apt: an instrument for monitoring seafloor acceleration ... · clayoquot slope/bullseye/apt...

APT: An Instrument for Monitoring Seafloor Acceleration, Pressure,

and Temperature with Large Dynamic Range and Bandwidth

by Earl E. Davis, Martin Heesemann, Joseph J. Farrugia, Greg Johnson, and Jerome Paros

Abstract A simple tool has been developed to facilitate the study of interrelatedgeodetic, geodynamic, seismic, and oceanographic phenomena in marine settings. Itincorporates quartz pressure and triaxial acceleration sensors and a low-power,high-precision frequency counter. The sensors are housed in a 6-cm outside-diameter,1-m-long pressure case that is pushed vertically into the seabed with a submersible orremotely operated vehicle, with no profile remaining above the seafloor to causecurrent-induced noise. The mass of the tool is designed to match that of the sedimentit displaces to optimize coupling. Intrinsic measurement precision of the order of 10−8

of full scale (in this instance, a pressure range equivalent to 4000 m of water depth andan acceleration range of �3g) allows observations of pressure, acceleration, and tiltvariations of 0.4 Pa, 0:6 μms−2, and 0:06 μrad, respectively. Temperature variationsmeasured near the top and at the bottom of the instrument are resolved to better than0.1 mK. With the large dynamic ranges, high sensitivities and broad bandwidth (10-Hz Nyquist to drift-limited zero-frequency DC), ground motion associated withmicroseisms, strong and weak seismic ground motion, tidal loading, and slow andrapid geodynamic deformation—all normally studied using disparate instruments—can be observed with this single tool. Examples of data are provided from fourdeployments with connections to the Ocean Networks Canada Northeast Pacific tele-metred undersea networked experiment (NEPTUNE) observatory cable.

Introduction

Instrument Description

Many seafloor instruments have been developed over thepast several decades to observe ground motion and pressuresignals from seismic, oceanographic, and geodynamic sources,the most common being ocean-bottom seismometers and pres-sure recorders. These have been extremely useful for a broadrange of studies in seismically and tectonically active regions.In this article, we describe a new tool (referred to here as APT,for acceleration, pressure, and temperature) for ground-motionmonitoring. It expands seafloor observational capabilitiesbeyond those of cable-connected instruments with similar sen-sors, such as one underway off Japan (e.g., Mochizuki et al.,2017), by way of its low-power consumption for autonomousoperations. It incorporates a triaxial accelerometer designed byQuartz Seismic Sensors, Inc., and a pressure sensor built byParoscientific, Inc. Both are housed in a slim (6.03-cm outsidediameter), 0.53-cm wall titanium pressure case that is pushed1 m below the seafloor (Fig. 1a). These sensors use quartzcrystal oscillators loaded by a Bourdon tube for measuringpressure and coupled to masses for measuring acceleration(Paroscientific Technical Notes, a, b). Both also contain inde-pendent unloaded crystals that allow the temperature

sensitivity of the pressure and acceleration crystals to beaccounted for and that provide observations of temperaturenear the top and at the bottom of the tool for scientific appli-cations. The quartz sensors have extremely broad bandwidthby nature and, with precise frequency counting technology,they provide measurements over a large dynamic range.Because of their small size, low-power requirements, androbustness, the sensors are well suited for autonomous (bat-tery-powered) deployments. The acceleration sensor ismounted rigidly to the lower pressure-case end cap, and thepressure sensor is mounted below the upper cap with a pres-sure feed-through leading to an external port protected againstbiofouling by a Cu/Ni screen (Fig. 1b). Having no profileabove the seafloor, the tool is unaffected by bottom currents,and the length of the tool buffers the acceleration sensor fromthe effects of bottom-water temperature variations. The massof the tool matches closely that of the sediments displaced,providing good coupling (e.g., see Sutton et al., 1981). Aprototype of the tool without a pressure sensor was deployedon the outer Cascadia accretionary prism in September 2015(Fig. 2) and connected to the Ocean Networks Canada (ONC)Northeast Pacific telemetred undersea networked experiment(NEPTUNE) offshore fiber-optic cable system to allow real-time data transmission and long-term operations without

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Bulletin of the Seismological Society of America, Vol. 109, No. 1, pp. 448–462, February 2019, doi: 10.1785/0120180132

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battery power. This instrument included six lithium thionyl Dcells to power the instrument for a period of several months inthe event of an extended cable connection failure. In the sec-ond version of the instrument, a pressure sensor replaced thespace occupied by the D cells; smaller batteries were mountedas part of the electronics chassis.

During the first deployment, seafloor pressure observa-tions were provided by a separate bottom pressure recorder(BPR) located 70 m away (Fig. 2). This instrument is oneamong many pressure recorders deployed for long-termautonomous and cabled seafloor and borehole pressure mon-itoring. The BPRs use Paroscientific quartz sensors and theoriginal precise-period counter (PPC) system designed andbuilt by John Bennest of Bennest Enterprises, Ltd., originallyfor use in borehole hydrologic experiments (e.g., Davis et al.,2010). These counters use a stable frequency reference oscil-lator to count variable frequency sensor cycles over a user-specified counting interval (typically 800 ms), and they deter-mine fractional cycles at the beginning and end of each count-ing interval with a capacitor charging circuit keyed by theleading edges of the reference and sensor square-wave outputs.Averaging multiple fractional count determinations allowsmeasurement of sensor frequency with a precision of ∼1 ppb.The full dynamic range of the Paroscientific sensors spans 10%of the sensor frequency; thus, with the PPCs, pressure varia-tions (and now accelerations) are determined to ∼10 ppb offull scale. Signal period counters that operate on the same prin-ciple are now being built by RBR Ltd., and they were used forthe APT instruments described here. Refinements achieved in

this design resulted in greater precision andlogging capabilities (130 million samples),higher sampling frequencies (program-mable from 1 sample per hour to 20 samplesper second vs. a maximum of 1 sample persecond in the original PPCs), less powerconsumption for autonomous battery pow-ered deployments (160 mWwhen operatingcontinuously, and less when operating inter-mittently, e.g., 4.5 mWaverage at 1 sampleper minute), and greater flexibility for exter-nal interfacing, including options for serial(RS232 or 485) or Ethernet communica-tions and for network time protocol (NTP)time synchronization for cable-connecteddeployments (see Data and Resources fordetails).

