OPERATION MANUAL - Scintrex

Ocean (nearest distance from Toronto is about 800 km). The frequency ofmicroseismic signals recorded in Toronto is in the range of 1/4 to 1/7 Hz.Highest SDs in the Scintrex lab were recorded during earthquakes (about 1.5mGal). However, we were notified by our clients of much highermicroseismic noise levels: SD in the range of 1.5 mGal during a typhoon inTokyo, 0.3 mGal in Northern Italy, up to 0.5 mGal during cyclone inAustralia.On a past survey a CG-3M meter (equivalent to the CG-5 sensor) recordedSD in the range of 2.5 mGal on the 1.5 in thick ice in the shallow PrudhoeBay, North Slope Alaska. To reduce the measurement noise in situations likethese we devised a different way of gravity signal processing which wenamed the SEISMIC FILTER.In short, the difference between processing of 1 second gravity signalsamples of the seismic filter and the standard filter is as follows:1. The sampling of the gravity signal is not interleaved with sampling of thecalibration signal (voltage reference). In stead, a group of calibrationsamples is taken before and after the gravity signal measurement,2. Gravity samples are not equally weighted as in the standard filter- a specialweighting function is used to suppress the edge effects.3. The rejection threshold has been increased from 4 x SD to 6 x SD. The leadtime of 5 seconds, before the rejection is enabled, has remained unmodified.The value of a rejected sample is replaced by the value of theprevious sample. The reason for increasing the rejection threshold is that,during microseisms, the seismic noise level varies greatly. If themeasurement starts during occasional low noise intervals the rejectionthreshold is set too low, and subsequent higher level seismic noise isrejected, resulting in numerous rejections. This defeats the benefits of theSEISMIC FILTER.Note:The rejection is designed to eliminate noise spikesand not the microseismic noise. In addition, it isworth noting that the seismic filter very effectivelyreduces measurement errors due to noise spikesB-12CG-5 Manual - part # 867700 Revision 2