Some Causes for Bad Sonic Logs and Some Editing Options

Some Causes for Bad Sonic Logs and Some Editing Options

Introduction

The objectives of this report are modest - to help you to identify bad sonic logs and to present methods of "cleaning-up" sonic data for input into Synthetics programs.

Poor quality sonic logs make creation of conventional synthetic seismograms difficult or impossible. Cycle-skipping is the most obvious symptom of bad sonic logs. Less obvious, but more widespread is the problem of acoustic signal attenuation resulting in anomalously high sonic values (low apparent velocity).

This report presents;

* Some reasons for poor quality sonic logs,

* Criteria for recognizing bad data, and,

* Some methods that can be used to "clean up" the original data.

Two methods are presented for calculating velocity from resistivity logs.

Causes of Bad Sonic Logs

Conventional wisdom holds that the sonic log is relatively unaffected by poor hole conditions. Most log analysis programs default to using the sonic for porosity determination where hole conditions are bad. In rugose hole, the sonic log may appear to be satisfactory with perhaps some cycle-skips. However, these first impressions of sonic log accuracy may be misleading.

Poor sonic logs can be caused by low transmitter strength, "road noise" and attenuation of compressional sound waves.

Low sonic transmitter strength will result in less than optimal receiver signal amplitudes. Under extreme conditions this will result in cycle-skipping. Records of transmitter signal strength are not kept, so it is not possible to determine how important this effect is in causing poor quality sonic logs.

Road noise is caused by tool movement along the borehole that generates a high frequency noise component that is superimposed onto the normal acoustic signal. These noise spikes travel along the tool body and can result in early stopping of the sonde timing clock. Road noise results in random spikes of varying amplitude on the sonic log. The far sonic detectors are more affected by road noise than the near detectors because of the reduced signal amplitude with increasing travel time.

Attenuation (decreased amplitude) of the compressional acoustic wave is probably the major cause of poor sonic logs. Attenuation results in the signal at the receiver crossing the threshold amplitude later than for a stronger signal. This results in "dt-stretch" where the apparent formation velocity is less than the true velocity. The magnitude of the "dt-stretch" can be up to 6 micro-seconds per foot (Tittman, 1986). Put in the perspective of porosity, over-estimation of transit time by 6 usec/ft in sand corresponds to porosity estimates that are about 4.5 porosity units too high.

Severely attenuated sonic signals may have compressional wave amplitudes lower than the detector threshold value. This results in the first compressional arrival not being detected. Instead, later, higher amplitude arrivals such as the Shear arrival are detected. This appears on the sonic log as the familiar and easily recognised cycle-skips where the transit time is obviously much too high and often has a "spiky" appearance.

Sonic "strech" is more common than cycle-skipping but is rarely recognized or documented.

Some causes of compressional signal attenuation are;

1) Low formation velocity. The sonic wave travels from the transmitter to the borehole wall through the mud. Some energy is refracted vertically and travels along the borehole wall. Energy is continually refracted back to the borehole where eventually some is detected at the receivers. Energy loss is therefore a function of formation velocity with slower (longer travel time) formations causing more signal attenuation.

2) High porosity. High porosity formations have poor grain-to-grain coupling with the result that acoustic enery is transmitted to the much slower fluid. The energy arriving at the receiver has diminished amplitude. This effect is most common at shallow depths.

3) Shale content. In laboratory experiments, Gardner et. al. (1968) noted a substantial decrease in signal amplitude with the addition of small amounts of clay to the formation. The reason for this attenuation has not been adequately explained but may explain why very dense shales have anomalously high transit time values.

4) Thin beds. Reflection and refraction of acoustic energy occurs at boundaries of beds with different velocity. This results in a reduction of signal amplitude. Attenuation is a function of both the velocity contrast between beds and the number of beds in the travel path.

5) Alteration of formation near the borehole. Drilling fluid often causes alteration of minerals (especially clay) near the borehole. The effect of this is to create a zone with lower velocity than in the virgin formation. Since the diffracted sonic waves travel close to the borehole wall, acoustic energy is attenuated by factors 1 and 2 above. Energy is also lost by refraction at the interface between the altered and virgin formation. Mud solids (clay) may be introduced into permeable beds with resulting signal attenuation by factor 3 above.

6) Eccentering of the sonic tool. Sonic receiver signal amplitude falls rapidly as the sonde moves away from the borehole axis (Tittman, 1986). This results because wave fronts travelling different paths undergo destructive interference.

7) Transmitter-receiver spacing. Sonic signal strength falls with increasing distance between the transmitter and receiver. This is an argument for not using long spaced sonic tools.

8) Borehole rugosity. In rugose hole it is not possible to ensure that the sonde is always centered in the borehole. The result is severe attenuation of the acoustic signal as in 6 above. Energy is also lost by diffraction at "angles" in the rugose hole. The transmitter-receiver distance is effectively much longer in rugose hole than in smooth hole. Since hole rugosity is often a result of alteration of the formation by drilling mud, several of the above mentioned factors contribute to signal attenuation.

9) Fractures. When an acoustic wave reaches a fluid filled fracture, part of it reflects back into the rock and part changes to a fluid wave in the fracture. When the fluid wave reaches the opposite fracture wall, there is further reflection loss and conversion back into compressional, shear and Stonely waves. (Schlumberger, 1987). Both reflection and mode