Data Reduction

Sensor signal frequencies are con-verted to engineering units using calibra-tions carried out by Paroscientific, Inc., forpressure sensors, and by Quartz SeismicSensors, Inc., for acceleration sensors.Because of the imperfect orientation of

the tools (Table 1), data have been rotationally corrected, firstby applying a matrix transform with factory calibration val-ues to orthogonalize the sensor’s axial components (seeParoscientific Technical Note, b), then by minimizing hori-zontal-axis values through rotations about the x and y sensoraxes in sequence, and finally by azimuthally rotating to opti-mize the fit of iteratively rotated horizontal APT waveformsof microseisms and earthquake surface waves to thoseobserved by nearby buried broadband seismometers, wherethe orientation is known to better than 1° (see Table 1).

Spurious values (spikes), originating with the periodcounters but of as-yet unknown cause, occur in the datarecords with a frequency of occurrence defined in the periodof operation of the first APTof ∼10−6 (20 occurrences over a9-month period of mostly 1 sample per second operation). Inthe current version of the APT, these are searched for in realtime with a 20-point running window. Isolated values outsidea predefined threshold (relative to previous and subsequentvalues) are replaced by linear interpolation and flagged.

Relative time is determined in all instruments with anonboard oscillator, with accuracy limited by the offset fromits nominal frequency (≤ �1:5 ppm from 0° to 40°C) andby changes with age (≤ �0:5 ppm=yr after 1 yr of operation).In the case of autonomous deployments, offsets must be deter-mined by comparison with time checks made at times of deploy-ment, recovery, and/or submersible visits. Improvements tointernal clock accuracy for autonomous seismic applicationsare being explored. In initial cable-connected use, clock offsetswere defined by comparison to shore-based time stamps.Although uncertainties arise from cable transmission latencies,

(a) (b)

Figure 1. Photo of the first acceleration, pressure, and temperature (APT) tool beingtested in a salt water tank. (a) An ROV manipulator handle and a cable to a shore-basedOcean Networks Canada (ONC) junction box can be seen at the top of the tool.(b) Construction details of the current-generation APT (deployed in 2017 and 2018)are shown in the cut-away schematic. See Data and Resources for further details.The color version of this figure is available only in the electronic edition.

APT: An Instrument for Monitoring Seafloor Acceleration, Pressure, and Temperature 449


over long periods of time and with redundant checks, theireffects could be made small. After construction of the first proto-type tool (referred to as APT1), timing accuracy was improvedby adding an NTP client. Earlier time checks allowed the drift ofthe onboard clock to be verified as steady (6:5 × 10−7) and to becorrected for with an uncertainty of < 50 ms over the earlyAPT2 deployment period. Regular NTP time checks are nowoperating in all instruments at a refresh rate of once per minute.

The NTP client and the transmission control protocol/Internet protocol (TCP/IP) port (used only in cabled-modeoperations) are by far the most power consumptive compo-nents; they bring the total power consumption up to 1.5 W.Both are located near the top of the instrument just beneaththe pressure sensor. The associated temperature perturbationat the pressure sensor is large (1.2 K), but comparison ofthe records of this sensor and of the nearby BPR and a con-ductivity/temperature/depth (CTD) instrument shows thatthe offset is constant. Thus, temperature variations recordedby the APT pressure sensor can be considered reliable.Temperatures measured by the accelerometer at the bottomof the tool also may be offset (although far less than the offsetseen at the top of the tool), and temporal variations can beaccepted as reliable.

Preliminary Testing

Frequency Counter Evaluation

Before construction of the first APT instrument, com-parative tests of different types of frequency counters andsensors were performed in the laboratory and in the seismicvault at the Pacific Geoscience Centre (PGC). First, three dif-ferent frequency counter technologies were evaluated: (1) aBennest PPC, (2) an RBR counter, and (3) a Paroscientificnano-resolution frequency counter. Initial tests of theBennest PPC and the RBR counter, done by measuring thesignal frequency of a stable oscillator, showed peak-to-peaknoise levels of ∼3 ppb of the nominally 35-kHz output fre-quency and a root mean square deviation of less than 1 ppbwhen making measurements at 1 sample per second. Later, acomparison of all three counters was done with input from apressure sensor insulated to reduce temperature variations inthe laboratory. Results of these tests also demonstratedfrequency counting resolution in the neighborhood of 1 ppb(better by a factor of three in amplitude in the case of theParoscientific counter) when sampling at 1 sample per sec-ond. (Fig. 3). Thus, all were considered appropriate candi-dates for application to the APT instrument. Although theParoscientific counter was capable of higher sampling ratesthan the former two and displayed lower noise (as would

Table 1Nomenclature, Locations, and Other Details of Instruments Used in or Relevant to This Article

ONC Site/Subsite/Instrument Type/Station Code Latitude (°) Longitude (°) X Tilt Y Tilt Azimuth Depth Date Deployed

Clayoquot slope/Bullseye/Prototype APT 1/NC89.Z1 48.6710 −126.8470 3.8° 3.8° 21° true 1255 m 15 September 2015Clayoquot slope/Bullseye/APT 2/NC89.Z1 48.6709 −126.8481 −14.3° −1.4° 17.3° true 1258 m 14 June 2017Clayoquot slope/Bullseye/BPR 48.6708 −126.8480 – – N/A 1258 m 6 September 2009Clayoquot slope/Bullseye/Güralp CMG-1T/NC89 48.6705 −126.8488 – – 12° true 1256 m 17 September 2009Clayoquot slope/Bullseye Nanometrics TitanEA/

NC89.W148.6705 −126.8477 – – – 1259 m 8 June 2017

Barkley Canyon/Node/APT/BACND.Z1 48.3459 −126.1580 – – 320° true 643 m 22 June 2018Barkley Canyon/Upper slope Güralp CMG-1T/NCBC 48.4275 −126.1752 – – 0.7° true 396 m 7 September 2009Cascadia Basin/EastAPTCBC27.Z1 47.7567 −127.7316 – – 310° true 2656 m 24 June 2018Cascadia Basin/West/Güralp CMG-1T/NC27 47.7623 −127.7579 – – 4.2° true 2656 m 22 May 2014

ONC, Ocean Networks Canada.

