Tidal phenomena at inland boreholes near Cradock

Abstract

Observations begun in 1905 and carried on at intervals until the present year on a group of wells on a farm at Tarka Bridge, Cradock District, are described in detail.The wells have not been bored very deep, the deepest being 225 feet, but it is obvious that the bores connect with deeply extending fissures, as the waters issue at temperatures of about 80° accompanied by large quantities of natural inflammable gas (methane), while sulphuretted hydrogen is present in notable quantities in solution in the water. The wells are 2,700 feet above sea-level and over 100 miles from the coast.Measurements of the pressure at which the water issues show a remarkable fluctuation, in some respects analogous to the tidal fluctuations of the sea.A series of direct measurements covering several days established the fact that there was a real fluctuation both in the amount of water discharged and in the well- pressure. Continuous records were then obtained over longer periods by means of clock -driven, self -recording apparatus in order to study the precise nature of the fluctuations. The longest continuous record obtained extends over a. period of fifteen weeks. This graphical record shows that the semi-diurnal fluctuations attain a maximum amplitude at fortnightly intervals at times corresponding to the times of new moon and full moon throughout the fifteen-weeks' period.This record further demonstrates the fact that the mean daily water pressure rises with each fall of barometric pressure, and falls with each rise in barometric pressure as recorded concurrently at the farm by means of a barograph instrument. The time scale on this fifteen -week record is about 11 inches per week.Records obtained for shorter periods on a time scale of 14 inches per day were found to be much more suited for detailed critical examination and analysis.. In particular, the record for a certain fortnight during . which the barometric pressure was very steady (and consequently its interfering effect almost negligible) was selected. The times of all the . turning-points were carefully determined in terms of South African official time. The heights of all the turning-points of the curve were also determined in inches.Similarly, the co-ordinates of all the turning-points for that fortnight were determined on the tide gauge records of the South African ports of Cape Town, Port Elizabeth, East London, and Durban.The original Tarka Bridge record for the fortnight was then subjected to a process of harmonic analysis for the purpose of determining the periods of the principal harmonic components of the curve. The particular method used was described by Chrystal as the method of " Residuation," (Trans. Roy. Soc., Edinburgh, vol. xlv., part 2, pp. 385 -7). This method is applicable to comparatively short curves and involves no assumptions as to periods or the causes operating to produce the curve. One by one the various simple harmonic components are disentangled from the compound curve with their periods unaltered but with their amplitudes considerably reduced. In this way components were isolated from the Tarka Bridge curve having the following wave periods :-

1. 12 hrs. 27 min. [probably divisible into 12 hrs. 26 min. and 12 hrs. 0 min.] 2. 23 hrs. 57 min. 3.. An anharmonic residuum which appeared to be the vertical inversion of the barograph curve for the fortnight.For general comparison all these measured data were plotted in parallel lines on the same time scale, and the general resemblance of the well curve to the curves of the coastal tide records demonstrated.The original Tarka Bridge record for the fortnight was then subjected to a process of harmonic analysis for the purpose of determining the periods of the principal harmonic components of the curve. The particular method used was described by Chrystal as the method of "Residuation," (Trans. Roy. Soc., Edinburgh, vol. xlv., part 2, pp. 385 -7). This method is applicable to comparatively short curves and involves no assumptions as to periods or the causes operating to produce the curve. One by one the various simple harmonic components are disentangled from the compound curve with their periods unaltered but with their amplitudes considerably reduced.In this way components were isolated from the Tarka Bridge curve having the following wave periods :- 1. 12 hrs. 27 min. [probably divisible into 12 hrs. 26 min. and 12 hrs. 0 min.] 2. 23 hrs. 57 min. 3.. An anharmonic residuum which appeared to be the vertical inversion of the barograph curve for the fortnight.Component No.2 was obviously not a simple harmonic function. It seemed to me to be composed of several harmonics of approximately diurnal period, but on the scale on which the analysis was being conducted the practical limit of the method had been reached. Accordingly no finer dissection was attempted.The above results may be compared with the well-known principal harmonic components of marine tides. 1. Principal Lunar semi - diurnal tide -period 12 hrs. 25 min. 14* sec. 2. Principal Solar semi-diurnal tide -- period 12 hrs. 3. Three diurnal tides with periods - 23 hrs. 56 min.; 24 hrs. 4 min. ; 25 hrs. 40 min. 9.5 sec.GENERAL REMARKS: The foregoing results seem to establish beyond question that the fluctuations in these wells are to be attributed directly or indirectly to extra- terrestrial causes, but the precise nature of the connection is not by any means clear.The wells are situated over 160 miles from the coast, at an altitude of over 2,700 feet above sea-level. High water at Tarka Bridge occurs about 14* hours after high water at East London, while the lag in the case of low water is nearly 15 hours.The principal conceivable theories to account for the phenomena would appear to group themselves in three classes :- A. Theories depending on the direct gravitative influence of the sun and moon on the land or the underground water. B. Theories depending on the action of the marine tides on the coast loading and distorting the land. C. Theories depending on the action of marine tides in periodically reducing the freedom of outflow of underground water through submarine springsNo attempt is at present made to state or discuss these theories. It is felt that a satisfactory theory can be arrived at only by the co-operative discussion of the subject by astronomers, geologists, and hydraulicians.Tidal wells are known in many parts of the world, but practically all are within 3 or 4 miles of the seashore and at no considerable altitude. The records from the Orisino Bore in Australia do not show the periodicity of marine tides.One case is reported at Lille in France, 40 miles from the coast, but at no great height above sea-level. The evidence supporting the tidal claim of this well is not quite satisfactory.It is believed that there is no other record of an inland well showing fluctuations of true lunar periodicity.