Neutrally-buoyant borehole investigation tools and methods

A method of acquiring seismic data downhole, comprising:

• lowering a substantially neutrally-buoyant tool into a borehole, wherein the substantially neutrally-buoyant tool comprises a section of cable at least about 300 m in length or greater and at least one sensor mounted on or within the cable or both, wherein lowering comprises mounting at least one of a weight or a tractor to the cable to assist in lowering the cable, and positioning the substantially neutrally-buoyant tool in the borehole such that the at least one sensor is substantially capable of moving along with a seismic wave that passes through the borehole fluid;
• firing a seismic source resulting in movement of the borehole fluid in response to a seismic wave;
• and
• acquiring data relating to the seismic wave using the at least one sensor which moves together with the borehole fluid.

A system for borehole for detecting seismic signals downhole, comprising:

• a downhole seismic signal acquisition tool comprising:
• a section of cable having a length of at least about 300 m or greater,
• a plurality of sensors for measuring seismic signals mounted on the cable, within the cable, or both;
• and
• a deployment device chosen from a weight or a tractor;
• wherein the acquisition tool is substantially neutrally-buoyant in a borehole fluid.