486 



Subsurface Geologic Methods 



A second type of instrument, called the "Log-0-Graf, graphs the 

 time horizontally and the depth vertically. The chief advantage claimed 

 for this type of instrument is that the resulting graph, when made on the 

 same vertical scale as the electric log, may be directly correlated with it 

 without redrafting (figs. 226 and 227) . 



A third instrument known as the "Martin-Decker rate-of-penetration 

 recorder" is based on the hydraulic principle. A fluid container on the 

 kelly is connected by a hose to the recording mechanism (fig. 228) . Low- 

 ering the kelly changes the hydrostatic pressure, which is recorded on a 

 polar chart (fig. 229) . 



Two transparent templates inscribed with drilling-rate curves are 

 provided. To reduce the drilling-rate curve on the chart to feet per hour, 

 the template is placed over the chart and rotated until a drilling-rate 

 curve coincides with the curve on the template, which may then be read 

 directly in feet per hour (fig. 230). 



Figure 230. Martin-Decker recorder. 