Figure 2. Regional (inset) and detailed maps showing the loca-tions of APT and broadband seismometer instruments connected tothe ONC/Northeast Pacific telemetred undersea networked experi-ment (NEPTUNE) cable system (see Table 1 for details and sitenaming), the latter showing locations of the first and secondAPT installations at the Clayoquot slope site (labeled here andin Table 1 as APT1 and APT2), the buried Güralp broadband seis-mometer NC89, the Nanometrics accelerometer, and the bottompressure recorder (BPR) used for comparisons in this article.Locations of gas-charged groundwater vents at Clayoquot areshown for context (Römer et al., 2016; M. Scherwath, personalcomm., 2018). The location of the earthquake discussed later inthe article is shown in the inset as a star. The color version of thisfigure is available only in the electronic edition.

450 E. E. Davis, M. Heesemann, J. J. Farrugia, G. Johnson, and J. Paros


have been the case for a counting system developed aboutthis time by Webb and Nooner, 2016), it required severaltimes more power, too much for extended autonomous oper-ations. Thus, continued developments adopted the RBRcounter technology along with its integrated logging system.

Sensor Evaluation

Two different types of sensors—both built by QuartzSeismic Sensors, Inc.—were evaluated during tests in thePGC seismic vault: (1) a triaxial accelerometer similar to theone used in the APT tool described here and (2) a prototypebiaxial tilt sensor. The triaxial accelerometer has a dynamicrange of �3g, and if 1-ppb frequency counting is achieved,acceleration can be resolved to 0:6 μms−2 and tilt to0:06 μrad. Although less robust, the tilt sensor is designedto apply its sensitivity over a range limited to ∼� 10°, or∼� 0:2g acceleration, thus providing greater precision formeasurements of tilt (i.e., ∼4 nrad with 1-ppb counting res-olution) and horizontal acceleration. Data from both sensorswere compared to measurements from a Güralp CMG-1Tbroadband seismometer, which was being tested in parallelbefore its deployment on the ONC NEPTUNE cabledobservatory. The output of this feedback-controlled induc-tively balanced mass seismometer includes three-axis veloc-ity (with linear response over a bandwidth from 40 Hz to∼0:03 Hz) and mass position data, which, at periods longerthan the seismometer velocity bandwidth, are linearly relatedto acceleration. At very long periods (e.g., tidal), the horizon-tal axes readings are geometrically related to tilt (Davis et al.,2017). All test data were synchronized using GlobalPositioning System and TCP time servers, with the exception

of tests with the prototype RBR period counter that ran on itsown independent clock.

Signals observed during tests carried out in the PGCvault with the accelerometer and tilt sensor included tidaldeformation, seismic waves, and ubiquitous microseisms.Tidal tilts of ∼1 μrad were observed by all sensors (Fig. 4),although the accelerometer signal appears to have been influ-enced by the temperature variations present in the vault (trueby varying amounts for all sensors). Tidal signals of greatestamplitude were aligned in a north–south direction,perpendicular to the local shoreline which lies ∼50 m northof the vault. A direct relationship to local ocean tide suggeststhat the origin lies with local ocean loading.

Arrivals from an earthquake on the Gibbs fracture zoneprovided a fortuitous test of the sensitivity of the various sen-sors at seismic frequencies (Fig. 5). A good match among allsensors was seen up to the frequency limit imposed by the tiltsensor sampling interval (using an RBR counter running atonly 1 sample per second at this time) and down to 0.01 Hz(spectra for all sensors are indistinguishable in Fig. 5c). Below0.01 Hz, the seismometer and quartz sensor spectra diverge,possibly as a result of noise introduced into the quartz sensorsby large thermal noise at this time. At higher frequencies, sig-nals from microseisms and local site noise recorded by thequartz accelerometer and the broadband seismometer continueto agree well up to near the 20-Hz Nyquist frequency.

A comparison of pre-earthquake signals recorded atnight with no cultural sources (lower curves of Fig. 5c)revealed more fully the practical resolution of the quartzaccelerometer and tilt sensor. Signals recorded by the tilt sen-sor agreed well with those recorded by the seismometer overthe full frequency range from near-Nyquist (5 Hz, defined atthis time by a sampling rate of 10 samples per second) down

Figure 3. Raw data determined with three types of period coun-ters connected to a single Paroscientific pressure sensor. Time isreferenced to the beginning of this bench test on 29 August2014. The several-minute-long variations that are coherent amongthe counters reflect variations in laboratory pressures. Incoherenthigh-frequency variations reflect intrinsic counter noise at levelsindicated. Period values are given as a fraction of the sensor signalperiod of 28 μs. Full-scale pressure variations range over 10% ofthis value; hence, the noise levels reflect pressure variations rangingfrom 4 to 13 ppb of the full range of the sensor. Absolute periodvalues vary among instruments and among channels of a singleinstrument by typically 1%. The color version of this figure is avail-able only in the electronic edition.

Figure 4. Tidal tilt variations (offset for plotting convenience)observed with the mass position data from a Güralp broadband seis-mometer, with a quartz tilt sensor, and with a quartz accelerometersituated on a stable pier at the Pacific Geoscience Centre (PGC),along with ocean tides observed in Patricia Bay with a BPR con-nected to the ONC/VENUS near-shore cable system. Sensors andinstruments are described in the Introduction. Signals roughly fourhours after the beginning of the record seen by the tilt and accel-eration sensors are from an earthquake on the Charlie Gibbs fracturezone in the North Atlantic Ocean (illustrated in Fig. 5). The colorversion of this figure is available only in the electronic edition.



to the limit of the seismometer (360-s period). Resolutionwith the quartz accelerometer was less, although its noise(≥ −130 dB in power relative to 1 ms−2) was understand-able in light of the limits imposed by its greater dynamicrange (six times that of the tilt sensor).

Although the tilt sensor was seen to be clearly superiorfor seismic recording, the limits to resolution of the

accelerometer were thought to be accept-able for many applications, particularly inlight of typically high noise levels on theseafloor (as seen in examples that follow).Advantages of the accelerometer includeits robustness and large dynamic range,properties that make it serviceable as aseismic strong-motion instrument, insensi-tive to deployment orientation, and cali-bratable along all axes using the Earth’sgravity field.

