Bedrock Topography Of Western Cape Cod, Massachusetts, Based On Bedrock .

Study Design and Data Collection   3Study Design and Data CollectionThis effort included the compilation of geologic boringand seismic refraction survey data and the collection andanalysis of the HVSR data for areas with little or no bedrockinformation. The HVSR method was applied at a total of 164sites on and near the MMR.Boring Logs and Seismic Refraction SurveysGeologic borings were drilled on and near the MMR inan effort to characterize the contaminant plumes originatingon the MMR and obtain data on the hydrogeologic frameworkof the aquifer. The locations of 559 borings reported to havehit bedrock in the area are shown in figure 1–1 (appendix).These sites are concentrated near the contaminant plumescaused by chemical spills, fuel spills, leachate from landfills,and firing ranges used for artillery and small-arms practice(Massachusetts National Guard, 2012). The lithologic log anddriller’s report for each of these locations were checked, andan estimated altitude of the bedrock surface was calculatedfrom the reported depth to bedrock and land-surface altitude.A set of 463 additional borings that were drilled to at least200 feet below land surface but reportedly did not hit bedrockwas also compiled (figure 1–1 in appendix). The boring logswere obtained from the environmental databases maintainedby AFCEE and ARNG.Three seismic refraction surveys were done in Falmouthin 1958 (Oldale and Tuttle, 1964). These inline refractiontraverses were made with a 12-channel portable refractionamplifier and oscillograph. Each survey was 1,100 feet long.These surveys recorded signals that are characteristic ofgeologic settings where unconsolidated sedimentary depositsoverlie crystalline bedrock. Additional seismic refractionsurveys were conducted on western Cape Cod in 1968 (Oldale,1969). Based on the data collected in each of these surveys,the average depth to bedrock was calculated over the length ofthe survey line.Analysis of Ambient Seismic Noise by theHorizontal-to-Vertical Spectral-Ratio MethodThe HVSR method is based on a relationship between theresonance frequency of ambient seismic noise as measured atland surface and the thickness of the unconsolidated sedimentsthat overlie consolidated bedrock. A spectral analysis ofthe ambient seismic noise from the earth’s surface is usedto determine the fundamental resonance frequency for themeasurement site. Ambient seismic noise is composed ofmicrotremors caused by ocean waves, wind, rainfall, andanthropogenic activities such as traffic and industry (Ibs-vonSeht and Wohlenberg, 1999). The HVSR method works best atlocations where the subsurface can be approximated by a twolayer model consisting of a layer of generally homogeneous,unconsolidated sediments overlying a consolidated bedrocklayer. The HVSR method is effective in areas where thereis a strong contrast in acoustic impedance between thesediment and bedrock (Lane and others, 2008; more detaileddescriptions in Nakamura (1989) and Geopsy (2011)).Cape Cod’s geologic structure, with unconsolidated glacialsediments on top of granitic bedrock, produces a very distinctcontrast in acoustic impedance at the sediment-bedrockinterface; this structure makes Cape Cod an ideal setting inwhich to apply the HVSR method.At each measurement site, a single, broadband, threecomponent seismometer was used to record ambient seismicnoise from the earth’s surface. The seismometer was placeddirectly on the ground or on a metal plate with spikes (fig.2A). The spikes on the bottom of seismometer or the metalplate were firmly driven into the ground to ensure that theseismometer was coupled well with the earth. A bullseyespirit level and north arrow built into the instrument and anexternal compass were used to level the instrument and alignit with magnetic north. The seismometer was then connectedto a laptop computer, a global positioning system (used fortiming only), and a battery. The seismometer was covered witha weighted 5-gallon bucket to reduce interference from windand weather (fig. 2B). Data collection was initiated by usingthe program Scream! (Guralp Systems, 2011) on the laptopcomputer. Data were collected for no less than 30 minutes ateach location. To minimize noise, the equipment was set upapproximately 30 feet from traffic when possible and wherepedestrians, including those collecting the data, were notwalking in the immediate area around the equipment; thesedisturbances would have created excessive noise that laterwould have had to be removed from the record.The raw seismic data were processed by using theprogram Geopsy version 2.7.0 (Geopsy, 2011). An examplefrom site 00MW0584 of the graphical representation,produced by the Geopsy program, of the raw seismic datais shown in figure 3. The horizontal scale represents time,and the vertical scale represents the amplitudes of the datacollected for seismic components in three perpendiculardirections: vertical (Z), north-south (N), and east-west (E).The raw data were divided into time intervals 60 secondslong that overlap adjacent intervals by 30 seconds each. Thesmall spikes in amplitude above the generally smooth traceswere caused by high-amplitude noise disturbances such as thenearby passage of individual vehicles or pedestrians as datawere being recorded. Data from these time intervals, whichare not colored in figure 3, were removed prior to furtherdata processing.The fundamental resonance frequency at each site wasdetermined by a spectral analysis of the horizontal and verticalcomponents of the recorded ambient seismic noise. Theamplitudes of the spectra of the vertical and two horizontalcomponents of the microtremors for each of the 60-secondtime intervals were calculated by the Geopsy program. Theaverage of the amplitudes of the horizontal componentswas divided by the vertical component, and this ratio (the

