Mechanical Caliper Logging Test
Index.
1 Scope
2 Principle of Operation
3 Apparatus
4 Calibration Procedure
5 Test Personnel
6 Test Procedure
7 Interpretation of Results
8 Report
9 Reference
1. SCOPE.
This module covers the general procedures necessary to
conduct caliper logging of boreholes,wells, access tubes, caissons, or shafts
(hereafter referred as boreholes) as commonly applied to geologic, engineering,
groundwater and environmental (hereafter referred as Geo-technical)
investigation.
Caliper logs are commonly used to: measure borehole
diameter, shape, roughness, and stability; calculate borehole volume; provide
information on borehole construction; and delineate lithologic contacts,
fractures, and solution cavities and other openings.
2. PRINCIPLE OF OPERATION.
Typically, the caliper arms are mechanically connected to a
linear or rotary potentiometer such that changes in the angle of the arms cause
changes in resistance. These changes in resistance are proportional to average
borehole diameter.
In some probes, the
voltage changes are converted to a varying pulse rate or digitized down hole to
eliminate or minimize cable transmission noise.
Different arm length can be used to optimize sensitivity for the
borehole-diameter range expected.
Vertical resolution of caliper measurements is a function of
the size of the contact surface (arm tip or pad); the response of the
mechanical and electronic components; and digitizing interval used.
The
theoretical limit of vertical resolution is equal to the width of the caliper
pad or tip. Selection of arm lengths and
angle, and tip diameter will affect sensitivity.
Shorter arms generally will provide more
detail of the rugosity of the borehole wall than longer arms. However, size of
caliper probe and borehole diameter may also determine arm lengths used.
The concepts of volume of investigation and depth of
investigation are not applicable to caliper logs since it is a surface-contact
measurement.
3. APPARATUS.
The Caliper Logging System consisting of:
3.1 Three Arm Caliper Tool.
The three-arm caliper provides a single continuous log of
borehole. The three-arm caliper has arms of equal length oriented 120° apart.
All arms move together, which provides an average diameter measurement.
Caliper probes are
spring loaded. The arms are retracted
and opened with an electric motor and retention spring as many times as needed
while logging a borehole. Caliper probes
closed by hand are held closed with an electric solenoid or weighted retention
ring that is released with a sudden drop.
3.2 Winch Assembly.
Winch Assembly including measuring wheel and electronic
depth encoder, tripod with line wiper, surface electrode and cables, cable head
and power source cables.
3.3 RG USB Micro logger.
The Micro logger needs only a PC, probe, and an RG winch to
provide high quality borehole logging With a USB interface for your laptop
computer for every situation.
4. CALIBRATION PROCEDURE.
- Calibration and standardization should be performed each time a caliper probe is suspected to, be damaged, modified, repaired, and at periodic intervals.
- Physical models of measured diameter that may be used to calibrate the caliper response may include rings or bars made of rigid materials that are not easily deformed and resist wear.
- Calibration can be checked by using casing of measured diameters log in the borehole.
- A Calibration bar is drilled and marked at a regular interval and machined to fit over the body of the probe. One arm is placed in the appropriate hole for the range to be logged.
- Calibration can be checked by using casing of measured diameter logged in the borehole.
5. TEST PERSONNEL.
The field testing is performed by an experienced technician
with at least 5 year experience in pile testing.
The interpretation of records is performed by the Engineer
having 10 years’ experience in pile testing interpretation.
6. TEST PROCEDURE.
- Caliper operation and calibration are checked at an interval consistent with project objectives. After calibration, the caliper arms are closed before lowering.
- Select and document the depth reference. The selected depth reference needs to be stable and accessible (for example, top of borehole casing).
- Determine and document probe zero reference point (for example, top of probe or cable head) and depth offset to caliper measurement point. The measurement point of a caliper is the end of caliper arms and it changes as the arms open and close with the sine of arm angle multiplied by length of arm. Measurement point varies less than a few centimetres.
- Select horizontal and vertical scales for log display.
- The caliper probe is lowered into the bottom while arms closed to avoid damaged of arms or to the side of the hole. The probe is typically lowered to the end of the hole, however a minimum of 30 cm above the bottom is required to allow for free opening of the probe arms.
- Selection of probe speed while lowering is based on knowledge of borehole depth, stability and other conditions.
- Open caliper arm(s).
- Select logging speed. A logging speed of approximately 4 m per min. is recommended for high resolution applications.
- Collect caliper data while the probe is moving up the borehole.
- When the probe reaches the top of the borehole, compare and document caliper measurement.
- Evaluate the field log quality and compare log with drilling and completion information.
7. INTERPRETATION OF RESULTS.
- A valid caliper log is essential in the interpretation of the logs that are affected by changes in borehole diameter.
- Caliper logs can be analyzed individually (that is, borehole volume).
- The caliper log should be depth correlated with the other geophysical logs as the first step to interpretation. This is especially important for logs that use the caliper data for borehole correction and depth adjustment.
- Other pertinent information including borehole construction (casing size), drilling history (hole size, drill method, penetration rate, core loss, fluid loss, etc.) and geologic information should be integrated with the caliper-log data.
- Interpretations based on changes in borehole diameter may be related to changes in drilling, mud cake, borehole construction, lithology and structure, fractions and solution openings, and stress-induced breakouts.
Caliper logs provide
useful information for borehole completion and testing:
a) Caliper logs are used to estimate the volume of borehole
completion material (cement, gravel,etc.) needed to fill the annular space between borehole and
casing(s) or well screen.
b) Caliper logs may be applied to correlate lithology
between boreholes based upon enlargements related to litho logy.
8. REPORT.
The report shall include the following information as a
minimum will be submitted within 3 days of testing.
Presentation of
caliper logs should be designed to meet project objectives. At a minimum, depth
(y-axis) and units of measurement (x-axis) scales should be
clearly marked.
Caliper logs are typically displayed with linear scales in
inches, millimetres, or centimetres.
9. REFERENCE.
ASTM D 6167.