Full Wave Sonic Logging – Multi Receiver (3 or 4) For Rock/Soil Properties
Multi receiver sonic logs or more recently referred to as Full Waveform Sonic (FWS) logs are collected in the fluid filled portions of the drill holes with an ALT model FWS50 sonic probe using a monopole transmitter and three or four different receivers at 0.6, 0.8, 1.0 and 1.2m respectively. A monopole transmitter frequency of about 15 kHz appears to resolve both compressional and shear waves in most borehole conditions These new tools allow for multiple settings during acquisition. One of the more advantageous we’ve been using is a longer time window. This second or long time window can be collected to help resolve reflected Tube waves. Field data is typically is displayed showing the Variable Density Log or VDL for each receiver, and the the associated travel time (dt) of the 1st arrival. The waveforms from each receiver are presented in a variable density log (VDL) display to highlight fractured intervals and to investigate the homogeneity of the rock mass around the drill hole
For mining applications, the processing of data from full wave sonic logging can be and has been complicated in the past, but recent advancements in computer processing has helped tremendously in cutting down time and increasing accuracy of requested data. Our standard processed full wave sonic logging data includes the compressional (‘Sp’ or P-wave), shear (‘Ss’ or S-wave), and Stonely (‘Sst’-wave) wave slowness (sonic Transit Time or “Delta-T”) recorded in uSec/ft and amplitude. Note that Sonic ‘Slowness’ is the inverse of velocity. .
A color VDL is made from the wavelets by assigning different color bands to the waveform amplitude values. The degree of discontinuity of the rock is reflected by the deviation from parallel banding in the FWS display. Decrease in amplitude of the Stonely or Tube wave is indicative of permeability. These Tube-waves are generally recognized as high amplitude coherent wave-trains. Because the Tube wave is coupled to the formation through the formation which it is traveling, it can perturb the formation across open fractures intersecting the borehole. This squeezing effect can generate Secondary or Reflected Tube waves which travel both up and down from the fracture location. These Reflected Tube waves can be diagnostic of the presence of open fractures and their amplitude related qualitatively to the length and width of the fluid filled fracture space; i.e., strong Reflected Tube waves are indicate of wider more permeable fractures some distance from the borehole.
Compressional wave transit time can be used to calculate porosity (using Wyllie Time-average Equation or Raymer-Hunt algorithm) and to provide additional information regarding lithology, consolidation, and the presence of discontinuities. Compression and shear wave velocity is calculated from of the respective slowness or transit times. These are combined with compensated density measurement to calculate rock elastic moduli.
