In this month’s Wireline Workshop Bulletin, we continued the discussion of sonic logs and spontaneous potential and how they are applied in geotechnical applications:
” The use of very precise and accurate P‐wave and full waveform sonic data for geotechnical analyses was discussed last month. The logs are used to generate an empirically based measure of uniaxial compressive strength (intact rock strength), Poisson’s ratio and the dynamic moduli of elasticity.
The sonic log is valid for IRS because it does not require calibration (it should always be accurate in reasonable hole conditions) and its measurement is sensitive to porosity; primary porosity, not fractures. The tool measures the transit time of a compression wave that is refracted through the formation. It discriminates the first arrival, which will be the one that has found a route past borehole effects and most fractures; hence intact rock strength. The IRS log will not be perfectly accurate but the comparison of multiple logs using a range of empirically based formulae results in a fairly limited range of solutions. No version is a mile out.
The dynamic logs of formation elasticity generally overstate stiffness because they are based on measurement at the molecular level, the intrinsic elasticity not the bulk formation elasticity that will be affected by micro‐ pores and fractures that tend to increase flexibility. The overstatement is usually in the order of about 10 to 15 percent. So a reduction by a nominal 10% might be reasonable. It won’t be a mile out.”
There was also a good contribution by a Geotechnical Logging company from South Africa:
“However, the frequency of data from the sonic log assists us (within a mining environment) at conceptual level studies at the variation in specifically rock strength and the other parameters with increasing depth, and with this we determine macro geotechnical zones within the overburden rock masses that are expected to behave similarly. This process is used to pick bench horizons, explosive charge positions, weathered material limitations among other variables. We don’t necessarily look at one specific data point and take that as the rock mass strength, but rather use the sonic‐based UCS log as an indicative tool.
The sonic logging tends to be more cost effective for the amount of data obtained, but we always back this up with field testing and laboratory analyses.”
–Jaco van Vuuren, Associated Rock Mechanics Services (ARMS)
Processing Full Waveform Sonic Data in WellCAD (Part 1) by Timo Korth of Advanced Logic Technology (ALT)
This section is very useful to keep on hand in case a log analyst only processes sonic logs infrequently. It can be used as a How-To for Sonic processing within WellCAD.
“In Part 1 we will cover the workflow from data import to deriving a slowness and velocity value for the compressional wave (P‐wave). In Part 2 (next bulletin) we will deal with the Velocity Analysis and Reflected Tube wave processes.
Our goal is to compute the time needed for the P‐wave to corresponds in a first approximation to the RX1 – RX2 separation and having the travel time for this interval we can compute a velocity (and slowness).
The general workflow to derive the P‐wave velocity consists of the following steps:
- Data import with correct time sampling rate
- Data pre‐processing to reduce noise
- Picking of the P‐wave intercept times
- Quality check and eventually correction of arrival times
- Computation of transit time, slowness and velocity
- Depth shifting of the results to the correct measurement point”
There is also a well-written, thorough article on the SP, the Spontaneous Potential log
This is often one of the most misunderstood logs in our industry, even though it is one of the oldest. The Bulletin does a great job of detailing some of the interesting history behind the discovery of this log in 1927, and follows up a an analysis of the measurement itself and it’s potential applicatiosn to mining and groundwater industries:
” The electrical potential, in the order of millivolts, is caused by the natural separation and migration of positive and negative ions in the borehole environment. It is the result of the normal division of sodium chloride (NaCl or “salt”) molecules in water into positively charged ions of sodium and negatively charged ions of chlorine.
It is important to distinguish between fluid flows (limited to permeable formations, shale is impermeable), ionic flows (ions will pass through shales, but some more easily than others), and electron flows (net electronic current flow).
In oilfield logging, the log may be used to calculate the resistivity of the formation fluid (Rw). In mineral logging, we are only interested in the SP log as an indicator of shaly and permeable zones and their boundaries, where their description in terms of SP adds to the characterization of the formation for correlation purposes.
The combination of SP with natural gamma or resistivity logs works well as a lithological characterization and correlation tool. Below, a zone of sandstones, shale and siltstone is described by natural gamma, resistivity and SP logs.”