A statewide task force studying illegal immigration got a preliminary snapshot today of the undocumented population in Texas, and what it learned contradicted some popular ideas.

The most surprising to some people was that undocumented workers are less likely to go back and forth between the United States and Mexico than are legal aliens from Mexico.

It has been assumed that the United States and Mexico share a porous border and that undocumented workers who come to the United States frequently return to their families in Mexico.

But the tentative conclusions of a survey prepared for the task force by V. Lance Tarrance and Associates, a respected public-opinion firm based in Houston, are to the contrary. According to findings presented today, an overwhelming majority of illegal aliens surveyed said they had crossed into the United States no more than once in the past five years.

More than 700 aliens have been interviewed in the survey, which is still in progress. The survey firm acknowledged that the margin for error is great at this stage, but Tarrance said his firm expected the general conclusions released today to hold up with further interviewing.

The survey found the greatest concentration of undocumented workers in Dallas and Houston, rather than in agricultural areas of south Texas.

Frank Newport, who presented the results, speculated that one reason was that the major cities contain more job opportunities. But he also cautioned that the sampling was done at a time when there are fewer agricultural workers in the country and that additional interviews might alter that conclusion, as well as show a greater incidence of border crossings.

Newport called the data presented today "very preliminary." They are based on surveys of Hispanic-surnamed workers in seven industries in seven different parts of Texas. Newport estimated that the final results would show between 10 and 30 percent of them are undocumented, although he said he leaned toward the 10 percent figure as the more likely.

In a separate report, two University of Texas professors told the panel they estimate the illegal alien population in Texas at between 600,000 and 900,000, with 750,000 their best guess.

The professors used both the 1980 U.S. census and the 1980 Mexican census to reach their conclusions, the first time Mexican statistics have been used in such a study.

The portrait of the undocumented workers drawn today showed that they are much younger than other Hispanics in Texas and considerably less educated. More than half speak little or no English, the preliminary results show.

But undocumented workers are more likely to be married than are other Hispanics, and about 60 percent of those married are accompanied by their spouses.

Only a fraction of the undocumented workers receives such federal assistance as food stamps, according to the preliminary findings, which also show that the illegal aliens came to the United States to make money, while other Hispanics came for other reasons, such as to be with relatives.