Compaction monitoring instrument system

An apparatus insertable into a borehole through a geologic formation for determining the distance between the surface and a subsurface marker, comprising:

• an elongated member having a lower end for insertion within the borehole;
• a mechanism for selectively deploying and retrieving a selected length of said elongated member within the borehole;
• a housing connected to said elongated member lower end, wherein said housing comprises a first housing section and a second housing section attached to said first housing section;
• a first detector engaged with said first housing section for generating a signal responsive to the marker;
• a second detector engaged with said second housing section at a selected distance from said first detector for generating a signal responsive to the marker;
• and
• a calibrator for identifying an initial attached orientation between said first and second housing sections, and for identifying deviation from said initial attached orientation following detachment and reattachment of said first and second housing sections to indicate changes in said distance between said first and second detectors.

An apparatus, attachable to the lower end of an elongated member for insertion into a borehole, for determining the distance between first and second subsurface markers, comprising:

• a housing section attached to the lower end of the elongated member, wherein said housing encloses an interior having a first end and a second end;
• a first detector within the first end of said housing interior for generating a signal responsive to a marker;
• a second detector within the second end of said housing interior for generating a signal responsive to a marker;
• a spacer between said first detector and said second detector;
• a first flexible retainer engaged with said first detector for permitting thermal expansion of said first detector toward said spacer;
• and
• a second flexible retainer engaged with said second detector for permitting thermal expansion of said second detector toward said spacer.

A method of assembling an apparatus for insertion into a borehole through a geologic formation to detect the location of a subsurface marker, comprising the steps of:

• placing a first housing section engaged with a marker sensing first detector proximate to an attachable second housing section engaged with a marker sensing second detector;
• attaching said first housing section to said second housing section;
• and
• identifying the deviation of a calibrator engaged with said first and second housing sections, wherein said calibrator identifies an initial attached orientation between said first and second housing sections under selected conditions.

A method of calibrating a subsurface marker detection apparatus having first and second detectors and a calibrator, comprising the steps of:

• placing a first housing section engaged with a marker sensing first detector proximate to an attachable second housing section engaged with a marker sensing second detector;
• attaching said first housing section to said second housing section;
• identifying the calibrator deviation between said first and second housing sections from an initial calibrator orientation between said first and second housing sections under selected conditions;
• and
• correcting the distance between said first and second detectors by calculating a correction factor from the deviation identified by said calibrator.