Overall, the results provided strongjustification for continuing the design,construction, and deployment of the firstAPT instrument using the RBR counterand logger and the Quartz seismic systemstriaxial accelerometer. For instrumentsdevoted primarily to seismic observationsand when leveling can be assured, a tiltsensor could easily be used or even addedfor the horizontal axes. For cable-powereddeployments when external power is avail-able, further small gains in resolutioncould be realized through use of aParoscientific or other higher precisioncounter.

Initial Results of SeafloorDeployments

After lab and vault testing and evalu-ation, the first APT instrument, built at thePGC (referred to in Fig. 2 and Table 1 asAPT1), was deployed in September 2015.Installation was done with the remotelyoperated vehicle (ROV) Jason at theClayoquot slope NEPTUNE site on theCascadia subduction zone accretionaryprism (labeled NC89 in Fig. 2). The BPRand seismometer at this site are located∼70 and 140 m west–southwest of theAPT1 position, respectively. A 1-m-longpush core was taken with the aid of anomnidirectional level to evaluate the sedi-ment bulk density and to create a pilot holefor the APT installation. After initialinstallation in the hole created, it wasbelieved that the coupling of the tool tothe sediment was poor, so the APT was

removed and pushed in directly (with no pilot hole) a fewmeters away. Azimuthal orientation relative to the ROV’sheading was determined visually, and inclination was deter-mined from the values of each of the three components rel-ative to 1g (Table 1). The seafloor at the site is flat andunderlain by accreted sediments, superimposed slope depos-its, and a ubiquitous gas hydrate bottom-simulating reflector

Figure 5. (a) Details of teleseismic surface waves and (b) relatively high-frequencysite noise observed in the PGC vault with the quartz accelerometer (similar to one usedlater in the APT instrument) and the collocated Güralp broadband seismometer (hori-zontal components are shown; see Fig. 4 for context). (c) Spectra of 1-hr-long recordsincluding the seismic arrivals and of nighttime background noise (avoiding cultural sitenoise), with background and earthquake spectra for the quartz accelerometer labeled 1and 2, for the broadband seismometer labeled 3 and 4, and for the quartz tilt sensorlabeled 5 and 6, respectively. The characteristic lower and upper background spectraof Peterson (1993), labeled 7 and 8, are shown for context. In this and all other figures,acceleration values from the Güralp seismometer are computed from velocity channeldata (unless the mass position channel or the strong-motion acceleration channel isspecified). Spectra shown here and in figures that follow were computed with aSigmaPlot macro that applies a Hanning taper. The color version of this figure is avail-able only in the electronic edition.



(Riedel et al., 2002, 2006). Numerous seafloor gas vents arepresent nearby (Römer et al., 2016; see Fig. 2). The instru-ment was connected to a NEPTUNE scientific junction boxat the site and began transmitting data to shore shortly there-after (15 September 2015) at 1 sample per second. In March2016, the sample rate was increased to 6 samples per second.Sampling at highest rate possible with this prototype tool,12 samples per second, was not done to avoid conflict withmilitary regulations.

In June 2016, the first APT instrument was disconnectedwhen its junction box was moved, and operation of the toolwas terminated. Over the following year, the technology wastransferred to RBR Ltd. who designed and produced toolsthat had enhanced capabilities, including a 20-Hz samplingrate, and that would be made commercially available. One ofthese new tools was deployed ∼70 m west of the originalAPT site—closer to the NC89 seismometer and less than20 m—from the Clayoquot site BPR (labeled NC89 inFig. 2) and connected in June 2017 (referred to in Fig. 2and Table 1 as APT2). After evaluation of data from this tool,additional instruments were acquired and two were deployed

and connected in June 2018, one at theONC/NEPTUNE Barkley Canyon site andthe other at the Cascadia Basin site(labeled NCBC and NC27 in Fig. 2,Table 1), both within a few kilometersof buried broadband seismometers identi-cal to the one at the Clayoquot site.

Tidal Deformation

An early comparison of signals fromoceanographic and earthquake sourcesrecorded by a Güralp broadband seismom-eter and by the Quartz Seismic Sensorsaccelerometer—after being packaged inthe prototype APT1 tool and installed atthe ONC/NEPTUNE Clayoquot slope sitenear seismometer NC89—is summarizedin Figures 6 and 7. Without the perturbingcontributions of varying temperature andgroundwater levels that contaminated thevault test results, tidal period signals aremore clearly documented by the offshoreaccelerometer data (Fig. 6) than they werein the land-based data. Signal amplitudesmatch those determined using seismom-eter mass positions, although there is ashift in phase of unknown origin. As dis-cussed by Davis et al. (2017), the verticalsignal at this site matches expected gravityvariations arising from body and oceanattraction terms, but the horizontal acceler-ation is believed to reflect formation tiltresulting from ocean pressure loading.The amplitude of tilt is surprisingly large,

given that the very long wavelength of open-ocean tides pro-duces a spatially uniform load (unlike the situation at thecoastal location of the PGC vault) and that the local topo-graphic relief is small. The direction of maximum tilt is ori-ented in a direction across the strike of the subduction zoneaccretionary prism, suggesting a subseafloor structural origin(Davis et al., 2017). The cause of the tidal signals is notimportant for this article, however; what is important is thatthe observations demonstrate that changes in tilt can beresolved with the APT instrument at a level of a fractionof a microradian and more generally that oceanographicloading produces perturbations that must be taken into con-sideration in any search for geodynamic signals.

