Step 1: Classify the four logging tools by their physical measurement principle:
Neutron, sonic, and density logs are all classed as "pad" or short spaced nuclear/acoustic tools that rely on particles or waves which attenuate rapidly in rock, while resistivity tools measure the formation's electrical response using either induction currents or focused current electrodes.
Step 2: Compare typical investigation ranges from petrophysical practice:
In standard well logging practice, neutron and density tools typically investigate only a few centimeters to about 10-15 cm into the formation, sonic logs investigate roughly a similar shallow range close to the borehole wall, whereas deep resistivity tools (deep induction or deep laterolog) are engineered to read several tens of inches to a few feet into the formation.
Step 3: Explain why resistivity tools read deeper:
Deep resistivity tools use larger electrode or coil spacing, which spreads the current path or electromagnetic field further from the borehole, allowing the measurement to bypass the shallow invaded (mud filtrate flushed) zone and sense the true, uninvaded formation resistivity.
Step 4: Cross-check with the purpose of deep resistivity logging:
Deep resistivity logs are specifically run to estimate true formation resistivity (Rt) away from borehole and invasion effects, which is only possible if the tool has a large depth of investigation. This purpose built design confirms that resistivity is the deepest reading tool of the four.
Step 5: Rank all four tools by depth of investigation:
Ranking from shallowest to deepest: density and neutron are the shallowest, sonic is slightly deeper but still shallow, and resistivity (particularly deep resistivity) is by far the deepest reading of all standard logging methods.
Step 6: State the final choice:
Therefore, resistivity logging gives the highest depth of investigation for the marine formation.
Final Answer:
\[ \boxed{\text{Resistivity}} \]