) are best suited for landfill seismic work. Placing the source off of the surface of the landfill, preferably on adjacent competent ground or a bedrock outcrop, can improve seismic source efficiency.

Vertical Seismic Profiling The traditional vertical seismic profiling (VSP) method, established about two decades ago to calibrate seismic velocities during oil and gas exploration, combines surface and borehole techniques. The method uses surface seismic sources (vibroseis, explosives, weigh drop, shot guns, etc.) with downhole geophones that are placed within specially drilled wells.

Unfortunately, known downhole seismic sources (such as those based on weight drop, hydraulic pressure, etc.) are not efficient for landfills. Underground air guns similar to those used in marine seismic work are more appropriate, since the boreholes can always be filled with water at least at the time of the experiment.

A variation of the air gun is the seismic drum, which produces a water shock wave within a water-filled borehole. The drum uses a two- to four-inch diameter PVC pipe, capped at both ends and filled with oil or water and securely fixed to a seismic blasting cap. After the drum is lowered into the water-filled borehole, it is detonated at a predetermined depth. The explosion produces an oil or water shock wave in the borehole equivalent to the air-pressure wave generated by an air gun in marine seismic. A modification of this drum has been used in dry non-cased holes, filling the drum with pellets that produce both shock waves (high frequency) and pressure waves (low frequency). The use generally does not damage the borehole walls.

Using the seismic drum in a borehole is key to the success of the VSP. Borehole availability allows the placement of the seismic source directly in the strata that would provide the best wave records at the geophones. The stratigraphic unit or group of units can be determined by initiating seismic shocks at various points inside the borehole.

Boreholes are usually scattered around landfills, the artifacts of groundwater baseline monitoring efforts or routine monitoring requirements in operating landfills. At least one borehole can be used without jeopardizing the landfill and while there is a chance that the borehole could be damaged during the use of the seismic drum, it rarely occurs.

The borehole should be uncased, but seismic drums have been successfully applied in cased boreholes as well. With either condition, water in the borehole improves the transmission of seismic energy from the drum and into the walls of the borehole. Dry wells should be filled with water until the seismic work is accomplished. If the wells are needed after the seismic survey has been conducted, the water can be removed with a standard well-purging pump.

Using the seismic drum in a borehole and placing a geophone array in the surface of the landfill allows the mapping of the interfaces between materials of different densities. These interfaces can be mapped in the native ground and attributed to rock type, stratification, fracture, faulting or weathering.

The materials in a landfill can be identified in a similar manner. A borehole adds significant flexibility and precision to the process. Incrementally adjusting the depth of the seismic source in the borehole and taking repeated readings through a fixed array of geophones on the landfill surface helps to map the depth of each interface. The final product is a cross-sectional view of the landfill under each geophone array. Compiling a series of these cross-sections forms a three-dimensional view of the landfill's components so that standard volumetric calculations can be conducted.

Geoelectric Techniques VSP interpretation can be enhanced and confirmed by applying geoelectrical exploration methods concurrently with the seismic effort. Geoelectrical exploration methods, like seismic ones, are surface applications which are non-invasive and do not disturb the landfill's integrity.

Geoelectrical exploration methods, which exploit the different electrical characteristics of materials, can be definitive in areas where moisture, a good conductor of electricity, is known to collect, including the contact between a daily soil cover and a subsequent layer of waste. Another example is the surface of a liner, synthetic or otherwise, which actively collects and guides leachate to collection points. Seismic surveys alone cannot always discern these features, especially if the landfill interior is complex, so geoelectrical methods are highly recommended in conjunction with VSP.

For instance, misea-la-masse (MALM) can be applied to the same uncased boreholes used for the VSP surveys. In this process, a current electrode is lowered into the borehole and pressed against the wall at a predetermined elevation using an inflatable packer. A second current electrode is solidly grounded into the surface near the borehole collar, then a direct current is applied to these electrodes and an electrical field is induced in the subsurface around the well. Voids and dry materials, for example, will present high resistance to the electrical field. Moisture zones, water tables, aquifers, metallic waste and similar conductive materials will conduct the field well.

The strength of the induced electrical field is measured by two potential electrodes placed a certain distance apart and thrust into the ground. The spacing between the potential electrodes dictates the relative lateral resolution desired. After reading and recording the electrical resistances with a sensitive ohm meter, the potential electrodes are moved away from the borehole, and another reading is taken. Systematically reading the resistance of the subsurface between these electrodes determines the electrical character of the material a certain distance below the surface. Repeating the process with increasingly larger spacing between the potential electrodes will provide a detailed record of the electrical character of the landfill at various depths into the subsurface.

Using multiple uncased boreholes will enable a practiced geophysicist to canvass a wide area above and adjacent to the geophone lines used to conduct the VSP. Superimposing the MALM profiles with those derived by VSP will enhance the definition of numerous subsurface features and generate a tomographic map of the underground structure between boreholes.

For instance, the cross-sectional figure on page 48 depicts a complicated bedrock geology, including a coal seam and an abandoned surface coal mine pit underlying an existing landfill. Combining VSP and geoelectrical methods can enable a practiced geophysicist to make determinations of the complex make-up of the subsurface beneath and adjacent to a landfill. In this example, researchers suggest that geophysical techniques can de-termine:

* The various layers of waste in the landfill and an ap-proximation of the landfill history;

* The orientation of the intermittent earth covers placed on the landfill;

* The types of mine spoil;

* Site geology, including various stratigraphic horizons;

* Bedrock stress relief fracture zones;

* The presence of a confined aquifer;

* The shape of the original liner; and

* A high-moisture contact zone below the mine spoil and portions of the landfill liner.

These interpretations can be made without drilling through the landfill. Geophysical techniques can save time and money while bringing the information hidden beneath the surface to light.