July 13, 2019
FWS (Full Waveform Sonic) borehole surveys can be an alternative to Borehole shear-wave seismic surveys in certain cases.
Borehole Shear Wave surveys use 1 or 2 borehole geophones that are ‘clamped’ against the PVC cased borehole wall. Ideally the holes are dry. Seismic shear waves are generated near the borehole collar using a hammer against a plank held in place under the wheels of a truck.
FWS can be used to determine Poisson’s Ratio by measuring Vp compressional and Vs shear Wave velocities down the borehole in a single self-contained borehole probe. The method requires the borehole to be filled with water. The main disadvantage of FWS compared to borehole seismic surveys is that the borehole needs to be uncased and as small diameter as possible. This might be difficult through soft soils so the borehole should be logged immediately after the casing is pulled by the drill crew. In some cases the drillers may need to fill the borehole with water to allow the survey to commence.
The ALT QUL40-FWS tool with a 4MXA-1000 (500m) motorized winch is used for the FWS survey. A Makita motor-generator with Panasonic Toughbook laptop is used to log the data. A typical FWS tool configuration has 1 transmitter and 3 receivers, as displayed below.
More specifically, the ALT QL40-FWS tool is specifically designed for the water, mining and geo-technical industries. The QL40-FWS implements a high energy source generated by a ceramic-piezoelectric transducer that excites the formations in such a way that waves of different frequencies are developed and propagated. Real time analysis and processing of the full waveform are performed by the tool to enhance the picking of the different wave propagation modes. The tool can only be operated in a fluid-filled hole. Logging speed depends on tool configuration and acquisition parameters.
The FWS Seismic data are typically post-processed as follows using the FWS Module of WellCAD version 5.2 or later. All data are preserved in raw <*.tfd> and processed <*.WCL> digital formats:
1)A low cut, low pass, high cut, high pass filter to the frequency spectrum is applied.
2)Stack traces 5x.
3)Determine 1st Arrivals using a ‘standard’ method which uses a blanking-factor and threshold factor. Also use an ‘advanced’ method which computes the ratio of the average amplitude values of the ‘small’ (signal window) and the ‘large’ (noise window). The transit time at the first sample for which the signal to noise ratio is larger than the specified ‘ratio threshold’ is returned in a Well log.
4)A stand-off correction of 0.0766 m and fluid slowness of 689.655 microseconds per metre (i.e., 1450 m/s) can be applied to correct for the water column around the probe in the borehole. This is typically only required when analyzing the results with single receiver.
5)Velocity Analysis using ‘Semblance’ is completed for each receiver on the stacked traces (step 2 above).
6)The ‘P-Slowness’ (reciprocal of P-Velocity) is calculated as follows: (Rx2-Rx1)/0.2. However, this assumes the first arrivals are P-waves and not first arrivals through the water column which has a velocity of 1450 m/s.
7)If the S-wave is detected, the ‘Approx S-Slowness’ can be drawn manually on the Velocity Analysis output of step 5.
8)The ‘Adjust Extremum’ process can be completed to produce a more accurate ‘S-Slowness’ result from step 7.
9)Poisson’s Ratio can be computed using the ‘Velocity Analysis – max’ (S-Slowness) and P-Slowness.