Schmucker, 1970 (Scripps) - MTNet

Schmucker: Geomagnetic Variations 105
Let us consider first the surface conductivities along the profile Tucson­
Sweetwater. West Texas contains a series of Permian basins and platforms
with up to 3000 m of rather undeformed Permian sediments. Green and List
(1963) reported for the Fort Worth basin a total conductivity of 0.24 . 10-6
emu· cm which is equivalent to 60 m of seawater. They inferred this value
from magnetotelluric soundings in conjunction with bore-hole measurements.
We may assume similar conductivities in the Delaware and Midland basins
between the stations Sweetwater and Cornudas.
Inserting this value of T and f = 1 cph in (6. 5) gives 2'ITj/H = 0.005 + i • 0.05
for h'" = 200 km, which suggests that superficial eddy currents contribute
here only a minor out-of-phase component to the internal bay field. Corresponding
estimates for New Mexico and Arizona are difficult. The southern
Arizona Rockies consist mainly of Precambrian rocks of presumably high
resistivity. Further eastward between Lordsburg and Cornudas the profile
crosses a number of Laramide uplifts, exposing Paleozoic and Precambrian
formations, various Cenozoic rift valleys filled with Quaternary deposits, and
interspersed young intrusives. In summary, we have to expect a variable
superficial conductivity distribution, but the mean total conductivity of the
whole area should be smaller than in West Texas.
We conclude therefore that the predominantly in-phase Rio Grande anomalyof
bay-type disturbances has a deep-seated cause. We could postulate,
for instance, distinct conductivity contrasts in a certain subsurface level--an
interpretation that is favored by Caner et al. (1967)--or assume that less
pronounced lateral nonuniformities prevail in a large portion of the upper
mantle. With the second concept in mind we apply the evaluation method of
section 6.6 to the Rio Grande anomaly and derive from the in-phase part of
the anomalous surface field at 1 cph the hypothetical surface of a perfect substitute
conductor.
Figure 48 shows the projected in-phase transfer values for this frequency.
Evidently, the step model from figure 39 provides us with a suitable 1st approximation
for the disturbed deep conductivity distribution. It explains
properly the increasing Z -variations east of the Rio Grande, but it predicts
a rather smooth decrease in Z toward Tucson contrary to the observed sharp
reduction and partial reversal of Z between COR and LAC. A corresponding
disagreement exists between the observed and calculated hp-values for the
anomalous part in D.
In order to improve the fit we develop the difference between the empirical
zp-values and the theoretical zp-values of the step model into a series of
three spatial harmonics with 2'IT/ka = 1080 km as fundamental wave length,
using 25 grid points. The corresponding difference harmonics for hp follow
by interchanging the sine and cosine coefficients according to (3.34). This
assures us that the harmonics describe a field of internal origin and that
their depth dependence is exp(nkaz) as indicated in (6.20).
The difference field is extended in this way downward and superimposed
on the field of the step model, yielding with Q" = 1 - 2Q ;::: 1 the slope of the
internal field lines at any subsurface pOint (6.21). An entire field line is