Regarding 8888 tie files:
  • For the first time on this job Grav99 (CG6) showed a large drift between beginning and end 
    of the day (145μGal). This was highly unusual and a tear was suspected to have occurred at
    some point during the day.
  • When the gravimeter operator was asked if anything had happened, the partial ripping of one
    side of the shoulder strap on the gravity bag was invoked. That did cause the gravimeter to
    partially fall, though it was caught before hitting the ground.
  • Though this event should have triggered a visit to the previous station, to investigate the
    possible tear or offset that could have resulted from it, this wasn't done, nor was the event
    and its time & location recorded in writing.
  • The investigation of the GPS tracks of the gravimeter operator and recalling by both the RTK
    and gravimeter operator of the approximate time of the event indicated that station 1350 was
    the most likely location, though 1250 was also a potential suspect. This conclusion came from
    a few points:
      — the event was after lunch (after station 1100);
      — it required some time at the station, because some “sowing” took place to remedy the tear;
        ○ 1150:     too short between arrival & departure
        ○ 1200:     too short between arrival & departure
        ○ 1250: possible
        ○ 1300:     too short between arrival & departure
        ○ 1350: likely the location
        ○ 1400:     too short between arrival & departure
        ○ 1450:     too short between arrival & departure
        ○ 1500:     not it → there was some wandering around to find a place to read, unlike the
                    place of the event, where a rock was immediately selected.
        ○ 1550:     too short between arrival & departure
        ○ 1600:     too short between arrival & departure
        ○ 1650:     too close to the end of the day
  • FOLLOW UP from the next day:
    — It was decided that stations 1250 to 1400 would be reoccupied.
        ○ 1250: consistent with minimal drift since the morning check-in (repeated within 5 μGal)
        ○ 1300: idem (repeated within 8 μGal)
        ○ 1350: Odd ball. It is consistent with a 100 μGal drift since the previous reading & the
                morning check-in, and consistent with a 50 μGal drift with the following station &
                the evening check-in.
                → The gravimeter was possibly still unstable and settling when the original reading
                  was collected following the tear.
        ○ 1400: consistent with minimal drift between the reading and the evening check-in (repeated
                within 3 μGal).
    — Pairs of virtual check-ins were created just before and just after station 1350 to make the
      original results of those four stations consistent with these new measurements.
      