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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 66 (1982)

Issue: 10. (October)

First Page: 1697

Last Page: 1697

Title: P-shooter--A Fast Seismic Source for Shallow Exploration: ABSTRACT

Author(s): Gildas Omnes, Philippe Robert

Article Type: Meeting abstract

Abstract:

The P-Shooter is a new, light-weight source developed by CGG in collaboration with Shear Wave Technology Inc. Though energy is provided by a mass falling vertically, the source differs from a simple weight-drop system by its coupling device and the additional acceleration provided by a spring. It is designed to work at a very fast rate.

In operating mode the weight travels up and down a vertical tower and is guided between two rails. During the drop it is accelerated by a spring until it hits a baseplate that is coupled to the ground by part of the weight of the vehicle. After the impact and before any rebound can occur, the weight is picked up by a cable and lifted back to the top of the tower ready for a second drop.

The weight is adjustable between 250 and 500 lb. The spring provides a maximum pull of 800 lb. The combined effect of the spring and a 500-lb weight provides a total energy estimated at 9,700 joules and an impact velocity of 30 ft/sec. By comparison, the same figures for a free fall drop would be only 5,400 joules and 23 ft/sec. The time interval between two successive drops is about 3.5 seconds.

Good coupling is very important; on the P-Shooter it is achieved by a special baseplate, which is separated from the impact surface by a ball joint. This allows the baseplate to keep good contact with the ground. The baseplate is decoupled from the rest of the vehicle through rubber springs, and a hydraulic system applies a constant downforce to it. The coupling baseplate also prevents damages to a road surface.

The whole system is mounted on the back of a 4 by 4 truck. The tower can be moved forward, backward, and to some extent laterally, so that it can be vertical even when the truck is on a slope.

The following seismic properties were observed during experiments carried out in Colorado.

1. Signatures recorded on downhole phones, from 400 to 1,000 deep (120 to 300 m), near the Colorado School of Mines have an amplitude spectrum peaking at about 45 Hz, with a 12 dB attenuation at 100 Hz and a fairly steep drop above 100 Hz.

2. Bandpass analysis on a stacked section confirms that the spectrum extends up to 120 Hz at 500 ms two-way time.

3. The source is perfectly repeatable when a sufficient amount of downforce is applied to the baseplate.

4. On a 1,200% stack obtained in southeastern Colorado and shot with 10 drops per station, penetration is good down to 500 ms, below which signal strength becomes weaker.

Assuming that 20 drops per station would be enough in most areas for targets at less than 1 second, and given a rate of 3.5 seconds per drop a shot point can be completed in 2 minutes (including moving time). This means that on a normal working day, production may approach 200 shotpoints per day. With the 55-ft (17-m) spacing common in this type of work, production would be 2 mi (3.2 km) per day.

The P-Shooter is particularly appropriate for surveys that do not require resolution above 120 Hz. Since it is an inexpensive source usable with a fairly small crew, it can give good quality shallow data at a relatively low cost per mile.

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Copyright 1997 American Association of Petroleum Geologists