Introduction

Our basic survey train consists of a Tropari  instrument, an instrument container, non-magnetic buffer rods, rod couplings and an impact foot.

In addition a coupling, substitute or adapter is used to connect the basic survey train to the drill rods or other lowering fixture, we will refer to this item as the "connector".

A - Typical survey train.

B - For basic survey trains used to survey boreholes with a diameter significantly larger than the survey train itself, centralizers are required.

Wireline Surveying with the Inner Tube Sub

General

The principle in surveying with Tropari Instruments in wireline drilling is to raise the drill rod train 10 meters (30 feet) so that the complete survey train can pass through and extend beyond the bit. This will place the Tropari beyond the magnetic influence of the drill bit and rods.

The connector (inner tube sub) is identical in all dimensions to a core-lifter casing for the wireline size being drilled, except that it has a pin thread protruding on the bottom end which threads into the top of the non-magnetic survey train. It is important to remember that the inner tube sub must be specific to the wireline system being used so that it will thread properly onto the inner tube being used.

The inner tube sub is threaded onto the bottom end of the inner tube (core barrel) in place of the core lifter casing. The normal inner tube assembly used in drilling lowers the Tropari survey train. When the assembly locks in place at the landing ring, the complete survey train will have passed through the bit. Since the train has to pass through the bit, the non-magnetic rods, container and centralizer must have a smaller diameter than the core.

The inner tube sub method with the Tropari instrument is capable of surveying holes drilled at any angle.

Pre-survey Procedures

Prior to the retrieval of the core after a coring run, lift the rods 10 meters (30 feet) to provide the space below the bit to accommodate the survey train. Then, with a watch, time the descent of the overshot from the collar to the spearhead. Mark the wireline at the collar using a rag or flagging tape and slowly lift the assembly until the marker is just entering the wireline winch drum and then stop. Mark the wireline again at the collar. When this mark comes off the wireline winch when the survey train is being lowered, the rate of decent should be reduced so that the end of the survey train will ease gently through the restriction at the bit.

The instrument was designed to provide accurate readings for long periods of time if it is not stressed by impacts during each survey operation. Aside from bottoming out, the movements during the seating of the survey train at the bottom of the hole are the most wearing.

Figure 1

(Not to Scale)

The Survey Train

Figure 1 illustrates the assembled elements of the survey train. All the elements, except for the connector, must be nonmagnetic.

Stock nonmagnetic rods are available in E (diameter 35mm, 1.375 inches) and A (diameter 41.2mm, 1.625 inches) sizes.

If the core size is significantly larger in diameter than the nonmagnetic rods used in the survey train, one or two centralizers can be used to improve parallelism between the axes of the instrument and the borehole.

The survey train is 7.2 m (23.5 feet) long and with a single centralizer, 7.5 m (24.5 feet) long.

In many cases, the rod draw (boom) of the drill is not long enough to accommodate the entire pre-assembled survey train and the core barrel assembly. In these cases, the train may be assembled in convenient sections and hand fed into the drill rod using hitch pins to hold the section at the top of the rods awaiting additional sections (see instruction PC007 Rev 4).

When the survey train is complete, the connector (inner tube sub) is threaded onto the inner tube of the core barrel assembly in place of the core-lifter casing. In underground environments, restricted head-room may require assembly of most of the train at the top of the drill rods.  

Step by Step Wireline Surveying Procedure

Click HERE for a more detailed photographic view of this procedure.

1. Unthread the sections of the instrument container and remove the two (2) foam shock absorbers. If they are not dry and resilient, replace them.

2. Set the required amount of time on the Tropari. Click here for detailed instructions on how to set time on a Tropari. The time required to lower the instrument to the survey depth was obtained during the pre-survey procedures.

3. Place the “timed” Tropari into the instrument container between the two (2) shock absorbers making sure that the arrow on the outer frame points into the borehole. An o-ring or an o-ring and a fiber gasket are used at the container join to provide waterproofing. Tighten the container using the open-end wrenches provided and only arm leverage of one wrench working against the other. DO NOT use pipe wrenches as they destroy containers.

4. Pull the remaining hitch pin and begin lowering the survey train.

5. Watch closely for the first marker to come off the wireline winch and start slowing down the descent rate immediately!

6. Once the survey train reaches the bottom of the hole, leave the Tropari stationary and wait for the set amount of time to pass. When locking time is over, the survey train can be retrieved. Again, use the hitch pin in the hole just below the connector, to hold the survey train and allow it to be detached from the core barrel assembly.

7. Remove the lower section of the survey train at the top of the instrument container, and take it to a dry spot to open and take your readings from the Tropari. Wipe away any water that could run into the container once it has been opened.  

Rod Lowering Survey Trains

Drill rod lowering or insertion techniques are used where instrument retrieval is uncertain in steep holes, or if gravity cable lowering is not possible.

In shallow or horizontal holes, the rod train will sag midway between the connector and the end centralizer. This error is 0.2 degrees or less for all hole sizes smaller than P. An additional centralizer midway in the survey train will limit this error in the larger hole sizes.

When surveying using the conventional “Rod Lowering” surveying method, the survey train is pre-assembled the same as it is in the wireline method. The connector in this case will be a drill rod box thread to A pin thread sub (adapter).

The time it takes to lower the survey train to the bottom of the hole is critical in calculating the running time to be set on the Tropari. To prevent running the rod train into the bottom of the hole, leave 10 meters of drill rod on the stack that was drawn out of the hole.

Figure 2

(Not to Scale)

Figure 2 illustrates the assembled elements of the open-hole survey train.

Cable lowering is restricted to holes with inclinations steeper than 45 degrees.

Since the magnetic effects of the steel cable swivel and the cable itself are smaller than the effects of the drill rods, the instrument only needs to be separated from the steel elements by several meters of non-magnetic elements.

Additional elements below the container (instrument is inside) are for connections and weight for faster lowering in water filled holes.

Other arrangements of the train elements are possible to reduce weight - for example, if hand manipulated rope lowering is used in short holes.

Typically, cable lowering techniques are used where the drill is no longer on the site or, as a fast surveying method in conventional drilling is desired.

In the latter case, some attention should be paid to the integrity of the rocks penetrated by the borehole. Rock fragments may wedge the survey train at the depth making the retrieval difficult.

If a wedged situation occurs, do not break the cable by using excessive force. Once the cable is broken, the best retrieval techniques can no longer be used.

Contact Pajari Instruments for details.  

MAGNETIC CAKE EFFECTS

Drill rods rotating in the hole frequently deposit a grease-rock powder-steel powder cake on the walls of the borehole. This cake is frequently magnetic and affects the azimuth readings of all compass based instruments. We recommend that surveys be carried out at the bottom of the hole as drilling progresses for the most accurate results.