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The U.S. Army Corps of Engineers is determining the feasibility of constructing a new barracks building on the U.S. Army Presidio of Monterey in Monterey, California. Due to the presence of an endangered orchid in the proposed area, invasive techniques such as exploratory drill holes are prohibited. To aid in determining the feasibility, budget, and design of this building, a compressional-wave seismic refraction survey was proposed by the U.S. Geological Survey as an alternative means of investigating the depth to competent bedrock. Two sub-parallel profiles were acquired along an existing foot path and a fence line to minimize impacts on the endangered flora. The compressional-wave seismic refraction tomography data for both profiles indicate that no competent rock classified as non-rippable or marginally rippable exists within the top 30 feet beneath the ground surface. Introduction A new barracks structure is being planned for construction on the Presidio of Monterey, located in Monterey, Calif., in woodlands between existing barracks and the Post Exchange (PX). Planning for excavation of the barracks foundation requires knowledge of the depth to what may be shallow granodioritic bedrock. However, permits have not been issued for invasive access with drill rigs that will harm the endangered flora present in the area: an orchid (piperia yadonii) that lies dormant for much of the year as a fragile tuber in the soft forest floor materials. To determine depth to bedrock and progress with the construction-planning procedures, the U.S. Geological Survey (USGS) designed, acquired (using a handheld hammer source), and analyzed two seismic refraction profiles. This report presents the results of the seismic-profile surveys across the existing woodlands. Because the U.S. Army Corps prefers that English units (also called Imperial units or foot-mile-pound units) are used instead of SI units, distances and weights were recorded in and are reported here in English units. Geological Background A geologic map (fig. 1) by Clark and others (1997) shows the site location within Cretaceous-age (145 to 65 million years ago (Ma)) porphyritic granodiorite (unit Kgdp on the map). Pleistocene (2.6 Ma to 11.7 thousand years ago (ka)) marine coastal terrace deposits (units Qctm and Qcth) are present above and below the granodiorite outcrops.

u.s. geological survey open file report/open-file report

Measurement of near-surface seismic compressional wave velocities using refraction tomography at a proposed construction site on the Presidio of Monterey, California