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In cooperation with the ARMY NATIONAL GUARD and the AIR FORCE CENTER FOR ENGINEERING AND THE ENVIRONMENT

Bedrock Topography of Western Cape Cod, Massachusetts, Based on Bedrock Altitudes from Geologic Borings and Analysis of Ambient Seismic Noise by the Horizontal-to-Vertical Spectral-Ratio Method

By Gillian M. Fairchild, John W. Lane, Jr., Emily B. Voytek, and Denis R. LeBlanc

Suggested citation

Fairchild, G.M., Lane, J.W., Jr., Voytek, E.B., and LeBlanc, D.R., 2013, Bedrock topography of western Cape Cod, Massachusetts, based on bedrock altitudes from geologic borings and analysis of ambient seismic noise by the horizontal-to-vertical spectral-ratio method: U.S. Geological Survey Scientific Investigations Map 3233, 1 sheet, maps variously scaled, 17-p. pamphlet, on one CD–ROM. (Also available at https://pubs.usgs.gov/sim/3233.)

Figure captions

Figure 1. The area of the map presented in this report is on western Cape Cod, Massachusetts, and includes the Massachusetts Military Reservation and all or parts of five surrounding towns. The study area is on unconsolidated glacial drift deposits that consist of glaciofluvial sand and gravel outwash overlying glaciolacustrine sand and silt, with sandy moraines near the northern and western coasts. The underlying bedrock is primarily granodiorite (Oldale and Barlow, 1986; Oldale, 1992).

Figure 2. The horizontal-to-vertical spectral-ratio (HVSR) method uses a single broadband, three-component seismometer A, to record ambient seismic noise from the earth’s surface. Ambient seismic noise is composed of microtremors caused by ocean waves, wind, rainfall, and anthropogenic sources such as traffic and industry (Ibs-von Seht and Wohlenberg, 1999). The HVSR method works best at locations like Cape Cod where generally homogeneous, unconsolidated sediments overlie consolidated bedrock (Lane and others, 2008). At each measurement site, the seismometer B, was placed firmly on the ground to ensure sufficient coupling with the earth, and data were collected for a minimum of 30 minutes.

Figure 3. The graphical representation of the raw data to obtain the depth to bedrock is shown for site 00MW0584 (location shown in fig. 5). The graph was prepared by using the computer program Geopsy version 2.7.0 (Geopsy, 2011). The horizontal scale represents time, and the vertical scale represents the amplitudes of the data collected for seismic components in three perpendicular directions: vertical (Z), north-south (N), and east-west (E). The raw data were divided into time intervals 60 seconds long that overlap adjacent intervals by 30 seconds each. The small spikes in amplitude above the generally smooth traces were caused by high-amplitude noise disturbances such as the nearby passage of individual vehicles or pedestrians. Data from these intervals, which are not colored, were removed prior to further data processing.

Figure 4. The fundamental resonance frequency at a site was determined by a spectral analysis of the horizontal and vertical components of the recorded seismic noise. The amplitudes of the spectra of the vertical and two horizontal components of the microtremors for each of the 60-second time intervals were calculated by the Geopsy program. The average of the amplitudes for the horizontal components was divided by the vertical component, and this ratio (the HVSR) was plotted as a function of frequency. The colored lines in this graphical output from the Geopsy program correspond to the similarly colored time intervals shown in figure 3. The fundamental resonance frequency in this example from site 00MW0584 is 1.06 Hz defined by the peak in the average HVSR plot (average and standard deviation shown by the solid and dashed black lines, respectively). An empirical equation (see accompanying pamphlet) developed for Cape Cod by Lane and others (U.S. Geological Survey, written commun., 2011) was used to estimate the sediment thickness from the fundamental resonance frequency. The resulting thickness was subtracted from the land-surface altitude to obtain the bedrock-surface altitude.

Figure 5. The bedrock-surface altitudes from the HVSR survey, seismic refraction surveys, and geologic borings that reportedly hit bedrock were plotted on a map of western Cape Cod and hand contoured with a 25-foot contour interval. This interval is consistent with an estimated uncertainty in the bedrock-surface altitudes from the HVSR method of about 10 percent of the depth to bedrock, or about 20–30 feet over much of the area. The altitudes from the borings have an estimated uncertainty of about 5 feet. The location of the map is shown in figure 1. MMR, Massachusetts Military Reservation.

Figure 6. The topographic map of the bedrock surface beneath the area near Snake Pond was prepared with a 10-foot contour interval because of the closely spaced data points obtained mostly from geologic borings in this area. The location of the map is shown in figure 5.

Figure 7. The topographic map of the bedrock surface beneath the area near Ashumet Pond was prepared with a 10-foot contour interval because of the closely spaced data points obtained mostly from geologic borings in this area. The location of the map is shown in figure 5.

Figure 8. The interpreted representation of the bedrock surface in figure 5 is similar in its general features to Oldale’s 1969 map shown here and the map produced by the Air Force Center for Environmental Excellence (AFCEE) (2006) (not shown). In all three maps, the bedrock surface generally slopes downward to the southeast toward Nantucket Sound. The maps similarly show lobes of shallower bedrock extending from the vicinity of the Cape Cod Canal to the east toward Barnstable and to the south toward Woods Hole. The bedrock surface is more than 47 feet below NGVD 29 (sea level) throughout the mapped area and therefore does not crop out at the land surface. The general trends in altitude shown on the small-scale map (fig. 5) may reflect preglacial drainage patterns in the bedrock surface. The large-scale maps in figs. 6 and 7, however, show a surface whose altitude varies considerably over a small area. Small-scale variations, including closed depressions in the bedrock surface, are consistent with a glacially eroded bedrock surface (B.D. Stone, U.S. Geological Survey, written commun., 2011).