between the joined sensor packs which can be several feet in separation. Tere are many bespoke requirements for
microseismic monitoring systems, ranging from continuous recording, fast sampling rates and high temperature tolerances; however these are all redundant if the event signal is unheard! By employing multiple phones within a sensor pack, the sensitivity is maximised, signal/noise improves and stacking uncertainty is reduced, all safeguarding the accurate event pick times that hydraulic fracture mapping demands.
Enter 14 or ✔ at www.engineerlive.com/ihss
Fig. 2. Cross normalised seismic trace recorded on Dual Geophones (traces 1 and 3) vs Quad Geophones (traces 2 and 4). The magnitude spectrum shows a ~6dB differential in signal to noise over the recorded bandwidth between the Dual (Blue) and Quad (Red) sensor packs.
at the surface, the background electronic noise (En) within a receiver can be enough to mask a microseismic arrival. To improve on this signal to noise ratio, technology has evolved to stack the phones within the sensor pack component of a downhole receiver, with the latest current technology achieving four phones on each component (12 in each receiver satellite).
As the thermal noise En output voltage
can be expressed as the square root of 4kTBR (B Boltzmann’s Constant, Tk Temperature in Kelvin, B Bandwidth and R Geophone Resistance) the sensitivity of a typical geophone is effectively doubled when stacked but the En noise only increases by the square root of the resistance. Fig. 2 emphasises this with a recorded
correlated vibroseis data sample, the recorded controlled source traces show the amplitude of the quad sensors are near double that of the dual sensors. Te advantages of multiple geophones within one sensor pack (Fig. 3) goes further in terms of guaranteeing perfect stacking. Standard industry borehole receivers can only accomplish four geophones per axis by mechanically joining two independent ‘dual’ geophone satellites together within the monitoring well. Tis can often result in stacking errors, where the incoming signals between the satellites may be different due to inconsistency receiver/casing coupling, either due to mechanical discrepancies between the receiver locking arm drive mechanism, cement irregularities, and increased distance
William Wills is a Geoscientist with Avalon Sciences Ltd, Somerset, UK. www.avalonsciences.com
Fig. 3 Avalon Sciences Microseismic Quad Geophone Sensor Pack (4 geophones on each axis). Each geophone can record and stack the same exact seismic signature due to sharing identical borehole coupling and measured depth increasing sensitivity.
Deep profiling from a stationary vessel S
urveyors from the Russian survey company Peter Gas used some lateral thinking to avoid time-
consuming sound velocity (SV) profiling activities during a AUV multibeam pipeline survey in the deep waters of the Black Sea. A Hugin AUV deployed from the offshore support vessel GSP Prince surveyed the 500 nautical mile route, with about 50 per cent of the pipeline to be laid in water as deep as 2000m. The survey team wanted a fast and economical way
to gather sound speed profiles in support of the deep ROV operations, and decided to use the Oceanscience UnderwaySV profiler. Developed in partnership with Valeport (UK), the UnderwaySV uses the latest RapidSV ‘free fall’ sound velocity probe. By deploying the RapidSV profiler from a stationary vessel, deep sound speed profiles can be collected much faster than using conventional methods based around a hydrographic winch CTD or sound velocity instrument. The Valeport RapidSV probe free falls at over 5m/s
deployment, allowing the probe to drop with little or no drag from the tether line. The high speed winch allows fast recovery of the
profiler, greatly reducing the overall profiling time for a high quality sound speed cast. This time saved equates to a more efficient survey job. The maximum profile depth achieved was 1730m, and the cast was completed in about 35 minutes from start to finish! This result smashed the Oceanscience profiling depth record, previously standing at 1563m, held by NOAA’s National Data Buoy Center. Where deep water CTD or SV profiles are needed,
reaching 1000m depth in only 3-4 minutes. The key to this down-cast profiling speed is the innovative XBT-style tail spool attached to the probe. The tail spool is loaded with up to 1000m of high strength tether line before each
and the vessel does not have the hydrographic winch capabilities, the Oceanscience UnderwaySV or UnderwayCTD are now proven options for fast, deep profiling from a stationary vessel.
For more information, visit www.oceanscience.com
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