Exploration Geophysics - Volume 15, Issue 4, 1984

The advantages of magnetic gradiometry as an adjunct to total field mapping are generally recognized and a few aircraft have been equipped with gradiometers. These gradiometers are derived from high-sensitivity total-field magnetometer systems that are in themselves subject to certain errors that can usually be tolerated in conventional surveys. However, in a gradiometer, where very large total-field values are differenced, these errors can, in many cases, greatly exceed the basic accuracy required of the system. There are two principal sources of error in inboard gradiometry systems. The first, and most significant, results from the inevitable magnetic interference of the aircraft or from the inability of currently available compensation systems to deal with the magnetic interference adequately. Passive methods of compensation are not sufficiently comprehensive for gradiometry and the active compensation systems currently in use, which were designed for military applications, cannot guarantee compensation at zero frequency (dc) or at the very low frequencies of interest to the geophysicist concerned with long-wavelength anomalies. The second source of error is the frequency-counting technique usually employed to convert a Larmor frequency to ambient total field. The counting process is somewhat analogous to digital sampling at a relatively low rate and as such affords little protection against aliasing from higher frequency interference sources, including components at aircraft maneuvering frequencies. This paper, using examples, illustrates the two types of error. A list of design criteria is presented and several techniques are described for realizing these criteria. Finally, compensation and survey line results are shown for a three-axis gradiometer system in the National Research Council of Canada’s Convair 580. This aircraft uses nonoriented cesium magnetometers, one in each wingtip and one at the tip of the tail fin. Compensations over the entire normal maneuver envelope of the aircraft on all headings give typical standard deviation errors of 3 mγ/m from dc to 1 Hz. Thus, the system is capable of measuring gradients down to nongeologic background levels.

A remarkable property of Fourier transforms, especially applicable to inclined continuation of potential geophysical fields, is the principle of “complex frequency scaling”. Briefly stated, let f(x) be the (gravity/magnetic) field due to a two-dimensional structure along a principal profile and F(ω) be its Fourier transform. The field along a profile passing through the same reference origin and tilted by an angle δ in the counterclockwise direction (+X to −Z) is obtained by inverse Fourier transforming F[ω exp (−iδ)] for positive ω. The complex scaling property and proof of the resulting space-frequency domain relationship are presented, introducing the total field as the analytic signal of the horizontal component. The applicability of the complex scaling principle is illustrated by considering selected geometric models. This principle can be advantageously applied for continuation of two-dimensional potential fields onto inclined planes.

Optically pumped magnetometers are characterized by an optimal angle between their optical axes and the direction of the magnetic field they are sensing. Departure from the optimal angle causes a shift in the Larmor frequency with a corresponding error in the scalar value of the magnetic field being measured. To minimize this error, magnetometers are conventionally either mounted in multiple sensor clusters such that the errors tend to cancel, or they are mechanically oriented to maintain the optimal angle with respect to the magnetic field vector. Recent cesium vapor magnetometers using a split-beam technique have a sufficiently flat error characteristic that they can be flown in a non-oriented or “strap-down” configuration. This configuration has advantages with respect to conventional methods in terms of reduced size and weight and of greatly reduced cost. This paper describes two fixed orientations for a particular split-beam magnetometer and calculates the allowable maneuver envelope for all dip angles from 0 to 90 degrees. It is shown that the residual orientation errors can best be handled by the conventional type of magnetic interference compensation model that must, in any case, be implemented in digital form for high-sensitivity magnetometry or for any type of gradiometry. The National Aeronautical Establishment (NAE) of the National Research Council of Canada has flown strap-down magnetometers in a three-axis gradiometer array in a Convair 580 for several years. Results for the entire normal maneuver envelope of the aircraft, including 30 degree bank turns, have equalled or surpassed those obtained with oriented magnetometers. Several typical maneuver compensation results are presented that gave root-mean-square (rms) (one-sigma) residual errors as low as 0.03 gammas (Γ) for total field and 3.5 mgammas/m(mγ/m) for lateral gradient.

A characteristic of salt domes is that an enhanced heat flux is associated with them, due to the contrast in thermal conductivity between rock salt and typical sedimentary formations. We present a simple analytical model of heat flow in the vicinity of an isolated salt dome, with numerical evaluations for some sample cases. In addition to an enhanced surface heat flow, these models show that on the upper flanks of a salt dome a temperature higher than the regional trend is predicted, thus enhancing hydrocarbon maturation at these depths. On the lower flanks a temperature lower than the regional trend is predicted, thus inhibiting overmaturation. The magnitude of this temperature anomaly depends upon the position of the measurement point, both vertical and radial, relative to the salt dome; roughly speaking it is confined to within one salt dome radius of the salt-sediment interface. The magnitude of the anomaly can be quite large for typical salt domes. These basic ideas are corroborated by observing downhole measurements from a salt dome in the Gulf of Mexico. The good agreement allows us to bracket the height of that particular dome at between 9 000 and 15 000 m, and also permits us to determine that the total sediment thickness overlying the basal salt is between 13 200 and 19 200 m at the location of the salt dome. These results indicate that the presence of a salt dome can have a significant impact on the subsurface temperature distribution and, hence, on the thermochemical maturation of any source rocks or hydrocarbon accumulations in close proximity to the salt dome. In addition to providing trapping structures, a salt dome also provides an enhanced environment for hydrocarbon maturation on its upper flanks and for inhibiting overmaturation on its lower flanks. Such effects should be included when evaluating the hydrocarbon potential of a salt dome province.