4   Bedrock Topography of Western Cape Cod, MassachusettsABGPSComputerBatterySeismometerunder bucketFigure 2. Closeup photographs showing A, a single broadband, three-componentseismometer, and B, the seismometer and data-collection system as typically deployedin the field.

60 secondsFigure 3. Example from site 00MW0584 of the graphical representation of the raw seismic data. Site location shown in figure 5.East-WestNorth-SouthVerticalStudy Design and Data Collection   5

6   Bedrock Topography of Western Cape Cod, MassachusettsHVSR) was plotted as a function of frequency. The graphicaloutput from the Geopsy software is shown in figure 4 for theexample from site 00MW0584. Each colored line is the resultof this analysis of the data from the 60-second time intervaldelineated by the similarly colored vertical band in figure 3.For this study, the processing parameters, which are definedand explained in detail in Geopsy (2011), included Konnoand Ohmachi (1998) smoothing with a constant bandwidth of40, a cosine taper width of 5 percent, and a squared averagefor the horizontal components. An average HVSR spectralplot and spectral plots representing upper and lower limitsof one standard deviation were calculated from the plotsfor the individual time intervals (fig. 4). The fundamentalresonance frequency for the site is the frequency defined bythe peak in the average HVSR plot. For the example from site00MW0584, the peak is located at a frequency of 1.06 Hertz(fig. 4). The Geopsy program automatically selects the peakin the processed data and determines the average resonancefrequency and its standard deviation (fig. 4). The user also canmanually select the peak if the peak selected by the programis unsatisfactory.The depth to bedrock (or thickness of the unconsolidatedsediments composing the top layer of the two-layer model)can be estimated from the fundamental resonance frequencyby using the following relationship:bZ af r0 ,whereZfr0(1)is the sediment thickness in ft,is the fundamental resonance frequencyin Hertz (s-1), and a (in ft s) and b(dimensionless) are determined empiricallyfrom a nonlinear regression of dataacquired at sites where Z is knownfrom adjacent boreholes (Lane andothers, 2008).The constants a and b for this study (a 297 ft s andb -1) were developed by Lane and others (U.S. GeologicalSurvey, written commun., 2011) from a regression analysis ofknown bedrock depths from borings versus depths obtainedby the HVSR method at 8 sites in this study area and 29additional sites on the glacial deposits of eastern Cape Codand southeastern mainland Massachusetts. The 8 sites fromthis study area that were used to develop the empiricalequation are highlighted in bold in tables 1–1 and 1–2(appendix). The estimated depth to bedrock for the examplefrom site 00MW0584 is 279 ft. The altitude of the bedrocksurface was obtained by subtracting the estimated depth tobedrock from the land-surface altitude. At site 00MW0584,where the land-surface altitude is 50 ft above NGVD 29, theresulting bedrock-surface altitude is 229 ft below NGVD 29.A total of 164 sites were chosen for application of theHVSR method from areas for which little bedrock informationfrom geologic borings and the seismic refraction surveyshad been collected. These areas generally are near the coastin Bourne, Sandwich, Falmouth, Mashpee, and along theeastern edge of the map area in Barnstable. Sites with knownland-surface altitudes were selected to allow the calculationof bedrock altitude from the estimated depth to bedrock.The HVSR data were collected during 2008–10 by severaldifferent seismometers. The measurements were repeatedeight times at the Ashumet Pond public boat launch (site164 on fig. 1–1, appendix) and 15 times adjacent to building1146 (site 163 on fig. 1–1, appendix) on the MMR. Analysisof these repeated measurements was used to confirm thatthe instruments were collecting spatially and temporallyconsistent, comparable data.Description of Data SetsSeveral datasets were used to prepare the updatedtopographic map of the bedrock surface in this report. Datafrom sites where the bedrock surface was hit during drillingare given in table 1–1 (appendix). Seismic refraction dataare documented in the reports by Oldale and Tuttle (1964)and Oldale (1969). Data from the HVSR method are given intable 1–2 (appendix). Finally, data on the maximum possiblebedrock altitude at 463 sites where geologic borings weredrilled to at least 200 ft below land surface but reportedlydid not hit bedrock are given in table 1–3 (appendix). Thelocations of the borings and HVSR measurement sites areshown in figure 1–1 (appendix).Bedrock Topography of WesternCape Cod, MassachusettsA topographic map of the bedrock surface beneathwestern Cape Cod was prepared by using data from thegeologic borings, seismic refraction surveys, and HVSRmeasurements. The map was prepared by manuallycontouring the bedrock altitudes estimated from the datacollection methods.Interpretation of Bedrock-Surface TopographyThe bedrock-surface altitudes from the HVSR survey,seismic refraction surveys, and geologic borings thatreportedly hit bedrock were plotted on a map of western CapeCod and hand-contoured with a 25-ft contour interval (fig.5). This interval is within the range of estimated uncertaintyof about 10 percent of the depth to bedrock (20–30 ft) in thebedrock-surface altitudes estimated for much of the area fromthe HVSR method. Bedrock-surface altitudes in two smallerareas near Snake Pond (fig. 6) and Ashumet Pond (fig. 7) withclosely spaced data points were contoured with a 10-ft contourinterval. The bedrock-surface altitudes in these two areas weremeasured mostly from borings; the altitudes from the boringswere estimated to have a range of uncertainty of about 5 ft.

