THEORY

Currently in use borehole seismic probes with guided hold-down most often have one arm creating rigid contact of the probe case with a wellbore wall. Such position of a probe in hole generates anisotropy of its azimuth sensitivity, i.e. dependence of sensitivity from an approach direction of a wave in data logging point. It speaks that at advancing of wave in the direction of an axis X on contact of a tool with a wellbore wall a strain "compression - tension" is occurring, described by elastic modulus Я1, and at advancing of wave along axis Y a shear strain is occurring with elastic modulus Я2. It is known, that elastic modules Я1 and Я2, corresponding to these strains for the same medium are connected by a relation

c1 = c2 (1 + ╣),

Where ╣- Poisson▓s ratio (for geological mediums ╣ = 0,2:0,4).

Distinction of elastic modules on different directions on contact of tool with medium also generates different sensitivity on these directions.

It means, that such probe can qualitative register only vertical component of the wavefield excited near the head of vertical well.

For adequate recording a full oscillations vector both from offset shot points and in inclined well and, accordingly, for expansion of a range of decided problems, the probe with constant azimuth sensitivity is necessary.

The probes with a centering hold-down have this advantage.

The operative embodiment of a probe with centering hold-down tool SPAN-3 is developed and tested in "Compass plus" Ltd.

Its trials have shown full advantage over the best probes with one-lever hold-down tool already when recording a vertical component of a wave field.