Instruments \ Seismic \ Seismographs \ 

Description:
Before you is one of the most unique and historical objects in the SEG museum. It is a seismograph invented and developed in Germany by Ludger Mintrop, a distinguished pioneer in the development of exploration geophysics. Very likely it is responsible for the first discovery by the seismic method. This was the discovery of the Orange saltdome in the Texas Gulf Coast. Except for a battery to power an electric lamp, it is entirely mechanical.

This seismograph was designed for the seismic refraction exploration method. The traveltime of seismic waves from a dynamite blast, in a borehole several feet below the earth's surface, to the seismograph is recorded. Velocity of these waves increases with subsurface depth and, thus, the greater the distance between the source (that is, the dynamite blast) and the seismograph, the greater the penetration depth of the waves.

Any abnormal change in velocity along the travel path of the waves will alter the traveltime and, thus, indicate an anomalous rock type. The mechanical seismograph was employed in the 1920's and early 1930's (the initial period of geophysical petroleum exploration) almost exclusively for the detection of salt domes in the Texas-Louisiana Gulf Coast region. Here, salt has penetrated overlying sedimentary rock layers to form domes. Porous rock layers that are pierced and bent upward by the rising salt are excellent traps for upward migrating fluids that are halted when encountering the nonporous salt. Seismic wave velocity in the salt is as much as two and one-half times that in the surrounding rock layers. The reduced traveltime of seismic waves traversing the salt dome was readily detected in the seismic refraction method, especially for shallow domes that penetrated to one or two hundred feet below the earth's surface. By 1930 the majority of salt domes in the Texas-Louisiana Gulf Coast region penetrating to shallow depths had been discovered by the refraction method, by the torsion balance (also exhibited in the museum), or by a combination of the two.

The Mintrop mechanical seismograph consists of the two units shown, a detector of seismic waves on the right and a recorder of these waves on the left. The detector consists of a metal ball suspended by a spring.

When the detector is planted on the earth's surface, vertical movement of the mass due to arrival of seismic waves deflects horizontally a mirror. The mirror is attached at the apex of an inverted cone which is an upward extension of the mass. A beam from a light source in the recorder directed to the mirror is reflected back to moving light-sensitive paper; thereby, recording motion of the detector's ball and, thus, the seismic waves. The instant of the dynamite blast was radioed to the recorder and onset time of the seismic wave was measured from timing marks on the recording paper.

Commencing in the early 1930's, mechanical seismographs were supplanted by electronic recorders and electromagnetic transducers, called seismometers, which are several times more sensitive. These were used in the seismic reflection method which proved much more effective than the refraction method in the search for petroliferous subsurface structures.

The Mintrop mechanical seismograph surely is a most important event in the colorful history of exploration geophysics. It was the first commercial seismograph and its successful application was the catalyst for initiation of seismic exploration world wide.

For more information, see the article:

The Mintrop mechanical seismograph, The Leading Edge, Sept., 1996, p. 1049-1052.