Harvard Professor Explains New Method of Detecting Oil Fields and Minerals--Electricity Replaces "Divining Rod"

The following article was written expressly for the Crimson by Professor D. H. McLaughlin, instructor of mining engineering in the Harvard Engineering School. He explains the development of "geophysical prospecting," a new field in mining engineering to be discussed at the next meeting of the American Institute of Mining and Metallurgical Engineers, to be held in New York City early in February.

"Geophysical Prospecting", a new development in the technique of mining and geological engineering, is to be one of the most active subjects presented for discussion and study at the meeting of the American Institute of Mining and Metallurgical Engineers in New York City early next February. All means of securing evidence concerning conditions existing in the solid rock ahead of actual exploration by operations are of course of vital interest to men in this field and geologists and others interested in the development of natural resources have steadily been working on the improvement of methods for estimating the nature of bodies concealed in the crust of the earth. No methods have been found or are likely to be found that will give a final and complete answer to this problem. encountered in the complex conditions that exist in nature, but with constant study more and more ways of scanning critical evidence of various sorts are being developed, so that today one "guess" about what is ahead of the pick is likely to be considerably more accurate than in the past, and there is reason to hope that there will be still further improvements in the future. This hope is to some measure reassuring, for otherwise with the rapidly increasing rate of depletion of our natural sources of supply and the increasing difficulty of finding new rich deposits as the area of unexplored regions gets smaller and smaller, we should be hard put to it before many decades to maintain the steady output of metals and oil needed for our industries.

Masses Vary in Conductivity

Irregularities in the earth, such as masses of rock of lower or higher density than the surrounding material, or of higher or lower electrical conductivity influence the natural physical fields of the earth or artificial physical fields that can be found on the ground in question. The normal fields can be calculated accurately, and by measuring the departures from the normal, evidence is secured that is of possible service in estimating the properties of the mass concealed in the earth that is responsible for the departures.

A body of sulphide ore, for example, generally has higher electrical conductivity than the surrounding rock. If an electric current is passed through the ground, the flow of electricity will tend to concentrate in the material which offers least resistance to its passage, and valuable evidence concerning the relative conductivity of the rocks that comprise the area can be gained by observing with suitable devices the distribution of current. A convergence of flow lines into a good conductor can be readily detected, and even the effects of better conducting material at considerable depth can often be estimated with a fair degree of assurance. Of course, all masses of rock in the earth that are good conductors are not necessarily bodies. Barren graphitic slates rock beds soaked with salt water, or basic dikes might give effects not unlike more valuable bodies and skillful geological observation and interpretation of the evidence in terms of what is reasonable to expect must be made before satisfying conclusions can be drawn.

Delicate Measurements Now Possible

One of the most delicate measurements made by the new methods is in the investigation of irregularities in the gravitational field of the earth by means of the torsion balance. Masses of density different from the surrounding rock, tend to deflect the direction of gravity and the change, though almost infinitesimal, from the standpoint of ordinary human experience, is capable of measurement by observing the amount of rotation of a light beam appropriately weighted and suspended on a thin platinum wire. Ore bodies themselves are rarely large enough to cause detectable variations in gravity, but large features such as heavier rock in cores of folds, dome-shaped masses of salt, or bleached cavernous ground can often be recognized by the anomalies caused by them in the gravitational field, and critical evidence bearing on the occurence of oil or ore bodies, may be secured for the use of the geologists.

Geophysical Methods Effective

The geophysical methods are not simple. They require thorough preparations in physics and geology, not only for laying out the work, manipulating the instruments and securing the readings, but above all in interpreting the results. They are in no sense "dividing rods," available for the use of the untrained man. They are merely instruments for the measurement of physical fields that, under favorable conditions, give results of significance and value to the geologist. The geophysical methods are of undoubted importance in increasing the effectiveness of geological observation.