Peak at 1.06 HzRatio of averaged-horizontal to vertical amplitudes (HVSR)Figure 4. Example of the Geopsy output graph of the fundamental resonance frequency determined by the spectral plot of the ratio of the averaged-horizontal to verticalcomponents of the ambient seismic noise at site 00MW0584. Site location shown in figure 5.Frequency in Hertz (Hz)Upper and lower limitsof one standard deviationMeanEXPLANATIONBedrock Topography of Western Cape Cod, Massachusetts    7

8   Bedrock Topography of Western Cape Cod, Massachusetts70 30'850,00070 0CoCaCAPE CODBAY-22541 45'-200-12 -15 1755 0pe-125-175-225-15-7550-175-1557-1-12-1 75-1750-15200-225-225-200-225-42 NAltitude of bedrock surface-175-275-200High -50 feet0-250-20-255-27 0-30 0-30-2755-32-30-225-250-275-350-200-200KEUCNT NDUSONA5-15-12-400-25 ,000-1505BUZZAR-175-200-250-225-200-17DS BAY-100Low -450 feet75-350-250Altitude contours—relative tosea level, interval is 25 feet,datum is NGVD 29-225Town boundaryMassachusetts Military Reservation boundaryWoods Hole41 30'RDEYAVIN UNDSO0015,0004,00030,000 FEET8,000 METERSPond shorelineOcean coastlineLocation of well 00MW0584on eastern shore of Johns PondBase from USGS and MassGIS sources, North American Datum of 1927 (NAD 27)Altitudes relative to the National Geodetic Vertical Datum of 1929 (NGVD 29)Figure 5. Topographic map of the bedrock surface beneath western Cape Cod as interpreted from horizontalto-vertical spectral-ratio measurements, seismic refraction surveys, and geologic borings, western Cape Cod,Massachusetts. Location of map shown in figure 1.

Bedrock Topography of Western Cape Cod, Massachusetts    9data points. Finally, the contours were checked for consistencywith the maximum possible altitudes at the deep boringsthat did not hit bedrock; the contours required only minimaladjustments during this final check.The contours were initially drawn by adhering strictly tothe estimated altitudes, but were then adjusted slightly whereappropriate within the range of uncertainty described above tosmooth the surface near anomalous features based on single70 32'70 30'870,000-170-120-160862,5000-16 50-1 0-14-160-160-150-1400-1341 ltitude contours—relative tosea level, interval is 10 feet,datum is NGVD 29Pond shoreline0-1441 42'Low -185 feet-1500-17-15Altitude of bedrock surfaceHigh -100 feet-170-120EXPLANATION4,000 FEET0-131,000 METERS41 ure 6. Topographic map of the bedrock surface beneath the area near Snake Pond as interpreted mostly from geologicborings, western Cape Cod, Massachusetts. Location of map shown in figure 5.-160-1500-13-140