Earthquake Seismic Waves

Since the deployment of the first APT in 2015, a varietyof local earthquake and teleseismic signals have beenrecorded. An example spanning a broad frequency bandand dynamic range provided by ocean background signalsand teleseisms from an Mw 7.1 earthquake off Hondurasis shown in Figure 7. An excellent match between the

Figure 6. Tidal-period acceleration and derived tilt (sin−1�−a=g�) observed with thefirst APT deployed at the ONC Clayoquot site and with the nearby buried broadbandNC89 seismometer as derived from the mass position data (see Fig. 2 and Table 1 forlocations). Total tilts resolved from the horizontal axes data are in a direction parallel tothe dip of the subduction prism structure; the cause of the difference in phase is unknown(see Davis et al., 2017 for details). The color version of this figure is available only in theelectronic edition.



waveforms (Fig. 7a) and spectral content (Fig. 7b) of signalsseen by the APT and of those seen by the nearby NC89broadband seismometer provided confidence that the effortsto couple the instruments to the formation by way of pushedinsertion (in the case of the APT probe), burial (in the case ofthe seismometer), and instrument–sediment mass matching(both instruments) had succeeded. As seen in the vault testresults (Fig. 5), the limit imposed on resolution by the APTaccelerometer counter, about −130 dB relative to 1 ms−2, isapparent in the background oceanographic record before theearthquake at submicroseismic frequencies (< ∼0:05 Hz),although the contrast between the Güralp and APT noise lim-its is not as great. As frequency declines, the signal levelclimbs in a manner similar to that seen in Figure 5, butthe site signal levels recorded by the APT are typically only10 dB or less above those recorded by the broadband seis-mometer. This limit to resolution is seen to be insignificantrelative to the earthquake teleseismic signal levels. The pri-mary factor that limits detection of seismic signals at higher

frequencies (e.g., generated by small local earthquakes) isseen to arise from oceanographic noise. This is clear bothfrom the intrainstrument comparison of signal levels of eachof the components and from the comparison of pressure andvertical acceleration, which shows a relationship followingthat expected for the acceleration of the overlying water massat the Clayoquot site under the influence of the groundmotion (∼1:3 Pa=μms−2) over much of the seismic andmicroseismic frequency band.

Microseisms and Infragravity Waves

Representative waveforms and spectra of signalsrecorded during a local storm by the second Clayoquot APTare shown in Figures 8 and 9. As in the case of teleseismicarrivals, a good match is found between the oceanographi-cally sourced waveforms recorded by the APT and seismom-eter (Fig. 8a) despite the separation of the instruments andpossible differences in local site characteristics, and in this

Figure 7. (a) Waveforms and (b) spectra of seismic arrivals from an earthquake in the Caribbean Sea off Honduras recorded by the secondAPT instrument and the NC89 broadband seismometer. Seismic-wave arrivals are split into early high-frequency body waves and later low-frequency surface waves. Spectra, including those for APT pressure, are computed for 1-hr periods during and before the earthquake. Upperand lower background spectra of Peterson (1993) are shown as dashed lines. The color version of this figure is available only in the electronicedition.



instance, there is little difference between signal levels any-where above a frequency of about 0.01 Hz (Fig. 9a). Nodifferences are seen in the comparison of pressures recordedby the APT and the BPR (Figs. 8b, 9b). Where coherencebetween acceleration and pressure is high within the micro-seismic band (periods centered at ∼8 s; Fig. 9c), the relation-ship between acceleration and pressure yields informationabout the spatial scale of coherence of double-frequencyocean-wave loading at the seafloor, with the longest periodmicroseisms behaving the same as long-period seismic sig-nals (i.e., 1D loading at 1:3 Pa=μms−2). In the infragravitywaveband (Fig. 8d), coherence between acceleration andpressure is also seen (at periods in this case centered at∼1min; Fig. 9c), showing that during this and other times

of strong ocean wave energy, the resolu-tion of the APT may be adequate for com-pliance determinations.

A more comprehensive view of thepractical resolution of accelerationmeasured with the APT instruments isprovided by a comparison of sepectra,expressed as probability density functions,of data from the APTs and nearby Guralpbroadband seismometers at the threeONC deployment sites (Fig. 10; seeFig. 2 and Table 1 for locations). The dataspan intervals ranging from about twoweeks (late June to early July 2018 forthe most recently deployed BarkleyCanyon and Cascadia basin instruments,Fig. 10a,c, respectively) to nearly 1 yr(for Clayoquot, Fig. 10b). Several thingsare apparent in this comparison. Despitethe data window available for theCascadia and Barkley Canyon sites beingin an oceanographically quiet season (e.g.,Thomson et al., 2014) and the nearly fullyear of recording at Clayoquot beingdominated by quiet intervals, signal levelsfall near the upper bound of backgroundsignals as defined by the standard charac-teristic curves of Peterson (1993). Thisnoisy condition is well known to be char-acteristic of the northeastern Pacific Ocean(e.g., Webb, 1998). As seen in time-spe-cific spectra presented in other figures ofthis article, background signals are strong-est at microseismic frequencies. Much lessenergetic infragravity waves are present inthe vertical-component seismometer data,but these signals generally fall below theinherent resolution of the APTs as definedby the frequency counting accuracy andfull-scale range, that is, −125 dB in powerrelative to 1 ms−2. Infragravity wave sig-

nals climb above this only occasionally (e.g., Figs. 8 and 9).Another comparison that might best illustrate the inher-

ent resolution of the APTs as they are currently configured ismade in Figure 11. These spectra represent a brief (12-hr)interval (within the longer one used in Fig. 10b) whenoceanographic background signals were at a particularly lowamplitude. Choice of this period allowed not only an evalu-ation to be done when site noise was minimized but also acomparison with a new instrument, a Nanometrics TitanEAaccelerometer, one of several now buried much like theGüralp broadband instruments and commissioned by ONC/NEPTUNE at each of the locations listed in Table 1. To com-plete the comparison, results from the strong-motion sensorof the Güralp seismometer are also shown. At frequenciesabove 0.1 Hz, data from the Titan, APT, and Güralp (velocity

Figure 8. Waveforms of microseisms and infragravity waves observed during astorm on 1 December 2017. Vertical acceleration seen by the second APTand the broad-band NC89 seismometer (as the time derivative of velocity) are compared in (a), pres-sure observed by the APT and the nearby BPR are compared in (b), APT verticalacceleration and pressure are compared to one another with infragravity signals removedin (c), and with microseismic signals removed in (d). The color version of this figure isavailable only in the electronic edition.



channel) agree well (with a small exception of the verticalcomponent of the Titan at 0.12 Hz), but noise levels exhib-ited by the Güralp acceleration sensor (used for strong-motion detection) are understandably high. Below 0.07–0.1 Hz, instrument noise of the Titan and APT causes theirsignals to fall above the site noise level defined by the Güralpvelocity channel. Low-amplitude signals in the infragravitywaveband that are seen during this calm period by the ver-tical component of the Güralp seismometer are not resolvedwith the APT, which is limited by its frequency counter res-olution to about −125 dB, or with the Nanometrics instru-ment, which is characterized by an inherent noise leveltypically 5 dB greater than that of the APT. Noise character-istics are simple, with APT, Nanometrics, and Güralp accel-erometer powers rising with falling frequency at rates of 1.5,2.5, and 4.5 dB/octave, respectively.

