A generic term used to refer to the specific resistivity well log which usually consists of short normal, long normal, lateral, and SP curves. Often used incorrectly to refer to borehole electric logs of other types. Compare electric log.
The electrical survey (i.e., normal and lateral formation-resistivity measuring systems) is suitable for use in wells drilled with relatively fresh mud.
In the early years of the development of electrical resistivity measuring well-logging tools. several different devices were employed utilizing a number of different electrode spacings. These devices were used in making the electrical surveys often referred to as “ancient” resistivity surveys, or sometimes ancient well logs. The curves on these resistivity logs were simply referred to as first curve, second curve, third curve, and fourth curve. The first, second, and third curves usually could be recorded simultaneously while logging upward during the depth-controlled survey. The fourth curve was recorded usually while the tool was being run into the hole. From time to time attempts were made to standardize the electrode spacings of the respective devices used within certain geographical regions. But, it was difficult to arrive at a common standard because of the diversity of rock types, bed thicknesses, environmental conditions, and customer (or user) preferences. It was not until 1947 that the API recommended spacings (for different devices) were adopted throughout the oil and gas industry. From that time on, the standard API spacings were offered to the industry, except where customers specifically asked for spacings tailored to meet their specific requirements. After 1947, the spacings were to appear on the log heading and the curve types and spacings were standardized (with few exceptions) as follows:
<html> <table width=“100%” border=“0”>
<tr>
<td width="25%"><div align="center">
<strong>First<br />
Curve</strong><br /> <hr />
</div></td>
<td width="25%"><div align="center"><strong>Second<br />
Curve</strong><br /> <hr />
</div></td>
<td ><div align="center"><strong>Third<br />
Curve<br /> <hr />
</strong></div></td>
<td width="25%"><div align="center"><strong>Fourth<br />
Curve<br /> <hr />
</strong></div></td> </tr>
<tr>
<td><div align="center">SP </div></td>
<td><div align="center">16" normal</div></td>
<td><div align="center">64" normal </div></td>
<td><div align="center">18'8" lateral</div></td>
</tr>
</table> </html>
Between l932 and the late 1940s, before the API standards were adopted, the electrical survey could have consisted of the following curves and spacings for various geographic locations.
<html> <TABLE BORDER=0 CELLSPACING=3 CELLPADDING=3 WIDTH=“100%”> <TR ALIGN=Center VALIGN=Bottom>
<TH WIDTH="20%">REGION<HR></TH> <TH WIDTH="20%">FIRST<BR>CURVE<HR></TH> <TH WIDTH="20%">SECOND<BR>CURVE<HR></TH> <TH WIDTH="20%">THIRD<BR>CURVE<HR></TH> <TH WIDTH="20%">FOURTH<BR>CURVE<HR></TH>
</TR> <TR ALIGN=Center VALIGN=Top>
<TD ROWSPAN=4 WIDTH="20%">Gulf Coast</TD> <TD WIDTH="20%">SP</TD> <TD WIDTH="20%">8" normal</TD> <TD WIDTH="20%">16' lateral</TD> <TD> </TD>
</TR> <TR ALIGN=Center VALIGN=Top>
<TD>SP</TD> <TD>10" normal</TD> <TD>18'8" lateral</TD> <TD> </TD>
</TR> <TR ALIGN=Center VALIGN=Top>
<TD>SP</TD> <TD>16" normal</TD> <TD>64" normal</TD> <TD>16' lateral</TD>
</TR> <TR ALIGN=Center VALIGN=Top>
<TD>SP</TD> <TD>16" normal</TD> <TD>64" normal</TD> <TD>18'8" lateral</TD>
</TR> <TR ALIGN=Center VALIGN=Top>
<TD> </TD>
</TR> <TR ALIGN=Center VALIGN=Top>
<TD ROWSPAN=7>West Texas and New Mexico</TD> <TD>SP</TD> <TD>16" normal</TD> <TD>55" normal</TD> <TD> </TD>