5002,0001,000 METERS4,000 FEETPond 23-19-130-140-150-2230,00041 38'Altitude contours—relative tosea level, interval is 10 feet,datum is NGVD 29240,000-1200-130-160-170-1240-190080-190 0-1 -200-220-230-18170- 0-200-220-210-260-2500AshumetPond-1700-150 -16-18-1700-1970 32'Figure 7. Topographic map of the bedrock surface beneath the area near Ashumet Pond as interpreted mostly from geologic borings, western Cape Cod,Massachusetts. Location of map shown in figure 5.00-160Low -280 feetAltitude of bedrock surfaceHigh -120 feetEXPLANATION0-210-14070-1-1-150-14041 40'80-130-1-17-1600-150-140-170 34'0-21-13-1-15-170 -160 50-240-230-26000-2-1700-18 0-17-230-220-210850,00010   Bedrock Topography of Western Cape Cod, Massachusetts

Bedrock Topography of Western Cape Cod, Massachusetts    11Figure 8. Topographic map of the bedrock surface beneath Cape Cod, Martha’s Vineyard, and Nantucket, Massachusetts (fromOldale, 1969).

12   Bedrock Topography of Western Cape Cod, MassachusettsComparison to Earlier Maps of theBedrock TopographyThe interpreted representation of the bedrock surface infigure 5 is similar in its general features to Oldale’s 1969 map(fig. 8) and AFCEE’s 2006 map. In all three maps, the bedrocksurface generally slopes downward to the southeast from theCape Cod Canal toward Nantucket Sound. The maps similarlyshow lobes of shallower bedrock extending to the east towardBarnstable and to the south toward Woods Hole. The bedrocksurface is at least 47 ft below NGVD 29 (sea level) throughoutthe mapped area and therefore does not crop out at theland surface.The bedrock surface presented in this report showsmore topographic detail than the earlier maps owing to theincrease in the quantity of data available for use in drawing thecontours. The general trends in altitude shown on the smallscale map (fig. 5) may reflect preglacial drainage patternsin the bedrock surface. The large-scale maps (figs. 6 and 7),however, show a surface whose altitude varies considerablyover a small area. The same variability could be characteristicof the entire study area, but more data would be needed totest this hypothesis. Small-scale variations, including closeddepressions in the bedrock surface, are consistent with aglacially eroded bedrock surface (B.D. Stone, U.S. GeologicalSurvey, oral commun., 2011).SummaryThe HVSR method was used successfully to obtainestimates of the bedrock-surface altitude for western CapeCod, Massachusetts, in areas for which altitudes fromborings drilled to bedrock or from seismic refraction surveyshad not been estimated. The HVSR method was effectivein determining sediment thickness on Cape Cod owing tothe distinct difference in the acoustic impedance betweenthe sediments and the underlying bedrock. The HVSR datafor 164 sites were combined with data from 559 borings tobedrock to create a spatially distributed dataset for the studyarea that was manually contoured to prepare a topographicmap of the bedrock surface. The interpreted bedrock surfacegenerally slopes downward to the southeast as was shownon previous maps by Oldale (1969) and AFCEE (2006). Thesurface also has complex small-scale topography, as was alsoshown in the AFCEE (2006) map. The updated map reflectsthe data available through October 2012 from borings tobedrock and better spatial resolution from the HVSR datain areas outside the MMR for which less information on thebedrock altitudes had been collected.References CitedAir Force Center for Environmental Excellence (AFCEE),2006, Massachusetts Military Reservation Plume ResponseProgram, final Chemical Spill-23 wellfield design report:Jacobs Engineering Group, Inc., A4P–J23–35BC06VB–M23–0003, variously paged.Geopsy, 2011, Geopsy Project: Geopsy, accessed February 24,2011, at http://www.geopsy.org/.Guralp Systems, 2011, Scream! Software for Wind

Topographic map of the bedrock surface beneath the area near Ashumet Pond as interpreted mostly from geologic borings, western Cape Cod, Massachusetts . John W. Lane, Jr., Emily B. Voytek, and Denis R. LeBlanc: Introduction: The bedrock surface of western Cape Cod defines the : lower boundary of groundwater models that are used to predict the .