In summary, the resolution of the APT in the seismicfrequency band is generally adequate in this environment,

with seismic detection thresholds set pri-marily by oceanographic noise that is to acertain extent exaggerated by site charac-teristics (e.g., high-signal levels atfrequencies within and above the micro-seismic band associated with sedimentaryshear modes). Resolution of verticalmotion associated with infragravity waveswill be limited to oceanographically ener-getic periods (e.g., Fig. 9).

Data from a Recent LocalEarthquake

On 22 October 2018, a series of earth-quakes on the Sovanco transform fault thatbounds the Pacific and Explorer platesprovided one more useful illustration ofthe utility of the APT instrument. The firstearthquake of the series was an Mw 6.1event ∼200 km distant, and althoughrecording has not yet been made routine,20 samples per second data were acquiredfrom the second APT deployed near theNC89 broadband seismometer. A com-parison of records at the time of this eventis shown in Figure 12. As expected fromFigure 11, high-frequency instrumentnoise can be seen riding on top of approx-imately 7-s period background micro-seisms in the case the APT and more soin the case of the Güralp accelerometer,which displays long-period noise as well(Fig. 12a). The high-frequency noisepartly masks the small Pn arrival (attenu-ated by the thick accretionary prism sedi-ments) in the APT data and fully precludesits detection by the Güralp accelerometer

(Fig. 12b). Later compressional and shear waves seen byall sensors track one another well, but ∼40 s into the seismicwavetrain, the large amplitude surface-wave signals recordedby the Güralp seismometer are clipped (Fig. 12c) and dis-torted by adjustments activated by the mass-centering elec-tronics. This behavior can be dealt with by use of the Güralpstrong-motion sensor data, but the lack of distortion and clip-ping by the high-dynamic range quartz sensor of the APTmakes such corrections unnecessary.

Temperature Variations

An example of temperatures measured at the seafloor bythe nearby BPR and at the bottom of the tool by the accel-erometer temperature compensation crystal is shown inFigure 13. Seafloor temperatures at this site vary over tidalto month-long periods, and these variations diffuse into thesediment section. The longer term variations observed at thebottom of the APTare attenuated by roughly a factor of 5 and

Figure 9. Comparison of (a) spectra of acceleration and (b) pressure variations cal-culated for a 1-hr segment of data spanning the records shown in Figure 8. Upper andlower background spectra of Peterson (1993) are shown as dashed lines. Correlationbetween APT acceleration and pressure seen in Figure 8 is confirmed by coherencebetween acceleration and pressure near unity within microseismic and infragravitybands (c). The color version of this figure is available only in the electronic edition.



(a)

(b)

Figure 10. Comparison between north- (upper panels) and vertical-component (lower panels) probabilistic power spectral density func-tions for APTs and nearby Güralp CMG-1T broadband seismometers (right and left panels, respectively, with the seismometer velocitiesdifferentiated to accelerations) at (a) Barkley Canyon, (b) Clayoquot slope, and (c) Cascadia basin sites. Standard high- and low-noise modelsof Peterson (1993) are shown as thick gray lines. Mean power spectral densities are shown as thick solid black lines. Probabilistic powerspectral density functions were computed following the work of McNamara and Buland (2004). The probability density functions wereconstructed for the APT and broadband seismometer over the period 22 June 2018–8 July 2018 using (a) 731 and 715 1-hr segments,respectively and (b) 528 and 535 1-hr segments, respectively. Probability density functions in (c) were constructed for the APT with dataover the period 2 August 2017–8 July 2018 using 9858 1-hr segments and for the broadband seismometer with data over the period 20October 2017–8 July 2018 using 11,478 1-hr segments. Low-rate broadband seismometer data were used to prevent data availability conflictswith military data-diversion schedules. (Continued)



delayed by ∼1 week relative to those in the bottom water(Fig. 13a). The full frequency-dependent phase and ampli-tude relationship between the seafloor and sediment temper-atures constrains the thermal diffusivity of the interveningsediment section (Davis and Villinger, 2018). Tidal-periodvariations are also present at depth (Fig. 13b); although small(�0:2 mK), they are much too large to be a consequence ofthermal diffusion from the seafloor. Their highly regularcharacter and close correspondence with seafloor pressure(Fig. 13b) suggest that they are a consequence of adiabaticheating and cooling under the influence of tidal loading(Davis and Villinger, 2018).

Secular Acceleration Variations

Resolution of secular geodetic signals is severely limitedby sensor drift. Drift exhibited by loaded quartz crystalsprobably arises from crystal aging, outgassing, and creepunder load and is difficult to avoid (e.g., Paros andKobayashi, Tech. Note G8101). Drift of the acceleration sen-sors over the full history of the first APT record is illustratedin Figure 14, along with the computed total gravity. Severalfirst-order aspects of this long-term record include (1) Initialdrifts of the x and z sensors decreased rapidly, reversed direc-tion, and then became generally linear after the first fewmonths after deployment. The initial drifts may have beena consequence of creep under load because of the changein the orientation of the tool, which had been kept horizontalfor several months before deployment or to the change in

ambient temperature from the laboratory and ship to the sea-floor. Physical equilibration of the tool in the sediment mayalso have contributed to this early transient, although thiscontribution is probably small, given that a similar transientis seen in the total gravity. (2) The sign of the long-term driftexhibited by the y axis is opposite that exhibited by the x andz axes. This may be noteworthy, but the cause is unknown.Drifts of all three channels of the second APT deployed at theClayoquot site were positive. (3) The magnitudes of drift dis-played by each channel after 1–2 months is large but roughlylinear (∼2:5 mms−2=yr). Drifts exhibited by each channel ofthe second tool were lower (0:1–0:8 mms−2=yr) for the hori-zontal channels and 0:45 mms−2=yr for the vertical. Long-term characteristics of the third and fourth APTs deployedmost recently at the Barkley Canyon and Cascadia basin sitesare not yet determined.

