The Dual Laterolog (DLL) provides two resistivity measurements with different depths of investigation into the formation: deep (LLd) and shallow (LLs). In both devices, a current beam 2 ft-thick (A0) is forced horizontally into the formation by using focusing (also called bucking) currents (A1-A2, A'1-A'2); two monitoring electrodes (M1, M2, M'1, M'2) are part of a loop that adjusts the focusing currents so that no current flows in the borehole between the two electrodes. For the deep measurement both measure and focusing currents return to a remote electrode on the surface; thus the depth of investigation is greatly improved, and the effect of borehole conductivity and of adjacent formations is reduced. In the shallow laterolog, instead, the return electrodes which measure the bucking currents are located on the sonde, and therefore the current sheet retains focus over a shorter distance than the deep laterolog.
The Dual Laterolog has a response range of 0.2 to 40,000 ohm-m, whereas the DIT has a range of 0.2 to 2,000 ohm-m. The DLL is recommended for igneous environments (e.g., oceanic basalts and gabbros) because the resistivities can be higher than the upper limit of what the DIT can measure (e.g., Hole 735B). However, in upper crustal environments (seismic Layers 2A and 2B), the resistivities are usually low enough that you can use the DIT. This was the case in data from, for example, Legs 104 and 152 as well as Holes 395A and 504B.
The DLL is usually run in combination with the Natural Gamma Ray Spectrometry tool (NGT), but may be run with the Triple Combo or alone.
The depth of investigation of the laterolog depends on the resistivity of the rock and on the resistivity contrast between the zone invaded by the drilling fluid and the virgin (uninvaded) zone. The vertical resolution of both LLd and LLs depends on the geometry defined by the focusing electrodes: this is about 2 ft (61 cm).
Porosity estimate
Because of the inverse relationship between resistivity and porosity, the dual laterolog can be used to compute the porosity of the rock from Archie's equation if the sediments/rocks do not contain any clay or if the contribution of surface conduction to the signal is negligible.
Fracture Porosity Estimate
This can be estimated from the separation between the deep and shallow measurements based on the observation that the former is sensitive to the presence of horizontal conductive features only, while the latter responds to both horizontal and vertical conductive structures.
For the LLd the borehole effect is small for hole diameters up to 16 in, while the LLs provides good readings in holes not exceeding 12 in. Corrections are available for holes up to 20 ft in diameter.
The LLd and LLs curves are usually displayed on a resistivity logarithmic scale, along with the gamma ray log.
Output plot of DLL data
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Temperature Rating:
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350° F (175° C) |
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Pressure Rating:
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20 kpsi (13.8 kPa) |
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Tool Diameter:
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3.625 in (9.21 cm) |
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Tool Length:
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30.6 ft ( 9.35 m) |
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Sampling Interval:
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6 in (15.24 cm) |
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Max. Logging Speed:
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10,000 ft/hr |
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Vertical Resolution:
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2 ft (61 cm) |
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Depth of Investigation:
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(see discussion in "Description" section) |
| LLD |
Deep Laterolog (ohm) |
| LLS |
Shallow Laterolog (ohm) |
As noted above, the DLL is usually run in combination with the Natural Gamma Ray Spectrometry tool (NGT), but may be run with the triple combo or alone. Obviously, combining the DLL with the Triple Combo will save an additional run.
Stuck/lost tool information
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