MASW to Investigate Anomalous Near-Surface Materials
at the Indian Refinery in Lawrenceville, Illinois
Richard D. Miller, Choon B. Park, Julian M. Ivanov, Jianghai Xia,
David R. Laflen, and Chadwick Gratton
Summary
The shear wave velocity field, calculated using the Multi-channel Analysis of
Surface Waves (MASW) method (Park et al., 1999; Xia et al., 1999), and disturbances
observed in the groundroll wavetrain transformations (phase and amplitude) were
used to help identify variability in the lateral continuity of near-surface
layers beneath asphalt roads within the Lawrenceville Refinery, settling ponds
in and around the refinery property, surface remediated burial pits, and containment
berms (Figure 1). Depths of investigation extended from a few feet below ground
surface (BGS) to as much as 100 ft BGS on line 5. Anomalies interpreted within
the soft sediments along all five lines are related to lateral changes in material
properties. Confirmation and/or enhancement of interpretations of these data
will require subsurface sampling. When comparing results from this study to
previous investigations using this surface wave imaging technology at sites
around the country (Miller and Xia, 1999a; Miller and Xia, 1999b; Miller et
al., 1999), it is interesting to note that in most cases interpreted zones of
anomalous material possess a unique velocity pattern somewhat universally consistent
with all features of the same kind (subsidence, fractures, bedrock, etc.). In
most cases, low velocity closures within the shear wave velocity field are indicative
of “anomalies.” Each profile collected in association with this
feasibility test at the Lawrenceville Refinery was designed with a specific
imaging objective in mind. The success of each profile can only be determined
by a drilling program specifically designed to evaluate these findings. Calculating
the shear wave velocity field from surface wave arrivals can generally be accomplished
with a high degree of accuracy regardless of cultural noise or obstacles. Data
for this study were acquired in and around areas with occupied personal residences,
railroad noise, site demolition activities, and normal industrial background
noise (60 Hz, running motors, vehicle activity, etc.). A wide range of surface
conditions required adaptations be made to optimize source and receiver coupling
to cement and asphalt. Care was taken to insure no data carried artifacts related
to surface features or background noise and that all data were acquired with
special attention placed on the spread location relative to surface materials
and structures. Comparisons of data characteristics recorded from geophones
with steel baseplates to those with spikes revealed no significant difference
in wavetrain properties or calculated dispersion curves. Unlike recording concerns
prominent when using other types of acoustic waves, surface waves seem to have
only limited dependence on changes in receiver coupling. Non-source noise recorded
on surface wave data reduces the quality of the dispersion curve but does not
usually prevent an accurate and robust inversion. MASW provides shear wave velocity
profiles that accurately (15%) represent average shear wave velocities for a
particular subsurface volume (Xia et al., 2000). Velocities measured during
this study ranged from just over 200 ft/sec to around 2500 ft/sec. Localized
changes of over 700 ft/sec (300%) across distances less than 10 ft were common
around areas with observed structural damage. Velocity inversions laterally
consistent over significant distances are evident within the upper 10 ft along
most lines and likely relate to stiffer clays or partially cemented sediments
in close proximity to sand or gravel zones. The sensitivity of shear wave velocities
to changes in sediment makeup within this alluvial setting allowed even subtle
changes in nearsurface material properties to be identified on 2-D cross-sections.
Uniquely locating localized zones of anomalous subsurface sediments (fill, sludge,
or rubble) and/or objects (such as pipes, trenches, or old landfill materials)
was possible with data from lines 1, 2, 3, and 4. Localized zones of lower velocity
material can easily be picked out on lines 1 and 4. Line 3 provides a glimpse
at an erosional surface beneath the base of the lower velocity fill materials.
It takes very little imagination to interpret the well-defined low velocity
zones extending down to depths of almost 10 ft at burial pits interpreted along
line 4. Line 5 presents a reasonable depiction of the dike and relatively coherent
sediments from the base of the dike down to about 100 ft BGS. Bedrock or a significant
increase in velocity is evident at depth on lines 1, 2, 3, and 5. Each type
of target imaged during this survey possesses a unique signature with each of
the different imaging methods used. Interpreting these data requires incorporation
of drilling, borehole measurements, and other geophysical soundings. Interpreting
changes in lithology with this technique has routinely involved correlating
high velocity gradients and measured velocities to ground truth. Velocity fields
along these five profiles possess relatively uniform increases in shear wave
velocity from the surface to the maximum depth of the survey. Anomalous (relative
to surrounding materials) high and low velocity closures, likely indicative
of extreme lateral variability in material properties or foreign materials,
are evident within the unconsolidated sediments along most of the lines. Several
localized changes in shear wave velocity are strong candidates for drill investigation.
Coherent layers (bedding, changes in lithology, structural features, etc.) were
interpreted based on velocity gradients, consistent changes in velocity contours,
and overall velocity trends. Several possible explanations exist for each of
these velocity phenomena. With the inherent nonuniqueness of these data, precisely
located borings would be necessary along each of these lines to increase the
confidence and/or modify these interpretations. Large velocity gradients in
the shear wave velocity field are likely indicative of changes in lithology
(i.e., alluvial/glacial contacts, alluvial/bedrock, glacial/bedrock), while
localized lateral changes (contour closures) in the shear wave velocity within
the unconsolidated section were considered evidence of infilling or altered
native earth. Mapping the surface of lithologic contacts using shear wave velocity
data combined with drill data will result in a significantly higher resolution
subsurface map than grid style drilling alone at this site. Advantages of mapping
variations in the shallow stratigraphy with the shear wave velocity field calculated
from surface waves using MASW include sensitivity to velocity inversions, ease
of generating and propagating surface wave energy in comparison to body wave
energy, being oblivious to cultural noise (mechanical or electrical), and sensitivity
to lateral changes in velocity.
Full Paper KGS-2000-04.PDF 4.52MB