The Hudson Bay Lithospheric Experiment - University of Bristol

BASTOW ET AL.: HUBLEalso financially and logistically prohibitive, soinnovative station designs are required.HuBLEbatteriespower regulatorseismometerrecording unitWith the goal of constraining better the reasonfor the existence of Hudson Bay, and the Precambrianprocesses that shaped the Canadianshield, the Hudson Bay Lithospheric Experiment(HuBLE) was deployed in the summer of 2007by personnel from the University of Bristol, incollaboration with the Geological Survey ofCanada. Nunavut, the homeland of the Inuit,is the most sparsely populated region ofCanada; centres of population andinfrastructure are few and farbetween. Wherever possible,seismograph stations weredeployed in safe compounds,such as airports and weatherstations with mains powersupply, and in small communitysettlements in Nunavut.Elsewhere, vast tracts of wildernessmeant that remote, independentlypowered recording sites had tobe designed within the financial limitations ofthe project. Transport to these locations was bychartered light aircraft with large tundra tyresto permit landing and take-off from relativelyflat and well-drained glacial deposits (figure 4).Figure 4 shows a completed HuBLE seismographstation in northern Hudson Bay. Eachsite was equipped with a Güralp CMG-3TDbroadband seismometer, recording at 40 samplesper second. Güralp DCM data recorderswere used at the stations, which were poweredby up to six solar panels (providing 100–140 Wsatellite modem4: HuBLE-UK remote station construction. 20–40 W solar panels on a steel frame recharge3 100 Ah batteries that power the seismometer and recording equipment. The GPS antennaprovides continuous accurate timing information. Remote communication with the stations isvia an Iridium satellite modem antenna that was scheduled to operate twice weekly. All fieldequipment is provided by NERC’s seismic equipment pool, SEIS-UK (box 1).‘‘ Theremoteness of theHuBLE network wasa hindrance during thefield campaign, buta blessing for thedata analysispower) and three 100 Ah deep-cycle batteries.Each remote site was equipped with an Iridiumsatellite modem that provided state-of-healthdata from the stations. Data retrieved using themodems included: station up-time; digitizer,seismometer, and recorder baud rates; externalhard disk usage; seismometer levelling information;GPS timing information; power supply.Using modems over the Iridium networkprovides pole-to-pole global coverage of bothshort message and short data burst services.For the CMG-DCM, this allowed the UK basestation to pull off weekly reports of thestate of health of the remote system.Where problems were diagnosed,’’GPS antennasolar panelslow-latency two-way communicationfor reconfigurationof the remote systems wasalso used via simple terminalinteraction. All seismicstation equipment are maintainedby SEIS-UK (see box1), part of the Natural EnvironmentResearch Council’s geophysicalequipment facility in Leicester.Where major physical damage to a station(such as cable damage from polar bear attacks)was deduced from the satellite modem data, theservicing team was able to focus its visits onthe problem stations, leaving those operatingsmoothly for the following year. This strategysaved thousands of pounds in plane chartertime. Arguably the greatest benefit of the satellitemodem system, however, was the abilityto carry out instrument configuration repairsremotely from the UK. In the summers of 2008and 2009, for example, two stations suffered1: SEIS-UKSEIS-UK – the UK’s Seismic EquipmentFacility based within the University ofLeicester – is one of three nodes that makeup the NERC Geophysical EquipmentFacility (GEF). It maintains a large anddiverse pool of seismic instrumentationand associated field equipment for onshorerecording of both earthquakes andcontrolled seismic sources. The equipmentis available for use free-of-charge toUK-based academics via a single loanapplication and subsequent peer review.To date the majority of SEIS-UK projectshave been basedoutside the UK, fromEthiopia to HudsonBay, and haveinvolved substantialcollaboration withnon-UK academics and institutions. Thefacility supports around 12 field projectsa year, providing expertise and trainingin the use of the field equipment andassociated data management systems.A processing system for continuousseismic data is fully supported, includingearthquake detection and location.In-house servers with more than 20 Tb ofstorage provide a rapid and convenientroute to data processing while removingthe need for users to maintain expensiveand time-consuming computing facilitiesof their own. SEIS-UK also manages thearchiving of data at the IRIS DMC fromwhere data become publicly available.winter power-outages from which they could notrecover. The seismometer and recording module(figure 4) had defaulted to different communicationbaud rates but, by logging into the stationremotely, the necessary corrections to therecording parameters were made. An additionalbenefit of the satellite modem systems was theability to unlock and reorient seismometers thathad moved significantly or ceased to operate –common in spring-time when shallow permafrostin the Arctic can begin to melt. Carryingout station repairs in this way, as opposed towaiting until the summer service runs, improveddata yield from the experiment by ~20%. Thecost of running the modems, the equivalent oftwo or three transatlantic flights per year, wasexcellent value for money.Scientific insights from CanadaWhile the remoteness of the HuBLE networkwas a hindrance during the field campaign, itwas a blessing for the data analysis. The qualityof seismic signals from seismic stations installedso close to basement rock, far away fromA&G 6.23