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Mapping Top-of-Bedrock and Soft-Soil Zones Beneath High-Traffic Areas Using 2D ReMi

Publisher —
Environmental and Engineering Geophysical Society (EEGS), 2009 Symposium on the Application of Geophysics to Engineering and Environmental Problems (SAGEEP) proceedings.
Zonge, 2010. Updated results summary.
The Leading Edge, March, 2013, pp. 936-941

Authors —
Phil Sirles*, Zonge Geosciences, Inc., Lakewood, Colorado;
JB Shawver*, Zonge Geosciences, Inc., Minneapolis, Minnesota;
Satish Pullammanappallil, Optim, Inc., Reno, Nevada;
Zoran Batchko, PB Americas, Orange, California.

Paper — [pdf] 2009 SAGEEP Mapping Top-of-Bedrock
Update — [pdf] Sirles 2010 mapping soft soils Honolulu
Article — [pdf] 3203_TLE_sirles_Passive near-surface seismic data where all else fails

Summary
In August 2008, Zonge Geosciences conducted a seismic survey under subcontract to Geolabs, Inc. in the greater Honolulu, Hawaii area for contribution to their ongoing geotechnical assessment of the proposed Honolulu High-Capacity Transit Corridor Project (HHTCP).

Objectives of the seismic investigation were two-fold:
1) Determine depth-to-bedrock; and,
2) Determine the lateral variability of the soils deposits.

To meet these objectives, Zonge completed a 2D ReMi survey. The ReMi method was selected because of its ability to acquire seismic data in very noisy environments and to distinguish between soil and bedrock based on shear-wave velocity. The method is not affected by the presence of a shallow water table, which was important as much of the survey area was very near the coast.

Site conditions along the surveyed portion were very similar where seismic data were acquired in traffic lanes, medians or along sidewalks. Line locations were selected depending on crew safety, day of the week, and lane closure feasibility and accessibility. Geolabs provided geologic and geotechnical data in the form of boring logs and shear-wave velocity data, which assisted with the seismic interpretation.

This project demonstrated that, in addition to meeting the required objectives, the ReMi method can be used when basaltic bedrock is shallow (e.g., 10 feet bgs or 3 m), and when it is quite deep (200+ ft bgs or 60+ m bgs), with a typical refraction seismic setup. Quality of the ReMi data ranged from good to very good, which is directly attributed to the use of a backhoe and the tidal (low-frequency) energy present near the coast.

Abstract
Two-dimensional seismic refraction microtremor (2D ReMi) data were acquired, processed and interpreted for the Honolulu High-Capacity Transit Corridor Project – Waipahu to Aiea – Ewa, Oahu, Hawaii. The objective was to image the depth-to-bedrock and determine the lateral and vertical extent of soft-soil conditions. Approximately 2.66-line miles (4.28 km) of 2D ReMi data were acquired along 12 separate lines. Data were acquired along the Farrington and the Kamehameha Highways. Line locations were determined based on a variety of geologic settings and the need for subsurface
information between, below and beyond geotechnical borings. Results indicate the seismic and geologic/ geotechnical data could be integrated to yield valuable information beneath the area investigated. The 2D seismic survey reveals that basaltic bedrock can be encountered as shallow as 5 feet (1.5 m), to as deep as ~230 feet (70.1 m) beneath the existing highways. Additionally, the seismic survey results provide good information regarding the presence, lateral variation, and extent of soft soils.