1. Log Correlation
Correlation can be defined as the determination of stratigraphic or structural units that are equivalent in time, age or stratigraphic position.
Preparation of subsurface interpretations (maps or cross sections) is based on 2 sources, wireline logs and seismic sections.

2. General Log Measurement Terminology
The following log depth measurements is important for converting log depths to depths used in mapping.
KB Vertical distance from Kellybushing to Sea level
MD Measured depth (From KB to TD or any point in between)
TVD True vertical depth from Kelly bushing to any point in subsurface
SSTVD Subsea true vertical depth. Vertical distance fro sea level to any point in
subsurface
Vertical wellbore A well drilled 90 deg to horizontal reference.

3. Rotary Drilling
The drill string is attached to the “kelly” a square rod that slides through the Kelly Bushing. The kelly bushing rotates the Kelly, drill string and bit.

4. Errors in Log Correlation
1. Incorrect well location or elevation information is a common source of error.
2. Incorrect stratigraphic tops are another possible source of error.
3. A third source of error is borehole deviation, i.e., when the well is not drilled vertically, but at some angle to the vertical.

5. Electric Log Correlation Procedures and guidelines
Electric log correlation is recognition of patterns on log.
Correlations must keep in mind geologic principles, depositional environments and tectonic setting.
A variety of logs must be represented.
Guidelines for log correlations are:
1. review major sand or carbonate bodies using sp or gamma ray
curves.
2. First correlate shale sections.
3. Use color pencils to identify correlations points.
4. Begin correlation at the top of the log, not in the middle.
5. Correlate the entire log.
6. Don’t force correlation.
7. In faulted areas, first correlate down the log, then up the log.