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Non-Invasive Determination of the Location and Distribution of DNAPLs by Seismic Reflection

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Tech ID: 2306
Project Overview
The proposed research is a study to determine the location and distribution of subsurface DNAPL contamination at various DOE sites by use of two and three dimensional high resolution seismic reflection data and borehole geophysical surveys. The specific objectives of the research are as follows: 1) Subsurface imaging of geologic sinks where DNAPL can pool; 2) Direct detection of DNAPL by use of seismic reflection amplitude versus offset (AVO) methods; 3) Generation of porosity and permeability maps based on borehole seismic reflection information to construct two and three dimensional models for delineation of preferential pathways for subsurface contaminant transport. The proposed seismic reflection AVO DNAPL detection technology has been successfully utilized by the offerors at the DOE Savannah River Site. At the SRS M-Area Seepage basin DNAPL, composed of mostly trichloroethylene (TCE) and tetrachloroethylene (PCE), occurs in free phase and dissolved phase forms in weakly consolidated Tertiary age sandstones and gravels. Most of the contamination pools are atop an aquitard locally known as the "green clay" at depths to 150 feet. Utilizing existing borehole geophysical logs and P and S wave velocity measurements, seismic modeling indicated that TCE saturated sediments would exhibit a seismic AVO anomaly provided that at least four feet of free phase product were present and the seismic data had a dominant frequency of at least 120 Hz. The model results, borehole data, and preexisting seismic reflection data were integrated to design three 2-D seismic reflection profiles. The first profile, M-1, was located in such a manner that the profile would cross a known pool of free phase DNAPL. The other two profiles, M-2 and M-3, were located so that each profile would cross areas of suspected high concentrations of DNAPL. The seismic models indicated that an AVO anomaly associated with DNAPL in this area would be manifested by a large amplitude increase at far source-to-receiver offsets. Thus a reconnaissance method to detect a potential DNAPL anomaly in this area was to examine the near and far offset section, but not on the near offset section. On seismic line M-1, over the known pool of free phase DNAPL, a high amplitude reflection occurs at the depth of the known DNAPL pool. At this location several feet of free phase DNAPL, consisting of mostly TCE, was bailed from well MSB-22. The Smith-Gidlow fluid factor stack also shows an anomaly in this area. These results were sufficiently encouraging to warrant a test of the AVO method. On profile M-1 at SP-83 occurs an AVO anomaly similar to that which is associated with the known pool of DNAPL. This anomaly was drilled and TCE was found at the predicted depth of 90 feet. Seismic lines M-2 and M-3 were examined for potential AVO anomalies similar to those found on lin M-1. On line M-2 occur three such anomalies. To date, the anomaly between SP 340 and SP 370 has been drilled and DNAPL was found to be present over a 20 foot interval corresponding to the seismic anomaly. At this location aqueous concentrations of DNAPL were found to be in excess of 110,000 ppb and the concentration of DNAPL in the soil was found to be in excess of 346 µg/g. At the Savannah River Site M-Area Seepage basin seismic reflection AVO anomalies were found to be associated with subsurface concentrations of DNAPL composed of primarily TCE and PCE. The seismic modeling results indicated that near-far offset stack sections would provide a quick reconnaissance tool to identify potential DNAPLs.

Technology Description
 The seismic reflection method provides a non-invasive means to acquire spatially dense subsurface information. Typically, a two dimensional high resolution seismic reflection survey may have data points only a foot apart. A three dimensional seismic reflection survey, because of the greater cost involved, may have data points every 5 feet throughout a regular grid. Vertical resolution of a typical high resolution seismic reflection survey is three to five feet. These data, combined with existing borehole information, can provide a detailed picture of the subsurface. Seismic reflection surveying has been used since the mid- 1920's to map subsurface geology - primarily for petroleum exploration. However, the use of the method for engineering and environmental applications did not begin in earnest until the early 1980's. The principles of reflection seismology are the same for both the petroleum and environmental fields. The major difference between the applications is scale. In the seismic reflection method the arrival time and amplitude of elastic waves, generated by an artificial source and reflected from subsurface layers, are recorded and analyzed. The ability of seismic data to resolve geologic features is governed by the spacing of the sensors (geophones) on the surface, the frequency of the reflected signal, and the velocity structure of the subsurface. The reflected seismic waves are picked up by transducers known as geophones. A geophone consists of a coil suspended by a damped spring in a magnetic field. It is coupled to the land surface by a 2 to 3 inch metal spike on the bottom of the geophone casing. As a seismic wave passes the geophone, the casing moves and creates a voltage proportional to the velocity of the waves. This voltage signal is carried via a cable to a recording instrument or seismograph where it is amplified, filtered, digitized, and stored. The recorded signal is further characterized in a processing center to create a seismic profile that is a vertical time image of the subsurface layers. For a typical seismic survey several hundred geophone locations are required to ensure adequate subsurface coverage. Seismic reflection data acquisition can be done as either two-dimensional or three dimensional recording. This information is integrated with borehole data to model subsurface flow to ultimately design an effective remediation program. To validate the interpretation made on seismic reflection data, the results must be integrated with borehole geologic and geophysical information. Once seismic data are correlated to borehole geophysical logs and converted to depth, the reflection amplitudes are related to selected aquifer properties such as porosity. This information is then integrated into two and three dimensional structural models to delineate preferential pathways for subsurface contaminant transport. The methods of AVO analysis are based on the simple idea that changes in the fluid content of a reservoir (or aquifer) can cause a large enough change in the Poisson´s ratio that a significant change in seismic reflectivity as a function of offset occurs. From this information seismic models can be constructed to determine whether for a given site there is a likelihood that a seismic AVO anomaly can be detected from the DNAPL saturated sediments.

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