As in the case of the early transients, the long-term sec-ular trends cannot be caused by settling of the tool. Whereasreal changes in tilt would be seen primarily by the horizontalaxes, secular trends observed are similar among all threeaxes. Furthermore, the total gravity value computed fromthe three components displays a secular trend that is nearlyidentical to the trend of the vertical channel—the expectedbehavior of an instrument with a sensor drift that is smallcompared with g and that is installed with little inclination.This confirms that the cause of the drift must arise from thesensors or the period counters. Laboratory tests at RBR usinga rubidium clock as a source have shown that the counterreference crystals provide counting stability to within

(c)

Figure 10. Continued.



0.5 ppm per year or 0:3 mms−2 per year when converted toacceleration. This is generally smaller than the annual driftexhibited by the APT accelerometers. Thus, much of theobserved drift must arise from the sensor crystals themselves.

These rates are comparable to drift rates exhibited bypressure sensors studied by Polster et al. (2009) and of sen-sors used in ONC NEPTUNE BPRs early in their operatinghistories. However, after 2–3 yrs of NEPTUNE operations,annual rates of BPR drifts stabilized at 0.15–0.25 ppm(again, stated as a fraction of the crystal frequency).Hence, the accelerometer sensor drifts seem large, and theywill limit the detection of secular geodetic changes to

∼0:5 mms−2=yr in gravity and 0:1 mrad=yr in tilt (in thecase of the second APT). With trends removed, however,transient signals of amplitudes greater than those associatedwith oceanographic loading should be easily resolved.

A technique for determining the composite drift of thethree axes (the total g value) involves periodically rotatingthe tool without changing its gravitational position, with thegoal of exposing each axis to ∼1g (J. M. Paros, “TriaxialAcceleration Assembly and In-situ Calibration Method forImproved Geodetic and Seismic Measurements,” U.S.Patent 9,645,267 B2, 9 May 2017). A singular example ofsuch an operation carried out at the time of the installation ofthe second APT instrument is shown in Figure 15. Doing this

Figure 11. Signal levels observed during an oceanographicallyquiet period with four independent instruments located at the ONC/NEPTUNE Clayoquot slope observatory site, including the APTand buried Güralp CMG-1T broadband seismometer (differentiatedvelocity output) discussed extensively in this article, the strong-motion acceleration sensor in the Güralp seismometer, and a buriedNanometrics TitanEA strong-motion accelerometer (see Fig. 2 andTable 1 for locations). Power spectral densities in this figure weregenerated using the Welch method. APT data were demeaned andfiltered using a high-pass harmonic reduction filter at 0.001 Hz.Other data from Incorporated Research Institutions forSeismology (IRIS) were decimated to 5 samples per second andbandpass filtered similarly. Upper and lower background spectraof Peterson (1993) are shown as dashed lines. The color versionof this figure is available only in the electronic edition.

Figure 12. Comparison of observations of accelerationrecorded by the APT and the buried Güralp broadband seismometer(velocity channels converted to acceleration = HHZ and HHN,acceleration channels = HNZ and HNN) at the ONC/NEPTUNEClayoquot slope site (a) before and (b,c) during seismic-wave arriv-als from an Mw 6.1 earthquake on the Sovanco transform fault200 km to the northwest. Minor ticks are shown at intervals of10 s in the upper and lower panels and 1 s in the middle panels.The time of the Pn body-wave arrival is indicated with an arrow.Clipping of the Güralp surface-wave velocity signal in (c) resultsin zero computed acceleration values. The color version of this fig-ure is available only in the electronic edition.



repeatedly with the existing design cannot be done withoutphysically removing the instrument from the seabed. Effortsto do this by automated rotation of the sensor within aninstrument are being made by W. Wilcock (personal corr.,2018), but application to a tool having the form factor ofthe APT would not be realistic.

Deformation Transients

During the first APT deployment, a small number ofacceleration anomalies stood well above oceanographicand seismic signals and instrument drift (Fig. 14). Some ofthese include a change in the total g value; their origin isunknown but likely to be instrumental. The abrupt change inearly April provides one example; this change in g originatesexclusively from the vertical axis. Other anomalies are notaccompanied by a change in total g and are likely to reflectformation tilt. Arguments supporting this supposition, madeon the basis of aspects of the example shown in Figure 16,include (1) anomalous values of acceleration are seen simul-taneously on all three channels but dominantly along the

horizontal axes (as would be expectedfrom tilt); (2) total tilt determined from thecombination of the horizontal axes followstilt determined from the vertical channel,and as a corollary to this, the total gravityvalue does not change (see Fig. 14); (3) notemperature anomaly is seen at the time ofthe “event”; one would be expected if theevent were related to physical disturbanceof the tool; and (4) acceleration (tilt) returnsto a value identical to that preceding the“event.” It is possible that this and othertransients (e.g., the one spanning earlyApril–May, also prominent in the horizon-tal channel data) are associated with hydro-logic activity at or below the fluid and gasvents in the area (Fig. 2), although no coor-dinated signals were seen at the seismom-eter located ∼140 m away, suggesting thatthe signal is very local in origin. Whateverthe cause, it seems clear that with the redun-dancy provided by the triaxial observations,signals associated with deformation can beconfidently resolved when they rise abovethe ∼1 μrad level of oceanographicallyinduced tilts.

Summary

A new instrument has been developedthat is capable of resolving a broad rangeof seismic, oceanographic, and geody-namic signals. Its salient characteristicsinclude robustness, ease of deployment,low-power consumption, small size, high

precision, large dynamic range, and broad bandwidth. Ithouses a triaxial quartz accelerometer with each axis havinga range of �3g and a pressure sensor having a range of40 MPa, mounted inside a slim 1-m-long pressure casedesigned to be pushed into seafloor sediment. It can beadapted easily for other deployments, for example, in bore-holes, and in low-profile monuments on hard substrates. Thetool uses high-precision (1 ppb) period counters to determinethe output frequency of the pressure and acceleration sen-sors. Each sensor also includes a temperature sensitive crys-tal. Acceleration, pressure, and temperature variations areresolved at levels of 0:6 μms−2, 0.4 Pa, and 0.08 mK, respec-tively. With sampling rates up to 20 samples per second, thetool functions well as a seismometer, with noise levels beingin the range of −120 to −130 dB (power relative to 1 ms−2)over a frequency band ranging from 0.01 to 5 Hz, higher thanthose characteristic of broadband seismometers but onlyslightly higher than characteristic seafloor site noise and sig-nificantly lower than noise characteristic of other types ofsensors (Fig. 11). The total frequency range, which spansfrom the Nyquist frequency to drift-limited DC and the large

