M/V American
with
Piggy-back shallow-water barge

Delta Exploration Incorporated
Jackson, Mississippi, USA

- ELECTRONICS and INSTRUMENTATION -

The Texas Instruments DFSIII digital seismic recording system.

A closer view of the main system units

Roll-along switch used for shooting with (static) bay-cables

As shallow-water shooting progressed, bay cable sections (containing hydrophone groups)
were removed from the near end of the cable and "swung" (and plugged-into) to the far end of the cable.
For each shot the roll-along switch (selecting 24 hydrophone groups) was indexed-down
one position, thus simulating a cable moving over the seabed.

Format & System Controller Module

Internal view of the Format & System Controller Module

Binary Gain Amplifier (BGA) Module

Internal view of the Binary Gain Amplifier (BGA) Module

Seismic signals fade rapidly in strength with time elapsed after the shot - as echoes from
deeper & deeper subsurface layers are received. Hence the seismic signal amplifier must automatically
increase it's gain with time to keep the amplified signal amplitudes within a useful range for viewing, plotting
and subsequent computer processing. The BGA module performed this function.
Each amplifier channel independently managed it's gain in binary (multiple of 2) steps.
Each BGA module had 12 amplifier channels so 2 modules were required for a 24-channel hydrophone cable.

Some of the DFSIII electronic circuit boards.

Note the early plated-through-hole pc board technology. No modern surface mount parts here. The
wires of the components push through the boards and are soldered on the underside of the board.
Some round-can integrated circuits with heat sinks can be seen.

Cable Depth Indicator (Top)
Pressure transducers in the Streamer cable sent depth data to these indicators.

Motorola VHF radio & Seismic Source Synchroniser (2nd from top).
This received a time-break from the DFSIII Format module and sent it as a coded
VHF radio signal to a receiver on the shooting boat where it triggered the 3000V blaster
to detonate the charge of Aquaflex detonating cord.

Lamba 115 volts ac to 12 volt dc power supply for the DFSIII (bottom).
The DFSIII was designed to work on 12-volt batteries in the field.

SDW-300 24-trace Camera

This camera created 24 parallel "wiggle-traces" of seismic data on a paper strip.
The wiggles were created by 24 mirror galvanometers which needed occasional adjustment.

The bottom section contains the tank & pump for the Freon-toner mixture.

The camera needed frequent cleaning of hardened toner deposits, also toner & paper replacement.

Motorola X-Band Range Positioning System (RPS)

This system transmitted radar pulses to transponders on shore - which returned the pulses.
The radar pulse transit time was measured and converted into range data to each shore station.
The transponder return pulses could be monitored on the marine radar set.
In Gabon, the heavy morning coastal mist blocked the radar signals until the sun dispersed it.

The X-band radar transponders. Originally developed by Motorola
as FFI (Friend or Foe Interrogation) transponders for aircraft used in the Vietnam war.
The transponders were installed on shore in high places of known location -
which included high masts, trees, rooftops of buildings, hilltops, etc.
The shore stations had large batteries which allowed unattended operation for days.

Bench-testing a radar transponder.

Some of the instrument-room test instruments
(frequency meter, pulse counter, oscilloscope, Mororola radio VHF test set)

The 3000 volt dc blaster - connected to the firing line through a high-voltage junction box.
This voltage was needed to initiate the ICI detonator underwater.

The on-board generator powering the DFSIII system was found to be too
noisy in shallow water operations - the acoustic noise was swamping
some of the weaker seismic returns. So I installed a battery system to
power the DFSIII while recording on-line, with change-over switches and
battery charger (see photo above)

Testing the Seismic Source Synchroniser

(Sent coded radio command to the shooting boat to trigger detonation of charge)

(Photos - Robert J. Horne)