</TR> <TR ALIGN=Center VALIGN=Top>
<TD>SP</TD> <TD>16" normal</TD> <TD>64" normal</TD> <TD> </TD>
</TR> <TR ALIGN=Center VALIGN=Top>
<TD>SP</TD> <TD>18" normal</TD> <TD>28" limestone curve</TD> <TD>13' lateral</TD>
</TR> <TR ALIGN=Center VALIGN=Top>
<TD>SP</TD> <TD>18" normal</TD> <TD>28" limestone curve</TD> <TD>18' lateral</TD>
</TR> <TR ALIGN=Center VALIGN=Top>
<TD>SP</TD> <TD>10" normal</TD> <TD>28" limestone curve</TD> <TD>19' lateral</TD>
</TR> <TR ALIGN=Center VALIGN=Top>
<TD>SP</TD> <TD>10" normal</TD> <TD>32" limestone curve</TD> <TD>19' lateral</TD>
</TR> <TR ALIGN=Center VALIGN=Top>
<TD>SP</TD> <TD>10" normal</TD> <TD>32" limestone curve</TD> <TD>24' lateral</TD>
</TR> <TR ALIGN=Center VALIGN=Top>
<TD> </TD>
</TR> <TR ALIGN=Center VALIGN=Top>
<TD>North Texas</TD> <TD>SP</TD> <TD>16" normal</TD> <TD>24' lateral</TD> <TD> </TD>
</TR> <TR ALIGN=Center VALIGN=Top>
<TD> </TD>
</TR> <TR ALIGN=Center VALIGN=Top>
<TD ROWSPAN=5>Oklahoma</TD> <TD>SP</TD> <TD>18" normal</TD> <TD>14' lateral</TD> <TD> </TD>
</TR> <TR ALIGN=Center VALIGN=Top>
<TD>SP</TD> <TD>18" normal</TD> <TD>16' lateral</TD> <TD> </TD>
</TR> <TR ALIGN=Center VALIGN=Top>
<TD>SP</TD> <TD>16" normal</TD> <TD>64" normal</TD> <TD>15' lateral</TD>
</TR> <TR ALIGN=Center VALIGN=Top>
<TD>SP</TD> <TD>16" normal</TD> <TD>64" normal</TD> <TD>19' lateral</TD>
</TR> <TR ALIGN=Center VALIGN=Top>
<TD>SP</TD> <TD>16" normal</TD> <TD>64" normal</TD> <TD>24' lateral</TD>
</TR> <TR ALIGN=Center VALIGN=Top>
<TD> </TD>
</TR> <TR ALIGN=Center VALIGN=Top>
<TD ROWSPAN=4>Rocky Mtns</TD> <TD>SP</TD> <TD>18" normal</TD> <TD>16' lateral</TD> <TD> </TD>
</TR> <TR ALIGN=Center VALIGN=Top>
<TD>SP</TD> <TD>16" normal</TD> <TD>64" normal</TD> <TD>15'8" lateral</TD>
</TR> <TR ALIGN=Center VALIGN=Top>
<TD>SP</TD> <TD>16" normal</TD> <TD>64" normal</TD> <TD>19' lateral</TD>
</TR> <TR ALIGN=Center VALIGN=Top>
<TD>SP</TD> <TD>16" normal</TD> <TD>64" normal</TD> <TD>24' lateral</TD>
</TR> <TR ALIGN=Center VALIGN=Top>
<TD> </TD>
</TR> <TR ALIGN=Center VALIGN=Top>
<TD>Kansas</TD> <TD>SP</TD> <TD>16" normal</TD> <TD>64" normal</TD> <TD>16' lateral</TD>
</TR> <TR ALIGN=Center VALIGN=Top>
<TD> </TD>
</TR> <TR ALIGN=Center VALIGN=Top>
<TD ROWSPAN=3>California</TD> <TD>SP</TD> <TD>20" normal</TD> <TD>12' lateral</TD> <TD> </TD>
</TR> <TR ALIGN=Center VALIGN=Top>
<TD>SP</TD> <TD>20" normal</TD> <TD>20' lateral</TD> <TD> </TD>
</TR> <TR ALIGN=Center VALIGN=Top>
<TD>SP</TD> <TD>10" normal</TD> <TD>8.5' Iateral</TD> <TD>19' lateral</TD>
</TR> <TR ALIGN=Center VALIGN=Top>
<TD> </TD>
</TR> <TR ALIGN=Center VALIGN=Top>
<TD>Northeast U.S.</TD> <TD>SP</TD> <TD>16" normal</TD> <TD>64" normal</TD> <TD>24' lateral</TD>
</TR> <TR ALIGN=Center VALIGN=Top>
<TD> </TD>
</TR> <TR ALIGN=Center VALIGN=Top>
<TD ROWSPAN=2>Canada</TD> <TD>SP</TD> <TD>16" normal</TD> <TD>64" normal</TD> <TD>18'8" lateral</TD>
</TR> <TR ALIGN=Center VALIGN=Top>
<TD>SP</TD> <TD>16" normal</TD> <TD>64" normal</TD> <TD>24' lateral</TD>
</TR> </TABLE> </html>
Quite often, today's user of the old or “ancient” electrical surveys will discover that the curve type (e.g., normal, lateral, or limestone curve) and the spacing used (e.g., 16“, 64”, 18'8“, etc.) will be missing from the log heading. In that event, the user must determine what kind of devices and what spacings were used for the second, third. or fourth curves by a careful examination of the behavior of each curve through different bed thicknesses and as the curve approaches surface casing. The preceding table might provide some guidance.