Subsurface Heavy-Metal Detection with the use of a Laser-Induced Breakdown Spectroscopy (LIBS) Penetrometer System

摘要

U.S. Army Engineer Waterways Experiment Station (WES) investigators have designed, fabricated, and demonstrated a cone penetrometer sensor for subsurface in situ field screening based on LIBS. The LIBS probe (patent pending) presented here is one of a suite of contaminant and geophysical probes developed by the WES Site Characterization and Analysis Penetrometer System (SCAPS) program. The LIBS probe includes a laser as part of the optical LIBS sensor section for metals analysis and a cone and sleeve soil classification module at its tip. Soil classification data are acquired during the push, while LIBS data are recorded during probe retraction, after shedding a sacrificial sleeve. Optical design concerns are discussed including the recessed window geometry to avoid output window damage and associated problems. Calibration, peak identification and peak confidence software are discussed. Although producing lower signal strength, the non-imaging optical receiver utilized is based on a simple design. Its performance is less dependent on soil conditions and produces a more consistent optical signal when compared to an imaging optical receiver design. Field results from four sites are discussed. Some spectral samples from a site indicate contaminant material that is not mixed with soil, as evidenced by spectra with a total lack of characteristic iron peaks in the background. Soil grain size and soil moisture are discussed in the context of empirical calibration and their effect on LIBS measurements.