Figure 13. Temperatures recorded with the temperature compensating crystals ofthe APT acceleration sensor at the bottom of the tool ∼1 m below the seafloor alongwith seafloor temperature and pressure recorded with the nearby BPR (see Fig. 2).(a) Long-term sediment temperature variations are attenuated relative to and lag thosein the bottom water. (b) Short-term sediment temperature variations (with 10-min aver-aged values superimposed on raw data) exhibit a clear tidal component that is in phasewith seafloor pressure (smooth curve). The color version of this figure is available onlyin the electronic edition.



dynamic range also allows the tool to serve in monitoringstrong ground motion, tidally induced deformation, and geo-dynamic deformation associated with seismic, tsunamigenic,

and slow slip on faults. The high sensitiv-ity of the temperature compensation crys-tal in the accelerometer has permittedobservations of tidal adiabatic heatingand hence determination of sedimentthermodynamic properties. Plannedimprovements to the tool include additionof (1) a thermistor at the top of the tool toallow bottom-water temperature monitor-ing, (2) an auxiliary module for in situpressure calibration, and (3) an exchange-able memory and battery module forextended autonomous operations. Use ofa quartz tilt sensor for horizontal channelswould provide increased resolution forseismic studies but with some loss ofdynamic range, ruggedness, and deploy-ment simplicity.

Data and Resources

Pressure and temperature data fromOcean Networks Canada (ONC/NEPTUNE) bottom pressure recorder(BPR) data are available at the ONC dataportal http://dmas.uvic.ca/DataSearch.Velocity and mass position data fromthe seismometers are available at theIncorporated Research Institutions forSeismology (IRIS) data portal http://service.iris.edu/irisws/timeseries. Pressure,temperature, and acceleration data fromthe APT instruments will soon be availablealso at the IRIS data portal. Data for thisinvestigation were last accessed in October2018. Details about the Güralp CMG-1Tseismometer beyond those described inDavis et al. (2017) can be found at http://www.guralp.com/documents/1OBS.pdf.Details about the current generation APTinstrument can be found at https://docs.rbr-global.com/apt. All websites werelast accessed on October 2018.

Acknowledgments

The authors thank the National ScienceFoundation for supporting the development of theprototype instrument as part of a Grant (OCE-1459265) to Laura Wallace for tilt and pressure mon-itoring at the Hikurangi subduction zone, NewZealand. The authors also thank Robert Macdonaldof Biologica Ltd., Chris Foreman of ForemanCNC, and Robert Meldrum of Pacific GeoscienceCentre (PGC) for responding quickly to the design,

fabrication, and assembly of mechanical and electronic components ofthe first acceleration, pressure, and temperature (APT) instrument. JohnBennest developed the original 1-ppb precise-period counters (PPCs) forthe PGC that have been used in seafloor and borehole pressure monitoring

Figure 15. Pressure and acceleration records acquired at the time of installation ofthe second APT instrument. The instrument was held with �x then �y axes orientedvertically up (∼10–20 and 20–35 min, respectively) before the tool was inserted partially(40–45 min) and then fully into its final position (post-50 min). The computed totalgravity amplitude shown in the lower panel probably contains early transient reactionof the sensor from its stored to deployed thermal state and orientation. The color versionof this figure is available only in the electronic edition.

Figure 14. Full histories of acceleration and computed total gravity data for the firstAPT deployment. Raw data plotted in the background show periods of enhanced infra-gravity and microseismic signals associated with local storms and swell from distantsources, short-lived signals from earthquakes, and occasional spurious spikes; 10-min average data (dark lines) show tidal components (see expanded plot in Fig. 6), sec-ular trends related to sensor drift, and transient anomalies. The color version of thisfigure is available only in the electronic edition.



http://dmas.uvic.ca/DataSearch



http://service.iris.edu/irisws/timeseries

http://service.iris.edu/irisws/timeseries

http://www.guralp.com/documents/1OBS.pdf

http://www.guralp.com/documents/1OBS.pdf

https://docs.rbr-global.com/apt

https://docs.rbr-global.com/apt

systems since 2004 and that later provided the foundation of the RBR Inc.instrumentation described here. Deployments were carried out by the Jasongroup of the Woods Hole Oceanographic Institution and the Hercules ROVoperators. Ocean Networks Canada (ONC) hosted the APT instruments onthe NEPTUNE cable system, and John Dorocicz helped with data manage-ment and processing. One of the authors (Greg Johnson) serves as thepresident of RBR Ltd., which designed and now manufactures the currentgeneration of APT instruments purchased and deployed by ONC. Anotherauthor (Jerome Paros) serves as president of Paroscientific, Inc., and ofQuartz Seismic Sensors, Inc., which manufacture the pressure and acceler-ation sensors used in the APT instruments.

References

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Pacific Geoscience CentreGeological Survey of CanadaP.O. Box 6000Sidney, British ColumbiaCanada V8L [email protected]

(E.E.D.)

Ocean Networks CanadaUniversity of Victoria2474 Arbutus RoadVictoria, British ColumbiaCanada V8N 1V9

(M.H., J.J.F.)

RBR Ltd.95 Hines RoadOttawa, OntarioCanada 2K 2M5

(G.J.)

Paroscientific, Inc. and Quartz Seismic Sensors, Inc.4500 148th Avenue NERedmond, Washington 98052

(J.P.)

Manuscript received 30 April 2018;Published Online 15 January 2019

Figure 16. Example of a several-day-long tilt excursionobserved during the first APT tool deployment. Tilt resolved fromthe combination of the horizontal components (x� y) agreesclosely with that determined independently from the vertical com-ponent. This consistency is supported by the observation that thetotal gravity vector remains unchanged for the full duration ofthe event (Fig. 14). The color version of this figure is available onlyin the electronic edition.



http://dx.doi.org/10.1029/2018JBG016151

http://dx.doi.org/10.2204/iodp.pr.328.2010

http://www.paroscientific.com/pdf/G8101_Root_Causes_of_Quartz_Sensors_Drift.pdf





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