Microseismic Testing and UTC (Universal Time Clock)

By Nathan Waldman | Tue, 22 Jun 2010

Microseismic testing is a relatively new and powerful tool now being used in the development of oil and gas shale deposits.  This tool provides an understanding of the fracture geometry, azimuth and length thereby leading to better frac design.   Seismometers are placed at fixed surface locations around the well to be fraced or in surrounding wells.  During the fracturing process, the time for the compression and shear waves that are generated and their magnitude monitored and recorded.  

If there are wells adjacent to the well to be fractured, it is also beneficial to install precision pressure recorders on the surrounding well to detect the presence and magnitude of pressure responses in the observation wells.  The precision pressure recorder must be capable of detecting a 0.01 psi pressure irregularity and also be able synchronize the arrival of the pressure response to a common time base (UTC) with an accuracy of no less than 10 seconds but ideally less than 5 seconds.  UTC, which stands for Coordinated Universal Time, differs from Greenwich  Mean Time (GMT) in that it doesn't change for Daylight Savings Time.

Some field instrumentation can continuously update its internal clock using GPS signals.  However, most self powered field data recorders rely on an internal clock crystal for time keeping.  These instruments present multiple sources of error relative to UTC which can render them unsuitable for microseismic testing work.

Possible sources of instrument clock error include the inherent inaccuracy of the clock crystal, the time standard used to program the field recorder, and the lag between setting the clock time when the "Set Clock" command is given and when the instrument clock starts functioning.  Each of these error sources are discussed in turn.

Crystal Error

A high quality uncompensated clock crystal will resonate at 32.768 kHz with an accuracy of +/- 20 parts per million which is equivalent to +/- 1.73 seconds per day.  Therefore, the magnitude of possible error in indicated time is equal to the number of days between the date when the clock was set and the date a measurement is made times the actual measured clock crystal time shift.   Each instrument dependent on an internal clock crystal must have its specific accuracy determined with a precision frequency counter.  If the clock crystal accuracy cannot be determined, that instrument will not be suitable for microseismic work.

Time Standard

Field instrumentation that does not have an internal mechanism for synchronizing its internal clock with a GPS satellite signal must have the clock manually set.  The reference time during the manual time set may be from the operator's watch, a computer clock or, in ideal circumstances, from a known time standard such as www.time.gov .  If not the latter, the error between the computer clock or wristwatch and UTC time can be several minutes plus the time zone difference between local time and UTC.  Therefore field instrumentation to be used in microseismic testing must be capable of self synchronizing with UTC or have a means of accurately compensating for errors in the Time Standard and clock crystal drift.     

Programming Lag

The third source of clock error in field instrumentation is the time lag between programming the instrument clock and when the instrument clock actually receives and implements the clock programming instruction.  Depending on the instrument, this time lag can be on the order of several seconds.  If the lag cannot be quantified with respect to UTC, the instrument will not be suitable for microseismic work.

Although the present generation SPIDR does not have the ability to self synchronize with GPS time, it does have a high quality clock crystal whose repeatable parts per million accuracy can be determined.  In addition, the programming record generated for every rental allows the SPIDR to be time coordinated with UTC and on return from the rental, the time is again coordinated with UTC to determine if the time shift during the job is consistent with pre-determined clock crystal accuracy.